Potterton Profile gas boiler

This boiler is well worth keeping if you have one, in my opinion. It was a development of the well known Potterton Netaheat and being a simple, basic, well-made boiler that’s reliable and easy to install, and equally easy to maintain, it was phenomenally successful. It was only discontinued because the government changed the Building Regulations in 2005 making slightly more fuel-efficient condensing boilers mandatory, and this is not a condensing boiler.

It has a cast iron heat exchanger, electronic ignition (so no permanent pilot light) and a fan-powered flue, so extended horizontal flues are possible and the boiler does not have to be installed against an outside wall. The Profile is also ‘room sealed’, which means the combustion process inside is completely isolated from the atmosphere inside the house, so should a combustion fault develop and dangerous carbon monoxide be produced, it cannot escape from the boiler into the house and put the occupants at risk.

It’s quite unfortunate that owners of these excellent boilers are sometimes advised to replace them on the grounds that its ‘old and inefficient’, or that parts are unavailable, neither of which is true.

Inside the Potterton Profile:

Here is the boiler with the front removed showing the heat exchanger, the fan and the gas control valve etc at the base.

Ignition and operating sequence:

When the external controls (programmer, room and/or hot water cylinder thermostat) call for heat an electrical signal is sent to the boiler asking it to light, then the ignition sequence is as follows:

1) The control board received the ‘call for heat’ signal and checks to see if the boiler temperature control is calling for heat. If it is, the ignition sequence proceeds.

2) The fan starts, which draws fresh combustion air from outside and blows it into the boiler sealed case. The combustion air passes over the burners, up through the heat exchanger and out through the flue tube to outside again.

3) The air pressure switch operates. As it would be dangerous for the gas flames to ignite inside the boiler in the event of failure of the fan, there is a safety device to test and check that the fan is running before the pilot flame is turned on, the air pressure switch, or APS for short. The APS tests for the small difference in air pressure present between the inlet and the outlet of the fan when it is running. When the APS detects this difference in pressure, it tells the control board the fan is running and the pilot flame can be safely lit.

4) The pilot flame lights. Quite a lot happens here. The control board detects the fan is running, it opens the pilot channel on the gas control valve which feeds gas to the pilot light assembly, and it delivers a stream of sparks to light the pilot gas. Whilst delivering the sparks it looks for the presence of the lit gas flame in the spark gap using a process called ‘flame rectification’, and when it detects that the pilot flame is alight and burning, it stops the spark.

5) The main burners light. Once the control board detects the pilot flame, it opens the main burner channel on the gas control valve and gas flows to the main burners. The previously lit pilot flame ignites the main burner flames and the boiler heats up.

6) All the while the flame should be alight, the control board uses flame rectification to monitor the pilot assembly for the presence of combustion. If combustion ceases to be detected for any reason, the control board shuts off the gas on both channels of the gas valve then goes through steps 1 to 4 above in order to attempt to re-light the boiler.

7) The boiler sooner or later reaches the temperature set on the boiler control thermostat, the thermostat stops calling for heat and the control board shuts off the gas and the fan, and waits for the next time the boiler temperature control thermostat calls for heat, and the process begins again.

So by understanding the ignition and operating sequence, fault tracing becomes quite straightforward. This boiler is easy to diagnose and fix!

If yours needs fixing and you’re within driving distance of me on the M4 corridor, feel free to call or text me on 07866 766364.

Mike