What happens when you heat water? It starts expanding. This process is called thermal expansion and it continues until the water’s temperature reaches its boiling point, at which point water evaporates, leaving the water heater safe from pressure build-up.
However, what would happen if water never reaches its boiling point? This happens in all water heaters. These units receive enough heat to raise the water’s temperature, but the amount of heat is nowhere near enough to help the water cross its boiling point.
In such a scenario, the pressure build-up will jeopardize the safety of the heater. That is because heaters aren’t designed to accommodate water’s thermal expansion. They can only heat the water before pushing it out. A build-up of pressure might expose them to issues like bursting.
Old water heaters dealt with this problem by pushing excess water back into the mains, which would absorb the pressure. But with changes in plumbing practices and city laws, today’s heaters aren’t permitted to push excess water (and pressure) back into the city water supply.
Here’s how a water heater expansion tank can prevent these issues:
What is a Water Heater Expansion Tank?
A water heater expansion tank is a small tank installed next to your water heater’s supply pipe. It is built for handling water’s thermal expansion as it heats up inside the water heater.
As the water pressure starts reaching dangerous levels, excess water from the heater begins flowing into the expansion tank. This relieves water pressure in the whole system.
Do I need an expansion tank on my water heater?
If you’re noticing that your heater’s relief valve is leaking water, or your plumbing fixtures’ washers are going bad rapidly, it might be time to install an expansion tank. It might be the only (cost-effective) way of saving your home’s plumbing system from costly damage.
An expansion tank is also needed if your plumbing system is ‘closed’.
Closed plumbing systems are those in which the backflow, which usually runs between your house and the city water supply, is blocked, leaving no avenue for the water pressure to relieve itself. To help the increased pressure leave the heater, an expansion tank will be needed.
You may also have to install an expansion tank to pass your city’s plumbing inspections.
Take, for instance, Frisco, Texas. The city’s local authorities require all new homes to have a PRV and expansion tank. Similar is the case with Little ELM, TX, where all the installed water meters come with check valves, creating a closed-loop system.
What is the Purpose of an Expansion Tank on a Water Heater?
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The main purpose of an expansion tank is to prevent damage to the water heater. As water is heated inside a heater, it starts thermally expanding. The increase in pressure, if left unattended, can damage the water heater, joints in supply pipes and even the valves in plumbing fixtures.
An expansion tank prevents all this damage by not letting the water pressure in the heater reach levels at which it could damage the heater. It does that by giving excess water an avenue to escape the heater, bringing down the pressure inside the whole system.
How does an expansion tank work?
Expansion tanks work by balancing pressure throughout the system. One expansion tank is divided into two sections by a rubber diaphragm. One of the sections is connected to the water heater and contains water. The other isn’t connected to anything and has dry air.
As the pressure inside the heating system increases, the diaphragm is pushed down. This creates space in the tank for excess water from the heating tank to enter. Subsequently, the pressure in the heating system is released, and the whole system (the heater, pipes, fixtures) are saved from being damaged.
What size expansion tank do you need?
Expansion tanks vary in capacity and size, ranging from models that could hold as little as two gallons to large units that hold more double that size.
To determine what size expansion tank you need, understand that most people with a standard 50 gallon heater will need a 2 gallon expansion tank.
Frequently Asked Questions
Where should a water heater expansion tank be installed?
Experts suggest installing the expansion tank directly above the water heater. You can use a tee-fitting installed in the cold water pipe for the installation. Most users choose to install their expansion tanks horizontally. But you can also install them vertically if ground space is limited.
How close should the expansion tank be to the water heater?
The expansion tank should be at least 18-inches away from the water heater’s cold water inlet fitting. The expansion tank shouldn’t be installed too close to the heater, or else the leakages from the heater might end up damaging the tank or the area surrounding it.
How much does it cost to replace a water heater expansion tank?
A water heater expansion tank costs somewhere between $40 and $200, depending on the model’s capacity. You might want to add another $50 to $200 as labor costs. That means you should be prepared to spend close to $500 if you’ve decided to replace your water heater expansion tank.
How do I know if my water heater expansion tank is bad?
To check if your water heater expansion tank is bad, push the air valve (Schrader Valve) located on the bottom of the tank. If pushing the valve results in the release of air, then there’s nothing wrong with your tank. If no air (or water) comes out, your water heater expansion tank is bad and needs replacement.
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Water heater expansion tanks provide you with the best of both worlds. They save your water heater system from costly damage while also enabling your fixtures to work properly.
And with more and more cities now adopting closed-loop plumbing systems, now is the right time to install a water heater expansion tank in your home, especially if you’re a new homeowner and want to pass plumbing inspection at the first attempt.
I graduated with a degree in Chemical Engineering and have written for a number of nationally recognized publications in the home improvement space. My skills include fluid mechanics and process engineering and I have worked on numerous projects, including in waste water flow rate calculation and heat balance of steam rollers in the paper industry. My goal as a technical writer is to make complicated topics easy to understand for the average person.