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Potterton Puma 80e Combination Gas Boiler Faults, Problems and Fault Finding

It was clear when we arrived that the 3 bar pressure relief safety valve had been letting water pass out of the system and out of the building through the pressure relief safety pipe. This could be simply a fault with the pressure relief valve but much more commonly it's caused by the failure of the expansion vessel in the boiler or the loss of charge pressure from the expansion vessel.


Almost all combi boilers are sealed system boilers. When water is heated in a sealed system it will expand as it cannot be significantly compressed. Without an expansion vessel in a sealed system the pressure would rise rapidly and pipes or joints (or heat exchanger or radiators) would burst so sealed system boilers contain an expansion vessel which accommodates the expansion and a 3 bar pressure relief valve to allow water to pass out of the system if the pressure rises too high.


The expansion vessel has two chambers separated by a rubbery diaphragm; one side of the diaphragm is connected to the radiator system pipework and the other side is pumped full of nitrogen or air. When the expansion vessel is manufactured the air side is pumped up to about 1 bar (15 psi) and this pushes the diaphragm right across to the other wall of the vessel. As the heating system is filled with water (usually up to a pressure of 1 bar or a little more) the water enters the other side of the expansion vessel and pushes the diaphragm back towards the middle. Now when the radiator system is heated up the water has a space into which it can expand, squashing the air on the other side of the diaphragm. There is an increase in pressure but usually of only up to a bar.

Potterton Puma 80e Combination Boiler Fault

Customer reported that the boiler was locking out and the pressure kept dropping

The Potterton Puma 80e and Puma 100e boilers and the Puma 80 and Puma 100 permanent pilot combi boilers all use a similar expansion vessel and 3 bar pressure relief safety valve.


The boiler problems detailed in the article above apply to the Puma 80, Puma 80e,

Puma 100 and Puma 100e combi boilers.



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Changing the internal expansion vessel wasn't going to be easy as there was insufficient clearance above the boiler; we'd have had to take the boiler off the wall. We decided to fit an external 18 litre expansion vessel, connected to the central heating return pipe close to the boiler.


Once a 3 bar pressure relief valve has been operated, the valve will often refuse to re-seat properly and will drip water out through the pressure relief pipe, again losing pressure. We don't use the safety valve to drop the system pressure unless the valve has already been letting by. Dropping the pressure by draining water from a drain cock may be more effort but it saves changing the safety valve. The Puma boiler has a small bleed valve which can be used to drop the pressure. It's fitted on the flow pipe just below the diverter valve assembly.


The pressure relief safety valve on this boiler was slowly letting by so we changed it too and then raised the system pressure to just over 1 bar. The boiler then operated normally, with a rise in pressure from cold to hot of only about 0.3 bar. That rise in pressure will vary depending the size of the heating system and the size of the expansion vessel. It also depends on the rise in temperature of the system from cold to hot but typically it's up to a bar.


Finally, where we've not been able to replace an expansion vessel straight away and the customer needed the heat, we have allowed air into one of the radiators (half full of air was usually OK). The air in the radiator then temporarily took up the expansion, allowing the boiler to run.

After re-pressurizing the water side on this boiler using the filling loop, and firing the boiler, the pressure rose rapidly and again forced water past the pressure relief valve. We dropped the system pressure back to zero by releasing water and checked the charge pressure on the air side of the expansion vessel. The connector is a standard Schraeder valve like a car tyre valve and you can use a tyre pressure gauge or a foot pump with a gauge. The charge pressure was zero so we pumped it up to 1 bar. Sometimes that's all that's needed if the diaphragm is still intact and the valve is good. (Over years the air can pass across the membrane as it is slightly permeable.)


This diaphragm had clearly failed as the vessel wouldn't hold charge pressure for any length of time. If, when you test the pressure, you get water at the Schraeder valve it indicates that the diaphragm has failed. If the diaphragm is split you can often hear air bubbling into the heating system as you try to pump up the expansion vessel.