Welcome to the forums! In quick reply, yes, a biomass boiler can work as a backup. For reference, we have an oil boiler as part of our ASHP system, and it’s configured to assist during defrost cycles and when temperatures drop below -1°C (which isn’t very often). In those instances, the heating switches over to oil as the ASHP becomes less efficient.

One point to consider is the buffer tank – 1,500 litres is quite large. I’m not entirely sure how that impacts efficiency or whether it would need to be bypassed in a bivalent setup. I’ll reach out to some installers I know for further insights and advice.

Posted by: @vinceprince

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We have a biomass system which works fine. Looking at possibilities to improve efficiency and reduce costs. Limited usable roof space for solar.

Would an ASHP work well with the boiler as back up? We have a 1500l buffer tank.

House is victorian stone built in SE Scotland and as well insulated as we can make it!

Thoughts?

Rather than being a buffer tank, the 1500l vessel would more likely be a thermal store. Do you now if it contains a heating coil? What is the make and model?

@derek-m good point. 

Maybe some of photos of the plant room would be helpful, @vinceprince 

I'll get some pics when I am back. It has input from the boiler and output for the heating and hor water circuits, but the boiler initially bypasses the tank if heating system is cold and then heats the tank when heat circuit reaches temperature. Heating will run off the tank when boiler reaches set temperature and shuts off.

@vinceprince our company supplies heat pumps, and have been doing biomass systems for much much longer. What you're describing is called a hybrid or bivalent system and is totally doable. However there are some design considerations that it's important to be aware of to keep the running costs of your heat pump low, and to make sure your biomass boiler isn't constantly cycling on and off. 

There are several ways to do it - you can have an either/or system, or one where one tops the other up. You will then have different logic as to when to use each. For example outside temperature may well be a factor. 

I'm happy to help with some possible schematics but would need details of the system (thermal store, type of hot water provision, heat emitter details (rads/underfloor, etc)), as well as what you're hoping to achieve from it. My email is sune@firepower.co.uk  

My view

Make sure you heating system can run at a low temp, ideally the lower the better.

Get a separate DHW heat pump suitable unvented cylinder, run this in the normal manner via diverter valve from the ASHP. This allows the big buffer to heated to a temperature more suited to a good CoP from the heat pump and dedicate this to heating only.

Once temperature gets in the sub zero range fire up the biomass.

Another option is keep the cold water feed to buffer coil, and use this as preheated water to an unvented cylinder. This would make reheats quite easy.

Then look around for a heat pump at a bargain price as size is quite flexible.

@vinceprince you'd be well advised to discuss this with @sune. He's one of the best out there.

Took some pics, a bit crowded in there at the moment!