I recently posted about running my UFH using a Willis heater. Plumber on board with this in theory but hasn't encountered before. We were wondering if it'll need an small expansion vessel? 

Shopping list so far is;

Manifold

Grunfos Pump

Willis Heater

Timer.

 

YES

 

Regardless of how you heat it, it is a fixed sealed volume of water that will expand when heated so an expansion vessel is required.

allow 1litre  on the expansion vessel for every kW of heat from the heat source

Thanks for the quick replies. I thought so. 

The obvious next questions is how big. 

TonyT you say 1 litre per kW heat (which would be 3l with a 3kW willis pump?)

I'd assumed you'd calculate it from the volume of water in the system (though I'd need to work out the length of pipe first!)

It's about 100m2 of standard-spaced 16mm per pipe. The 

Use a 12 litre expansion vessel and drop the precharge pressure to 1.5 bar when you install it. 
 

also get a Wilo pump - Toolstation have them on offer and they are virtually silent. https://www.toolstation.com/wilo-yonos-pico-circulating-pump/p75983

 

https://www.arrowvalves.co.uk/media/wysiwyg/pdfs/evcpdatasheet.pdf