Repeated complete pressure loss

[Mjward]

Can it be anything other than a leak?

The reason I ask is that I have had 4 plumbers/heating engineers out over the past 3 months and none are able to figure this out. System boiler powering a mixture of radiators and UFH. No detectable leaks by the rads, by the manifolds or really anywhere in the system. Yet the (10yr old) boiler will happily drop from 2 bar of pressure to 0 in a 2 week period. At wits end to solve it

 

[Chuck]

It's a leak in the sense that the laws of physics tell you that water must be leaving the system. If the radiators and pipework have been ruled out there are several other things that could be happening.

IMO it's important to positively demonstrate whether the problem is internal or external to the boiler. This is done by pressuring the system and then closing the service valves to isolate the boiler. There needs to be two pressure gauges to do this

If you have a vented hot water tank, don't forget to check for leaks in the DHW circuit including the heat exchanger coil inside the cylinder. It's not common but these can fail.

Possibilities inside the boiler include:

(a) failed expansion vessel. This causes the system to overpressure when hot and loose water via a pressure relief valve (PRV).
(b) A faulty PRV that has not reseated properly.
(c) a faulty automatic air release valve (AARV) dripping.
(d) faulty heat exchange (HX) allowing water from the radiator circuit to escape into the flue and away down the condensate drain.

(a) & (b) are usually checked by putting a balloon over the pressure release outlet (outside) and checking for water collecting.

(c) usually results in a puddle of water in the bottom of the boiler and/or dripping.

(d) is trickier to establish and depends on the arrangements to deal with condensate.

Although careful observations from the homeowner can help diagnosis, assuming it's a gas boiler, these faults need to be fixed by a registered gas engineer.

 

[Mjward]

A thoroughly comprehensive and incredibly helpful response Chuck. You've suggested a whole raft of things there that I'll get the engineer to investigate. Many thanks!

 

[Mjward]

In fact thinking more about it and connecting dots...re d) this white pipe beneath the boiler leads out to a soakaway which in a completely separate issue has an improper connection along its route causing water to leak out. But...I have been surprised just how much water has leaked out of it. Could this be related or for what you described must it be escaping via the flue itself?

 

[Chuck]

A HX leak can let water into the flue system and, like the condensate, it then runs downwards and out via the condensate drain ('white pipe'). So, what you are looking for is water dripping / running down the white pipe when the boiler is not operating. (When the boiler is operating the products of combustion are cooled by the HX and water vapour condenses out as 'condensate', which will mask the leak.)

It may help to know that condensate BTW is colourless and clear and fairly acidic. System water containing corrosion inhibitor is usually tinted yellow / brown and should normally be a bit alkali.

There is another level of leak-finding available from specialist firms who use acoustic / ultrasound detection, thermal imaging, tracer gases, fluorescent dyes, etc. but they can be quite pricey. In my experience most leaks can be found by careful observation and being systematic but this takes time. For a professional plumber time is money, so the right point to get a specialist in is sooner than if an enthusiastic home-owner is DIYing.

 

[Mjward]

A HX leak can let water into the flue system and, like the condensate, it then runs downwards and out via the condensate drain ('white pipe'). So, what you are looking for is water dripping / running down the white pipe when the boiler is not operating. (When the boiler is operating the products of combustion are cooled by the HX and water vapour condenses out as 'condensate', which will mask the leak.)

It may help to know that condensate BTW is colourless and clear and fairly acidic. System water containing corrosion inhibitor is usually tinted yellow / brown and should normally be a bit alkali.

There is another level of leak-finding available from specialist firms who use acoustic / ultrasound detection, thermal imaging, tracer gases, fluorescent dyes, etc. but they can be quite pricey. In my experience most leaks can be found by careful observation and being systematic but this takes time. For a professional plumber time is money, so the right point to get a specialist in is sooner than if an enthusiastic home-owner is DIYing.

I'm wondering if the most practical thing is to replace the boiler. I say that as it's almost 12yrs old and on the radar for replacement. I was planning to get system powerflushed but I know my gas engineer includes that with boiler install. Given several of the issues you flag as potential reasons for the pressure drop are directly boiler related, this seems like a prudent measure. If it turns out to still be an issue after, I've got a new boiler I needed to get soon anyway and at least a few potential causes have been eliminated. Does that make sense as a plan?

 

[ShaunCorbs]

I would pressure test the heating system first close / disconnect the boiler and pressure test the system at 4 bar over a couple of days wrong time of the year to do it really

 

[Mjward]

I would pressure test the heating system first close / disconnect the boiler and pressure test the system at 4 bar over a couple of days wrong time of the year to do it really

The problem is it's a rental property and the tenants (rightly) are giving me an ear bashing about the various issues. Unfortunately we had tenants in for 2 years that didn't flag anything and so the new tenants are unfortunately discovering all sorts of issues that have been left.

 

[Mjward]

Grrr... the moneypit continues. In December paid out for a well-reviewed heating engineer to do the below works but unfortunately the tenant has reached out and said the problem is still there, getting pressure loss to the point the heating is cutting out and she needs to top it up at the boiler. Engineer ruled out a HX leak. Will get him back to reassess

1) Power-flushed full heating system to British Standards.
2) Dismantled and cleaned flow gauges to UFH manifold to enable system balancing.
3) Supplied and fitted an additional 18 litre heating expansion vessel to primary return pipe work, also replace boiler internal vessel (as completely flat)

 

[Chuck]

3) Supplied and fitted an additional 18 litre heating expansion vessel to primary return pipe work, also replace boiler internal vessel (as completely flat)

If the system has been operating without the proper expansion volume there is a chance that the boiler's PRV is not re-sealing properly. Your plumber may well have checked this as part of the 'ruled out HX leak' work but your list doesn't explicitly mention replacing it or performing a pressure test.

 

[Mjward]

I'll need to check as he did mention doing something to the boiler prv but I'd be lying if I said I was able to understand what.

 

[Mjward]

Realised I never came back on this.

After the initial plumber attempted several (expensive) remedies, including the last dart of adding leak sealant to the system, the problem remained. I called in a leak detection company, at further great expense, and very quickly noticed water damage to plasterboard beneath the boiler. Added some dye to the system and all was revealed, a damaged 10 quid filling loop connection was the source of the pressure loss. 2 months later and pressure has held.