How to turn a lovley old house with solid walls into a warm efficient home



air pump performance

At the time of writing I have used the pump through two winters. The first was a particularly mild winter (2013/14) but the second was fairly typical (2014/15). Using the pump couldn't be easier. Once set, it comes on and off when needed without any input from me. It supplies water for my heating system, which it supplies to the buffer tank and it also supplies hot water to my DHW tank for showers, baths etc. I have been experimenting with temperatures and keeping logs to try to asses the most efficient way to use the system. The two golden rules of heat pumps are that the lower you can keep the water temperature output, the better, and you do not want to set the pump so that it short cycles - ie it switches on and off for very short periods. I have looked at several sites for advice and it is generally considered that if a pump needs to switch on and off 6 times in any one hour period then it is short cycling.

The control panel is very straight forward. The only basic user settings are to set your desired target temperature for each tank. Though a traditional boiler might raise the temperature of your DHW tank to scalding temperatures - I now my old Rayburn did! It is in your interest to never take water temperatures higher than you need with a heat pump. That said, obvioulsy the system has to produce what is required. I set my DHW tank to a target of 48 degrees. This may not sound high, but 48 is too hot for a shower. Because the DHW tank is heated via an indirect coil, obvisouly the heat pump will produce water hotter than 48 in order to heat up the tank. I have watched mine and it often reaches 60 as it raised the tank temp. You also set the low temperature for the tank - ie. at what temperature do you want the heat pump to kick in and heat the tank up again. This is most likely set by your installer, but you have the freedom to fiddle if you prefer your own setting. The default for mine was 5 degrees below target, so in my case that would be 43. However, I found that from initial start up the pump takes time to get the water hot enough to begin transgering heat to the tank, and once it has got the water up to temp it heats the tank very quickly. I figured, therefore that I might as well make the most of the time whilst it was hot so I have set mine to a 7 degree drop. I did try to monitor if this was more efficient but to be honest, if it is then it is small beer.

Air source heat pump controller, eco, green deal, renovationIt is worth noting that when running DHW at only 48 degrees it is important to raise this temperature to 66 degrees periodically to prevent the risk, though small, of legionaires contaminating the water. It is most efficient to do this with intermittent use of the immersion heater as raising you DHW to such a level with the heat pump would be inefficient even if it could do it. With the use of a 7 day immersion heater timer this is easily automated.

You have the same settings for the buffer tank. Target temperature and how many degrees below target before the pump kicks in and reheats the buffer.

air source heat pump, rhi tariff, eco, homeNaturally, I have been trying to monnitor electricty usage. I have a designated electricity meter that measures the heat pump usage only, so I can be confident that my usgae figures are accurate. I also have a dedicated meter monitoring the use of the immersion heater that is required periodically to boost the tank temperature.

I used the pump to supply both DHW and CH through the winter and to supply DHW only in the summer. I took electricity meter readings on the first of every month, and confirmed these with my electricy bill. I have not published all the spreadsheet data here as usage will be very specific to any particular home, so what I use is of little specific consequence.

99% of my 'fuel' is now electricity (I do have a wood burning stove I can use when I wish) I compared my fuel usage to my earlier usage when using oil and as you would expect my total fuel bill has been slashed. However, I have done considerable work to the house so this is not all down to the heat pump. Indeed, unless I actually ran a modern oil boiler in my refurbished property it will remain difficult to say categorically whether the pump is cheaper to run than a boiler.

My home is a large 4 bedroom detached house built in 1804 with solid 9" brick walls and was previously a sieve into which I poured heat that quickly disappeared and left you feeling cold because of draughts. In the winter of 2014/15 which I consider fairly typical, my entire electricty bill - heat pump and all other electic in the house was around £800. I think this is pretty good, and is covered by my RHI payments, so effectively, for the next 7 years I expect to have my bills paid by the RHI scheme.

I write in more detail about my CH system design in another article but for interest, last year I wanted to see how low I could run my CH water whilst remaining warm. In the end I had it set to 28 degrees. This kept a good base temperature to the home. For an evening in the lounge I often threw a few logs on the woodburner for a top up. I found this combination worked well because UFH in a large concrete slab will not respond quickly.

© Christopher Thompson