Firebird combi pressure

[FrozenNorth]

I've had no end of problems with this machine and it's only 3 years old! To cut a long story short, I'm having problems with the pressure which keeps dropping, boiler cuts out and the much-used but crazily-placed reset switch is getting worn out, just like it's owner....
We've had the filling loop replaced, a new pressure relief valve fitted, an alleged expert from Firebird has visited, I've checked as many internal joints as we can with no signs of leak or salting and a whole load more besides. Unless anyone has some really bright ideas could someone answer me a simple question?
Whereas I'm not best happy to have a hidden leak somewhere and as the boiler is outside, is there such a thing as a valve that replaces the filling loop and automatically holds a 1.5 bar pressure in the system?
Thanks

 

[MANDATE]

You've not mentioned the expansion vessel!
I would say it's most likely the source of your problem.
It's supposed be charged with air at about 0.8bar when fitted. This pushes the diaphragm to one side, which means its all 'air' in the vessel.
In operation you pre set the water pressure say to 0.8bar, so vessel still the same.
Now with heating on, the water expands as it gets hotter and the extra volume has to be accommodated. This expands against the diaphragm and compresses the 'air' and reduces the air volume (lets say by half for example).
The 'air' compressed, now has an increase in pressure. (lets say it goes up now to 1.8bar. That's a 1bar increase which is about what is expected.
With heating off it will return to the original pre set figure.

Now suppose the 'air' charge has leaked over time and it's only 0.4bar.
When you pre set the water pressure to 0.8bar the diaphragm will be pushed over until the forces on each side are equal(having only 0.4bar opposing it to start with) and the volume of'air' will be much reduced. (lets say vessel is now only half full of 'air').
When you operate the heating now, the vessel still has to accommadate the same extra volume, but because the 'air' has been halved the resulting pressure is doubled.
Water pressure would now rise from 0.8bar to 3.6bar.
The PRV is set to open at 3bar, so it discharges water, pressure drops to zero and boiler will not start again, because of pressure switch.
So extra water is added to get back to the 0.8bar when the boiler works again.
Even when the vessel has been checked and corrected, its highly likely the PRV will need replacing.
If you're starting with a water pressure of 1.5bar, the diaphragm could be in a position where there is almost no 'air' to compress.
Hope this might help.

 

[FrozenNorth]

Thanks Mandate, think I understand - is rectifying this something an amateur could attempt, would he be better advised to get a heating engineer to do or would the vessel need replacing?

 

[MANDATE]

The vessel would only need replacing if the diaphragm was split/ruptured and water could pass from the water side to the air side.
It appears adding another vessel in a convenient position is sometimes a better option than replacing the existing one.
Other than that, its a matter of pumping some air in and testing the pressure.
A car foot pump with gauge is OK or even a bicycle pump and air pressure gauge.
What is important is that the water taking up the volume where 'air' should be has to be somehow discharged. So water pressure must first go to zero and then some draining off to take place, or you could open the PRV by hand.
If you pump air to about 1bar you may find it will drop a little as water is discharged, if it does not then like the car its easily adjusted
When you refill with the filling loop its wise to keep the pressure about 1bar.
In operation it should go up to about 2bar. Obviously if it approaches 3bar you most likely have not discharged enough water.
I reckon you'll manage it and find a vast improvement!
Do let us know how you go on

 

[FrozenNorth]

Cheers - I'm working today so won't attempt it until this evening (assuming I've got the bottle!) but I've a couple more questions. First, it appears that I've two expansion vessels but there only seems to be one place to put air in. Also, there's no trace of water at the pressure valve or anywhere else around the boiler which I would have expected - does this sound right? Last (just curious) the vessels have no lagging so could the temperature affect them? It's been mighty cold but I suppose that's stating the obvious!
Thanks again

 

[MANDATE]

With two expansion vessels, it would suggest the original one was faulty and considered beyond repair so adding the second one was the easy option. In which case I'd just concentrate on the second one.
Each vessel has got to have a schrader type valve in order to charge it.
The PRV normally allows water to be discharged through a pipe outside, so there will not be any sign of water inside even if the PRV was not sealing correctly.
The only concern I would have regarding temperature, is if the PRV was passing water and ensuring the discharge pipe was not frozen.

 

[FrozenNorth]

Thanks again Mnadate - the pressure has dropped in the expansion vessel but it's going to have it's service brought forward a month so I'll let the engineer fine tune it, hopefully before the snow comes again! In the meantime I've been keeping the pressure at just under 1 bar and it's been behaving itself for five days now with only two small top ups.
Guess it's a really fine balance between the two pressures?

 

[MANDATE]

Sounds like you've corrected the problem of the expansion vessel if its operated for five days and not opened the PRV. that's got to be a big improvement.
So with pressure set to 0.9bar it should rise to around 1.9bar.
Your two small top ups may be the result of bleeding, but if it continues to need top ups, I would suspect the PRV may not be sealing fully. A very small amount of dirt on the seal could allow a small amount of water to pass. I don't think you'd see the evidence cause it would dry up quickly.

Thanks for letting me know you've had some success