Amount of leakage from central heating system

10 Dec 2010
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United Kingdom
How much leakage is needed from a central heating system before it loses pressure?

We have a Saunier Duval Thema Condens f30e combi boiler (fitted 2006)that loses pressure in matter of hours after refilling the system. System feeds a 3 bed Victorian house (8 rads).

We've tested the prv by putting a bag over the release pipe outside but this doesn't fill with water. We therefore assume we have a leak in the pipework of the system.

However we can't find a leak anywhere, even after having the floorboards up. How much of a leak are we talking about to depressurise the system in just a few hours? Eg, a cupful of water or a bucketful? I'm envisaging quite a large quantity of water, which I simply can find no evidence of.

Could there be any other cause of the depressurisation?

Many thanks
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Not much as it happens and if the heat exchanger is leaking internally you would not see is at all as it will go down the condensate pipe.No leaks at the boiler?The amount lost is variable from a litre or more to drop pressre to zero.
Have you bled all the system? if you have lots of air in rads you may just be adding a little water only to have the air escape which reduces pressure very quickly.
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I've checked/done what you both suggested, in particular:

1. I left the boiler on but didnt use the HW or CH and put a bag over the end of the condensate pipe. It didnt fill with any water and the boiler still lost pressure. The bag did fill when you put the HW/CH on and it starts condensing etc. I repeated the same exercise (again) with the prv outlet pipe too to be sure. Again, no water. Incidentally, I filled the loop to over 2 bar on purpose to see if the prv worked and it did - it started expelling water over 2 bar. Therefore, can't see significant leakage at boiler.

2. I've bled the rads. A lot of air came out of the upstairs rads. This air must have got in when we had some pipework moved last month in our kitchen refit. With the exception of the rad I discuss below, the upstairs rads now get hot all over but the boiler still loses pressure after about 2 hours. It seems to hold pressure a bit longer when the HW is on though.

However, there is one upstairs rad that doesnt get hot at all, not even the flow and return pipework just below the connection into the rad.

Im wondering if when the pipework was moved (professionally!) this rad has not been connected back up. To explain further, we had the main upstairs flow and return and hot and cold water feeds rerouted and tidied up as they leave the ground floor.

Incidentally, he replaced the bathroom tap at the same time (mixer for a mixer) and now we cant get any hot water from the tap - when you turn the mixer to hot, no water comes out at all.

Can you think of anyway the above bits of information might explain the pressure loss? Eg, the plumber connected the wrong pipes up.

I appreciate that the next steps are to get the plumber back but I do like to inform myself as much as possible before I do.

Thanks for any further help

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