If you are just starting your journey to buy a tankless water heater, you are likely asking yourself, “What size tankless water heater do I need?”
It is probably the most important question when wondering how to choose a tankless water heater.
Before that can be answered, it takes a change of mindset.
People are used to thinking in terms of capacity. Like, a hot water tank for 5 people will require at least a 60-gallon tank.
Since tankless water heaters supply endless hot water, you don’t have to worry about capacity, but instead, think in terms of water flow rate. Gallons Per Minute (GPM) is the key when it comes to sizing tankless water heaters.
The lowest price you can get for the highest GPM is easily the Takagi T-H3-DV-N that will give you a whopping 10 GPM. You may not need that big of a tankless water heater so make sure you read the whole article.
| # | Preview | Product | Rating | Price | |
|---|---|---|---|---|---|
| 1 |
|
Rinnai V65IN Tankless Water Heater, Large, V65iN-Natural Gas/6.5 GPM | 650 Reviews | $680.40 | Check It Out On Amazon |
| 2 |
|
Rinnai Outdoor Tankless Hot Water Heater, V65EN-Natural Gas/ 6.3 GPM | 622 Reviews | $680.40 | Check It Out On Amazon |
| 3 |
|
Camplux Tankless Water Heater Natural Gas,High Efficiency Residential Natural Gas Water Heater with... | 21 Reviews | $329.99 $302.07 | Check It Out On Amazon |
| 4 |
|
Rinnai RU199iN RRU199iN, RU199in-Natural Gas/11 GPM | 215 Reviews | $1,666.50 | Check It Out On Amazon |
In this article:
How many gallons per minute (GPM) do I need?
The flow rate of the number of fixtures running at the same time is your Gallons Per Minute (GPM). So, you don’t really have to think of how many gallons you use over the course of the day. Only the gallons used at the same time.
For example, let’s take the morning as that is when most people are taking showers or doing laundry.
Let’s say that you have a two bathroom house with 4 people. In the morning you have two showers running and somebody put in a load of laundry at the same time.
You have to figure out how many gallons each of those fixtures uses per minute and add it up.
How many GPM is a shower? How many GPM is a dishwasher? Or washing machine?
You can check your specification sheet on your manual for the fixtures. Or, you can take a look at this handy cheat sheet below to get an idea.
- Shower – 1.5 GPM for low flow to 3.0 for standard
- Kitchen Faucet – 2 to 3 GPM
- Bathroom Sink – 0.5 to 1 GPM
- Dishwasher – 1.5 to 3 GPM
- Washing Machine – 2 to 3 GPM
- Bathtub – 4 to 6 GPM
Take a week or so and write down the times that you have multiple fixtures running at the same time and which ones they were. At the end of the week, you will see some patterns and understand exactly when your peak water demand is and how many GPM you will need from your on demand water heater to cover that.
- Exclusive Water Savings technology conserving up to 1, 100 gallons per year
- 157, 000 BTU (gas consumed per hour)
- Exclusive Hot Start Programming minimizes cold water burst by staying in...
- Recirculation pump ready kit ready
- For use outdoors with freeze protection up to -30 degrees
Tankless water heater sizing calculator
Before we get to how to use the calculator to size a tankless water heater, let’s talk about the factors that go into understanding what size inline water heater you need.
We already covered GPM.
Now you need to understand what temperature rise means.
Temperature rise is how you will figure out what the actual GPM that the tankless water heater is.
When you see the GPM rating of a tankless, it is the best case scenario. In certain circumstances, you won’t get that water flow and still have the water come out hot.
What happens when the GPM you need is higher than what the tankless water heater is capable of is you end up with endless lukewarm water.
This is why sizing a tankless water heater is so important.
To determine your temperature rise, you need to figure out the temperature of the water coming into your home.
Take a look at this groundwater temperature map to get
As you can see, the temperatures vary wildly by location.
Now here is the fun part. In the sizing calculator below, enter the temperature that you expect your incoming water to be.
Then in the next field enter how hot you want your water setting to be for outgoing hot water.
Hit the button to calculate it and there is your water rise temperature.
Once you have this temperature rise number, you can look at the specification sheet of the tankless water heater you want to buy and see what the GPM is rated for based on that number.
How to determine how big of a tankless water heater I need?
Let’s recap.
What size tankless water heater depends on your GPM at peak hot water demand. And your temperature rise which is determined by your incoming water temperature subtracted from your hot water setting.
As an example, I will show you what came out for me. I live in Boston so my groundwater temperature is around 47°F.
I have small kids at home so I keep my water set around 105°F. This is a safe temperature and it also will keep my gas bill down. My temperature rise then is 58°.
Our peak water demand is a shower, washing machine and faucet running at the same time so I calculated my GPM as 6.
Since I live in Boston I should be looking at a gas tankless water heater since they work best in cold climates. I wrote a really detailed article reviewing the best gas tankless water heaters for different sized households that you should read by clicking here.
So, I need to know what size tankless water heater is best for a family of 4.
Looks like the Rinnai RUC98iN from Amazon is going to be best to suit my needs. It is listed as being 9.8GPM, but since my factors are not the best case scenario, it drops down to 6.7 GPM. This will probably rise in the summer as my incoming groundwater temperature will also be a bit warmer.
If you live in a cold climate that is going to be a huge factor in which tankless water heater to go with. Read up on my full article on how to use a tankless water heater in a cold climate with 8 simple tips.
- INDOOR installation only. Fuel Type: NATURAL GAS.
- Concentric or PVC venting option
- Up to 0.96 Energy Factor/Up to 0.92 Uniform Energy Factor
- The space-saving design allows Indoor Installation type
- Ultra low NOx emissions
How to choose a tankless water heater
There is more to just the size of the tankless that you need to think about. There are a few different kinds to choose from. Each has its own purpose that it is designed and engineered for.
Here are a few things to consider before buying.
Gas vs Electric
It’s my opinion that most households should use a gas tankless. Gas tankless water heaters are best for most homes since they deliver the highest GPM. To get hot water to the whole house using only one unit, then this is going to be the way to go.
Electric units are good when you don’t have access to natural gas or propane.
The benefit of electric tankless heaters is the initial cost. They cost much less to buy than a gas version.
How Much BTU?
BTU is a measure of how much energy is needed to heat water.
The formula goes like this: 1 BTU is equivalent to the amount of energy it takes to raise 1 lb of water by 1°F. The higher the BTU, the more water it is capable of heating. To make sure you are getting the water hot enough you need to have a high BTU for high water demand.
If you need to provide a whole house with hot water and your peak water demand is high, then you’ll need up to 200,000 BTU.
If you only need a couple of low flow fixtures at once, then you can go with lower BTU since it will consume less. Something around 140,000 BTU is more than enough for a small household with low water demand.
Whole House vs Point Of Use
Most people will opt for one unit to provide hot water for the whole house. In many scenarios, this makes the most sense.
However, there is a case to be made for having multiple, small tankless water heaters at each point of use.
For example, if you have two bathrooms, you can put a unit under the bathroom sinks and it will provide hot water on demand for the sinks and showers in each and even a washing machine. Then another, larger one can go in the kitchen to provide hot water for the kitchen sink and dishwasher.
These point of use tankless water heaters can provide a few GPM which will be enough for most applications and they don’t cost nearly as much to buy or install.
I wrote a buyer’s guide to the best point of use tankless water heaters that you can read by clicking that link.
Installation
The cost to install a whole house gas tankless heater can be quite expensive and may play into the decision as to which type of tankless heater to buy.
The factors that contribute to the cost are the distance from the heater to the fixture and where you need to vent. In some cases, you may be drilling through a brick wall which increases the cost. You can read all about your installation options by clicking that link.
Electric units cost far less to install since they don’t need to be vented. There is no exhaust since the heating element is a type of anode.
You’ll need an electrician to help, though since they need to be hard wired.
Can you install a tankless water heater yourself? In most cases, you definitely can. It’s a matter of following the instructions. But, it may make sense to hire a professional to make sure you don’t void your warranty if you install it incorrectly.
FAQ About Tankless Water Heaters
Technically, no, it can’t run out of hot water. When you have the right sized tankless for your hot water demand, you will have endless hot water. However, with that said, if your tankless heater can’t keep up with your demand, you will have and endless supply of lukewarm water as it will have to lower the temperature to keep up. This is why it’s so important to buy the right unit for your needs.
No, they don’t. If your unit is close to your fixtures then you will get hot water faster than with a tank system placed far away. However, if you want to get instant hot water you will need a hot water recirculating pump.
If you take care of your tankless water heater and do yearly flushing of the heat exchanger, then you can expect your tankless to least up to 20 years.
Yes, you can use a tankless water heater even with well water, but you will need to use a sediment filter to make sure it doesn’t get ruined. You should also use a filter to deal with hard water from a well.
Are tankless water heaters worth it?
After sizing a tankless water heater and evaluating your needs, it may become clear that a tankless water heater is not the way forward for you. They are not for everybody after all.
In the right circumstances, a tankless water heater is the best way to go. If you have heard negative reviews, then you probably are dealing with somebody who didn’t properly evaluate their needs and got one that didn’t offer the GPM necessary. If they had sized their tankless heater well, then they would have gotten the one with a higher GPM and more than likely would be very satisfied.
I hope that this article on how to size a tankless water heater was helpful and will put you onto the best one for your needs.
Any questions? Drop a comment below and I’ll do my best to help you out!
9
- 9Shares
YourH2Home is your go to site to get the most up to date information on all your water system needs.
I have a Family of 7, We all take Showers and we have 2 Sinks. What size Tankless Hot Water Heater do I need? Do you have a Preference?
Hi Darrell, sorry for the late reply. It is hard to say which I prefer without more information. With 7 people in a house, though I am sure you need something robust. In my opinion the Rinnai RUC98iN will probably take care of your needs. Just click that link to go to Amazon to check it out. It is a whole house gas unit. You may not need one with quite that much GPM, but again, without more information, it’s hard to say. What is the average temperature like where you live? Do you know the temperature of the groundwater?
I live in Indiana and have a two story house with a basement. My water heater is in the basement. How well will a tankless gas heater do for running two showers on the second floor, pumping 2 floor up?
Jeremy
There won’t be an issue with getting the water to your second floor. It will be just like with a tank system as far as that goes. Now, as far as running 2 showers it depends on the factors I outlined in the article. IE what your incoming water temperature is and what the GPM of the model you are looking at. 2 showers at once shouldn’t be an issue for most tankless water heaters. Using the calculator it looks like your temperature rise is 53°F so you can likely get up to 7gpm. That puts you in a good spot for just about any tankless since you’ll only likely be needing 4 GPM at the most if you are only using 2 showers at the same time.
Looking at building a new place and there’s just the wife and myself. We live in Wisconsin so we get some pretty extremely cold winters. What would you recommend with regard to Btu’s and GPM?
Since you will only have 2 people using the hot water that opens up a lot of possibilities. You don’t need to go for the biggest tankless with the highest GPM. For me to answer that I really need to know what your temperature rise is and how many GPM you need at any one time. In Wisconsin I probably wouldn’t go less than 6 GPM and would definitely go with a gas tankless. Check out this list of the best gas tankless here https://yourh2home.com/tankless-gas-water-heater-reviews/. Any one of those is going to get the job done for you.
Hello, my temp rise is 40, I have a small home with 1 bathroom so most I would possibly run at once would be a washing machine, shower and a faucet. I need an outdoor gas unit. Any suggestions? and say I need (for example) a 6gpm unit and I put in a 9gpm unit. Would that cause a problem or would it just heat faster and give me more flexibility on hot water demand?
Hi Donaven
You won’t have any issues if you have a bigger unit than you need. But, it also won’t heat the water really any faster. I always say that it is better to have it and not need it than need it and not have it. So I say go for the bigger one since you never know. As far as which one I recommend, I would say it’s a toss-up between the Takagi T-H3S-OS-N and the Rinnai RL Series HE+. Either one is going to give you more than you need but also the flexibility just in case.When you click on those you can also choose ones with lower GPM too if you don’t want to spend for the higher GPM one. Let me know what you end up with and how it works out!
Hope this helps,
Nick
Need to replace my 15 year old propane rinnai 2532 tankless water heater that supplies my infloor heating. The current one is 8.4 gpm for approximately 1200 sq feet of floor. Have another 8.4 same model that runs the water for the rest of the house. Funds are minimal, hoping that I can use a smaller unit to run the floors. Any suggestions or help would be deeply appreciated.
Hi Jennifer
You can definitely use a smaller one to run the floors, but you’ll need to make a closed loop. And add a recirculating pump to keep the water flowing. If you need it to also supply hot domestic water then that changes things, but it sounds like you already have the other one for that. Take a look at this article that addresses infloor heating specifically, https://yourh2home.com/water-heater-vs-boiler-radiant-floor-heating/
I hope that helps!
Hi. We have two people in Colorado (near Denver) and we have solar (with more capacity than we currently use) so I am considering swapping out our 15 year old gas tank heater for an electric tankless. I figure 6 GPM is about right. What would you suggest? Any help would be appreciated. Thanks.
I am not too sure how the electric will work with solar. Before I can recommend one you’ll have to let me know what kind of breaker you have. These electric tankless have quite a big draw and your system might not be enough for it. Take a look at this article about the best electric tankless to get an idea of which one, if any, will be able to work for you. Let me know if you have any other questions and thanks for reaching out!
I would like something for the detached laundry room only. I wash a couple loads a week. Any suggestions
In this case you just need a small point of use tankless water heater. I would recommend an electric one as they are far less expensive and should still give you more than enough GPM for what you need. You can take a look at the ones I recommend here https://yourh2home.com/best-point-of-use-tankless-water-heater-reviews/. Thanks for reaching out and let me know which one you end up with!
I live in Canada my high gpm demand would be around 7-8gpm, inlet water temp around 37f in coldest months, would it be viable for me to install an electric tankless heater? we currently utilize a 60 gallon electric tank heater that seems adequate to our needs. The space saving of install and efficiency has triggered my query of the tankless units as my current tank is reaching end of life.
To replace a 60 gallon tank system with electric is not going to work. You will be lucky to get 1GPM out of it with the kind of temperature rise. What you can do is buy several electric ones and use them at the point of use. One in each bathroom, one in the kitchen, etc. As far as space saving, even the biggest gas ones are a fraction of the size as a tank anyway. Especially a 60 gallon one. Would you not consider a powerful gas one that will get you closer to 5 GPM with your temperature rise?
I am purchasing a house in New Orleans that needs a new water heater – the old one is in a detached garages . 2 bed, 2 bath. There is two of us living in the house .
Would you suggest a whole house unit vs a point use unit? Thanks
Hi Kazli
Since you are living in an area with warm groundwater and only have two people living there I would recommend going with a whole house. You just don’t need one with a high GPM. If you get a point of use, you’re only going to get hot water in that one spot and will either need another one for the rest of the house, or have a tank system to get hot water elsewhere. Use the calculator to see how many GPM you need. I doubt you will need one bigger than 6.6 but check just to make sure. Then go check out my list of the best gas ones or the best electric tankless to see which one is best for your needs and budget. You don’t need to buy the biggest and most expensive one based on the size of your household. Thanks for reaching out and let me know if you need any help!
Hello! First of all thank you for taking the time to give advice on tankless hot water heaters!
We are converting a 600sqft garage into a mother in law apartment in Central Florida, with a shower that has 2 shower heads running at once, 1 bathroom sink, 1 kitchen sink, 1 dishwasher and 1 clothes washer. For 2 people, the only things I can see running at the same time are the 2 shower heads and maybe the dishwasher at the same time. From all the articles I’m seeing, that would max at 7.5 GPM but when I actually look at the units available that seems towards the high end of the spectrum, and I have a hard time believing a tiny garage apt would need all that! Any advice would be appreciated.
Hi Blair, thanks for reaching out! 7.5 GPM for 3 fixtures running at the same time definitely sounds very high to me. Have you looked at the manual for those fixtures to see the exact GPM? If you don’t have the manuals then I would run a test to see exactly what your GPM is. You can turn on the faucet and let it run into an empty gallon milk jug or any other vessel that has the volume in gallons. Then you literally time how long it takes to fill. The dishwasher obviously can’t be measured like that though. Take a look around online and see if you can find the exact GPM it uses.
Just guessing, but I would think that even without using low flow fixtures that it shouldn’t be more than 6 GPM. I could be way off there so definitely check to be sure. If it does prove to be 7.5 then finding the right tankless that covers that plus some to be on the safe side shouldn’t be a problem. In Florida, you have warm groundwater so you get a high water flow rate from a tankless. Check out this article to see which unit will be right for you once you’ve confirmed your water demand. I suspect that you could even use an electric tankless for a small apartment, but again you need to confirm your demand first. Here is my list of the best electric tankless.
Thanks again and good luck. Let me know how things turn out!
Nick
My family owns a summer home in Cape Cod, MA that has two bathrooms with showers only (no tubs), kitchen sink, dishwasher and washing machine. The house is not used in the winter, only late spring through fall. We are looking at replacing the gas water heater with a gas tankless heater. I calculate 7-8 gpm but not sure what BTU’s are needed. Can you offer any suggestions?
Hi Mark. Sorry for the late reply. Your BTU needs are not that great. You could easily get away with a unit that uses 140,000 because it looks like your max BTU at peak usage will be around 120,000. That said, you do want a good size unit to make sure your water is hot enough. Even in summer, the groundwater may be coming in cold so your temperature rise does affect the GPM you need in addition to higher BTU. And with the higher GPM units comes lots of BTU. Usually 180,000 to 199,000. Which is more than you need in that regard but you’ll ensure that you get the proper GPM based on the temperature of the incoming water.
Hope that helps!
If you have any other questions let me know! By the way, I love the Cape and am so jealous that you have a place! I used to spend time in Truro and Wellfleet and need to get there more often!
All the best, Nick
Hello, we’re replacing a 24 yr old natural gas tank water heater with a tankless. We’ve got 5 people total, 4 girls that like long hot showers. So looking run 2 showers(4 total shower heads) and either washer or dishwasher at the same time. We live in MI so a 78+ degree rise. Any suggestions on size? Also, we have well water, and I’ve read that we should hook up a softener to help make the water heater to last; do you agree?
Hi Amber. That’s quite a temp rise so you’re going to need the highest GPM you can get to cover your needs. Assuming you don’t have low flow fixtures, you’re going to need about 7GPM. Now, the highest GPM you can get is from the Takagi T3 at 11 GPM but with that kind of temperature rise it drops down to about 6 so you might make it if you are using low flow fixtures. Or, just keep the temperature lower than what you had when you came up with the 78° temp rise. As far as a water softener, you need to do ao hardness test first to see exactly how hard your water is. You may not need a softener, but it is probable. I would look at this electric water descaler so you don’t end up with soft water that isn’t pleasant to drink. I think it would be a shame to have healthy well water and then remove everything that is healthy about it.
I hope that helps!
Nick
Thank you for this information! Based on all the posts, etc. I think I would need at least 10 GPM – I have a family of 4 and three showers that would be running at once (mornings!) followed by shower #4 (that is always cold right now with a 50 Gallon hot water heater).
We are in NW Arkansas – so about a 55 degree ground temperature rising to at least 104 degrees.
Is there an Electric model that you would recommend ?
Hi Davie! I am so glad you found the guide useful. I put a lot of time into writing it so it is nice that it was worth it! Anyway, honestly, it is going to be difficult to find an electric that can handle 3 showers at once with cold incoming water. Though keeping it at 104° does help. I would suggest getting two different ones and putting them where you need them. You can see the big electric tankless reviews here or you can look at some small point of use ones that I reviewed here. If you opt for two you also get the benefit of having the tankless closer to the fixture so you get hot water faster and waste less water.
If you have any other questions let me know!
Nick
Thank you for the information! We are building a house that will have 3 dishwashers, 3 washing machines, and 6.5 bathrooms. 57 degree ground water. It is likely the dishwashers and washing machines will all run at the same time when the cleaning lady is there. There are 2 adults and 3 children that will live in the house. It’s unlikely we will shower when the dishwashers and washing machines are going, but it’s possible one shower could be going at the same time occasionally. It’s very possible 5 showers could be going at once in the morning or at night or maybe 3 baths and 2 showers. We were planning to have 2 75 gallon tank hot water heaters, but are considering tankless. Do you think we can make this work with 2 tankless? The smaller footprint would be very nice. If so, what model would you recommend? The builder likes Rinnai. We also want instant hot water and were planning on 2 circulating pumps with the tank hw heater. Is this feature available on tankless?
Hi Jared
I’ll be honest, I am not sure that 2 tankless will get the job done. You may need a third. You may be alright with two with high GPM, but with all of those fixtures, you may also have to plan for the worst case scenario and not what you expect as it could change. Takagi are very good tankless and do well with cascading, which is when you have multiple units attached to each other. Take a look at three that I think are pretty good by clicking this article. Out of those, the Takagi T-H3-DV-N is going to be your best bet.
Your builder is right to like Rinnai as they make fine tankless in which case you’re going to need the Rinnai RUR Series Sensei SE+ which gets 11 GPM (though not for your incoming water temp) Click that link to see it on Amazon with all the specs.
If you have any questions let me know!
Let us all know what you end up getting and how it works out for you!
Nick
Thanks Nick! I’ll check out the Takagi and let you know what we decide to go with. If we end up going with 3 tankless, do you still recommend that same model Takagi or Rinnai you mentioned or would you recommend a smaller model of each?
Hi Jared
What temperature do you want to set your water to? This way I can get a sense of the temperature rise and be able to advise you better. If you get the Rinnai that gets 11 GPM in the best case scenario, but you have a temperature rise of 47 then you will get around 8 GPM. I actually just reread your original post and realize that I messed up the calculations. I thought you had said that you would have the appliances and the 5 showers going at once. Ok, so now that I see my mistake, and you only need the 5 showers going at once, It looks like you’ll need a bare minimum of 10 GPM but probably more like 12 or more. You can definitely do it with two smaller units and not three. I would say to go for the higher GPM as you have the capacity if you ever do need to run the other appliances while many showers are being used at once.
I would go for the same Takagi I recommended as it costs much less than the Rinnai RUR Sensei and you still get 10 GPM in the best case scenario so you will get enough from two of those units to cover your needs. If you do opt for the Rinnai you definitely won’t be disappointed. We are in a temporary apartment while we house hunt and will be getting that Rinnai ourselves when we buy our house.
Sorry about the confusion!
Nick
Nick – thank you so much for your feedback! To answer your question, in the scenario where the dishwashers and washing machines are running, the temperature (based on what I’ve read online) would need to be up to 120 for the washing machine (so a temp rise of 63). For the scenario with the showers, it would need to be 104 (temp rise of 47). This may be a dumb question, but do you set your heater to the maximum temp you would need (in our case 120) and then regulate then temp down for other uses (i.e. showers) at the faucet?
We used to set our tankless to the heat we wanted for the shower so we didn’t have to add cold to it. This way it saved a bit on the bills since it didn’t need to heat as much. And that was hot enough for washing clothes too as we preferred a cooler temperature to preserve the clothes, too. But, when we wanted to use the dishwasher we would crank it up to 120° and turn it down when it was done. You have to be more mindful this way but we liked having lower bills and feel it is less wasteful going this route, as well. I like cooler showers though and my wife likes it a bit hotter than 104° but she compromised on this point and got used to it.
We have no children at home it is just my wife and myself our home is 1450 sq feet 2 showers dish washer and washer what size tankless water heater do We need.
Hi Lonnie. It’s hard to say without knowing what your temperature rise is. But, where it’s 2 people you don’t need anything too big. I like Rinnai and Takagi tankless heaters so take a look at these articles and see which one is going to be best based on your temperature. You can use the calculator in this article to figure it out. https://yourh2home.com/rinnai-tankless-water-heater-reviews/
https://yourh2home.com/best-takagi-tankless-water-heater-reviews/
I hope that helps!
Nick
OK. Let me know if I get this math correct (or wrong). I have been looking for a calculator to help in the choice process but never found one with real numbers, so I did the math myself.
If 1 Btu is required to raise a pound of water 1 degree Fahrenheit, and water weighs approximately 8.34 lbs/gallon, then you need 8.43 Btu’s per degree of temperature rise per gallon. Move 1 gallon per minute then you need 8.34 Btu’s per gallon per minute. Since Btu ratings on heaters are per hour, multiply by 60 (60 minutes in an hour) for Btu’s per hour, per gallon, per minute. This gives the amount of energy required to heat the water (assuming a 100% efficient heater). Divide the total number of Btu’s per hour required for your flow rate by the heater efficiency to find the required Btu gas input for your needs.
Temp rise x 8.34 x 60 x GPM required / Heater Efficiency = Minimum Btu/hr gas input of heater
Example in my area:
72° groundwater
115° desired water temp.
115-72=43° temperature rise
Assume:
2 showers @ 2.5 GPM, then 5 GPM required,
And a good gas condensing heater with 95% efficiency
43 x 8.34 x 60 x 5 / 0.95 = 107,586 Btu/hr minimum gas input
It works out to approximately 500 Btu’s per gallon per minute per degree F temperature rise. If you do the math for 50° temperature rise, you need approximately 25,000 Btu’s per hour per GPM of water required (which doesn’t take heater efficiency into account, but gives a good idea of the size of high efficiency heater needed). That’s the numbers I’ve been using when looking at possible heater options.
I would always assume a lower efficiency than stated by the manufacturer due to losses caused by varying input air temperature, exhaust venting types, scale buildup on heat exchanger, variations in Btu content of incoming gas, etc. Maybe knock off 5-10% of published ratings?
Again, if I did the math right.
Jim
Hey Jim
That’s exactly right. Your math is right on. I was actually working on making a calculator for that since my wife does some programming, but it’s more complicated than I thought to make one. If all you are needing at any one time are the showers, then you don’t need a tankless with a high BTU rating. Though the lowest I see for gas ones anyway is 140,000.
Anyway, Thanks for reaching out!
Nick
I’m building an outshower and having difficulty sizing a point of use propane tankless.
Here are my specs:
Rise 65°
Max GPM demand 3gpm (probably closer to 1.5)
The shower head will be connected to a SeaFlo 55 Series 12VDC 60 PSI 5.5 GPM Marine Water Pump with 2 Gallon Accumulator Tank. The pump is on demand with a Heavy Duty Pressure Switch. I will be pulling in water from a 300 gallon water container which will be above ground. It is mainly for warmer weather use but worse case scenario, the water could be exposed to temps as low as 40°. I would be extremely happy if I could get at least 105° water. All PEX runs are extremely short and all equipment in very close proximity. Any advice is much appreciated.
Hi Crow Bird! So, you have some good options for a tankless heater based on your temperature rise since you have low hot water demand. You should take a look at some reviews here that also will give you some numbers on how many BTUs you need and how much propane you will likely go through based on that. Here is the article. You’re also a go with just about any unit based on your PSI.
I think the area of concern, however, is with your pump. since the accumulator tank is only 2 gallons, I would wonder if the pump is going to be consistent enough. Many tankless water heaters will have the hot water cut in and out if the pressure isn’t consistent. If you are getting 60 PSI then that is good, but sometimes it drops every few seconds as it builds pressure back up. If you feel like this pump and tank set up will give you consistent water pressure then you should be fine.
If you need any help, let me know!
Nick
I appreciate your article. I am currently researching replacing a 50 gallon gas HWT with a tankless for my daughter and her family. They have 3 daughters and a son ages 8 to 2. With six total in the household I calculated they would need probably 7 to 9 gpm. It may be less now but will definitely increase as the kids get older. Our ground tempature is 62 degrees in Georgia. The house is a two story with a basement with 2 full baths on the second floor and a two additional full baths (one on the main floor and one in the basement. Need your opinion on what tankless natural gas unit I need to consider. Don’t know enough about the venting requirements on these units. The current tank is located in the basement adjacent to the furnace and has a 3 inch flue tied into the furnace flue
I am the son of a pipefitter and intend to remove the old hwt and install a tankless. Aporecuate your help. Thanks. Alex
Hi Alex
Your daughter is the perfect candidate for a tankless water heater. Assuming she keeps the temperature of the hot water under 110°F, which she should since her kids are small, then she can get away with excellent GPM since the incoming water temperature is pretty good. I would take a look at the RUC98IN. Check out the article here for the details. You’ll get just about 9 GPM from it. Plus it has a lot of bells and whistles to make it easy to use and very very efficient. Another good option is the Takagi T-H3-DV-N which you can see on Amazon by clicking the link. I don’t have an article about it yet but plan to have one soon as this one has some improvements made over the last model. It is cheaper than the Rinnai, too and gives you the same GPM.
Being that there seems to be a bit of a distance from the tankless to some of the fixtures you might want to also look into a hot water recirculating pump to avoid wasting water. At the furthest fixture, you may be looking at close to a minute for the hot water to arrive. https://yourh2home.com/recirculation-pump-pros-cons/
As far as venting goes, you can’t vent through the furnace. You should be looking at condensing units that will give you the GPM you need and they have concentric vents. It highly depends on where your tankless will be located as to where and how it needs to be vented. https://yourh2home.com/where-to-install-tankless-water-heater-requirements/
Hope that helps!
Nick
I live in North Carolina. Today is March 28 and very warm., 80 degrees. My ground water temp is 60 degrees today. It is just my husband and I. He has had a stroke and has memory issues and can’t help with decisions. We have only one shower, because of our age. I bathroom sink, kitchen sink and washing machine. I don’t have a bathtub. I would like the heater that mounts outside. What do you suggest?
Hi Betty. Sorry to hear about your husband.
To answer your question, you really don’t need one that is too big because it looks like the maximum GPM you need at any one time is probably less than 6. I don’t know if you need natural gas or propane, but I would definitely go with either a Rheem or Rinnai outdoor tankless. I wrote a comparison between the two brands here https://yourh2home.com/rinnai-vs-rheem-comparison/ with a section about outdoor tankless water heaters. Those two listed but if you look at their respective page on Amazon you can see that there are versions that have lower GPM at a lower price. I recommend the Rinnai RL Series HE+ that gives you 7.5 GPM and that you can see by clicking the link.
I hope that helps.
If you need more help then definitely reach out anytime!
Nick
In reference to my earlier .post, I would need propane. Don’t know if that makes a difference with size. Thanks so much for your help
Nope, makes no difference at all. Most gas tankless heaters have a propane or natural gas version of the same model that offer all the same specs.
HI Nick,
I am looking at buying a home that has a tankless Rinnai – its installed in an interior closet on the second floor. I think that it must vent out through the roof , which is a far distance as this house has a third floor. Do you see any issues with that set up? Thank you.
P.S. In the house i currently live in the tankless are set up in a mechanical room in the lower level of house and vent right to the outside – not going a far distance.
Thank you!
Hi I live in DE and are thinking of changing a 50 gallon electric to tankless system. We have 3 baths on first floor and kitchen and a bath on second floor. What would you suggest?
Hi Robin
The answer really depends on how many fixtures you are using at the same time. By the sound of it, 4 bathrooms likely means that your hot water demand is high, but if you are only using one at a time, then it isn’t that high after all. However, with that said, when you have that many bathrooms it could happen that they are all being used at once even if it happens only occasionally so you’ll need a high GPM tankless. This one from Rinnai on Amazon is what I would recommend in that situation.
Hope that helps!
Nick
Thank you for taking time to answer these questions. I own an old duplex in Denver that is on a slab and has a 700 sqft back half and a 700 sqft front half. There is 1 person living in the back and 2 people living in the front. Each unit has 1 shower and 1 stackable apartment style washer/dryer combo. I put a Navien NPE-180S tankless in the back unit that is rated at 8.4 gpm. Since I pay for utilities I would like to use this unit for both halves by tapping into the hot water line in the back and running it to the front but don’t want my tenants to have warm showers. I figure max running would be 2 showers and maybe 1 of the washing machines. Worst case scenario would be 2 running showers, 2 washing machines, and 1 kitchen sink. Will this work for me? Should I recommend that they don’t run their washing machine when taking a shower? I would also assume that the washing machines would run warm rather than hot water.
Hi Derek
Well, I just did a quick search on the groundwater temperature for Denver and it doesn’t look like that tankless GPM is going to cut it. You have to plan for the maximum hot water demand even if it doesn’t always happen. Otherwise you’re dealing with warm water for some or all of the fixtures. What you can do though is to put in a small point of use tankless in the bathroom or washing room where you think it will be needed most. That way you aren’t straining the Navien. Check out this article to see if you think this will work for your needs. Here’s the article.
I hope that helps
Nick
My wife and I are building our dream home. Its just the two of us in a 1535 sqft home. We are getting a tankless water heater, but are debating if we need one with a recirculating pump. The greatest distance from the water heater ( in the garage) to the furthest sink is 50 feet.(master bathroom sink). How long do you feel it would take for the hot water to travel 50 feet to our master bathroom sink/shower? And would the recirculating pump be worth it?
Robert
Hi Robert
I think a recirculating pump is definitely worth it in your case. 50 feet will probably take up to a minute to get hot water to your fixture. This will save a lot of water over the years. You probably won’t see much of a difference on your bill, however, but it is nice to have hot water right away and not waste any water. This article shows the top pumps so take a look and see which is best for you. Make sure it has a timer though so it isn’t running all day when you aren’t home.
All the best
Nick
Hi there. Wondering if you could help decide if what I already purchased is gonna work before we actually go ahead and install. We are a family of 3 living in LA, CA, so water temp in pipes is apparently around 62 degrees. Bought a gas tankless that has 180,000 btu and 8 gpm. Do you think that’s sufficent?