Boiler stops working after turning central heating off (until boiler is re-filled)

click86

I've had a problem with my gas boiler for a while and would be interested to know your thoughts or suggestions.

Firstly, albeit probably irrelevant, my gas central heating is controlled by a digital wall panel.

When I turn the central heating on, everything's fine.

Once I turn the central heating off (or if the house reaches the desired temperature), the system cools down and I lose all hot water.

To get the hot water back, I have to fill the boiler.

If I don't re-fill the boiler, the water stays cold and if I try to turn the central heating back on...nothing. The boiler stays silent.

When I re-fill the boiler, you can hear it kick back in. Hot water is then flowing as normal.

I've been round the house, no visible leaks from the radiators. All radiators get hot too.

Any ideas as to what the problem could be?

Thanks!

grumbler

Do you have to refill because the pressure drops?

Falafels

If you've got to repressurise the boiler every time the heating goes off, there's something wrong with the system! 

Jeepers_Creepers

Make and model, please.

Ok, you top it up to - what? - 1 bar or so? Right, the next time you turn on the heating, watch what that gauge does. I'm guessing it shoots up alarmingly towards 3 bar-ish. If so, then what's happening is that this 'safety limit' pressure of around 3 bar is opening the pressure release valve and depositing the excess water outside your house via a 15mm copper pipe.

Can you let us know what the pressure is doing as the boiler is switched on, please? Also, tomorrow when it's light, you could try loosely tying a plastic bag over the end of this pipe to see if it catches the expelled water - that will provide the evidence.  

(Water expands with heat, so the volume in your sealed system has to go somewhere. That somewhere is the 'expansion vessel' inside your boiler. If this vessel is faulty - which I suspect your is - then the water cannot be accommodated so the pressure rises until it 'blows' the PRV.)

grumbler

grumbler said:

Do you have to refill because the pressure drops?

I guess the answer is yes.

In this case, I think, hot water expansion vessel is either faulty or just needs charging. Normally it contains compressed air allowing water to expand when getting hot. If the air leaks, and the vessel is full with water, the pressure rises sharply with the temperature and the safety (pressure release) valve releases some water from the system. When the temperature drops the pressure drops too.

click86

Unfortunately, the pressure gauge is broken and constantly at 0. Which doesn't help identify the issue!

I'll try and locate the copper pipe tomorrow and try the plastic bag suggestion.

Would the water be expelled when turning the heating on, turning it off, gradually or once re-filling the boiler?

Jeepers_Creepers

The water would usually be expelled quite suddenly when the pressure reaches around 3 bar, and that would be after the boiler has been running for a good few minutes and is heating up the rads nicely. 

Oh, and the water being expelled will almost certainly be hot...

Make and model of boiler? 

click86

Jeepers_Creepers said:

The water would usually be expelled quite suddenly when the pressure reaches around 3 bar, and that would be after the boiler has been running for a good few minutes and is heating up the rads nicely. 

Oh, and the water being expelled will almost certainly be hot...

Make and model of boiler? 

Sorry, the boiler is 'Remeha Avanta Plus'.

Jeepers_Creepers

This is quite an old boiler with an analogue (moving pointed) pressure gauge? 

If the boiler is otherwise working ok, then worst scenario is that it needs a new expansion vessel (around £100?), and new pressure release valve (~£30) and a new gauge (£no idea). That and a couple of hours labour. So we are talking around £300-ish?

It might not need a new EV - the one you have might just need recharging with air. It might not need a PRV - the one you have might settle and close off again and not drip (they do tend to seep once triggered). You do need a pressure gauge - don't carry on without one. 

Remeha have their own service people, and I think I'd be tempted to go with them, at least for a diagnosis and quote. I don't think they do a 'fixed price repair', but you can always ask. 

If you are ok with taking the cover off the boiler to view the EV - it's usually only decorative - there is a simply check you can do on them. BUT, you need to look up maintenance instructions for your boiler first so you know what you are doing...

click86

That's right, it has the old style pressure gauge with the two pins.

I wouldn't feel comfortable taking the front off the boiler so it's definitely time to get a professional round.

I will still check the external copper pipe tomorrow though.

Thanks very much for your help on this. Really do appreciate it!

Le_Kirk

click86 said:

Unfortunately, the pressure gauge is broken and constantly at 0. Which doesn't help identify the issue!
I'll try and locate the copper pipe tomorrow and try the plastic bag suggestion.
Would the water be expelled when turning the heating on, turning it off, gradually or once re-filling the boiler?

How do you know when to stop filling if the pressure gauge is broken?  Time to get it replaced.