Combi boiler Q

[rick]

My son has just bought his first house - has a Ravenheat 780 CSI(T)AAA combi boiler. with RHVT 2 wireless thermostat.

He is complaining that pressure on boiler gauge is fluctuating sometimes at 0.5 bar then when hot it can be 1.5 or even as high as 3 bar. Plus he says it is often noisy, and sometimes sounds like water running/filling inside boiler, plus although temp control set to 21 degreee (wirless stat) house only gets to 17 degrees and boiler shuts off.

I did bleed the rads and checked wireless stat is correct day/time and programmed for sensible on/off periods.

The wireless stat works on 21 degree 'demand' and 17 degree night modes,

Any tips what to look for for combi issues ? ..... never owned a combi.

[The Natural Philosopher]

failed pressure vessel, or may need pumping up.

Its a combi :-)

[rick]

Can also add that the main index rad takes ages to get warm, even reducing lockshields on the other rads, and having this one wide open it take a long time to get hot ........... maybe just poor system design. Only time it will warm up quickly is if you crank all the other rads closed (did it for a test)

Variable head pump is on max speed.

[Tim Watts]

It is usually filled to 1 bar cold (well the few combis I have fiddled with). Yes, it will go up when hot. Does the guage have a "green" zone that indicates cold pressure?

However 3 bar is very high when hot - might be worth getting it checked.

Water running sounds *may* just be the pump.

[rick]

He advises sounds like blowing noise and water rushing ?

Is there a simple way to test combi pressure vessel .... on free standing ones, I have just operated schrader valve ... if water came out it was knackered.

I'll go check it's charge tomorrow.

[rick]

can I charge it (with air pump) without draining system down.

[Capitol]

Sounds like heat exchanger is blocked. New boiler time?

[John Rumm]

While the pressure will rise at the temperature goes up,it probably ought not go as high as 3 bar.

If the pressure goes too high, then the emergency relief valve will open and vent water outside. This can happen if the expansion vessel has failed or lost pressure. (if the latter, it can be pumped up again)

Is he having to top it up from time to time?

Check the filling loop, and make sure the tap has not been left permanently on. Sometimes people try to "cure" a problem where the boiler contentiously vents when hot by leaving it filling all the time (only works if the cold mains pressure is lower than the vent pressure)

That could just be a bad choice of stat location or a poorly balanced system.

If the pressure vessel does have a problem, then bleeding the rads will likely make for even greater pressure swings.

None of these are combi issues particularly, but sealed system issues. You can have a sealed system boiler without it also being a combi.

[John Rumm]

Yes, just let out a little bit of water out to release the pressure...

[rick]

I'll drop pressure to nil using rad bleed valve .. and leave them open (upstairs rads) while I check charge.

Googling around it seems Ravenheat boilkers has a pretty poor reputation :-(

[rick]

Not that old allegedly only installed 3 yrs ago I'll check date today ....... is heat exchanger blockage a general issue on Combi's

Is it worth trying an flush/cleaner product ?

[rick]

Yes says its dropping to zero overnight ......... have checked all rads valves - no leaks

Filling loop def closed ...

I can move stat anywhere - currently in room which is coldest and not heating up .... range is OK as no comms errors

The system is about as balanced as I can get it ... Index rad fully open (furtherest s=away & largest) All others lockshields reduced to lowest setting that will allow heat required (know it should be 11 degree across F&R)

Not lifted floorboard to look at pipe arrangements so best I can do for now on balance. I could have reduced flow to this rad due to pipe layout I guess,

[Roger Mills]

Yes. Make sure there's no pressure in the wet side - by letting some water out if necessary. Then check the air pressure with a car tyre pressure gauge - and top up with a foot pump to about 0.7 bar (10PSI).

Then open a valve on the filling loop to re-pressurise the wet side to about 1 bar. Then bleed all the radiators - topping up as necessary to maintain 1 bar in the system.

Then watch what happens to the pressure when the system heats up. It shouldn't go above about 2 bar when fully hot. Make sure that no water is discharging to the outside world through the pressure relief valve.

[harry]

Probably failed pressure vessel.

[John Rumm]

Its fair to say they are not the quality end of the market...

[John Rumm]

Not on combis specifically, but any modern boiler with an high efficiency HE - they have lots of small water pathways. The installer must flush the system very well prior to commissioning.

Possibly if that is the problem. However I suspect that if it were the problem you would see the boiler lock out with an overheat error code.

[John Rumm]

Probably the overpressure valve letting it out then. It could just be a valve with crud in it (once they have operated once, they don't always seal again properly), or more likely, its the expansion vessel. To be sure fix a plastic sandwich bag over the pipe and tie on with a rubber band - then you can see if any water comes out even when not there to see it.

If you need a stop gap expansion vessel, then just leave a pocket of air in one rad to soak up the expansion.

Also see:

[Andrew Gabriel]

This would not be my first thought from the listed symptoms.

In any case, if it was, running the system for a week with a desludge/descaler would be a first step.

[rick]

Update - pressure vessel fine .... just as well as would have had to take boiler off the wall to change it.

As it seems to run well - then lose a lot of pressure suddenly - Stripped out the PRV, it has some white crud in it. On reassembly when filling found auto air bleed was locked closed - so set that to open.

System then ran fine for 48 Hrs ......... 1 bar cold 1.8 bar hot

Then lockout - and again pressure down to 0.5bar

Re-pressurised .. runs for a day or so .........

The boiler is supposed to be only 3 years old ..... so certainly not due changing.

Only thing I could do is run a boiler 'cleaning' product in it and see if that clears anything out.

Other than that - I'm out of things to look at.

[The Natural Philosopher]

Have you re pressurised the pressure vessel?

You may need to do that more than once as well as re pressurising the system

And how did you establish that it was 'all right'