UFH causing boiler dry-fire safety cut-out

[Devila1]

Hi all
Brand new to these forums and would welcome any help.

I've recently moved into a house with wet UFH installed. It is a standard Polypipe system with manifold and zone pump (only 1 zone, but large). I've attached a diagram showing the set up and a picture of the mixer/pump combo on the UFH manifold (i.e. standard Polypipe set up). The boiler is a Baxi 133he Plus.

When the central heating and UFH both call for heat everything works fine (i.e. radiators get warm and UFH gets warm). The flow meters show that there is flow around the UFH circuit. However, when the central heating zone shuts down and the UFH still calls for heat, the boiler cycles a few times (produces heat for a couple of seconds) and then cuts-out with a dry-fire warning light.

Any ideas why this would happen? Is the Baxi output/flow rate too much for the standard Polypipe mixing valve?

Cheers
Andy

 

[Chalked]

Those valves tend to stick. It may need a new cartridge.
It also needs a motorised valve and an auto bypass

 

[Sparkgap]

The boiler probably has a minimum fire rate of about 5kW. Not sure if your UFH has individual circuit valves on the manifold but probable that when only UFH running it's well below minimum for the boiler and since flow rate is low boiler trips out. Certainly could do with an auto bypass valve to keep up boiler flow.

 

[Devila1]

Thanks, the manifold hasn't got any circuit valves fitted. So even when ufh thermostat off, boiler pump still pushes a bit of heat around ufh circuit! Not particularly efficient.
Baxi boiler manual says boiler has internal bypass, are you suggesting a second bypass loop to keep flow going?