In this post I am outlining whether using my excess solar generated energy to drive air to air heat pump heating = cheap/sustainable heating?
One of the ways I plan to address spiralling heating costs is by experimenting with using an Air to Air heatpump run off my excess solar PV generation (i.e. that above the base usage of always on devices, fridge freezer etc). The idea being that when it is sunny, which fortunately in the UK winter often coincides with cold weather, we pump as much heat as possible into the house during the day. Hopefully, with a well insulated house, then maintaining enough of that heat to last the evenings/ dark (supplemented with a woodburner).
Whilst much of the world, and most commercial buildings use air to air based climate control systems (air con + heating), in the UK we have traditionally shunned this approach for domestic use. Favouring “wet systems” i.e. radiator based central heating, or electric heating via. night storage etc.
Air to air versus other heat pump types:
| Heating type | PROs | CONs | Cost of CH + water install (Typical) |
| Ground Source heat pump | – Most efficient (Ground temp consistent) – Can sometimes work with legacy hot water tank and radiator systems – Ideal for underfloor heating – Can also supply hot water | – Needs a large garden – Most expensive install – Install can be slow and disruptive, borehole drilling etc. – Hot water produced is at a lower temp. than conventional boiler so may require supplementing with immersion heater | £15k+ |
| Air source heat pump (Air to Water) | – Can often work with legacy hot water tank and radiator systems – Works well with underfloor heating – Can also supply hot water | – May require upgrade wet system to larger radiators or underfloor – Expensive install – Requires suitable outside area with free space – Hot water is lower temp so may require supplementing | £10k+ |
| Air source heat pump (Air to Air) | – Can be fitted to flats/ small buildings – Cheapest install – Wall mounting of outside unit means it can be sited without inhibiting outside space usage – Range of unit types for installation flexibility | – Can be noisier – Requires seperate hot water heating – Wall units not super attractive – Blown air heating can feel cooler than ambient due to breeze | £2k+ heating £2k+ hot water |
Air to air (air conditioner) unit types suitable for heating
There are really 3 main types of air conditioning unit (air to air heat pump) available, with heating function, for domestic use:
- Portable air con units. Those with heating start from around £350 upwards
- Mini split systems. These have an externally mounted unit (the square box with fan type unit seen on commercial buildings) and 1 – 6 or so internal units for heating/ cooling distribution. These start at £500 for cheap single interal unit systems running to several thousand for state of the art domestic units with 5+ outlets.
- Wall mounted single room units. These look a little like a storage heater and start at around £800. e.g. Olympia Splendide
Before going down the route of permanantly fixed systems I have opted to experiment with a portable unit. The biggest downside of portable units is the requirement to fit a vent for the air duct. Typically these are provided with a window fitting which works for sash or sliding windows but not so well for casement windows, although adaptors are available for these. I have gone through a wooden external door with a cowled ventilation hole which is slightly more convenient.
Crunching the numbers
Measuring the output of our Solar Array, 13 panels, at this time of year (early October) we are getting around 5-6kW per day of generation in mixed conditions. On a good sunny day seeing >1-1.5kW per hour is common. On a gloomy day the generation may be negligable. Background energy usage for the fridge, broadband routers, pond pump etc. seems to be <0.2kW per hour.
Our portable air con unit claims to use around 1.1 kW. So in theory when it is sunny we should effectively be able to run for free. NOTE: Electricity we are not using personally is exported to the grid so technically the grid is making up for energy it cannot get from us with another source.
The heating output of this unit is around 3.2kW so really a 1:3.2 energy in versus out. For comparison a 2kW electric fan heater is 1:1 so in energy terms compared to a fan heater we are getting 50%+ more heat output for half the cost. As of 1st October 2022 my electricity cost per kW has bumped up to 37p. Therefore the equivalent cost of 1hrs output of 1kw of energy versus the fan heater is:
| Heating type | Input:Output | Cost per hour | Cost for 1 kW heat output p/h |
| Electric Fan heater | 2:2 kW | 74p | 37p |
| Portable Air con | 1:3 | 37p | 12.5p |
So with a difference of 37p per hour to run (or 25p per kW/h), and an additional kW of heat output, with 3 hours usage per day in the winter it could certainly add up to a good saving. Especially when I am utilising the excess solar on sunny day so that heating usage is effectively coming at no cost to me.
With a unit cost of £390 for the portable unit versus £20 for a fan heater it does mean around 1000 hrs or usage are required for break even. Probably a couple of years in real use but with gas and electricity prices only going one way this value equation could shift further in one direction. Also of note my FITs (Solar generation payments) are generation not export based. This means that I am good to use my generated energy without it lowering my export payments (paid at 1/2 generation rate).
So fine on paper but is it any good for heating in real use?
We are a few days into the test and so far so good. I can see by monitoring the electricity and solar generation meters that by picking sunnier parts of the day (luckily i work from home) I can quickly bring the ambient room temperature in the our main living/dining area (21ft by 12ft) up from 16-18 degrees to 22 degrees or so, and without incurring extra electricity cost. This takes around 20 minutes. With a well insulated house this heat is reasonably well maintained especially once we cook dinner, and start to use the area, generating our own heat.
So is air to air a good option?
As it turns properly cold outside ( we were down to 5 deg C last night but still 15 deg in the day) and the overall ambient temperature of the house drops it will be a sterner test but will post an update on what I have found in a few months.
Overall if I am satisfied that using air conditioning to drive my heating is effective i may consider installing fixed units, either wall mounted or a mini split.