Leaking microbore but no pressure loss

[MissMoppet]

We have 35 year old copper microbore underfloor that now leaks on one run, minutely, but enough to have to correct. However there is no pressure drop whatever showing on our Vaillant boiler. According to all the forums I've ever read this is impossible. Comments eagerly awaited.

 

[big-all]

heeelllooo and welcome miss moppet

maybe no "noticeable" drop

you have say 100 litres you remove a teaspoon full you wont notice you remove 1 litre you may notice you remove 10 litres you will notice

 

[MissMoppet]

Yes you may well have a point. It's enough leakage to buckle plastic floor tiles, but that could have been over several months. Thanks for your response.

 

[big-all]

have you topped the system up and how frequently ??

 

[MissMoppet]

Not had to top up for a year.

 

[big-all]

must have just started or water is from a different source but traveling down the pipe and dripping off like condensation on a cold pipe

 

[MeldrewsMate]

Is it a combi boiler?
If so I have known the secondary heat exchanger on Vaillants to pinhole between the mains water side and the heating side. If the mains pressure is low (less than 2 bar) then you may (stress MAY) have low pressure mains leaking into the heating system, effectively topping up the losses from the leaking valve.
Equally, and this also applies to system boilers, the topping up valve may not be shutting off tightly. Prove this by removing the 'temporary filling loop'.

 

[MissMoppet]

That sounds feasible, thanks.

 

[Phill Peck]

More likely again if it's a Vaillant the filling loop is passing and topping up the system for you