Boiler expansion vessels

The expansion vessel within a sealed system boiler takes the place of the feed & expansion tank in an open-vented heating system. It allows the system to be pressurised and maintain a fairly steady pressure while the water inside expands as it heats up. Having an internal pressure above that of the atmosphere helps to prevent air getting in and causing corrosion.


The way water behaves with temperature is shown in one way in the following graph, which shows the percentage change in density relative to 4 celsius, where its density is highest. At low temperatures, below 20C, changes are relatively small, but by 80C, the density has fallen by 2.8%:

Another way of putting this is that under the same pressure, the water occupies 2.8% greater volume than it did when cold.


Water isn’t, as you sometimes hear, incompressible, but it is pretty hard to compress. If you apply 1 bar of pressure, its volume shrinks by only around 46 ppm (parts per million). If you had 100 litres of water and wanted to pressurise it to 1 bar using a piston in a 22mm pipe, that piston would only have to move 14.5mm.


If instead of allowing the water volume to increase by 2.8% as it heats up to 80 C, we kept it in a pressure vessel and didn’t let it expand at all, the vessel would have to withstand 603 bar, that is, 603 atmospheres of pressure! (Not in my house, thanks!)


The expansion vessel is a practical solution for a sealed system. The water is allowed to expand, pushing a membrane behind which is pressurised air. This is a very neat solution: a vessel of only around 10 litres in capacity can cope with small to moderate sized domestic heating systems: no feed & expansion tank, and no pipework in the loft, at risk of freezing in winter.


However, everything involves a trade-off. Over time, as with a tyre, air escapes from the expansion vessel. (At worst, its internal membrane can split.)


As the internal air pressure (with the system cold) falls, the air is less able to resist the expansion of water, and the heated pressure rises. When it reaches (generally) 3 bar, the boiler’s pressure relief valve opens, allowing water out of the pressure relief outlet pipe. When the system cools, the pressure will have fallen. As the process repeats, eventually the system pressure will fall below what’s needed to operate the boiler, and it will decline to fire.


The remedy seems simple – re-pressurise using the filling loop – pressure restored – job done. Unfortunately, it isn’t so – the cycle will repeat itself – use of the filling loop will become more frequent, as will the discharges of water. Worse, the pressure relief valve spring may weaken over time so that it releases water at lower and lower pressures and needs to be replaced.


What needs to be done is to isolate and drain the boiler to release the water pressure, and re-charge the expansion vessel using, typically, a foot-pump. The main bugbear for me at the moment is the inaccuracy of air pressure gauges (and just because a gauge has a digital readout doesn’t make it more accurate). That noted, I’ve seldom come across an expansion vessel that didn’t need re-charging.


Here are some facts and observations, some resulting from calculation:

  • When an entire system has been drained and re-filled, some of the pressurising air will be in pipes and radiators and not just the expansion vessel. This air has to be released first.
  • When a system is pressurised, the expansion vessel’s internal air pressure equals the water pressure as indicated on the system pressure gauge (usually in the boiler).
  • For this reason, in order to pressurise a system at all, the water pressure has to exceed the expansion vessel air charge pressure (or the water isn’t pushing on the membrane).
  • If the air charge pressure is low, more water will enter the expansion vessel, leaving less room for water expansion due to heat.
  • So, allowing for gauge inaccuracy, charge the expansion vessel to the upper end of the range allowed by the manufacturer (usually somewhere between 0.5 and 0.9 bar, depending on the vessel).
  • Go easy on over-filling with water, and stick to the manufacturer’s recommended cold system pressure, commonly 1 bar.
  • Keeping the system water temperature low helps to keep the pressure rise down. It also helps to increase the efficiency of a condensing boiler (return water that’s too hot prevents condensation of the combustion water vapour). That’s achieved by radiator balancing, though people seldom want this done unless there’s an acute problem.
  • Since the volume increase in heated water is in proportion to the total volume of water, large systems will need an additional expansion vessel fitted external to the boiler on the return. To take an extreme example of 80 C water expanding by 2.8% and an 8 litre expansion vessel, a cold system containing 8 x 100/2.8 = 286 litres would expand by the entire 8 litres of the vessel. One manufacturer with an 8 litre vessel states a limit of 143 litres before an external vessel is needed, exactly half of this, allowing at most half the vessel to be taken up by expanding water (with a full air charge).
  • Different manufacturers have different criteria for specifying when an external expansion vessel must be used, so it’s best as always, to follow their instructions.