Slow leak on my pressured central heating system

[SurreybasedCumbrian]

Hi, new to this forum. I hope someone experienced out there can help me find the fix. We recently finished a building project and had a major re-design/move of all of our hot water (megaflo), boiler and central heating system. I am not a plumber nor do I know much about plumbing but I know enough about how the system is designed. During the building project we added 100m2 of wet underfloor heating to our existing central heating system (which is pressured/non-vented - it had been previously converted from a vented system in 2018). This is a NuHeat designed/installed system which integrates with our Nest controlled central heating radiators. Everything had to be moved and completely re-plumbed during the project into our new garage being built on the side of our house (converting existing garage). Project has completed fine and everything has been commissioned ok - its been ~ 6-7 months now running. My building is chasing me for the final part of the 5% retention payment (~£4K). But we have a frustrating slow leak on the system - the pressure will drop on the central heating loop to zero over about 7-10 days from its usual 1.5 to 2 Bar - hence not paying the remaining £4K until it is fixed. I didnt really pay much attention to the leak in the early weeks and months post commissioning as the project was still de-snagging and the plumber coming back to complete some minor pieces, etc. We have bled out all of the air in the system multiple times (rads and high points), and cannot see any visible water leaks anywhere - all rad valves fine etc. At the moment no evidence of water on ceilings etc as there was a bunch of pipe mods made in the ceiling/floor voids when the steels were going in. I am in discussion with the Builders plumber to try various things, and he is coming over this coming week to have a look. We isolated the boiler and the NuHeat system and the system pressure was still dropping so it seems almost certainly not the boiler (relatively new Worcester Bosch) or the NuHeat wet underfloor system. Our system is copper but the builders plumber ran much of the new stuff in plastic - particularly under the floor etc. Should I be worried about the integration of copper and plastic pipes? We checked the pressure of the expansion tank using a bike pump and it was at 2.5 bar - so ok? This issue is keeping me awake at night - the thought that somewhere perhaps water might be leaking and it could take several months before we see it. Any thoughts on further diagnosis, checks etc. Whats a reasonable pressure drop for my system - its large ~ 20 rads + UFH system (6 bed house). See photos.

Many thanks,
Nick

 

[cjg]

your expantion vessel is over presure, lock off boiler, drain, then take it to 1.2bar, then open up you c heating.

 

[ShaunCorbs]

your expantion vessel is over presure, lock off boiler, drain, then take it to 1.2bar, then open up you c heating.

Was taken with water / pressure in

 

[SurreyCumbrian]

Thanks… so an expansion vessel over pressure will result in a pressure drop.. where is the pressure drop going if so?… I will try what you say. I don’t see any water in the tundish drain by the over pressure valves.

 

[John.g]

If the system is pressurized to 2.5bar while cold then almost a certainty that it will rise to 3.0bar and lift the PRV when hot.

 

[SurreyCumbrian]

Was taken with water / pressure in

Yes… water/pressure in

 

[ShaunCorbs]

If the system is pressurized to 2.5bar while cold then almost a certainty that it will rise to 3.0bar and lift the PRV when hot.

Pre charge Jon with there still system pressure on

 

[SurreybasedCumbrian]

Thanks… so an expansion vessel over pressure will result in a pressure drop.. where is the pressure drop going if so?… I will try what you say. I don’t see any water in the tundish drain by the over pressure valves.

Just to say/update. This fix worked brilliantly. I dropped the pressure in the expansion vessel back to system pressure at 1.5bar using a bike track pump - previously it was overpresssured at 3.0 bar ish. Now no issues on pressure drop - the system is completely stable. Brilliant - many thanks.