I am thinking about doing it because it could overall use less CO2.

I envisage the ASHP being connected in the radiator loop just before the return to the boiler. So all I would expect it to do is help maintain the temperature of the system. As it is only a 3-5 or 7kw system it will only be supplementing the boiler, not replacing it.

The system I am looking at is here as you can see it seems reasonable priced.
link to remote site http://www.heatandplumb.com/acatalog/Activair_Air_Source_Heat_Pumps_3kW.html

I am not use that I understand the impact on the units COP being reduced because of going through my existing radiators. Surely the incorrect radiators will limit the heat transfer from the heated water, not how efficiently it heats the water in the first place. Anyway as I am just supporting the boiler I don’t believe this will be an issue anyway.

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Stuart