The problem with this question is there are too many variables for a perfect answer.
In some cases, a boiler is needed and only a boiler will do. In other cases, you can certainly use a water heater and it will also be more efficient than some boilers.
In yet other cases, the right tankless water heater will do a good job since it only heats the water when you need it and it provides it quickly.
Which one is best for you will depend on how big a space you need the radiant flooring to cover and if you need the hot water for other applications.
In this article, I will go over how each option should perform and in what circumstances so you can understand which one is going to do the job for you.
In this article:
Water heater vs. boiler
Many people use these terms interchangeably, but the reality is that they are very different. They both heat hot water, this is true, but their function is different.
- Hot water heater – A hot water heater is used to give you domestic hot water for your appliances like dishwashers and washing machines as well as fixtures like showers and sinks. They are either a tank or tankless style and can be gas or electric.
- Water boiler – A boiler is used to heat just the water as it boils the water and turns it to steam. These are for closed systems usually called hydronic systems and they are just for heating. They can be used with radiators or radiant floor heating.
There is a third type called a combi water heater that can actually do both. It is designed in a way to use different zones. You can send hot water to your heating system like an infloor heater or use to wash your hands.
So, which one do you need? You actually have a choice between them as you can use a water heater or tankless for radiant floor heating and not just a boiler.
Learn More!
Check out the article about the best thermostats for radiant floor heating to get the most out of your heating system!
Water heater for radiant heat
Water heaters are good for radiant floor heating because they maintain a low temperature. You really only need a maximum of 120°F for your floor.
That temperature is safe and comfortable as well as being efficient.
You will need to be aware of the incoming groundwater temperature as cold water will drop the overall water temperature. It may take a little longer for the floor to warm when using a water heater with water colder than 55°F.
To have your water heater heat your floors, you’ll need the right water heater and a hot water circulating pump which I will get to in just a minute.
When you are not trying to heat different zones, then a water heater will work. Where you get into trouble is when you have different zones that require less, more or no heat than the others. With a water heater, you are better off when you have one zone with the same temperature throughout.
What to look for
The water heater to use with radiant floor heating should be a high efficiency unit. The best kind is one that has a sealed combustion chamber and an external heat exchanger.
The reason for this is to have the two water uses separate. In the summer, water will stagnate for the floor heating and if it mixes with your regular water it won’t be good. Legionnaires disease is a real threat when water sits in pipes for too long. This keeps the two separate for their intended use.
What you can also do to avoid this is to set a timer on your hot water circulating pump to run 10 minutes per day to flush out any stagnant water. Or, set up an external heat exchanger for your ordinary water heater if you are not looking to upgrade. Then you’ll just need to run a loop from that to back into the water heater so the water doesn’t mix with your domestic use water.
Here are a couple of good water heaters that can handle your radiant floor needs as well as domestic.
- Uses indoor air for combustion; blower exhausts the flue gases
- Exclusive air/fuel shut-off device. Maintenance free - no filter to clean
- Long venting lengths up to 100 equivalent feet
- Rheum exclusive R-Tech (registered) anode rod provides long-lasting tank...
- Permaglas glass coating protects steel tank from corrosion and maximizes...
- Dynaclean diffuser dip tube helps reduce lime and sediment buildup while...
- CoreGard anode rode aluminum anode with stainless steel core protects tank...
- Durable, tamper-resistant brass drain valve
- Meets all current regulations set forth by the National Appliance Energy...
A major caveat is that many water heater manufacturers will void the warranty if you use it as an infloor heater. That is not its intended purpose even though you can use them effectively for that.
Another thing to remember is that they don’t usually have the BTU that can heat up a big house and supply the domestic water at the same time. If you only need one zone and not the whole house then these will work fine. You’ll also need an expansion tank for when the hot water heats up so it doesn’t end up coming out of the pressure relief valve.
Since it is a closed loop, the expanding hot water won’t have a release so you need the expansion tank to give it somewhere to go.
- Protects Your Heating Systems; Prevents Dangerous Pressure Build up;...
- Features a Solid Carbon Steel Construction with an Exclusive Polypropylene...
- Each Tank is Factory Leak Tested and NSF61 Certified
- Max Operating Pressure 75 PSI; Temperature 210°F
- Low Profile Tank Design with Full Membrane Depth Eliminates Stretching of...
Lastly, you need a hot water recirculating pump. I have an article about the best ones for tankless, but those on the list will also work great on a tank style water heater. You can read that here.
Below you’ll see a good one that has a timer and is easily installed on any water heater.
- TLC = Timer, Line Cord, 6-Foot
- Flow range: 0 - 9 U.S. GPM, Head range: 0 - 6-Foot
- Motors: Single phase, 115V. Supply Voltage:115-120 VAC, 60 hertz, Contact...
- Connection: 3/4-Inch M by 3/4-Inch FNPT.Maintenance-free, low energy...
- The Grundfos circulator pumps, Series UP, are specifically designed for...
If you already have a hot water recirculating pump but it doesn’t have a time, there is no need to replace it. Instead, use this Taco Smart Plug to control when you need the pump on. It even records your water use patterns so you know when you need it most.
- Smart Plug Lets You Upgrade Any Existing Hot Water Recirculation Pump With...
- Simply Connect The Smart Plug To Any 110V Power Outlet Then Plug The...
- Country Of Origin: United States
- Brand Name: Taco
Boiler for radiant heat
Since a boiler does one job, to heat in a hydronic system, they are ideal for the job. Many people don’t like to use boilers because then they need to have two systems. One for the heating and one for the domestic water use like appliances and fixtures.
The reality is that they are built for this purpose and will have a full warranty if something goes wrong since they aren’t being asked to do a job they aren’t designed for.
A key in finding the right boiler is one that doesn’t drop the temperature too quickly as that causes a caustic condensation to develop. A condensing boiler will release the exhaust through a flue at around 60°F so it is cool enough to vent with PVC. The acidic discharge is condensed to water and returned into the boiler to avoid a drop in efficiency as well as special venting being needed.
Below are a couple of boilers that are leaders in boiler technology. Check them out side by side, but I think the Rinnai is the better choice for efficiency and price.
No products found.
If you need to heat a floor in a garage that can get really cold when it is not in use, or are trying to heat a driveway, then you’ll need a boiler since you will need to use antifreeze fluid mixed with the water.
Did you know that the wrong thermostat can ruin your furnace? Read the detailed article here!
Combi boilers
A combi boiler is the best of both worlds and is how I suggest you heat your home and take care of your domestic water needs. They are specifically designed to do both jobs well and are very efficient.
They are small so the installation is easier as you have more options as to where to install them.
They are super efficient as they act almost like tankless water heaters and deliver hot water quickly without a lot of heat loss.
For the radiant heating, they have enough BTU to handle large areas and different zones if you set up a manifold. And most of them also have a circulating pump already built in so they save you some money on that plus less to install.
Below are the two that I highly recommend.
- Min gas input: 18K btu, max gas input: 199K btu
- Energy factor: 0.95
- Max flow rate: 9.2 GPM
- Max gas input for space heating: 120K btu
- 11:1 turndown ratio
- Rinnai Q205SN - Natural Gas Condensing Boiler
- Flexible venting options with approved Concentric, PVC, CPVC, Stainless...
- Stainless steel primary boiler heat exchanger for unmatched performance and...
- Compact wall-mounted design saves space over traditional boilers
- Features an integrated modulating pump, and outdoor reset sensor (in the...
Tankless water heater for radiant heat
Lots of people are turning to tankless water heaters to heat their floor and to supply the domestic water needs.
They take up very little space, are extremely efficient and can operate at temperatures lower than a traditional boiler without creating toxic condensation. They also maintain the right temperature thanks to high tech internal sensors so they can operate consistently at those lower temperatures.
I recommend a tankless water heater with at least a .90 energy efficient rating. You can go as low as .82 but I think you’ll see better savings with the higher rating versions.
There are natural gas or propane tankless water heaters available so you can find one that works with your fuel source. There are electric ones too but they will only be good for very small areas.
One big downside is that when you are heating your floor, the tankless is running non stop. This can shorten their life from 15 years down to around 5.
Also, when you have a high heat load where you need a lot of BTU/hour then a tankless with a high GPM is going to get the job done whereas you may need multiple boilers to handle the heat load.
You’ll need a recirculating pump unless the model you choose has an internal one already built in. Below is the unit I highly recommend as you get the highest Gallons Per Minute (GPM) out of any tankless heater. You will need a high GPM tankless if you live in a cold climate to make sure you can get the hot water you need at the right temperature.
- Professional installation required; call 84-GO-Rinnai for assistance.
- ENERGY STAR Qualified with Uniform Energy Factor of .93
- ThermaCirc360 technology provides faster hot water with a built-in...
- Save time with hot water whenever and wherever you need it, any size home,...
- Save energy and reduce utility costs by operating only when hot water is...
How big of a water heater do you need for radiant heat?
Unfortunately there are a lot of factors that go into the size of a water heater to use for infloor heating.
The average output needed is 25 BTU per square foot, but that could be higher or lower depending on your circumstances.
To get a bit more specific, here are some common load factors based on where you need it:
Basement (below ground room) 10-15 BTU/ hr/sqft
First Floor 15-20 BTU/ hr/sqft
Second Floor 20-25 BTU/ hr/sqft
Shop (slab on grade) 15-20 BTU/ hr/sqft
Garage 5-10 BTU/ hr/sqft
Then you need to multiply the load factors by the area you need to heat. The result will be the heat load. Here’s an example:
Basement @ 12 BTU/hr/sqft X 1000 sqft = 12,000 BTU/hr
Do the same for all of the spaces you need to heat based on the load factors above and then add them all up. That will give you the BTU/hour that you need to determine how big your water heater or boiler needs to be.
If you have over 60,000 BTU/hour needs then you should look into a high GPM tankless or get multiple boilers.
Is it legal to use a water heater for radiant heat?
Before you decide on which type of source for your radiant floor heat, make sure you find out if it is allowed to use a water heater and not a boiler. In some states like Michigan, you cannot. Any heater used for space heating has to be labeled as such and can’t be used for domestic hot water either.
Some localities require a closed loop system to avoid legionnaires disease.
Even in areas where it is permitted to use a water heater instead of a boiler, many manufacturers will void the warranty if something goes wrong when you use it as a boiler.
Conclusion
Hopefully, after reading this article on water heater vs boilers for radiant floor heating, you have a better idea about what you need.
These decisions are not easy to make as there are so many variables as you no doubt have gathered.
If you have any questions, then drop a line in the comment box below and I will get back to you as soon as possible.
YourH2Home is your go to site to get the most up to date information on all your water system needs.
Hi.
I like the idea of tankless water heater
or combi units but I think with hard water in Alberta, Canada there will be issues with scale and water deposits. A descaler or water softener would help, but that increases the cost.
Would you agree and are these TLWH reliable?
Hi Kortney
Yes, you’re right about the potential pitfalls of having a boiler if you have hard water. But, first, if you have hard water then all of your appliances are going to have a shortened life and not just your boiler or tankless. Your dishwasher and washing machine are both also vulnerable. Also, you may live in a hard water area but not have very hard water yourself. I would definitely do a hard water test first and foremost and see how hard your water is exactly. Then, if you do have hard water, I would get an electronic descaler that attaches to the incoming water pipes that will eliminate the scale without needing to replace water filters a couple of times per year. This one on Amazon is affordable and long-lasting.
I hope that helps
Nick
I’m told I can’t use a water heater to make a closed loop system to heat my shop. The code in Saskatchewan prevents it. What would the reasoning for this be? I read websites that suggest it’s actually a preferred method to a boiler.
Hi Tyler
The thinking is that if the water heater is ever going to be used for hot water use in a sink or shower etc that there is a risk of legionella which can lead to Legionnaire’s disease which is like pneumonia. Of course, if the loop is closed and it only circulates through the floor heater it shouldn’t be an issue, but that is likely the reason.
Excellent article, thank you it was very helpful.
I would like to combine our dhw heating with infloor heating of two smaller rooms. I estimated combined max heat load to be 15kbtu/hr
I’ve been concerned that the min heat release of a tankless heater would still be too high for a single room. I’m concerned it would cycle on/off numerous times for each thermostat cycle. Is this a valid concern?
Thinking a condensing tank heater would cycle less and last longer?
Gary
Hi Gary
You’re right that a tankless makes the minimum a concern. That’s the area in which a boiler is the better choice because when your heat needs are not that high, they can manage those low BTU situations. It does depend on the tankless, however. There are smaller ones that might be better that will not be cycling all day. Instead of a condensing tank heater, I would go with a boiler if you aren’t convinced about a tankless.
I know that doesn’t sound like much of a help, so if you have any other questions let me know and I will see if I can find some answers for you.
Nick
HI NICK,
LIKE THE ARTICLE. I HAVE BEEN RESEARCHING TANKLESS WATER HEATERS AND BOILERS. I HAVE A RINNAI FOR MY HOUSEHOLD HOT WATER NEEDS. I LOVE IT. THAT BEING SAID I DECIDED TO DO RADIANT TUBING IN MY NEW GARAGE FLOOR. I HAVE 2020 SQUARE FEET TO HEAT. THE GUY THAT GAVE ME A LAYOUT FOR MY DISTRIBUTION PANEL TOLD ME I NEED A TANKLESS BOILER AND CANT USE A TANKLESS WATER HEATER. ME BEING ME I DIDNT BELIEVE HIM SO I STARTED DOING SOME RESEARCH. SO HERE I AM. EVEN MORE CONFUSED. IN COMPARING THE TWO HEATERS I DONT SEE MUCH DIFFERENCE IN EITHER ONE. I SEE MODULATING, HYBRID, EFFICIENCY GOOD, EFFICIENCY BETTER, PVC PIPING, STAINLESS STEEL PIPING, CONDENSATION, NON CONDENSATION AND SO ON. I WATCHED A YOU TUBE VIDEO ON INSTALLATION OF A HEATER AND THE GUY USED A NATURAL GAS TAKAGI TANKLESS WATER HEATER. HE SAID IT WORKED GREAT. I WENT TO BUY THE SAME MODEL BECAUSE IT SAID OK FOR RADIANT FLOOR HEATING. I ASKED ABOUT USING IT ON A CLOSED LOOP WITH GLYCOL AND IF IT WOULD EFFECT THE WARRANTY. TAKAGI SAID IN THE PAST IT WAS OK BUT NOW THEY DONT RECOMMEND IT. NO WARRANTY IF USED THIS WAY. ALL I WANT TO DO IS HEAT MY 2020 SQUARE FOOT FLOOR WITH THE BEST. LESS EXPENSIVE AND MOST EFFICIENT WAY. ANY SUGGESTIONS SIR??
THANK YOU SIR,
MIKE
Hi Mike
You’re doing the right thing by looking into every possible avenue when it comes to heating your under floor hydronic system. But, there really are only two things that you need to deal with. One is what your local regulations are. Some states or counties don’t allow tankless heaters for this purpose at all. Even though they work great with a closed loop and using glycol. I have no idea why this is the case. It’s mainly because of the risk of disease but a closed-loop, dedicated line is not ever going to interfere with your drinking water. So, this is the first order of business. Look into what your specific state or county says about it. You could do it yourself but I am not recommending that if they say it isn’t allowed. Your call.
The second thing to decide is how comfortable you are with using a tankless with the warranty being invalid. The reason for them to say no is a simple case of CTA. Covering their you know what. So, you are likely to have it work out for you but if there is a manufacturing defect you’re kind of out of luck. However, it is very unlikely that this will be the case. Remember, the warranty only covers manufacturing defects which are very rare and since they are only meant to be used as a way to heat domestic hot water, they just don’t want to have to pay out if used “improperly”.
So, if I were you, I would go with that Takagi but you have to decide for yourself how comfortable you are with essentially having no warranty.
The bottom line is that they do work really well as a hydronic floor heater, cost less than a boiler even though they essentially do the same thing and are used by a lot of people, especially Canadians.
I hope this helps.
Let me know what you end up going with as I am curious how it works out for you.
Nick
Great article and an informative read. I’m planning to do exactly this in the coming months, partly as my main floor joists are exposed underneath and my basement slab needs to be removed and re-poured, so a perfect time to install radiant heating. As I’ve researched I’ve been smitten by the air to water heat pumps that are becoming available these days, and the hybrid tanks also available such as those made by Rheem. I would love to clear out all the gunk of the gas burning furnace and ducting in my basement, along with the water heater, also gas, and replace it all with one single tank. My house is pretty small, around 800 Sqft and similar basement, and I’m planning to insulate the heck out of it, below slab, walls, rim joists, attic space, and using spray foam wherever budget allows. My thinking is that with an extremely well insulated home, even with the cold winters here in Edmonton, I should be able make this work, and use a wood-burner as a backup heat source. The cold air that gets pumped out from the heat pump I will either duct out of the house (if I end up going with the hybrid tank) or I even considered venting it into a well insulated cupboard that could act as a white wine and beer store. By my calculations the BTU/hr should be well within the capabilities of the larger tanks available so should be able to provide all my heating and hot water.
The main question I had was regarding the return water coming off from the heat exchanged back to the water tank. At some point this loop of water will have to meet up with the mains cold water also coming into the water tank. When the heat exchanged is in use both the closed loop glycol is being pumped and so is a loop from the hot water tank. My query is how do you design the system so that when this loop is being pumped that only the slightly cooler water leaving the heat exchanged comes into the hot water tank and not the mains water supply which will be a lot colder. I hope that question makes sense to you.
Many thanks again,
Max
Hi Max
I’m not sure I understand or can visualize how the cold water is coming in. I suppose what you are saying is that cold water is going to come into the tank where the incoming water will be cold and at some point they will mix. If that is what you are saying, then you have to understand that the closed loop with or without glycol is going to be separate from the water that comes in for domestic use. That is why you have to choose your heater carefully as some are not going to have dual options. It will be one or the other. Some tankless heaters are not going to be able to handle closed loop for heating the floor plus the rest of the domestic water needs, as an example.
I know that i didn’t actually answer the question haha!! Here is a link to a diagram that might be helpful. Ignore that this is a wood fired system as the concept should still be the same. Also this has multiple zones which you probably won’t be dealing with.
Anyway, I hope this helps somewhat but if I didn’t understand correctly then let me know!
Nick
We have a sun room between the house and garage. It is 16′ x14′. It appears to be linoleum tile over a layer of concrete on concrete block. We are looking at installing radiant heat. What thoughts on how to best get this? If possible, lower operational costs is important as we are retired. Thanks for your help? John
Hi John
Luckily for you, it is possible to put a radiant floor heat system right over that linoleum and concrete. It involves some insulated panels and tubing and then you put a new floor over the top. I would stay away from a wood floor as it doesn’t transfer the heat well, however. Just keep in mind that the system is going to add a few inches of height to your floor so you may need to trim down your door if there is one that goes into the room. I recommend going with an electric system over a hydronic considering the space is small. In theory it is more expensive than using a hydronic system, but again in a space that small it hardly makes sense to put in the expense of a hydronic and it takes longer to heat the floor than electric so it is probably going to be a break-even type of scenario. And electric versions are much cheaper and easier to install yourself.
Hope that helps
Nick
Hi Nick, your article and the answering of people’s questions have been so informative.
I personally have a walk out basement with roughly 1000 sq ft of floor to heat. The big local plumbing companies all suggest I go with a boiler and seem to be negative towards a water tank. The house is heated by an electric furnace but since we have a wood stove burning all day and evening the furnace rarely goes on, hence the basement temp gets to around 13 Celsius and makes it difficult to stay warm when working in my office down there. (oh yeah, we do not have gas, only electricity on site)
Suggestions, especially for the running cost?
Thanks Bill
Hi Bill
Admittedly, I am not 100% on the best solution for you as I have always had gas for furnaces. (Except for when I was a kid and we heated our home with a coal furnace!) What I am inclined to think is that you may want to install electric under floor heating system rather than use a furnace. If you are only trying to heat the basement since you seem to be covered with the wood stove everywhere else, then this may be your best bet. Like I said, I am not sure but it is something to look into at least.
All the best
Nick
You are so kind, thank you