So less than 15 litres does not remove need for safety devices like pressure relief, it just becomes non notifiable for building regs.
So I think a PRV and probably expansion vessel would be needed. Biggest issue would be arranging a safe place to discharge the PRV - normally means go outside.
I think the idea could work if the Willis was permanently plumbed into the system with no isolating valves between the heater and the heating system PRV. I guess a flow restriction could be fitted to control bypass flow providing one side of the heater is fully open to the system. But I donât have a G3 ticket so might be talking rubbish.!!!
=> âwhat is the degassing effect of 80 deg heatâ
I expect it not important provided the temperature is significantly higher then end of heatpump dhw run, we only need to remove all the air that a DHW run will make into micro bobbles.
What the min pressure that a heatpumpâs water pump can be forced to run at? Eg is it possible to setup temp header tank at top of loft? (Tank can be small with single pipe connected to bottom, but must think about temperature of water that enters it.)
Personally I would be happy with a flow meter on Willis outlet and not plugging in to until confirmed flow.
Willis also useful if need to commission heatpump on very cold day in cold home.
(I assuming temp connecting Willis on spare UFH manifold manifold port or manifolds drain/fill with a radator flow meter.)
Needs something like 5% of flow in Willis as wish rest of system to be as cool as possible to maximise air absorption.
In my case I have a old unvented DHW tank and old CH pump, so could use tank in loft as temp connected open vented air boiler off point. (Would need filter to protect CH pipework from any scale in tank.)