2012 Heat Pump Cost Summary

Published January 26, 2013 by Sean

My Hitachi Yutaki heat pump has been running for a year now. Here is a summary of the running cost and comparison with the previous couple of years when all heating was electric storage heaters/fan heaters. First the basic data, costs are approximate, based on last years figures as entered in the imeasure website which I use to track this.

Total kWh Cost Per Month deg days
2010 7861 £1,089.00 £90.75 2092
2011 9389 £1,300.00 £108.33 1661
2012 6183 £856.00 £71.33 1970

Degree Days is a measure of the total time during the year when I might need heating (adding the time when the outdoor temperature is below 16°). Today is likely to come out at around 15, last week the total was 80 (232 kWh), and even in the summer there will be enough hours below 16° to clock up single digit degree-days for each week (but no need for heating) It doesn’t make for a perfect reference, but does give some measure of how cold each year is. The heat pump was installed at the start of December 2011 and suffered a few teething problems up to the start of Feb’12, in terms of the settings. During 2012, I believe I have improved the control of water heating and crucially mostly avoided the scenario where the heat pump is trying to raise the entire space-heating circuit to 60°C for up to 20 minutes at a time.

Here is a breakdown of 2012’s electricity use by 4 week intervals (ending on Mondays). Heating load is only for the external heat pump unit, not the control circuit.

2012_in_13_totals
Base load is reasonably constant (allowing for freezer, cooking, lighting, computing and pond pump) accepting that there will be variations due to holidays. There were roughly 4.5 months in 2012 where the heat pump was only supplying hot water, at a cost of very roughly £40 per year, making this a poor candidate for too much investment as a source for future savings. One big reason for improved efficiency is that the heating responds much more closely to the actual temperature compared with storage heaters which are pretty much set and ignore, regardless of the day to day temperature.

Future savings are more likely to come from the ability to better adjust the heating by room zoning (although the dependence of efficiency on flow temperature, and radiator area play a part here), and avoiding over-warming the house when I’m more than 30 minutes from the door. Current setpoints are 16° for non-heated times, and 21° when I’m in. The control is single zone, with weather compensation plus room compensation. Room compensation is a trade off for efficient ramp up (low), or high response to manual adjustments. Response times is something I hope to come back to in a later post if I can find some suitable daily profiles where I’ve not been in the house and complicating things.

Comparing 2012 calendar year against earlier data (as financial and calendar years) shows a saving of roughly 25% total, or a halving in heating cost compared with storage heater, stored DWH and electric shower. That’s maybe £300 per year, for a £8000 investment – but the shower is improved by an order of magnitude, and the old system was starting to fail. In general, the house is more comfortable too.

effective_2012

The plot above assumes a constant non-heating baseload of 3500 kWh per year, percentage vs 2012 labelled on the right.

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Comments (3)

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  1. Chris says:

    I’m very impressed with your “Annual Energy Load” graph on your web page “2012-heat-pump-cost-summary”.

    In respect of my 74m² apartment, I have historical energy consumption data, month-by-month, going back 13 years. What I don’t have is a similar archive of “Degree Days”.

    My local weather station is RAF Northolt. Given the recent “improvements” to the Met Office website, which seem to have seriously curtailed access to old data, can you suggest a current source of this information?

    The existing gas fired central heating and hot water system here is based on a 1994-vintage non-condensing boiler which is coming to the end of its useful life if only because of the matter of spares availability. At best, present whole-system CoP is 0.7. Hence my interest in your installation; I’ve already got the radiators and unvented hot water cylinder so I reckon that in terms of capital cost I’m already half way there.

    The gas boiler is the only gas appliance in the property. Looking at the wider picture, a switch to all-electric at the point in time when the gas boiler goes belly up would save the “pseudo standing charge” associated with the first so many cubic metres of gas being billed at a higher rate and it might also be cheaper in terms of annual service costs, currently £275 per year. Non-heating electricity use already takes up the higher rate units of electricity, so the heating charge would kick in at 13p/kWh. It might even pay me to scrap the gas boiler early! Determining whether that is in fact so is what underlies all this.

    CE

    Posted November 21, 2013 @ 6:07 pm (UK)
  2. Sean says:

    Either enter all your data on the imeasure website, and use their degree-day calculatons, or see if they link to the source in their help pages. I am fairly sure I found tables of monthly data (possibly from oxford uni). I never saw it on the met-office site…
    Ill update my annual data after the end of the month if I remember.

    Posted November 22, 2013 @ 12:13 am (UK)
  3. Chris says:

    It looks like the website http://www.degreedays.net/ has loads of downloadable data.

    Apparently there are statistical pitfalls, so best consult the associated tutorial at
    http://www.energylens.com/articles/degree-days

    Maybe two of these pitfalls merit a mention here?

    Firstly, the matter of BASELOAD. That term has a special meaning in degree-days calculations. It ISN’T just the energy used by equipment that’s on all the time. It IS the total energy used by all devices that are not switched on/off on a weather-related basis. The tutorial compares different ways of determining the Baseload, one of which is dependent on choice of appropriate Base Temperature but another that’s not. (Do both and I reckon you have a check-back on your choice of appropriate Base Temperature! Does anyone agree?)

    Secondly, there’s the matter of BASE TEMPERATURE itself, the trigger point for clocking-up a degree-day. Typical UK value is taken as 15.5°C. But I know that my central heating never needs to be switched on if the outside temperature is above 10°C, I think because the natural heat gain of the building is above average. Fortunately, Base Temperature can be adjusted within the degreedays.net online calculator.

    CE

    Posted November 22, 2013 @ 5:12 pm (UK)

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