Range tribune pressure issue

[Glenn14]

I have a range tribune indirect 210l which does heating, hot water and sunroom underfloor heating. The pressure keeps dropping when I have everything working but can take a few weeks until it starts. If I turn off the valves at the manifold for the underfloor heating it stops dropping pressure. After a couple of weeks I can turn back on the manifold valves and it will stay steading not dropping pressure but after about a week or so it will start to drop pressure. My plumber said if leak was in underfloor pipes then when I turned valves to manifold back on pressure should of dropped straight away As water should of leaked out of the pipes. This doesn't happen. What is going on, please help?

 

[MrTherm]

I would calculate the total heated sealed water volume and check if the current expansion vessel is sufficient to cope because by isolating the underfloor volume, the working volume would be less and therefore might be within the expansion vessel limits. A larger or additional expansion vessel may be required.

Check for slight seepage at the PR valve outlet (the old plastic bag trick) also check the primed air pressure of the expansion vessel.

 

[Glenn14]

Thank you for quick reply MrTherm my plumber says he's done that an all seems big enough and no leakage as asked about. It all seemed fine a few weeks ago but radiators in lounge were no getting very hotand thermostat was on 30. Plumber came and put something in system to clear limescale etc out of pipes. Came back 2 weeks later and cleaned something out and rads are getting really hot with thermostat on 23, but since them the pressure has been dropping again???

 

[MrTherm]

If you have gauge tapping points on your manifold, obtain and connect a suitable gauge then get system pressure running normal then isolate underfloor loop and monitor its pressure separately, this will at least verify if the u/f loop needs attention before looking elsewhere.