How do I find a (small) leak from pressurised system

[JImCPN]

I see that there was a post in November that refers to a LARGE leak, but ours is much smaller (needs topping up every ten days to two weeks, perhaps a litre over that period). It has been going on for about six months. The boiler is new since last year; however, to cover all bases, we called the installer out and, predictably, they found no fault. Interestingly, they had no suggestions as to what to do next. Most of the pipework is exposed and there is no sign of leakage anywhere. There is a small amount of piping boxed in and I (rather cleverly I thought, my wife will be SO impressed when I find the leak) invested in a cheap borescope and had a look. As far as I can tell, it is all dry - but I can't see everything and the quality is not brilliant.

It is certainly irritating .... and my wife has nightmares of the house foundations gradually dissolving. My questions are

1) How common is this sort of slow leakage and is it likely to get worse, assuming there is a small leak somewhere?
2) Are there other methods of leak detection? The previous post mentioned a specialist firm, but that sounds pricy - is it? What methods do they use? I just have a feeling that a really expensive borescope used by an expert may do the job ... but where do I get advice from without committing myself to a large invoice?
3) How common is this problem?

Jim

 

[CHRISX]

First of all re the main thread, I would consider 1litre/2 weeks or 50 litres/year a fairly bad leak, I keep a eye on a neighbour's (oil fired) sealed system and only top it up every 18/24 months from 1 bar to 1.5 bar (cold) it has a 10 litre EV with a pre charge pressure of 1 bar so in falling from 1.5 bar (cold) to 1.0 bar it will still only have used its reserve of 2 litres in this time.

Re pressure testing, assuming a cold pressure of 2.5 bar in both cases and assuming a 10 litre EV with a pre charge pressure of 1 bar then with the EV connected to the system it will contain ( the reserve) 4.3 litres of water at 2.5 bar so the system must leak/lose 4.3 litres before the pressure falls to 1 bar.
With no EV connected to the system, theoretically at least, if system has no entrained air then it should take practically nil litres to pressurize to 2.5 bar (once system is full) since water is incompressible so the pressure should fall very rapidly to 1 bar and also to ~ 0.4bar, the static head of the system, thereafter should fall at a very slow rate due to the system contents and lower pressure.

Hello John.g,

Thanks for replying with the explanation.

I had not taken into consideration the additional capacity that the Expansion vessel reserve volume has - however that expansion capacity is only pertinent if it is being used when the system water has expanded due to being heated.

Without wanting to seem pedantic just some further thoughts:

As a sealed Heating system PRV would be set at 3 Bar the system cold pressure should be closer to 1 - 1.5 Bar theoretically rising to about 2.5 Bar when the Heating system has heated up / system water had expanded so the Expansion vessel would have to accept a lower amount of expansion / reserve capacity than you calculated.

Something else that I would have been thinking when I wrote my previous reply to you was that if a sealed Heating system has a capacity of 100 litres when filled and has been fully vented to remove any air from the system it has only 100 litres.

There is not for example 104 litres of water even when the water is heated - although the water would have expanded by for example 4 litres when heated [not calculated] the system capacity would still have been 100 litres.

In my previous reply when I was trying to explain why I could not agree with what you had written I had it in mind that the Heating system capacity is static in terms of actual water content whether the Expansion vessel was isolated from the system or not - and as we would be checking the system pressure drop from cold - not when the system was being heated [EV isolated so Boiler off] there would be no difference in the system water volume and at a cold water pressure of approximately 1 Bar I could not see any difference in the rate that the pressure would drop with or without the Expansion vessel isolated from the system.

Chris

 

[John.g]

Hi Chris,

Not quite sure from the above but are you (still) of the opinion that there is no difference in the rate of pressure loss in a system under (cold) test whether connected to a EV or not?.

 

[CHRISX]

Hi Chris,

Not quite sure from the above but are you (still) of the opinion that there is no difference in the rate of pressure loss in a system under (cold) test whether connected to a EV or not?.

Hello again John.g,

From a combination of reading your explanation and what I was thinking when I wrote my first and second replies to you I have realised that when the initial pressurisation of a sealed Heating system is done - for example to the recommended cold temperature pressure of 1 Bar - that pressurisation is deflecting the Expansion vessel membrane so that deflection will be slowly returning back to or towards its pre-pressurisation form as part of the pressure drop when there is a leak from the sealed system.

I agree that would take more time to cause a pressure drop than a sealed system which had the same initial cold temperature pressure of 1 Bar and was on a pressure drop test with the Expansion vessel isolated from the system.

I have been a Heating Engineer, Plumber and Gas Engineer for well over 50 years and this is the first time that this subject has ever come up to me regarding whether a sealed Heating system pressure drop would be faster with or without an Expansion vessel isolated from the system.

I have often isolated the Expansion vessel on a sealed Heating system or isolated the Combination Boiler to prove that there was a leak somewhere on the pipework, radiators or radiator valves etc. but it has never occurred to me to be thinking along the lines of the speed of a pressure drop being different with the Combination Boiler or Heat only Boiler sealed system Expansion vessel isolated from the radiators and pipework.

Having written that - any sealed Heating system pressure loss investigation that I have ever done has involved just finding the cause so the particular test that we have corresponded about here regarding the speed difference of the pressure drop with or without the Expansion vessel isolated from the system was something that would not have been on my mind.

Thanks again for writing your explanation in reply to my initial comments.

Chris

 

[John.g]

Yes, all very interesting.

I suppose that when a system gas boiler is isolated and being tested that the EV is still "attached" so even more attention required to ensure no pressure loss if the test is very short term.

Generally, EV pre charge pressure is normally 0.75 to 1 bar even though theoretically 0.5 bar is enough for a system static head.
Also, knowing the actual pre charge and filling pressures is very useful as one can get a very accurate feel for the vol of water leaked, for example, a 10 litre EV with pre charge/filling pressures of 1.0/1.5 bar will have a reserve of 2 litres so if/when the final pressure eventually drops to 1.0 bar then 2 litres has been lost.
For interest, I attach a snapshot of the calcs I have used for decades, is is based on the fact that (air side) vol is inversely proportional to absolute pressure. So a pre charge/fill pressure of 1.0/1.5 bar gives a (10*2/2.5), 8 litre air side vol and (10-8), 2 litre water side (reserve) vol and so on.

 

[Ian118]

Do the radiator pipes downstairs come up out of a concrete floor ?

 

[John.g]

He says in post #1 that "Most of the pipework is exposed". It would be interesting to know if the leak was found or not.

 

[Lou]

(Just bumping a few older threads, they will drop off what is new soon)

Check Out Plumbing News!

 

[PRoger]

Not sure if replying to old threads is discouraged, but I'm having to top up my pressure roughly once every six weeks, have had the gas engineer round to look at the boiler and he couldn't find anything and there's no signs of anything on the visible pipes.
Because it's a slow drop, I was thinking leak sealer might be the best option? Is it something I would do myself or should get a plumber round? The boiler is a Worcester Bosch Gr8300iw 30 C if that makes any difference.

 

[Chuck]

Not sure if replying to old threads is discouraged,

I think it's better to start a new thread for a problem about a specific system. I may not be typical, but I don't have the time or patience to figure out which answers refer to which questions.

I was thinking leak sealer might be the best option? Is it something I would do myself or should get a plumber round? The boiler is a Worcester Bosch Gr8300iw 30 C if that makes any difference.

I consider leak sealer to be temporary fix to be used as a last resort, e.g. to buy time before the system is re-piped. The best option is to find the leak.

That said, adding sealant is an easy DIY job. Use a well-known brand, preferably the same one as the inhibitor you have in your system, and read the instructions carefully.

 

[John.g]

Not sure if replying to old threads is discouraged, but I'm having to top up my pressure roughly once every six weeks, have had the gas engineer round to look at the boiler and he couldn't find anything and there's no signs of anything on the visible pipes.
Because it's a slow drop, I was thinking leak sealer might be the best option? Is it something I would do myself or should get a plumber round? The boiler is a Worcester Bosch Gr8300iw 30 C if that makes any difference.

Did your engineer check the age of the EV and also then depressurise the system to check the precharge pressure, if the filling and precharge pressures are the same then any slight leakage will cause a large pressure drop (water side) but the system may only have lost a tiny amount of water.

If, for example, a 8L EV has been charged properly with say precharge/filling pressures of 1.0/1.5bar, then the system will have lost 0.56L in falling to 1.3bar, if precharge/filling pressures of 1.0/1.3bar, then the system will lose 0.66L in falling to 1.1bar.
You will have a fair idea when topping up, if the pressure rises very very fast then probably/possibly pre&filling pressures equal.

What is the pressure drop in those 6 weeks, any idea of the EV capacity?.

 

[PRoger]

Did your engineer check the age of the EV and also then depressurise the system to check the precharge pressure, if the filling and precharge pressures are the same then any slight leakage will cause a large pressure drop (water side) but the system may only have lost a tiny amount of water.

If, for example, a 8L EV has been charged properly with say precharge/filling pressures of 1.0/1.5bar, then the system will have lost 0.56L in falling to 1.3bar, if precharge/filling pressures of 1.0/1.3bar, then the system will lose 0.66L in falling to 1.1bar.
You will have a fair idea when topping up, if the pressure rises very very fast then probably/possibly pre&filling pressures equal.

What is the pressure drop in those 6 weeks, any idea of the EV capacity?.

I'm sorry I don't recall all the details, but the expansion valve has been replaced recently with the justification from the engineer that it might solve our problem. Pressure would usually be going down to something like half a bar before I notice and top it up.



Re: leak sealer, I'm supposing it doesn't make any difference where in the system it's added? Most of our radiators are some sort of "vintage" model so they might be uncommon fittings.