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

[gmartine]

Yes, introducing

Just logged in again to see this - is it a serious suggestion?! Its just that I succumbed to pressure and called a very helpful leak detection service who gave advice as follows

1) A top-up every two weeks is unlikely to be found, it is just too small. We don't want you to waste your money on the call-out and not find anything when we do.
2) At this level of leakage it is not going to cause damage (but I did read somewhere on other firms websites that it may re corrosion etc. Is this true?)
3) It is likely to get worse, especially as the winter approaches and we use the CH more.
4) Call us again if / when it does

She sounded knowledgeable and the company answered the phone within two rings. I'd love to get confirmatory thoughts that this advice is correct and then I can go back to my wife with a definitive answer *** with a view to using this firm in the future.

Yes, it's a very serious suggestion. What have you got to lose except a couple of quid on some scent.

The only issue is you're probably not running your central heating atm which means it won't circulate around the system as freely but certainly consider doing it when you do switch your heating on if the problem still persists.

 

[John.g]

Hello John.g,

I cannot agree with what you stated about the pressure drop falling faster because the Combi Boiler Expansion vessel was isolated from the Heating system - however I am open to further explanation if you feel happy to write it ?

Here are my views:

When the Combi Boiler Flow and Return isolating valves were turned off obviously the Boiler would be off and whatever pressure was in the Heating system radiators and pipework at that time would not drop any faster because there was no connection to an Expansion vessel.

If there was 1 Bar pressure in the system with the Combi isolating valves open and therefore the Boiler Expansion vessel membrane had that pressure applied to it that would not affect the speed of a pressure drop which was being caused by a water leak or an air leak within the Heating system.

However if the Heating has been on and the system pressure has risen to perhaps 2 bar with that pressure rise having been taken up by the Expansion vessel membrane deflecting inside the vessel that rise in pressure / deflection of the Expansion vessel membrane would cause a pressure drop to fall faster for a short period of time until the system pressure fell to the cold temperature pressure when the drop would be slower.

Whatever water pressure was in the system with or without the Combi Boiler / Expansion vessel being isolated will affect how quickly the pressure drop falls - the higher the pressure the faster the pressure drop.

As I mentioned above I would welcome your reply - especially if I find that I am wrong in what I have stated ?

Chris

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.

 

[SimonG]

Over pressurise the system then double check radiator valves, joints etc.

 

[John.g]

Also give all the lock shield valve gland nuts a nip up.

 

[grahamgorb65]

have you tried adding leak sealer to the system.

 

[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)

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