If you went for an ASHP, like the Daikin Altherma (assuming it meets the output you need) then this can be used instead of a gas boiler, with a direct electric element picking up top-up load and a legionella cycle where the heat pump alone cannot meet the required temperature.

If you're running radiators, and don't plan to change them for larger ones (about double the size), then at a higher heat output you'll only get a CoP of perhaps 1.8-2.5 rather than the 3-4 you'd expect running for lower temp u/floor heating.

Note, if you have solar hot water, Daikin don't like using this ASHP with a twin coiled cylinder - preferring a separate heat exchanger.
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Quisling

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

With respect to CoP, a heat pump has to work harder to raise the water to a higher temperature, so instead of taking 1kWh input electricity to raise the temperature by 35 degrees (for example) it'll take, say 2 to raise it by 55. Since radiators work at a higher temperature than underfloor heating the heat pump will have to work harder to raise the temperature higher. They work more efficiently running longer for lower outputs.

If you have a condensing boiler, they work most efficiently when the return temperature to the boiler is below a certain level (about 35C if I recall correctly for gas boilers). Using the heatpump to pre-heat the return to the boiler could therefore cause a decrease in efficiency of your existing boiler (assuming it's a condensing type).

Things to consider:
1. what is the CoP at the maximum output temp of the ASHP?
2. What is the max output temp (I guess this will depend on flow rate of your heating circuit?). Most heat pump systems don't go above about 55 - 60C max.
3. Does the ASHP use a refrigerant, if so then what type and can it be serviced by a Corgi engineer or do you need a specialist HVAC contractor to service it. Some ASHP's need specialist servicing.
4. Heat pumps contain a compressor which cuts in and out. Generally, in order to maintain a decent design life, the heat pump controls specify that the compressor shouldn't start/stop more than x times an hour - typically 3 times. A gas boiler (especially if a little oversized) will start stop more often. I'm not sure whether your planned installation would compromise the longevity of the heatpump? If that was a risk, you could use a heat store so that the ASHP doesn't short cycle, and then use the heat store to indirectly heat the boiler return.
5. Is there a risk that - hypothetically - the return to the boiler could end up hotter than the boiler flow - e.g. if the ASHP fired up prior to the boiler coming on in the morning. I'm not sure how your boiler's sensors/brains would react to such a scenario.

Carbon benefits.
Grid supplied elec is 0.422 kg CO2/kWh (source: Part L2, Bldg Regs).
At a CoP of 3 (optimistic best case if your heating up to 55C), you therefore create 0.422 kg CO2 to create 3kWh of heat.
Assuming your boiler is natural gas, then is generates 0.19 kg CO2/kWh, so for kWh it'll create 0.57kg CO2.
Your heat pump will create 0.422, so there is a carbon saving of 0.148kg per 3kW of heat output, or 0.049kg/kWh.
And that's IF you get a CoP of 3. If you get down to a CoP of 2.2 there is no carbon benefit, unless of course you have a renewable import tariff for your electricity.
I've heard from ASHP manufacturers saying that at high outputs (e.g. for radiators) the ANNUAL CoP (throughout the UK heating season, and assuming it's doing hot water and space heating) is as low as 1.8.

It sounds a laudable plan but I'm not sure it will give significant benefit.
Does your existing boiler have weather compensation on the controls? If not, can the manufacturer provide an upgrade? This might give as good a carbon benefit without going down the ASHP option. If your boiler is a Worcester Bosch then the answer will be no; I've recently investigated this for mine.

Regards
Colin
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Quisling

I do have a Worcester boiler so I guess the weather controls are out.

I am very dissapointed as that ASHP was so damn cheap. Is there no way I can use one?
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Stuart