Home / Services / Water Heater Expansion Tank Installation
Hot Water · Expansion ControlWater Heater Expansion Tank Installation
Water heater expansion tank installation is needed when the house plumbing is a closed system: a backflow preventer, check valve or pressure-reducing valve on the water service leaves heated water nowhere to expand. A 50-gallon heater makes about three extra litres of volume each time it heats from cold, and without a cushion that volume becomes pressure. The fix is a small air-charged tank on the cold line, pre-charged to match your house pressure and sized to the heater.
Water grows by nearly two per cent as it heats from tap-cold to 60°C. In an open system that extra volume pushes back out through the meter into the street main and nobody notices. Put a one-way device on the service — a backflow preventer, a check valve at the meter, or a pressure-reducing valve — and the water is trapped between it and your closed taps. Water does not compress, so pressure climbs with every heating cycle until something gives, usually the relief valve on the heater, which opens at 150 psi.
An expansion tank is a small steel vessel divided by a rubber diaphragm, with air on one side and house water on the other. As the heater warms up, the expanding water pushes into the tank and squeezes the air instead of spiking the pressure. It only works if the air charge equals the house's static water pressure. We measure that with a gauge on a hose connection, then set the tank with a tire-type gauge and pump while it is still empty of water; a tank left at its factory charge in a house with higher pressure is partly full of water before the heater ever fires.
Size follows the heater's volume and the supply pressure. A tank of about two US gallons covers most 40 and 50 gallon heaters at ordinary pressure; 60 gallons and up, or high street pressure, calls for the next size. It goes on the cold supply between the shut-off and the heater, rated for drinking water, and it is strapped or bracketed to the structure because a tank that has filled with water is heavy enough to break the fitting it hangs from. If static pressure measures above 80 psi, the plumbing code limit, a pressure-reducing valve comes first.
Expansion tanks wear out quietly. The air charge leaks down over the years or the diaphragm splits, the tank fills with water, and the pressure spikes return. Tap it with a knuckle: a healthy tank rings hollow on the air side, and a waterlogged one thuds like a full can from top to bottom. Whether your house needs one at all is answered separately in do I need an expansion tank on my water heater; this page is about putting one in correctly.
Signs an expansion tank is needed or has failed
- The relief valve on the water heater drips while the burner or elements are on, then stops
- A pressure gauge on a hose connection climbs sharply during a heating cycle and falls when a tap opens
- Taps surge for a second when first opened, or toilet fill valves hiss and weep
- An existing expansion tank sounds full and dull when tapped, or spits water from its air valve
- A backflow preventer, meter check valve or pressure-reducing valve was added and the dripping began afterward
How it works
A gauge with a telltale needle goes on a hose connection to record static pressure and the peak during a heating cycle. We also look for the check valve or pressure-reducing valve that closes the system.
The tank is pre-charged to the measured static pressure before it sees water, tied into the cold supply above the heater with an isolation valve, and supported independently of the pipe.
With the heater recovering from cold, the gauge should barely move. We check that the relief valve stays dry, mark the charge pressure and date on the tank, and show you the tap test. For a quote, tell us the heater size and whether the relief valve is dripping.
Common questions
What does an expansion tank installation cost?
There is no flat figure we publish, because the price depends on the tank size, how accessible the cold line is, the pipe material being tied into, and whether a pressure-reducing valve is needed as well. Added during a water heater replacement it is a small extra; as a separate visit it is mostly labour. Bands for other work are on the pricing page.
My relief valve is dripping. Is it the valve or the pressure?
Test before replacing anything. If the drip only happens while the heater is recovering and a gauge shows pressure climbing toward 150 psi, the valve is doing its job and the cure is expansion control. If it drips at normal pressure with the heater idle, the valve seat is fouled or worn; that repair is relief valve replacement.
Do houses on a well need one?
Usually not an extra one. A private well system in Kilbride, Campbellville or rural Caledon already has a pressure tank that absorbs expansion, provided nothing blocks the path between it and the heater. If a check valve or a treatment system with a one-way device sits between the two, the heater is on a closed section and needs its own expansion tank.
Can a failed expansion tank be re-charged instead of replaced?
Sometimes. Air seeps through the rubber slowly, so a tank that is only low can be isolated, relieved of water pressure and pumped back up to house pressure. A tank that pushes water out of its air valve has a ruptured diaphragm and is replaced. Checking the charge once a year, with the water side depressurized, catches both conditions early.
Is this the same tank that sits on a boiler?
No. They work on the same principle, but a boiler's tank lives in a closed heating loop at low pressure and is built of plain steel. A water heater's tank holds drinking water at full house pressure, so it has a lined shell and a higher rating. They are not interchangeable; the heating version is covered under boiler expansion tank replacement.