Wet ufh system and radiators off same combi boiler

[Bob Wilson]

We have just had an extention built with wet under floor heating in screed, but still have radiators in the old part of the house. The system has a zone valve that controls the radiators and the wiring harness seems to mean it doesnt need a second zone valve for the ufh part. Both the radiators and underfloor use a common boiler return.

The problem is this - if I use the underfloor heating at a high temp (45c) the radiators heat up even when the thermostat on that zone valve is closed. What seems to be happening is that the outlet from the ufh manifold is backing up into the radiators via the common return. Has the plumber forgot to fit a non return valve or something?

I can minimise the problem by using the lowest temperature on the manifold mixer valve because then the return is 24c and hardly noticeable, but I would like to run the ufh at 45c to heat the room quicker.

 

[DP]

Replumb so that returns are commoned at the boiler

 

[BlueLoo]

Forgive me if i have this wrong, there is only one zone valve?

Each zone should have its own valve i would think.

UFH-zone valve.
House-zone valve.

Common return is ok, but the feed needs to be valved.

Would be my opinion.

 

[Mr Chibs]

I have similar set-up and have two zone valves, one for house rads, the other for ufh in one room. Both can be controlled independently.

Can’t you turn the boiler up slightly And used the ufh blending valve to reduce water temperature?

FWIW, Just looked at my boiler rad temp 47°

 

[Madrab]

You should have a 2 port valve on the UFH prior to the UFH pump, that is controlled by the UFH thermostat that should be in the extension?? Depending on where they have plumbed the return from the UFH into the main return for the CH, then it sounds like you may be getting reverse circulation.

Bit of a no brainer ... get them back in to fix it, it's their responsibility!!