Boiler backup with no buffer tank/low loss header?

[JamesPa]

Having more or less settled on using Mitsubishi, Im now thinking about the system design.  Rads are becoming obvious - make them big to reduce flow temp, balance properly, use TRVs as limiters only not for temperature control (adjust the flow temperature as low as possible using weather comp).

 

Mitsubushi don't insist on a buffer tank provided there is sufficient system volume (which there is) and Id like to avoid buffer tanks/low loss headers a) because of the space involved and b) because of the flow temperature and hence efficiency loss across them.  However I want to have a backup system, at least for the first few years, and the obvious one is my existing boiler (which of course comes complete with pressure gauge and expansion vessel).  So I have been trying to come up with an arrangement of valves etc that means that, in normal operation, the heat pump sees exactly what it expects to see, but it can be switched out and a boiler turned on (manually) as a substitute.  I currently plan to retain my existing boiler controls in addition to the Mitsubishi, so that the controls for each are totally separate.  

 

In principle its not too difficult to come up with an arrangement, until you consider the case where both have to be running at the same time to avoid them freezing up.  But I think even this may be achievable.  I attach my draft diagram, the key is the three 'zone valves' which need to be wired so that either ZV2 or both ZV1 and ZV3 are disabled (and therefore shut), together with the one way valves preventing backflow when the bypass valves kick in (which should only happen if both turn on to avoid freezing)

 

I am not asking for anybody to comment on the diagram (but of course feel free) - but has anybody else tried a boiler backup without a mixing tank/low loss header or am I either a) breaking new ground or b) being stupid even thinking about doing such a thing?

[JohnMo]

All the ASHP manufacturers offer hybrid solutions, so may be easier copying their solutions for pipes and valves, instead of reinventing the wheel.

 

It could even be a benifit to use a hybrid controller to get the best performance out the system.

 

Not sure why they would both need to run to stop them freezing up.  I assume the ASHP will have antifreeze in it, so it would not freeze, as you have both the ASHP and boiler in the same system (sharing the same antifreeze mix) the boiler would not freeze up either.

 

Also if you are running weather compensation, the heating runs 24/7, but could be setback a couple of degrees only at night.  So even less chance of freeze.

[George]

For what it's worth, my space heating backup is a wood burner or plug in electric heaters and the hot water backup is an immersion heater in the cylinder.