How the two differ in kind
Two tanks, frequently in the same plant room, often the same shape and colour, both containing a bladder or diaphragm with an air charge behind it, and both fitted with a valve that looks like a tyre valve. They do unrelated jobs, and substituting one for the other does not work.
A THERMAL EXPANSION TANK exists because water expands when it is heated and because a closed system has nowhere to put the extra volume. The expansion is a small percentage of the system's contents, but water is nearly incompressible, so a small volume increase in a sealed system produces a large pressure rise — enough to lift a relief valve, stress fittings, and in a domestic hot water system cause the characteristic symptom of a relief valve dripping every time the heater runs. The tank gives that volume somewhere to go by compressing an air cushion. It is sized from the system volume and the temperature rise, it is usually small, and it stores no usable water.
A WELL PRESSURE TANK exists because a pump should not start every time somebody opens a tap. It holds water under air pressure and releases it as demand occurs, so the pump runs in longer, less frequent cycles. Its defining quantity is DRAWDOWN: the usable volume delivered between the pressure switch's cut-in and cut-out settings, which is a fraction of the tank's nominal size rather than all of it. Undersize it and the pump short-cycles — starting and stopping repeatedly — which is the single most effective way to destroy a pump and its switch.
What they share is the thing that most often goes wrong on both: the PRE-CHARGE. The air pressure behind the bladder has to be set to the right value, measured with the water side drained, and a tank whose pre-charge has leaked away or was never set correctly behaves as though it were not installed. On an expansion tank the symptom is a weeping relief valve; on a pressure tank it is a pump cycling every few seconds.
The factors that actually differ
Show
| Thermal expansion tank | Well pressure tank | |
|---|---|---|
| What it is for | Absorbing the volume increase when water is heated in a closed system. | Storing usable water so the pump does not start at every draw. |
| Governing quantity | System volume and temperature rise — how much the water expands. | Drawdown: the usable volume between cut-in and cut-out, which is far less than the tank's nominal size. |
| Usable water stored | None. It is a cushion, not a store. | This is its entire purpose. |
| Size | Small — sized to a percentage of the system volume. | Much larger, because drawdown is a fraction of nominal capacity and the pump's cycle time depends on it. |
| Symptom when it fails | The relief valve weeps or lifts whenever the water heats. | The pump short-cycles — starting and stopping every few seconds — which rapidly destroys it. |
| Pre-charge | Set to the system's static fill pressure, checked with the water side drained. | Set just below the pressure switch's cut-in, checked with the tank drained. |
| Where it connects | On the cold feed to a water heater, or on a closed heating circuit. | On the discharge side of the pump, with the pressure switch. |
| Why it became necessary | Backflow preventers and check valves. They close the system that used to expand back into the main. | Any pumped supply without a gravity storage tank. |
| Can one do the other's job | A pressure tank can absorb expansion incidentally in a well system, which is why some installations have only one — but this is a design decision, not a default. | An expansion tank cannot provide drawdown. It is far too small and is not intended to. |
| How it is checked | Tap it — a healthy tank sounds hollow at the top and solid at the bottom. Then measure the pre-charge with the water side drained. | Identical test, plus timing the pump's cycles under a steady draw. |
Which one, and when
Choose thermal expansion tank when…
- The system is closed — a check valve, a backflow preventer or a pressure-reducing valve stops water expanding back into the main.
- The relief valve on a water heater weeps or lifts when the heater runs, which is the classic symptom.
- A sealed heating circuit is being installed, where an expansion vessel is part of the design.
- A backflow preventer is being added to an existing supply, which converts an open system into a closed one.
Choose well pressure tank when…
- There is a well pump, or any pumped supply without gravity storage.
- The pump starts and stops rapidly under a small steady draw.
- The pump is being replaced and the existing tank is old or waterlogged.
- Pump cycle life matters, which it does, because short-cycling is what kills them.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why did my relief valve start weeping after a new backflow preventer was fitted?
- Because fitting it closed the system, and a closed system has nowhere to put thermal expansion. Previously, when the water heater warmed its contents and the water expanded, the small extra volume pushed harmlessly back into the incoming main. A backflow preventer, check valve or pressure-reducing valve stops that — so the same expansion now has to go somewhere in a sealed volume of nearly incompressible water, and the pressure rises sharply. The relief valve does exactly what it is designed to do and lifts. The fix is an expansion tank on the cold feed to the heater, sized for the system volume and temperature rise, with its pre-charge set to the static supply pressure. Replacing the relief valve, which is the common first attempt, treats the one component that was working correctly.
- What is drawdown and why is it not the tank's size?
- Drawdown is the volume of water a pressure tank actually delivers between the pump switching off at cut-out pressure and switching on again at cut-in — and it is a fraction of the tank's nominal capacity, commonly under a third. The reason is that the tank is part water and part compressed air at all times: water only leaves as the air expands from cut-out pressure down to cut-in, and the air cannot expand indefinitely. So a tank labelled with a large nominal volume delivers considerably less usable water than the number suggests. This matters because pump cycle time depends on drawdown divided by the draw rate, and sizing a tank by its nominal capacity rather than its drawdown is how systems end up short-cycling despite having what looks like a generous tank.
- Why is short-cycling so damaging?
- Because the wear on a pump is concentrated in starting rather than in running. Each start draws a large inrush current, heats the motor windings, and stresses the mechanical components; a pump that runs for ten minutes has done that once, while a pump that runs for four seconds thirty times has done it thirty times for the same water delivered. The pressure switch suffers the same way, since its contacts arc on every operation. A short-cycling pump can accumulate a normal lifetime's worth of starts in months. The usual causes are an undersized tank, a waterlogged tank whose bladder has failed so it holds no air, or a pre-charge set wrongly — and all three present identically, which is why the diagnosis is to check the pre-charge with the tank drained before buying anything.
- How do I check and set the pre-charge?
- With the water side depressurised, which is the step that makes the reading meaningful. Isolate the tank, drain the water side so there is no water pressure acting against the bladder, then read the air pressure at the valve with an ordinary tyre gauge. On an expansion tank the target is the system's static fill or supply pressure; on a well pressure tank it is just below the pressure switch's cut-in setting. Adjust with a pump or by releasing air, re-check, and only then restore the water side. Reading the valve with water pressure still on gives a number that tells you nothing, because the two sides are balanced across the bladder. If air will not hold, or if water comes out of the air valve, the bladder has failed and the tank is replaced rather than repaired.
- How can I tell which tank I am looking at?
- By size, by position and by what is written on it, in that order of usefulness. Size is the strongest clue: an expansion tank is small, typically sized to a modest percentage of the system volume, while a pressure tank is large because drawdown demands it. Position is the next: an expansion tank sits on the cold feed to a water heater or on a closed heating circuit, while a pressure tank sits on the pump's discharge alongside the pressure switch. The label and the rating plate settle it, and they also carry the maximum working pressure and temperature, which differ — a tank intended for a cold supply is not necessarily rated for hot water, and using one where it does not belong risks both the bladder material and the pressure rating.
- Can one tank do both jobs?
- In a well system, the pressure tank does absorb thermal expansion incidentally, because the system is not fully closed toward it and the tank has air to compress — which is why many well installations have no separate expansion tank. That is a design conclusion rather than a default, and it stops being true the moment a check valve or backflow preventer is fitted between the pressure tank and the water heater, which isolates the heater into its own closed volume. The reverse never works: an expansion tank cannot provide drawdown, because it is far too small and is not intended to store usable water. Where both functions are needed and the system has been sectioned by valves, both tanks are fitted, each on the part of the system it serves.
- How is an expansion tank sized?
- From the volume of water in the closed system, the temperature rise it will see, and the pressures involved — the system's fill pressure and the setting of the relief valve it is protecting. Water expands by a small percentage over a typical heating range, and that percentage applied to the system volume gives the expansion volume the tank has to accept; the tank then has to accept it without the pressure reaching the relief valve's setting, which is what brings the two pressures into the calculation. The common errors are underestimating the system volume, particularly in a heating circuit with long pipe runs and heavy emitters, and ignoring the pre-charge, since a tank whose air pressure does not match the fill pressure has less usable acceptance volume than its rating implies.
- What does a waterlogged tank mean?
- That the bladder or diaphragm has failed and the tank has filled with water, leaving no air cushion — so the tank is now a plain steel vessel doing nothing. On a pressure tank the symptom is immediate and unmistakable: the pump cycles every few seconds under any draw, because there is no stored volume between cut-in and cut-out. On an expansion tank it presents as the relief valve weeping again, having been fixed by fitting the tank in the first place. The diagnosis is the same in both cases: tap the tank, which should sound hollow toward the top and solid toward the bottom, and check the air valve — if water comes out of it, the bladder has failed. A waterlogged bladder tank is replaced; there is no field repair.