You are perfectly correct about the phase change from gas to liquid and liquid to gas as being the points where maximum energy transfer occurs.

I think we need to clarity terminology, since there seems to be a little confusion.

Certainly as far as qualification for RHI payments are concerned, what you describe as a 'fan coil' would be termed the indoor unit of an A2A ASHP or more commonly called an Air Conditioning Unit and would not qualify for RHI. Whilst it is feasible to use water as the cooling medium in the 'fan coil', it would not be as efficient as using refrigerant gas, since there would be no phase change occurring in the indoor 'fan coil' unit. Also without adequate protective devices it may be possible to freeze the water in the Evaporator, which would not be desirable.

For these reasons what is described as A2A ASHP only use refrigerant gas as the transfer medium.

The A2W ASHP as defined for RHI qualification uses the refrigerant gas to heat water, which is then used to transport the heat energy to the points of use.

As you have stated you could quite easily have the problem of puddles of water around your radiators or UFH if an A2W system was used for cooling.

To try to answer your question about the difference in efficiency between heating and cooling of an A2A ASHP.

I recently installed a A2A system, primarily for heating but also for cooling when required.

To quote the manufacturers specification:-

Cooling Capacity is 2.6kW with a Seasonal Energy Efficiency Ratio (SEER) of 6.2

Heating Capacity is 2.62kW with a Seasonal Coefficient of Performance (SCOP) of 4.2

Whilst the actual heating and cooling capacity is approximately the same, in heating mode it is expected to be less efficient overall compared to cooling mode. This is due to the fact that when the ambient air temperature falls below 5C, the efficiency of the ASHP starts to reduce, and can fall by as much as 50% under sub zero temperatures.

I hope that this answers your questions.

Regards,

Derek.

A fan coil is just like a car radiator in reverse - cold water goes in, fan blows room air over radiator, air cools, water heats up, warm water comes out.  Humidity that comes out of the cooler air is collected and piped to drain away. Having a fan means the air is mixed so you aren't relying on passive convection. It's like a fan oven has much better temperature uniformity than a convection oven.  You can also use a fan coil in heating mode, and for the same heat output you need a smaller size than a passive radiator.

So my question is getting at: what is the difference in efficiency (quantity of heat transferred) between a fan coil driven by an ASHP in cooling mode and a A2A unit which inside the box is doing a phase change of refrigerant from liquid to gas? Phase changes absorb more energy, but I don't have an idea what kind of scale how much more in this situation.

Let me see if I can explain the main differences as far as I understand them.

In the A2A ASHP, the compressor which is located in the outdoor unit, compresses and heats the refrigerant gas. This gas then flows through the high pressure pipe to a heat exchanger (Condenser) located within the indoor unit. This heat exchanger is basically a finned coil over which a fan blows air and thereby extracts heat energy from the refrigerant gas. As the gas cools, it starts to condense, and flow back to the outdoor unit via the low pressure pipe. In the outdoor unit the refrigerant, in liquid form, flows through a further finned coil heat exchanger (Evaporator), where it absorbs heat energy from the outside air and evaporates back into gas form. The refrigerant gas then flows back to the compressor where the cycle starts once again.

In the A2W ASHP, the compressed and heated refrigerant gas flows to a different type of heat exchanger (probably a plate type), in which the refrigerant gas and the central heating water are in good thermal contact with each other. The hot gas therefore heats the water, which in turn carries this heat energy into and around the building.

Similar to the A2A system the condensing gas then flows back to the evaporator where it is evaporated using heat from the outside air and the cycle starts again.

The main difference between the two systems is therefore the type of heat exchanger used as the Condenser.

The other difference is that the A2A can be safely used for cooling as well as heating, which is not the case with the A2W.

Regards,

Derek.

So I'm wondering how much of the heat transfer is due to the phase change, and how much is just the fan pushing more air through than you'd get from a passive system?

@Derek-m - Can't wait to read you full review. your thought on efficiency got my attention - I thought ASHP are highly efficient up to 5c and it might be better to move to gas boiler below 5c - did I get it wrong?

I forgot to mention why I think there is more emphasis put on insulation with reference to heating rather than cooling. Though I feel it is equally important for cooling.

In the cold of Winter ASHP's are not as efficient, and hence need all the help they can get. It would be the same as running an AC Unit with the windows open.

The other reason I decided to go for an Air to Air ASHP is that it will also provide AC if we have any hot Summer days.

Regards,

Derek..