Central heating slowly increasing in pressure

Kencollins

Hi all,

I have a central heating system that is causing me a bit of trouble. It is a pressurized system with two pumps. The hot water leaves the boiler and goes to a T piece, one pump on the hot side of the T for the hot water coil, and a return pump on the cold side just before the boiler for the radiators. All works fine with the relay system that I have set up. The rads get hot when the heating is on, the water heats when the hot water is on...perfect!

The problem is that the system is building pressure gradually. The hot water runs for 30 minutes in the morning to heat the water for the day. After 3 or 4 days the pressure is up from 1bar cold to 1.5bar cold. 3 or 4 days later it is now up to 2 bar cold. I release some water from the drain valve and the cycle starts again. I estimate about 2 - 3 liters.

Things I have tried:
Disconnect the fill loop
Replace the pressure vessel
Recharge and double check the pressure vessel
Add more corrosion inhibitor
Replace a leaky AAV
Replace a dripping PRV

Nothing has rectified the situation. Where do I go from here?

One thing that has my attention is that the cold water (and the pump feeds the hot water tank too) is pressurized to about 2.5 bar. There are 3 pumped showers in the house, and all works well.

Is it plausible that the heating coil in the tank has a pinhole and the hot water is back-filling the heating system? I have a new tank ordered as an unused thermostat port on the tank is weeping from the weld, but the plumber hasn't come to fit it yet (three months later...)

I would be so grateful if anyone has some suggestions!

Ken

Tails142

Yes I would think of the hot water tank leaking from the pressurised side into the heating circuit as being the culprit, if you have one weld leaking it is reasonable to think there may be two! When you have it replaced I suppose you will know.