Combi-Boiler at 3-bar!! How can I release some pressure???

[Cronky]

Hi,

I was having problems with my 'Sime 80c Format' combi-boiler - the water wasn't getting very hot (still isn't actually!). Anyway some smart ass thought that this might be solved by increasing the pressure to 3 bar (max!!!). Obviously this was a mistake.......but how can I now get the pressure back down to 1 bar??????

(...and any ideas further ideas on why the water won't get very hot - the heating is fine?)

Cheers!

 

[ChrisR]

You'll have to find a drain point and let some water out. Do not try to use the pressure relief valve, cos it'll leak afterwards.

cool hw - usually a sludged up secondary heat exchanger. Cleanable. Sime are bust now, by the way!

 

[oilman]

There is a small chance of the PRV leaking after you have used it to release the pressure, but then you'll just have to replace it, sorry.

The PRV should be checked at EVERY service, (annually) since the pressure in the system has to be relieved to allow the "air" pressure side in the expansion vessel to be checked. (Unless, of course, you have an open vented system). You need to check the diaphram in the expansion vessel is still sound. It's important to check the expansion vessel and it's diaphram are ok because the vessel will fill with water if the diaphram fails, then the only way the system copes with expansion is via the radiators and boiler, leading to premature failure.

 

[MANDATE]

I would have thought that bleeding a radiator would reduce the boiler pressure. Can anyone confirm this?

 

[oilman]

yes you're right, bleeding a radiator will reduce system pressure, if the diverter valve is set to heating, though the pressure will probably go down but slower if hot water is selected. However, there will be quite a lot of water to collect to reduce the pressure significantly. It would be easier to use the PRV (pressure relief valve), as there is a purpose made drain to the outside.

 

[ChrisR]

Bleeding a rad will always reduce system pressure - even if water is being drawn ( though it seems a bit strange to do that!) because the return side of the system is just as connected to the rads whatever the position of the diverter.

Do Not check the prv operation as part of a service. AFAIK it is not part of any manufacturer's instructions, though they are all quick to tell of the risk of opening it. It would be really asking for trouble.

Quite a number of combi boilers have drain valves underneath - tho I can't remember which.

 

[corgiman]

Bleeding a rad will always reduce system pressure - even if water is being drawn ( though it seems a bit strange to do that!) because the return side of the system is just as connected to the rads whatever the position of the diverter.

Do Not check the prv operation as part of a service. AFAIK it is not part of any manufacturer's instructions, though they are all quick to tell of the risk of opening it. It would be really asking for trouble.

Quite a number of combi boilers have drain valves underneath - tho I can't remember which.

Must say chrisr that I agree with Oilman and I have always checked the PRV as part of a service on a combi, sealed system unvented hot water cylinder (which is i manufactuerers requirement) and find that at best they leak and need a clean or at worst need a new washer and always use PRV to drain down boiler when changing say a diverter valve. WOuld strongly agree with your statement if it is directed at non qualified person if so ignore my last

 

[ChrisR]

Weeel, unvented HW is clean so there's no dirt problem, yes they must be checked. (though who checks the actual pressure?).

Many a combi prv doesn't have a washer, which is the seat (ha) of the problem, and a lot don't come apart to clean. I won't risk being called back to mend a problem I'd be accused of causing. (Similarly I wouldn't readily touch isolating valves cos they leak.) I used to check everything like Hi limit stats, but now I do just what it says in the manual. Don't think I've ever seen one which says check the prv. (Bet you prove me wrong!) If someone wants a stripdown service, including eg cleaning the diverter and renewing diaphragms, that's different.

Apparently the reason BG 's "service" became just a sniff of the POC's was because deeper servicing increased unreliability. Not a policy I'm happy with but it's credible.

 

[corgiman]

Weeel, unvented HW is clean so there's no dirt problem, yes they must be checked. (though who checks the actual pressure?).

Many a combi prv doesn't have a washer, which is the seat (ha) of the problem, and a lot don't come apart to clean. I won't risk being called back to mend a problem I'd be accused of causing. (Similarly I wouldn't readily touch isolating valves cos they leak.) I used to check everything like Hi limit stats, but now I do just what it says in the manual. Don't think I've ever seen one which says check the prv. (Bet you prove me wrong!) If someone wants a stripdown service, including eg cleaning the diverter and renewing diaphragms, that's different.

Apparently the reason BG 's "service" became just a sniff of the POC's was because deeper servicing increased unreliability. Not a policy I'm happy with but it's credible.

I agree totally with everu thing you said mate

I am really coming across on this esteemed forum as a pendantic sod who will bend over backwards to prove people wrong

And your right about BG manys a time that i have gone to a client who used to have their appliance serviced by them for them to say " well they ahave never done that" whaen i am only cleaning the heat exchanger

 

[oilman]

The reasons for operating the PRV:
1 is to ensure it is not stuck on its seat.
2 is to reduce the water side pressure to zero so the air side pressure can be checked and reset if necessary. this also allows confirmation of the integrity of the diaphram.

If I find a leaky valve is a leaky valve to replace.
If you do not test the valve, how do you know it is not stuck?
What is the purpose of a PRV?
What is the boiling temperature of water at the specified blow-off pressure of most PRVs?
What happens to water at this temperature if pressure release is uncontrolled?
Which was cheaper, a replacement PRV or .................

Statistics may protect you most of the time, but....

Grant UK is one manufacturer that recommends these tests.

 

[oilman]

oops, that is the diaphram in the expansion vessel. Sorry for any confusion.

 

[breezer]

as i am not a plumber or heating engineer we have to "make do" with BG

(long story but to be honest they are better than the others we had, lets leave it at that)

our boiler (combi) is going to be (annual) serviced in about 10 days.

so i know it is being done right, what should i look for as he drinks the tea?

I also know that you will say look for this do that, but in general, what are the Main things i should see if he does (and ask why not if he doesnt)

 

[oilman]

Hi breezer, is yours a gas or oil combi?
the checks I outlined for the expansion vessel is a must from my point of view, (but then I'm a bigot) whether oil or gas. If the expansion vessel diaphram has failed the expansion vessel will be heavy, and if you turn it upsidedown and depress the air valve, water will come out. If this is the case, it will not be working as an expansion vessel. Instaed the expansion of the water as the system heats up will force the radiators and boiler to cope with the expansion.

The expansion vessel air pressure should be as low as possible to just exceed the static pressure of your system. 0.5 bar for 17ft top to bottom (ok for 2 floors) 1 bar for 34ft (and pro rata). The pressure is needed so it can push the water up to the highest bleed point. The lower the pressure the greater volume of system water can be accomodated. You can never have a vessel that is too big.

If its oil, I can give you more info, if its gas there are better people to listen to.

 

[ChrisR]

Breezer - You should have a copy of the manufacturee's Installation and Maintanance instructions - by law. You can wave it at the BG man who will then probably go through the list within.. I think BG's wording on what they do is someting along the liines of a "Safety check". It could be argues that if the thing is burning OK then it isn't dangerous. SOme of the above would obviously disagree!

Normally there are various checks around the ignition and combustion dept, gas pressure; checking cleanliness of air/fanways is often not done. CAse seals are important, as is checking for leaks you can't see from outside. The slightest combustion irregulatity usually indicates something which needs seeing to before it gets worse.

The problem with PRV's is dirt - so if you replace one, then test the new one, it often gets a bit of dirt on its seat immediately so needs changing! The solution is to clean the whole system but that isn't part of a service any more than testing the prv is - unless specified. It isn't a servicing engineer's responsibility to m,ake the boiler safe, as such. It's to do what it says in the book. Some people would replace a thermocouple every year. I wouldn't unless it looked bad. Many of them outlast the boiler. Vokers ones are famous for not ever failing. Diverter diaphragms generally don't outlast the boiler - but again, most people don't change them at a normal service.

Oilman, do you test high limit stats? If so, how? If not, why not!?

 

[breezer]

thanks people, yes i do have the manual and oilman sorry but i forgot to mention its gas.