Air source heat pump modification

Hi we live in the south of France and currently heat our house with evacuated tube solar panels (search Google for smartfrance blogspot - can't post URL as Newbie )

As air source to water heat pumps are less expensive than water/water heat pumps is it possible to connect something like an old truck radiator to a geothermal loop and place that in front of the air source heat pump air inlet. In this way on a day with say -5 outside the air pulled through the radiator would be at +5C and that would help the COP. Anyone tried this?

I presume air source heat pumps are less expensive as they sell more of them?

The main thing which reduces the efficiency or air source heat pumps is that they have to defrost the outside air coil heat exchanger from time to time when the outside air is very cold.

The exact outside air temperature below which the unit needs regular defrost depends on some veriables & the manufacturer.
But 7 deg C is an aproximate rule of thumb.

As the coil gets covered in frost so the efficiency & heating performance starts to drop away.
Then at some point the system does a defrost by pumping hot gas through the outdoor coil.

A major benifit of ground loop is that the water from the loop should stay warm enough so defost is not required.
But the disadvantage is that the ground loop temperature is often colder than outside air temperature.

For instance today in the UK the ambient outside air was about 13 deg C but if a gound loop water temp was say 10 deg C then the air source system would be more efficient.

Therefore a system which could use ground loop water when thats hotest & then outside air when thats hotest would be super efficient but it would be very expensive & payback time would be longer.

If you have an air source HP & can find any way to preheat the air then that will improve the performance & COP.
If you can keep the air on coil temp above the temperature at which the coil starts to frost up that would make the best performance improvement.

Hi Thermatech, thanks for the reply. We have 2 Mitsubishi reversible aircons so I can experiment with them I will monitor the temperature of the air flowing into the air con unit and rig up a heat exchanger in front of the system. I will have to assume if I get the air temperature up that I have a better COP. We have several possibilities here to get the air temperature higher. Our swimming pool has a loop in its base. The pool is 11C today. We also have a ground loop under the pool. I can also dig some more ground loops if it works....

You could do with getting the performance data for your systems from the manufacturers technical data book.

Then you can check what the heat output & kw electrical input for your system should be for your indoor air temperature & any outdoor ait temperature.
You will need to do a correction for the lenght of interconnecting refrigeration pipe if its a split system.
You will also need to be able to measure the relative humidity or wet bulb temperature of the outside air as this has an impact of heating performance.
Heating performance data is often based on outside wetbulb air temp.

Dont forget that you will be using some extra electric kw to pump the water around.

If you can estimate that for large part of winter time you can keep outside on coil temp above 7 deg C DB or about 5 deg C WB then I should think it would be worth the time & effort to fix it up.
I suppose to some extent that will depend on the weather conditions you get at your location & exactly how much heat you can extract with your ground loop.

Hi Thermatech, by coincidence I found the technical manual yesterday on the web and it has all the figures etc in it. The acid test though is whether the units still manage to heat the room when the outside temperature drops. It is quite marked how the performance drops off once the outside temperature gets around 0C. That as you say must be due to freezing of the coils and the need to defrost. If I can prevent that it will be worth it. The 2 units are Mitsubishi SRK25_ZD-S and have 410a refrigerant. The cost of the recirculating pump is small and we have solar PV to offset that. I will only rig up something temporary as I would really like a system that uses the underfloor heating and the air to water system that is least expensive. So if I can keep that efficient with a secondary geothermal heat exchanger then great. Our climate is mediteranean so it is quite mild.