Central heating boiler losing pressure (again)

Granny_98

5 year old gas CH boiler, serviced December - boiler had been losing pressure - installers rang manufacturer - sent out engineer, who reinflated expansion tank. Installers rang them and saud it need a new expansion tank, a week later called back, supposedly fitted a new expansion tank ( wasn't here as long as first visit) 5 weeks later, boiler losing pressure again. (engineer said if it loses pressure again I've got a leak) boiler requires filling about twice a day, sometimes, if I'm sitting next to it, there is an audible "gulp" as pressure is lost. Boiler has a 10 year warranty, Is it still the expansion tank? I don't turn my heating off in the winter, just turn it down 21°C during the day and 18°C at night as I can't stand being cold (pensioner) 

BUFF

Granny_98 said:

 previous boiler a little copper pipe was the overflow - that is still there, but I also have a black drain pipe from the boiler to the outside drain - and this is where the water is coming out (overflow?) I put a plastic bag under the drainpipe to catch the water,

are you sure that isn't your condensate pipe?

Granny_98

Yes it is, I looked it up after writing it! Been editing while you replied! Will you please re-read for additional information! 

Bendy_House

Hi Granny.

With a boiler that loses pressure that quickly, it's usually easy to track the source of the leak - you can see the results from your checks right away. However, it's hard to do when the boiler is running all of the time...

That black plastic pipe is the 'condensate' waste, and that will have slugs of water (slightly acidic) coming out of it when the boiler is running - that's normal, and should be happening. And I wouldn't be surprised if you could hear these as 'glugs' from within the boiler (a small container inside the boiler fills up with the condensate until it 'overflows' down a pipe and empties in one go like the syphon in your toilet cistern.)

The one test you can do is to rubber-band that plastic bag over the end of the 15mm copper pipe instead, and remove it from the black plastic pipe (that shouldn't be covered!) If any water ends up in the bag now, then it's the same problem as before - water going out the safety discharge pipe due to the Exp Vessel issues.

If water comes out the black plastic pipe when the boiler hasn't been running for a while, then that indicates a leaking main exchanger inside the boiler - a major component, but should be covered by the warranty. But you cannot test this as your boiler is running.

Another test a plumber can do is to isolate the boiler from the radiators by turning two valves under the boiler. Again, tho', the boiler cannot be running for this test. If the pressure still falls, then the leak is within the boiler. If the pressure doesn't fall - but does when the valves are reopened, then the leak is in the radiator pipework.

The only thing you can try, then, is the plastic bag over the copper pipe - can you let us know how this goes?

Granny_98

Boiler was turned off last night and is still off, nothing from copper pipe, 100 ml from condensate pipe. I have kept heating off and replaced bags - the bags would hold about 2 litres. Pressure is 1.0 bar this morning. The only thing about the "glug" is the pressure plummets on the boiler - on my old boiler the heating would go off, this one doesn't, even when the pressure is rock bottom, the heating stays on (faulty gauge?) 

Granny_98

Background: It is a Worcester Greenstar 32cdi fitted Dec 2016 - residence is a bungalow, all pipework I think was plastic push fit originally fitted in 2002 under floorboards but is copper to radiators so not 100% sure. Floorboards since replaced with chipboard 😬

Bendy_House

Some strange things there, Granny.

As you suspect, most (all modern ones?) boilers would shut down when the pressure goes below a set figure, usually around 0.6bar or so - this is for obvious safety reasons. It's strange that yours doesn't. What make and model is it?

Once a boiler is turned off and has cooled down a bit, then condensate should stop coming out the plastic pipe. There might be a delayed 'slug' at some point soonish afterwards, depending on whether the wee syphon trap was virtually full in any case, and a few more drips from the main exchanger then tipped it over into emptying, but usually once the heating is off, the condensate stops coming out in these slugs. (It never does 'trickle' out steadily, but is always in slugs/glugs every few minutes or so).

So, if your bag has ~100ml in it after the boiler has been shut down, then that is very suspicious of a leaking mainX. There's still a chance it's as above - just a late 'slug' from the boiler, but if this happens regularly overnight, then I think you can safely assume there is a leak from the main exchanger (a leak from here ends up in the condensate syphon trap).

Can you repeat this test tonight?

Something that is a mystery to me is the way the 'glug' coincides with a drop in pressure. I don't understand this. The 'main exchanger' is the water 'jacket' that surrounds the burner and is heated up by the flames. Condensing boilers have a way of cooling the burnt gases and extracting more heat from them, and this means that some of the products of this combustion are condensed out into droplets inside that chamber. This trickles down to the bottom and end up in a syphonic trap which slowly fills up. Once it reaches the tripping level, it overflows out and 'sucks' the trap's contents after it down that plastic pipe - ie the trap fills slowly, but empties quickly in a 'slug' of water.

My puzzle is, the 'slug' may well make a 'glug' sound, but the leak - if it's from the mainX - shouldn't. The leak should be tiny and pretty-much constant, and inaudible. So I cannot figure out how the noise of the 'glug' coincides with the pressure dropping. The pressure dropping should - like the leak - be slow and steady.

The other main test that can be carried out - isolating the boiler - is a very good one, but is more involved, and you'd need to be happy about what you are doing. It would/should, however, indicate whether the leak is from the boiler itself, or from the rads and pipes.

Belenus

A faulty or leaking pressure release valve is another potential cause of the pressure dropping.

I am having that problem as detailed here in my thread.

Granny_98

Bendy_House said:

Some strange things there, Granny.

As you suspect, most (all modern ones?) boilers would shut down when the pressure goes below a set figure, usually around 0.6bar or so - this is for obvious safety reasons. It's strange that yours doesn't. What make and model is it?

Background: It is a Worcester Greenstar 32cdi fitted Dec 2016 - residence is a bungalow, all pipework I think was plastic push fit originally fitted in 2002 under floorboards but is copper to radiators so not 100% sure. Floorboards since replaced with chipboard

Once a boiler is turned off and has cooled down a bit, then condensate should stop coming out the plastic pipe. There might be a delayed 'slug' at some point soonish afterwards, depending on whether the wee syphon trap was virtually full in any case, and a few more drips from the main exchanger then tipped it over into emptying, but usually once the heating is off, the condensate stops coming out in these slugs. (It never does 'trickle' out steadily, but is always in slugs/glugs every few minutes or so).

So, if your bag has ~100ml in it after the boiler has been shut down, then that is very suspicious of a leaking mainX. There's still a chance it's as above - just a late 'slug' from the boiler, but if this happens regularly overnight, then I think you can safely assume there is a leak from the main exchanger (a leak from here ends up in the condensate syphon trap).

Can you repeat this test tonight?

I will repeat tonight - and leave a little while before putting bag on condensate pipe

Something that is a mystery to me is the way the 'glug' coincides with a drop in pressure. I don't understand this. The 'main exchanger' is the water 'jacket' that surrounds the burner and is heated up by the flames. Condensing boilers have a way of cooling the burnt gases and extracting more heat from them, and this means that some of the products of this combustion are condensed out into droplets inside that chamber. This trickles down to the bottom and end up in a syphonic trap which slowly fills up. Once it reaches the tripping level, it overflows out and 'sucks' the trap's contents after it down that plastic pipe - ie the trap fills slowly, but empties quickly in a 'slug' of water.

the first lot of water I collected - though heating was still on - was still quite warm - is that normal?

Granny_98

PS - I need a kitchen refit - having two radiators moved also - but need this boiler sorting before kitchen is done,  so work of some sort is in the pipeline anyway at some point in the near future

Belenus

Granny_98 said:

the first lot of water I collected - though heating was still on - was still quite warm - is that normal?

That is normal. The circulating water may be as hot as 70C degrees and it will take time to cool down.