Discussion ... ASHP(Air/Air) with Solar pv ....

I ran my 3.5 kw ASHP today for a full 12 hours raising the temperature of a 38 sqm (409 sq ft) room with the door open from 20C to 23C. Yes, it is nice and toasty so have just turned the heater off and closed the door for the rest of the evening. I had plugged the ASHP into a Energenie power monitor and recorded a total usage of 4.7 kWh which equates to 392w per hour over the 12 hour period. The heater was operated on the Eco Auto setting.

Unfortunately I have no means of measuring the heat output of the ASHP so some may consider the above information of little use but it is an indication that an ASHP can be run very economically in a large room and raise and sustain the temperature at a very comfortable level for an input of less than 400watts.

mmmmikey wrote: »

I've been running my ASHP for a couple of hours in the morning and about three hours in the evening for the last few days, which is plenty to keep the back of the bungalow warm, at about the same 400W power levels. The bonus for me is that at this time of year my E7 night rate ends at 08:30 so I can run the ASHP using cheaper night rate electricity in the mroning. The low power consumption also means that it is entirely practical to run the ASHP off the battery in the evening. The battery is charged with E7 electricity.

If you make the assumption that 400W electrcity in gives about 1kWh heat out, it's costing me about 2p/kWh at the moment for heating, making this cheaper than gas even on a kWh per kWh basis. And this is without taking into account the control benefits of only delivering exactly the right amount of heat when and where you want it. To say nothing of the environmental CO2 emmissions benefit, with a higher proportion of overnight electricity coming from nuclear and RE sources.

I actually bought the ASHP earlier in the year primarily to use for summer aircon, with any heating use a benefit. As it happens, it's going to save me a worthwhile amount as a shoulder months heating solution as well as making a nice dent in my greenhouse gas emissions.

So I’m not the only one who stands outside watching the meter to see what time it switches to E7.:).. in my case it is 08.36.

It’s my first year with ASHPs and second year with solar. Last year nothing got switched on if the sun wasn’t shining (until the wife protested) but this year I am going to try living a normal life. I got my first ASHP at the end of January and second end of April and am still working out how to get the best out of them. Like you, in the spring, I was operating the kitchen unit on the inbuilt timer to come on at 6.30 am (yes we do have PV at 6.30 from late April) to get the house warm for breakfast then only using them during the day and evening if we felt cold, letting the IBoost take any surplus. Yesterday was a bit of an experiment as It was cool enough to run a heater all day and had limited solar - just over 4kwh (almost typical winter day PV). As a result the IBoost only saw 0.17 kWh and as that had followed a couple of below average days I wasn’t sure if we would have enough hot water this morning for showers but we did - just.

The plan for this winter is to run the ASHPs most of the time (using as much free solar PV as possible) in preference to oil CH and top the water tanks up overnight with E7. That means the IBoost will only be useful on sunnier days but it is cheaper and more efficient (less heat loss) to heat the water overnight and use my ASHPs during the day. As we have a big house and only 2 ASHPs I will probably have to run the oil CH for an hour in the morning and one in the evening as well.

I noted that you were only assuming a COP of 2.4. Is that something you have calculated? The COP of my 3.5 kw heater is supposed to be 4.0 and the 2.0 kw one 4.3 but I have no means of checking if this is true in practice. ( I seem to recall reading they are more efficient when operating at low power and when the temperature differential to outside is not too large).

At an efficiency factor of 2.4 my daytime use would be slightly more expensive, at 6.25 per kWh, than oil at, 5.2p per kWh, but maybe cheaper once I factor in the cost of powering the CH pump. I like your point though about the targeted nature of the heat from ASHP as my wife loves to sit directly underneath the unit and have the vanes forcing the airflow straight at her.

Today with clear blue skies (at the moment) one ASHP is on, building up core temperature, along with the washing machine and the IBoost is still getting fed. Happy days.:)