Heating circuit loses all water yet can't find the leak

[old duffer]

Having made some pipe diam changes over the summer I thought it would be a good idea to fill the system and check it, ready for the winter.
On filling the circuit I noted that it seemed to be taking longer than usual to fill so checked for leaks and started to refill again.

Got the circuit to pressure (1 bar) and then watched it drop to zero within seconds so must be huge leak.
Now I can access all of the heating pipework and have checked..no leaks, no water

I then check that the external release value was working (I have a quarter turn 22mm value that dumps the circuit water outside) but there was no water at the valve so a substantial amount of water is being lost but I have no idea where.

I have checked the pressure relief value by closing the circuit values and over pressurising the combi so I know that works.
My only guess is that the filling loop non return valve is now letting the circuit pressure go into back flow but as the incoming pressure is 5-6 times higher that the circuit pressure I doubt it could get back.

Any suggestions for the next round of diagnostics?

 

[CBW]

Was the system completely empty? Did you vent the rads? What boiler?

 

[old duffer]

Was the system completely empty? Did you vent the rads? What boiler?

it was. Cant vent the rads as they are not getting pressurised
zen combi

 

[CBW]

it was. Cant vent the rads as they are not getting pressurised
zen combi

How long didn’t you give it to fill? It can take a while to pressurise depending on the incoming pressure.

 

[old duffer]

How long didn’t you give it to fill? It can take a while to pressurise depending on the incoming pressure.

it takes around 3 minutes to read 1 bar and 20 seconds to read zero...incoming is at 6bar

 

[terryplumb]

Is water flowing out the condensate pipework?
Are any pipes within floor void or buried in concrete ?

 

[old duffer]

Is water flowing out the condensate pipework?
Are any pipes within floor void or buried in concrete ?

No and No

 

[old duffer]

Is water flowing out the condensate pipework?
Are any pipes within floor void or buried in concrete ?

well they are in a void but easily accessible for checking...which is how I know

 

[Johntheo5]

Can't you just keep the filling valve cracked open to maintain 1bar while you have a detailed look around.

 

[Alec_t]

Can you hear water running while the pressure drops?
Have you opened any inspection chamber to see if any water is going down a drain?

 

[old duffer]

resolved. Open valve on a radiator.

 

[terryplumb]

resolved. Open valve on a radiator.

Ehhh ?

 

[old duffer]

Can you hear water running while the pressure drops?
Have you opened any inspection chamber to see if any water is going down a drain?

i had sound proofed a floor by adding a lot of mass (cement boards and insulation boards and 18mm chipboard from under the floor. I could not hear the gushing water until I went under the floor to inspect. Still, I know the sound insulation is working

 

[old duffer]

Ehhh ?

each radiator (16) can be isolated and drained without having to drain down the system. So there is a drain pipe and valve that comes off the return side so it dumps the water.

 

[terryplumb]

Non the wiser ! Where was the water going ,and why couldn't you see any signs of it ?