Unvented system pressure problem

[Palma8910]

I have an unvented heated system, installed about 10 years ago (oil fired boiler). Recently had some pressure loss which was solved by locating a leak. At the same time the expansion vessel on the radiator/boiler circuit was replaced (to eliminate possibility of a 10 year old diaphragm causing problems. Pressure release valves were also changed.
Since then the pressure at the boiler is 2 bar when running hot (1.5 bar up in bungalow loft) and this is steady (i.e. no apparent leaks). However, the pressure drops to zero when cold (takes about an hour). So far this has not stopped the boiler firing and the central heating rads have been fine.
Guy who did the job initially said the pressure fluctuation could be down to air in the system after the drain down - and that this should clear. It hasn't and it is now a couple of months down the line.

Questions:

1. Anyone any idea what is going on?
2. How can I correct the situation?
3. Will circulation pumps be damaged running on zero pressure?

Any assistance very gratefully received!

 

[SimonG]

1. Yes
2. Get the bloke back.
3. Depends on location of pumps.

 

[Palma8910]

Thanks for your interest Simon. Are you prepared to say what 'yes' means? Rgds Clive.

 

[Riley]

I think what Simon is alluding to is that we will not give DIY information relating to unvented cylinders as you need to be G3 qualified in order to work on them

 

[SimonG]

Sounds like its just a sealed heating system so will be ok with no g3.

Also sounds like the guy you got in was clueless. The ev is either duff or ok, you don't change it for the sake of changing it just because it's ten years old.

Also the prv, if it's passing then change it. You say prv valves? Plural?

Anything coming out the new prv? Are you topping pressure up when you find it on 0?

Same size ev fitted? How many radiators? Singles or doubles?

Is the new ev getting warm when the heating is running?
Is it piped with a flexi or hard piped?

 

[Palma8910]

Thanks for that clarification, I quite understand and have no intention of DIY! I was just hoping to get a steer as to what needs to be done - so I can make the necessary arrangements. However, if the take home message is that a G3 qualified plumber should know what to do to rectify the problem, then that is very useful.

 

[SimonG]

I would think it's either.

No precharge in ev.
Faulty new ev.
Undersized ev.
Blockage in feed to ev.

 

[Palma8910]

Thanks for your interest Simon. Answers:

1. Old ev was spouting water when valve depressed.
2. New ev identical, same model, old one worked fine so suggests not too small? Hard piped.
3. New ev slightly tepid at bottom when system hot (stone cold top half).
4. I topped up ev once and this resulted in hot pressure at boiler going from 1.5 to 2 bar, so have left alone. Hot pressure at ev is 1.5 bar presumably gravity effect?
5. Equivalent of 14 radiators (actually old fashioned skirting type) plus 2 double radiators plus two underfloor heating areas and 3 heated towel rails.
6. Three AAV's replaced, one on boiler, one in loft, not sure about the other.

 

[Palma8910]

Sorry forgot - no water from AAV's I can find.

 

[SimonG]

Was the diaphragm split then or just a bit of condensation?

Big system then. What size is the ev?

 

[Best]

Could be anything causing water loss and therefore pressure drop.
Leak inside oil boiler - crack opening when hot also possible. I saw one that boiler was leaking outside at base weld and water didn't show as evaporating while it leaked when hot.
You don't need G3 plumber for heating system.
The pressure for a bungalow usually only needs maximum 1bar system cold on oil boilers

 

[Palma8910]

Can't tell you about diaphragm, but it was more a mist than a flood.

 

[Best]

Doesn't take a massive loss of water to drop the pressure to zero.
If the relief valve is not passing water if system is getting up to 3bar for example, and no obvious leaks at rad valves, pipes, etc, then I would suggest you also have the oil boiler checked.

 

[Palma8910]

Thanks for suggestion. But I if there is a leak I do not understand why the pressure when hot has remained unchanged for 2 months. It takes 1-1.5 hr to drop to zero when system goes off, but goes back up to 2 bar in minutes when it comes on.
I don' t mean to prolonged all this, so thanks for your advice.
Regards.

 

[Best]

Thanks for suggestion. But I if there is a leak I do not understand why the pressure when hot has remained unchanged for 2 months. It takes 1-1.5 hr to drop to zero when system goes off, but goes back up to 2 bar in minutes when it comes on.
I don' t mean to prolonged all this, so thanks for your advice.
Regards.

I would think if the pressure is rising so high each time the system is heating, then there is no expansion available in the system. I would also be wary of the accuracy of the pressure gauge(s).
I hope nothing stupid has been done, - like a valve fitted on connection to expansion vessel? That is why the question of flexi or hard piped was asked.

 

[SimonG]

Aye, would be questioning accuracy of pressure gauges. Been caught by that before.