What thermal expansion is
When cold water is heated, it takes up a little more space. In a tank holding a good quantity of water, that extra volume has to go somewhere. In an open system, it can push back toward the supply. In a closed system, where something stops water flowing back, it cannot, so the pressure in the household plumbing rises each time the heater runs, and falls again as hot water is used.
That repeated rise in pressure strains the heater, its fittings and the rest of the plumbing, and often shows up as a temperature and pressure relief valve that drips or discharges now and then.
Why so many Connecticut homes are closed systems
Private well systems commonly include a check valve that stops water flowing back toward the well, which makes the household plumbing closed. On town water, a pressure-reducing valve or a backflow device at the meter can do the same. With private wells so widespread in eastern Connecticut and the hill towns, thermal expansion is a frequent reason for relief valve drips here.
The installers do not work on well equipment, pressure tanks or the water service. The expansion tank we arrange sits on the water heater side, on the cold supply to the heater.
How an expansion tank works
A water heater expansion tank is a small tank divided inside by a flexible diaphragm, with air on one side and water on the other. As the water heater runs and the water expands, the extra volume pushes into the tank and compresses the air, instead of raising pressure throughout the house. When hot water is used, the air pushes the water back out. The air side is set to suit the home's water pressure when the tank is installed.
On a private well?
Wells and expansion
If your well system has a check valve, your plumbing is almost certainly closed. Pressure also swings as the pump cycles. An expansion tank on the water heater side helps with the thermal part of that picture. Mention the well when you call.
Well homes are the most common candidates.
Signs you may need one
- The relief valve drips or discharges from time to time, especially after the heater runs
- A tap spurts or thumps when first opened after a quiet period
- A well system with a check valve, or a pressure-reducing valve on town water
- An existing expansion tank that is waterlogged or has failed
What to leave alone
- The relief valve and its lever; never cap or plug the discharge pipe
- The air valve on an existing expansion tank
- Well equipment, pressure tanks and pressure switches
How installation works
The technician checks the household water pressure and whether the system is closed, then fits the expansion tank on the cold supply near the water heater, supported properly so its weight does not hang on the pipe. They set the tank's air side to suit the home's pressure before filling, then check that the relief valve stops discharging once the system settles. If an old expansion tank is present but has failed, they replace it. The price is given before any work.
A small job, priced before it starts.
Sizing and setting the tank
An expansion tank has to be large enough for the amount of water the heater holds and the pressure in the household plumbing. It is also set before installation: the air side is adjusted to suit the home's water pressure, so that the diaphragm is ready to take expanding water rather than already squeezed flat. A tank that is too small, or not set to suit the house, will not do its job, and the relief valve may keep dripping. This is one reason the technician measures the household pressure first.
Expansion tanks wear out too
Over time, the diaphragm can fail or the air charge can leak away, and the tank fills with water. A waterlogged expansion tank no longer absorbs anything, so the relief valve may start dripping again, sometimes years after it was quiet. A technician can check whether an existing tank is still working and replace it if not. Please leave its air valve alone; adjusting it with the system under pressure does not fix a failed tank and can make the diagnosis harder.
Indirect tanks, heat pump units and expansion
Any storage heater on a closed system can need an expansion tank on its cold supply, including indirect tanks and heat pump water heaters. For an indirect tank, this is separate from the boiler's own expansion tank, which serves the heating side. When an indirect is installed or replaced, the installer checks the household water side and fits an expansion tank there if the system calls for it.
Support and placement
A filled expansion tank is heavier than it looks, and hanging it from a pipe without support strains the fittings over time. Installers mount it so its weight is carried properly, usually close to the water heater on the cold supply, where it can be reached for checks later. In a cramped cellar, the placement is planned so the tank does not block access to the heater's drain or anode.
What affects the cost of expansion tank installation
- Tank size for the water heater's capacity
- Space and pipe layout near the heater
- Whether an old tank needs removing
- Whether the relief valve also needs replacing
How long expansion tank installation takes in Connecticut
Fitting an expansion tank is usually a short job within a single visit, often done at the same time as a water heater replacement.
When an expansion tank is added at the same time as a new water heater, it adds very little to the job. Added on its own to an existing heater, it is still a modest visit, and the technician will usually check the relief valve and household pressure while there.
Expansion tank questions
Does every water heater need an expansion tank?
Not every one. It depends on whether the plumbing is closed, as it is on many well systems and on town water with certain valves at the meter. The technician checks your system and explains whether one is needed.
Why is my relief valve dripping?
Thermal expansion in a closed system is a common cause. Others include water that is too hot or a worn valve. Never cap or plug the valve; have a technician find the cause.
How do I know if my expansion tank has failed?
A failed tank fills with water and stops absorbing expansion, so relief valve drips may return. A technician can check it; please leave the air valve alone.
Is the expansion tank the same as the well pressure tank?
No. The well pressure tank is part of the well system and stores water for the pump. The expansion tank on the water heater side handles thermal expansion. We work only on the latter.
Where does the expansion tank go?
Usually on the cold supply pipe near the water heater, properly supported. The installer chooses the spot based on your pipe layout.
Can an expansion tank be added to an existing heater?
Yes. It does not need a new water heater; it is fitted to the cold supply of the heater you have.
Do tankless units need expansion tanks?
With little stored water, thermal expansion is much less of an issue. Some installations still include one depending on the system. The installer will advise.
Will it fix taps that thump when opened?
If the thump comes from pressure built up by thermal expansion, it often helps. Other causes exist, and the technician can check.
Does an expansion tank need maintenance?
Very little. A technician can check it during other visits to confirm it is still absorbing expansion and has not filled with water. Leave its air valve alone between checks.
Will adding an expansion tank change my water pressure?
It should not change your normal water pressure. It absorbs the extra volume created as water heats, which stops pressure climbing in a closed system every time the heater runs.