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sean

Combi boiler question...

The pressure in our combi has dropped so that it cuts out. I've found the tap underneath, but when I open it, instead of the sound of rushing water and the pressure rising nothing happens. Confused Is there anything simple I can check before calling a plumber? (The water supply is on and all the little valves under the boiler are open.)
Treacodactyl

Have you read the manual, if you don't have it it should be easily found online.

It will take water from the mains inlet so as long as hot water flows then it should fill. Have you opened the underneath tap sufficiently and have you got the right one?
Shane

The guy that installed our combi said that it's standard practice to disconnect the top up connection to prevent know-it-all householders from overpressurising the system. I managed to convince him to leave ours connected, which I'm pleased about because the system needed topping up a couple of times as air made it's way out.

Might be that yours has been disconnected a some stage.
sean

Yep and yep. To TD.

I'm going to have to take the cover off aren't I?
Treacodactyl

My combi contains all the parts for the filling of the heating system, I just turn a tap and it fills. IIRC some combis would need a separate filling loop so you should see some pipe work going between the cold and the heating - I assume you don't have this?

If your combi is like ours you can see all the filling stuff from underneath.
Shane

Our combi has a braided hose between the external supply and the fill connection on the boiler, with a valve at each end. The plumber said that they normally confiscate this to prevent people overpressurising the boiler (and presumably so they can charge you an £80 callout to top your boiler up). I got him to leave ours on and then had to mention to him that it's best to leave one of the valves open in case of thermal expansion, which worryingly took some explanation.

Apologies for the errant apostrophe in the word "it's" in the previous post. I have heavily chastised myself and shall whip myself with birch as soon as I get home.
Treacodactyl

Or you can buy a filling loop from somewhere like Screwfix for less than a tenner. They have a tap at one end and an isolation valve at the other which both need to be on to fill don't they? I assume you don't then have another tap in the boiler?

I often have to top-up my heating system even though there's no leak.
dougal

Treacodactyl wrote:
.... IIRC some combis would need a separate filling loop so ...

Those are the important words to search for together with your boiler's make/model.
That's what the flexible hose linking the 'sealed' heating circuit to cold (top up pressure) mains water supply would be referred to as in manuals/installation instructions - but *don't* expect to find it in the User Guide though...
James

Its actually illegal to keep the filling loop in place. I think its because if the mains water pressure drops (if thereís a broken main for example) and the valves on the filling loop fail for some reason, you'd get central heating water (with all its added chemicals) siphoning back into your drinking water system.
In reality, the chances of both these happening are tiny, so they're normally left on, but a CORGI plumber must recommend they're taken off... at which point you say 'No thanks, I'll take the risk' and he says 'OK'. ...Its just a game.


If your water pressure's slowly dropping, check you havenít got a small leak in any radiators (normally in the fittings). A slow leak might not even result in a wet patch- it could evaporate on the hot pipe before it reaches the floor. Have a look at the pipe junctions and look for dribble marks. Also check the tightness of the bleed valves at the top. Often tightening everything up before re-pressuring will sort things out.


------------------------------------------

From my memory , this is how I'd re-pressurise our boiler (a glow-worm):

1) Turn off the gas at the mains (big metal handle)

2) there are five pipes at the base of the combi- cold input, hot output, central heating in, central heating out & gas.
Each of these pipes has a gate valve (tap) just before it goes into the boiler. Find the cold water in tap on your combi boiler and close it. The coldwater input is usually on the far left as you look at the boiler.

3) the coldwater in has a flexi hose which passes into the the central heating return pipe

5) thereís a tap at either end of the flexi hose - make sure the tap at the "coldwater in" end of the hose is closed. Once you've done that, open the tap on the "central heating return" end of the hose.

6) Now re-check: You should have two taps closed - cold water input to combi boiler and the tap on the "coldwater in" end of the flexi hose. All other taps are open. The gas is turned off at the mains

7) Get someone to go outside and keep an eye on the pressure relief outlet pipe (small copper pipe sticking out from the wall and looped back onto the wall). - if you've gone wrong, water will spurt out of this pipe at the next stage. It wont do your boiler much good, but it wont be the end of the world either. Get them to shout if this happens (if this happens, stop everything, re-check you've closed the cold input.)

8 ) Very gently open the tap on the "coldwater in" end of the flexi hose, and keep a close eye on the pressure gauge. Do it slowly and stop very frequently. Open the tap for only a few seconds at a time, then allow the pressure gauge to stabilise before giving it another quick shot.

9) Once you've got to pressure (recommended at 1 bar, no more than 1.5 bar- I keep mine at about 1.2 bar), close both flexi hose taps, and open the cold water input to the boiler.

10) Turn gas back on at the mains.
Treacodactyl

James wrote:
Its actually illegal to keep the filling loop in place.


Many are built into boilers now so are always kept in place. Should the external filling loops be removed even if they have a double check valve? I suppose they should as I bet the central heating chemicals are more than a slight risk to health.
James

Treacodactyl wrote:
Should the external filling loops be removed even if they have a double check valve? .


Yes, thats my understanding
(although mine is still attached....)
James

sean, just another thought - was it working last night? in which case you must have lost pressure quickly.
Whats the pressure at the momment?
Barefoot Andrew

Just to echo a previous comment - my combi has two valves that need to be opened to re-pressurise. And I leave the hose permanently connected...

A.
sean

James wrote:
sean, just another thought - was it working last night? in which case you must have lost pressure quickly.
Whats the pressure at the momment?


Stopped working Sunday morning. We've been having to bleed a *lot* of air out subsequent to some other problems though.
James

If you get it pressurised as per the above method, take it to 1.5 bar, then bleed from furthest radiator (upstairs, other side of the house) back down to the closest. You'll need to do it a few times per day for a few days, closing the bleed valves tightly each time. Then your supposed to re-vist them a week or two later (if the central heatings on), and ideally again a moth later.
Shane

James wrote:
7) Get someone to go outside and keep an eye on the pressure relief outlet pipe (small copper pipe sticking out from the wall and looped back onto the wall). - if you've gone wrong, water will spurt out of this pipe at the next stage. It wont do your boiler much good, but it wont be the end of the world either...

...although once a relief valve has popped it can be extremely reluctant to reseat, causing a continually loss of water from the system - so worth checking the tailpipe for dribbles if you continually lose pressure (although it does sound more like air in the system is the problem in this case).

James wrote:
9) Once you've got to pressure (recommended at 1 bar, no more than 1.5 bar- I keep mine at about 1.2 bar), close both flexi hose taps, and open the cold water input to the boiler.

This is where I had a discussion with the plumber. The way I see it, once you've refilled your system you've got a length of flexi containing water at supply temperature, which is pretty cold, especially at this time of year. If you trap that water by closing a valve at each end (we have ball valves on ours, by the way) its temperature will increase until it is at room temperature, or even slightly more given that it has an active boiler right next to it. When water warms up, it expands, and there ain't a lot that will stop it - it will either bugger the valve seals or split the pipe. That's why I always leave one valve open - although I must admit I'd forgotten about the preventing backflow regulations. I would have thought that a double check valve would be good enough to satisfy the regs (it was last time I looked into it ten years ago) as long as the two check valves are of a different type.
Treacodactyl

Shane wrote:
That's why I always leave one valve open - although I must admit I'd forgotten about the preventing backflow regulations. I would have thought that a double check valve would be good enough to satisfy the regs (it was last time I looked into it ten years ago) as long as the two check valves are of a different type.


If there was likely to be a problem I would expect similar problems elsewhere in household plumbing. I've just installed a bath and as the taps have a shower attachment and it can reach the bath I've had to install a double check valve on each pipe. So, for example, when the cold tap gets turned off you'll have a short distance of cold water between the tap and the double check valve that's can't go anywhere and will warm by a fair bit.

While on the subject of backflow I wonder how many houses meet the regs? I gather it's double checkvalves if the shower can reach the bath or basin water and far more complex measures if it can reach the WC basin.
sean

Tap on the filler loops bust internally. Stupid plastic components. Plumber's refilled the system for now and left instructions *not* to bleed the radiators until the new tap's been fitted.
Treacodactyl

That's another one of my pet hates, you should fit shut off valves near every tap but I find they often don't work that well, i.e. don't shut the water fully off or they can break/leak, and it's far easier to turn the stop-cock off. Rolling Eyes

At least you've got your heating back.
sean

'Twas the HW that was more of a concern.
Treacodactyl

Ah yes, I forgot that 'feature' of a combi and not having a hot tank there's no backup.
James

Shane wrote:
. If you trap that water by closing a valve at each end (we have ball valves on ours, by the way) its temperature will increase until it is at room temperature, or even slightly more given that it has an active boiler right next to it. When water warms up, it expands, and there ain't a lot that will stop it - it will either bugger the valve seals or split the pipe.


Thats a fare point which I'd not thought of. With weak valves its probably not best to shut off a small volume of cold water next to a warm boiler.
But the alternative is to only have one valve between your drinking water and your heating system.

hmmm....the more I think about it, the more I realise there might be some sense in this law
James

Treacodactyl wrote:
That's another one of my pet hates, you should fit shut off valves near every tap but I find they often don't work that well, i.e. don't shut the water fully off or they can break/leak, and it's far easier to turn the stop-cock off. Rolling Eyes

At least you've got your heating back.


yeh, but they're great for locally regulating the cold water pressure, so you (I) dont turn the kitchen tap half an inch and spray the whole kitchen with cold water.

I agree though- they always seem to drip slightly when I take the taps off, so I end up turning off at the mains.
dpack

back in the mists of time non return valves were used to prevent backflow to the main (bylaw 30 regs?) from any appliance attached to a cold supply
once a combi has settled ie all non soluble gas at running temp has been expelled from the vents it should not need refilling ,but eventually they all do .
a loop from a supply with a non return valve is fine
the cold supply which is heated should already have a non return valve and so a loop from that is safe
flexi pipe can cope with thermal expansion within normal use and both valves shut
oldangrey

James wrote:
Its actually illegal to keep the filling loop in place


I have just had a new combi fitted by a corgi reg plumbing company, and they have left the flexi hose connected and even went to pains to show me what to do if the pressure drops. This boiler replaced one fitted about 3 years ago, it was so wasteful with water, needing to fill the kitchen bowl 2 times before getting hot water, it was of French design, the new one now fitted is of Italian design, WHERE are the British designed Boilers?? Crying or Very sad
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