Potterton Puma 80 Boiler Pressure keeps rising above 3bar

[ianm00]

I have a Potterton Puma 80 Combination Gas Boiler to provide hot water and central heating.

The problem is that the boiler will not maintain the recommended 2bar pressure. I have to keep pressurising the system every few days by letting more water into the system.

I cannot find any water leaks in the heating system.

I think I know why its losing pressure. When the heating is on and gas burner is burning the sytem pressure increases above 3bar. At 3bar the pressure relief valve opens to release the pressure by letting water out of the overflow.

I replaced this valve as initially I thought that this valve was leaking but I still have the problem.

Im guessing that the problem is concerned with the burner heating the water too much and causing the pressure to rise abnormally and then causing the relief valve to do its job!

Does anyone have any idea/clues as to why this would overheat? It must be faulty thermostat or something only Im not sure which one if any or maybe something totally different.

I have also checked the pressure in the expansion vessel, its about 0.75bar and only air comes out of the valve so I think the vessel is ok, Maybe the pipe has a blockage can this happen?

Thanks for any help from Boiler Engineers.

 

[kevplumb]

im sure to be shot for posting this but i would think 2 bar is when hot .pressure when cold 1 bar usually would explain why ?

 

[ianm00]

I set the pressure at 2bar when cold as that is where the red marker is. Also in the past the boiler has been set to 2bar pressure and it hsa worked fine.

So something has definitaly gone wrong with the boiler but what?

 

[kevplumb]

the red marker is the top not the bottom
if it has worked ok till now whats changed ?

 

[ChrisR]

You may have lost the precharge pressure in your expansion vessel. Lots on here about that - do a search. 2 bar cold is too high. Set it to 1 bar cold.

Also you may find that the presseure relief pipe - the one through the wall - keeps dripping. They tend to do that once opened. If so it will need attention because you will keep losing pressure.

 

[ianm00]

Kev,

The red marker I talk about is a pointer on the gauge set at 2bar. The gauge shows red at low pressure 0bar to 0.5bar, then green from 0.5bar to 3bar, then red from 3bar to 4bar.

What has changed? Nothing obviously something with the boiler has gone wrong thats why Im asking the questions to try and find out what is wrong!

Chris R
The PRV has been replaced and the sytem only lets water out at pressures above 3bar. I have tried with 0.5bar in the vessel & 1.0bar in the vessel and it still doesn't work.

Ian

 

[ChrisR]

The movable red marker is easily pushed about and I assure you is set wrongly, unless your house has about 3 or more floors above the boiler.

How many rads do you have, and are they older ones? These might add up to a system volume which is too big for the fixed pressure vessel in your boiler. What could have changed --well you might have bled some air from radiators, which would have been acting like a pressure vessel.

I wonder how you went about measuring the pressure vessel precharge - easy to do it wrongly. If you set the precharge at 0.5 bar then set the system pressure at 2 bar, the gas volume inthe PV will be very small - it's hardly surprising the pressure rockets when the water expands a bit. For the PV to be 50% gas, the precharge would have to be half the system pressure.

If your Pressure Relief valve EVER HAS let water out, it's a very good bet indeed that it's doing it whenever the boiler gets hot, if not all the time. They are all a bit like that. Hot water would evaporate very quickly so you'd not see it. But do not think about capping the pipe!

Over what period are you losing how much pressure? Can you tell if it does it with the boiler off?

 

[ianm00]

Chris R,

I have tried with the pressure in the vessel at about 0.5-0.75 bar. Measured using tyre gauge with the system pressure reduced to zero.

Then I pressure the system at 1bar as it appears 2bar is too high.
The pressure is not lost if I do not turn on the central heating.

I bought the house new about 8 years ago and the system has worked fine since then. No new rads etc have been added.

If I turn on the CH within about 5mins (not all rads are even hot) the boiler pressure rises to 3+bar which obviously causes PRV to open and hence I lose pressure when it cools.

 

[kevplumb]

is the boiler any noisier than normal?

 

[ChrisR]

Is it overheating - does the boiler stat work - ie will it run OK at a lower temperature?

-thinking along same lines as Kev perhaps

 

[ianm00]

No the boiler seems OK no noises etc.

I don't think its on long enough to overheat as the rads don't reach full temperature before the pressure is above 3bar.

Its a combi boiler and the hot water side works perfectly, its just the CH side.

 

[kevplumb]

just re read your first post if the boiler aint noisy and pressure rise is fast could be sludge blocking the expansion vessel pipe never known it but noy beyond the bounds of possibility
obviously the ex vessel aint doing its job so it might be worth a look

 

[ianm00]

Kev,

Yes I think it must be something to do with the expansion vessel. I think its presurised OK and the valve doesn't let water out only air so I think the diagphram in the vesselis OK.

The pipe is very difficult to get off of the expansion vessel but that is my next task to check for blockages.

Ian

 

[kevplumb]

sounds like the way to go the bore of the pipe is not very large you might be able to squeeze it see if its solid if it is blocked it will be hard sludge
prob easier to replace the pipe than try to clear it

 

[ChrisR]

Good thought.

Check the pressure at the Pressure/exp vessel when the system is at 3 bar. It should be way above the precharge level if the vessel is connected.