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How our air to air heat pump keeps the house warm and bills low

Posted by Keith Searle on 5 March 2013 at 9:04 am

Our entire house has been nice and warm since we installed our air source heat pump in December 2011, and on the odd hot day in the summer it has cooled the living room. We leave it switched on 24hrs per day and it keeps on blowing out warm air at about 35 degrees-even on below freezing days. After being off for the summer we switched it on again on 22 September 2012.

We have never used any additional heating, except for a 300 watt towel rail in the bathroom (and that’s a luxury my wife has always wanted). Even on below freezing days, it keeps blowing out warm air at about 35 degrees. Our house is open plan and the heat from the living room makes its own way upstairs where it keeps all the bedrooms warm too. 

The indoor unit is very quiet, and the only noise that the outdoor unit makes is the sound of the fan blowing air out, but you have to be standing very close to it to be aware of it. Our cat sometimes stares at the indoor unit for long periods, so it’s possible that she can hear something that we can't.

Since we installed it, our electricity consumption has gone down. Last year we got by on just under 7,000kW hours, and no gas at all. 

The heat pump is rated for 0.7kW. On the face of it 0.7kW, to heat a house? Could it possibly be big enough? In our experience: yes. The floor area of our house is 98 square metres and the volume is 225 cubic metres. The old gas fire our heat pump replaced was rated at 11.7kW. So 0.7kW replaced 11.7kW (although it doesn't heat water). A huge gain in efficiency I think.

After the gas fire  was taken out, for a time we heated the house using a 2kW wall-mounted heater. During last  December and January we used an average of 27kWh per day. In the previous winter when we used the 2kW electric fire we used 34kWh per day. 

I would love to know what our heat pump's COP (coefficient of performance) is. I think it must be about 3 in reasonable conditions, falling perhaps to 1.5 to 2 in very cold conditions.

Our installation was probably very simple. The outside unit is just about 150mm above the decking and the indoor unit is the other side of the wall about 1,500mm vertically above it. So,  no long pipe runs, and no need for ladders or scaffolding. We got three quotes,  one for £8,000,  one for  £2,100, and the one we accepted for £1,200, plus the lower VAT rate for qualifying renewables inhalations of 5%

We had recently had some building work done and in the process had lost a lot of pipe work and radiators, so to go to hot water heating would have involved installing new pipe work and new radiators. We knew from past experience that the old gas fireplace in the living room could adequately heat the whole house without using the radiators, so I was fairly confident that a new source of hot air in the living room could do the job. And so it has proved. 

Limited outdoor space means that a ground source heat pump would not be practical for us, and since we wanted hot air anyway I couldn't see the sense of heating up water to pump through radiators to ultimately heat air.The indoor unit is a Mitsubishi MSZ GE25VA and an outdoor unit is a MUZ GE35VA. 

For us, it has been an great success.  Maybe we just got lucky. 

About the author: Keith and his wife Kathryn live in their Superhome energy saving house in Steeple Claydon, Meadoway.  See for details of free open day events in March and September.

If you have a question about anything in the above blog, please ask it in the comments section below.

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