DIY Doctor UK DIY Forums

Hi All,

Hope someone can point me in the correct direction here with this strange pressure issue I'm experiencing with our heating system.

It's a Camray Oil fired combi boiler. System is installed for a bungalow with 9 radiators attached. Hot water works perfectly.

I've had trouble with it since last year on and off and a couple of engineers have looked at it but have been unable to solve it.

Basically the pressure gauge on the control panel shows 0. Most of the time this is no problem, however from time to time it seems to cause the pump to squeal loudly until its topped up again.

If I top the system up to pressure (roughly 2bar) and then run the heating, the pressure will climb until it hits 3 bar and the safety valve opens and dumps out some water.

Last night I topped the system up to just half a bar and then ran the heating. The pressure then climbed up to about the 2 bar mark, however I've just come home tonight and checked it and its back down at 0.

Later tonight I'll run the heating and see what the gauge does, but I'm just wondering if anyone can point me in the direction of what might be wrong.

Cheers,

Steve

Ok, so tonight after it's been running for a while the pressure has gone up to almost the 3 bar mark.

Other strange thing. I'm sure previously that when the thermostat called for heat, the boiler would just run until the desired temp was reached, then it would switch off until called again.

Now it appears to run for some time, then the burner switches off for a few minutes, although the 3 port valve remains open and therefore water still circulates in the heating circuit. Thereafter the burner will kick back in again and so on until the temp is reached and the stat stops calling for heat.

Any ideas?

Running a combi boiler at zero pressure risks serious damage to internal components so should not be considered ever as ok.

The system is fitted with an expansion chamber which should allow for the expansion as the water heats. If this is faulty, the pressure will build up as the boiler fires and the excess water will be vented out of the prv. The symptoms you describe are classic for this type of fault.

Has the pressure in the expansion vessel been checked? This is not an uncommon fault and an experienced engineer should spot it early on.

Thanks plumb bob!

Having read some other posts I was wondering if a faulty expansion vessel might be to blame.

Am I correct in assuming that if I dump off the pressure in the system I can measure the vessel with some sort of gauge?

Also, what would be the ideal pressure, or would you need more info on that?

You can dump the pressure by operating the prv manually.

Somewhere on the expansion vessel will be a car tyre type valve which allows you to connect a bike or tyre pump.

Checking the pressure is just a matter of connecting a gauge to the valve. Check the handbook for the correct pressure, but it is often somewhere in the 30's psi.

Let some air pressure out of the vessel manually (if there is any) to see what comes out. Be on the lookout for anything that would indicate water mixed with the air (water vapour, rust etc). This would indicate the vessel has failed.

Cheers,

I'll give that a go and see where I get to with it.

OK, so managed to check the pressure in the expansion vessel. 3.5psi.

Didn't seem to be leaking water into the air chamber so I've depressurised it to 20psi for now.

Not sure if that's enough but I didn't want to go wild with it as I don't have the manual.

No change on behaviour though. Stuck 0.5bar pressure on the water side and fired it up. Pressure crept up and up and up until the PRV operated, although it did so around the 2.5bar mark, so either the gauge is not very accurate, or the PRV opens a little early.

Any more ideas or do I need to try another engineer?

Sorry, you have confused me. The pressure was 3.5 psi and you lowered it to 20psi???

The maximum pressure should be 22 psi (I think) but would normally be between .5 and 1.5 bar (7 -22 psi)

"The Expansion Vessel is suitable for systems with a static head of up to 5 metres (16.5ft) i.e. the vertical
distance between Expansion Vessel and highest point of system, usually the top of bedroom radiators. If the static head is greater than 5 metres, then the charge in the vessel must be increased to equal this higher static pressure. The pressure can be increased if required by simply pumping up the vessel with a standard air type pump and checking the pressure with a tyre gauge with the system at zero pressure. A Schrader type valve is provided on the vessel for this purpose.

The air charge should not exceed a pressure of 1.5 Bar (22 p.s.i.)"

If the existing pressure vessel is in good working order but the pressure continually rises above 2.5 bar when hot, it may be indicating your system requirement is greater than it's capacity and an additional pressure vessel must be added.

Sorry, that was a typo, should have been re-pressurised, not de.

I pumped it up using a foot pump with a gauge on it until it was just over the 20psi mark. I then checked it with my digital pressure gauge I keep for checking the car tyres with and it was reading 22psi.

As you say the 22psi should be more than enough, considering its a bungalow so the highest top of any radiator would be within 1-1.5m max which would be the top of the towel rails in the bathrooms.