Losing water

[tonysab]

Hi there I have a combi boiler which is approx 15 years old, and over the past few years I have noticed that the water pressure gradually drops but over a period of probably a few weeks. On our Annual Service I always point it out and the chap always says something technical along the lines of he has done something which should fix it!
Anyway just lately I have noticed the problem seems to have got a lot worse, I can top up the water to about 3 on the dial (apologies I really am a total novice at diy and central heating!), and it will be back down to 0 within a day! I noticed last time that when the heating comes on it did rise slightly but when I checked it some time later when the heating was off it was back down to 0.
I have checked radiators etc for leaks with no obvious signs. Does it make a difference as to when the leak is occurring ie is is more or less likely to occur if the heating is on or off?
I would have thought that a leak as severe as this may show itself fairly quickly in terms of if it is not obviously coming from radiators presumably it will be under the floor somewhere upstairs or downstairs (the boiler is downstairs). However for example it is leaking under the downstairs floor how would I ever know?
Sorry if some of this sounds stupid I would be grateful for any advice, do you think it would be worth calling out BG on the service contract?

One more thing, since this has been happening the hot water takes an age to come through, even though it fires up from the pilot immediately.

Many thanks.

 

[wemyss]

Your own diagnosis of losing water is the only one really feasible in my opinion. Long retired I only worked on these boilers for a short period so I'm not fully up to speed on faults.
However a drop in pressure has to be a leak somewhere on the system. It takes very little loss of water to cause a pressure drop in a small capacity system such as a heating system so don't expect water to be gushing out. It needs fixing quickly as new water constantly entering the system is a sure recipe for rusting radiators and boiler.
If you have a service contract covering all parts of the system you should use it and insist on a cure.
And yes the pressure would rise slightly when the boiler is hot.
When these combis first came out I remember the by-laws insisted that the top up flexible pipe had to be removed after filling to avoid any feedback into the mains. However I never saw one that was removed.
wemyss

 

[Bahco]

You should call out BG if you have a service contract, after all you are paying for it.
Do not top up your system to 3 bar, should only need around 1 bar

 

[tonysab]

Many thanks for your advice. I shall try calling BG out in that case, I was just a bit worried they would say something along the lines of "it must be a leak under the floor somewhere we can't dig that up" and that would be that!

 

[ArtfulBodger]

At the moment it is you that is unwittingly causing the problem. By raising the pressure when cold to 3 bar, the pressure relief valve will operate within a very short space of time when the heating is on, dumping water outside and effectively zeroising the pressure.

If the PRV has operated before, then it is likely that it is now letting by and this is where your water is going.

 

[IanDB]

If the PRV has operated before, then it is likely that it is now letting by and this is where your water is going.

and it will be back down to 0 within a day

In order for the pressure to drop to zero in that period the water would be p***ing out. It's worth checking but I don't think you'll find that to be the cause.

 

[cowboyplumber]

Had one of these problems on a relations boiler - couldn't find leak anywhere - nearly drove me mad. Turns out to be diverter valve between dhw and ch. Feel the pipe on ch when hw is on or vice versa if either gets hot then its diverter valve.

 

[ChrisR]

If there were a leak between tap and system water, the system pressure would go high in most areas, not low, because the mains is usually 2 bar or more. MOst diverters would leak to atmosphere too, though 15 years ago I was only 6 so I don't know all there was then (or now!).
Heat exchangers can leak between the waters of course , but again the pressure would rise.

 

[tonysab]

Thanks again for your replies.

Cowboyplumber, sorry if I am being dense but can you just explain exactly what you mean when you say -

between dhw and ch??

Feel the pipe on ch when hw is on or vice versa ??

Many Thanks.

 

[brumylad]

Could it be the expansion vessel (big red disc at back of the boiler) has lost its air charge

System hot - expansion* relief valve operates (*or PRV if you prefer to call it that)
system cool - pressure zero ?

 

[ChrisR]

Tonysab - I tried to explain that a leak such as cowboyplumber describes is A) very unlikely and B) wouldn't give your symptoms.

brumylad's suggestion is probably right - once the prv had passed water it wouldn't seal anymore so you wouldn't get or see the high pressure.