Central heating system losing pressure

[dogfonos]

Would appreciate some guidance/advice to see if I'm on the right track...

I'm trying to rule out all possible causes of my central heating system (system boiler incorporating circ. pump & expansion vessel) losing water pressure. These are my thoughts (and observations) so far. Am I missing any other possible cause of pressure loss?

Leaking underfloor pipework to radiators. Who knows?
Leaking original pipework to radiators (original supply/return pipework runs around loft of bungalow and drops down to individual radiators). No visible leaks
Boiler safety valve leaking (connected to condensate drain). Can't tell - poor visibility
Heat exchanger leak into boiler casing or into condensate drain. No leaks into boiler casing
Other pipe/valve/pump leakage within boiler casing. No leaks into boiler casing
Hot water tank heating coils leaking into hot water. Pressure drops even when hot water port valve is closed so not leaking HW tank coils.
Gas/air or water leakage from expansion vessel. No water leaks. Air/gas leak? I'd guestimate the amount of top-up water I've added would probably have overfilled the expansion vessel by now so its probably not leaking expansion tank.

Thanks for your consideration.

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Background:

Had a new gas system boiler installed last summer as part of a new extension. Three new radiators were added to the system at the same time. As expected, the system lost pressure as air left through the boiler's automatic air vent and therefore required topping up with water 3 or 4 times over the first few months. All was fine after that - system held pressure well.

Unfortunately, we had a problem with the final few feet of supply & return water pipes feeding one of the new radiators because a section of the new supply/return pipes set into screed were close to the floor surface and made the floor far too hot to fit the final covering. As a consequence, the radiator, and a section of supply/return pipework, had to be temporarily removed and part of the floor screed dug up so that these speedfit plastic pipes could be set deeper. Following this work, the screed was relaid over the new pipes.

Problem is, the system now loses pressure at a faster rate than it did before. I expected a few top-ups initially but now I'm getting worried. Around 7 top-ups required since the work was carried out less than three weeks ago. At present, the system requires a top-up every 3 days.

Digging up the floor - again - to look for leaks is a last resort. Before I get that done, I'd like to rule out all other possible causes of pressure loss.
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[flameport]

Boiler safety valve leaking (connected to condensate drain). Can't tell - poor visibility

Tie a plastic bag over the outlet, any water will be trapped in the bag.
Surely it isn't really connected to the condensate drain? It should be an entirely separate pipe.

pressure drops even when hot water port valve is closed so not leaking HW tank coils.

Not relevant, the valve only closes one of the two pipes connected to the cylinder. If it is leaking there, the valve position makes no difference.
However if it is leaking there you would also find inhibitor in the water from the hot taps, and if the hot cylinder is supplied from a loft tank, it would overflow after a while of not using any hot water.

As expected, the system lost pressure as air left through the boiler's automatic air vent and therefore required topping up with water 3 or 4 times over the first few months.

That is not normal - any air should be removed within a matter of hours, not months.

speedfit plastic pipes

Are there any joins in these pipes in the floor? How exactly were those pipes installed in the floor - any other coverings over them?

 

[lonner]

Every time my old Halstead boiler lost pressure it was the PRV.
I replaced it about 5 times in about 18 years.
Nearly every time i had to work on it, the PRV would fail.

 

[dogfonos]

Not relevant, the valve only closes one of the two pipes connected to the cylinder. If it is leaking there, the valve position makes no difference.

Good point. Yes, only the supply pipe is valved. Seems to rule out leakage from the hot water tanks coils as I've not noticed any other effects.

Tie a plastic bag over the outlet, any water will be trapped in the bag.
Surely it isn't really connected to the condensate drain? It should be an entirely separate pipe.

I believe it is connected. It's an ATAG system boiler, iS24, and that's what the ATAG Instal. & Service instruction diagram indicates, as I understand it. The diagram ties in with what I see at the base of the boiler. The plastic condensate pipe is plumbed into the drainage system but I could probably dismantle and temporarily allow condensate to drain into a bucket.

That is not normal - any air should be removed within a matter of hours, not months.

Interesting. Even before the boiler change and the addition of new radiators (with underfloor feed/supply pipework), it was a pig of a system to bleed but I put that down to the Keston colander being on it's last legs and the fact that the supply/return pipes drop down to each radiator making air removal difficult.

Are there any joins in these pipes in the floor? How exactly were those pipes installed in the floor - any other coverings over them?

Lots of pipe joints, at a guess about 20 - 30 buried below insulation & screed. From earth upwards: Concrete base on hardcore/earth - ventilated air space - beam & block (to which the new C.H. supply & return pipes were fixed) - damp proof membrane with bituminous sealant - 100mm Kingspan insulation - 100mm concrete screed. No final floor finish as yet but it will be sheet vinyl, eventually.

The small length of pipework that required re-work is almost entirely in the old/existing floor because the radiator in question is a couple of feet inside the original part of the building. Pipes rise from below the new floor (approx 200mm below floor surface) to the old/existing flooring which, at that point, is screed directly onto hardcore/earth but channels cut to set the small length of pipe lower into the old screed were probably cut mainly into the original foundation concrete as they run very close to a wall. Very tough concrete which probably explains why the channels were cut so shallow when first laid. However, any appreciable water leak from these pipes could run down into the new floor structure.

If the radiator feed/supply pipes are leaking as I suspect, I could have one heck of an underfloor lake forming - perhaps that's what is meant by a 'floating' floor.

 

[flameport]

Lots of pipe joints, at a guess about 20 - 30 buried below insulation & screed.

Any joints there is bad news, if they are plastic speedfit then that is even worse.

 

[dilalio]

Isolate boiler from system pipework to determine if drop occurs at boiler or on system pipework.
If boiler is not at fault, the pressure will hold whilst isolated from the system and will drop immediately when flow and return valves are reopened.

Obviously, Boiler off during this test.

ATAGs have a combined condense and discharge, as you have stated.

 

[dogfonos]

Isolate boiler from system pipework to determine if drop occurs at boiler or on system pipework.

This is a new boiler to me and I haven't got familiar with it yet but, referring to the ATAG manual, I've managed to isolate the boiler from pipework & radiators after topping up/re-pressurising the system with water. There's a crude gauge on the boiler plus the boiler has it's own water pressure sensor which can be interrogated from the boiler's control panel. It's currently reading 1.1 bar. Be interesting to see what it's reading tomorrow. And then when I open it back up to the system.

Boiler's been off for a couple of days now - damage limitation

Any joints there is bad news, if they are plastic speedfit then that is even worse.

So why are (some) MCIPHE, Gas Safe, Plumbing & Heating Engineers currently using this method?

 

[denso13]

So why are (some) MCIPHE, Gas Safe, Plumbing & Heating Engineers currently using this method?

Hard to explain why some people can't be bothered doing things correctly.

 

[dogfonos]

Hard to explain why some people can't be bothered doing things correctly.

Bit late for me, but out of interest, what is the best method/technique for laying radiator feed & supply pipes beneath a concrete screeded floor? Indeed, is there a 'best' method?

 

[denso13]

what is the best method/technique for laying radiator feed & supply pipes beneath a concrete screeded floor?

For example, pipe in pipe if using speedfit http://www.johnguest.com/speedfit/product/pipe/pipe-in-pipe/ (other brands are available).

There should be no joints in the screed as a starter.