Thursday, February 10, 2011

Graph of Sunbox versus PV capture

9 Feb 2011: Blaise came round to collect more data from the datalogger, although we think it ran out of memory as November and December didn't seem to be there. Never mind.
   One thing that complicates the calculation of COP is that the underfloor heating circulating pump draws its power from the  GSHP, adding about 55 kwh to the monthly winter consumption figures. He tried to explain how to calculate the COP, but this is going to take a bit of work. Having datalogger sensors on the internal compressor helps us to be more accurate.

Blaise has found the recent set of graphs produced from my Peveril Metering spreadsheet very useful, especially the ones comparing Degree Days with Heat pump consumption, and the one showing the deep ground temperatures.

Here is another, comparing the capture of the Photovoltaic and of the Sunboxes. This will be updated monthly, and I hope that the SBs with their reflectors do better next summer. The mirrors were only installed in the autumn, and notice the brief rise in Oct'10. Thereafter, we had our cold cloudy Nov and Dec'10, so both curves went down. Due to the hill behind our house to the south, the PV capture is significantly reduced in the winter months. The Sunboxes are less affected by this, as they face south and are driven just as much by demand from the GSHP as they are by sunshine during those months.
The blue curve is compensated
by deducting DHW and floor pump

  Also, I have had to point out to everyone viewing the comparison of Degree Days and GSHP consumption that the GSHP figure includes hot water and floor pump, so here is another version of the graph with those overheads deducted.
  By removing those overheads, you see an even clearer distinction in the heating curves between Winter 09-10 and Winter 10-11.


  1. 55kWh for the UFH pump in one month? - is that correct? the pump doesn't run 24/7 does it? just as a rough guess - 60W running consumption for 14 hrs per day over 31 days = 26kWh

  2. Hi Chris, Its 125W as far as I know, so I assume for 14+ hours a day in the winter, which is 55 kWh per month in winter. I assumed that drops to less in November and March and less still in October and April when it is less than 14 hours. Only works when out door temp is below 15º.
    (Sometime I must get back on a very high wobbly stepladder just to confirm if it is 125W)

  3. The DHW is more than the floor pump, approx 65 kWh (apart from July when we are on holiday). According to the manufacturer we should be using 110 kWh/month for DHW, but we defy that by having showers instead of baths.

  4. yes but.... the ufh pump does not run continuously... the house does get up to temperature and as i remember you have no heating at all through the night?? perhaps its no where near 14hrs..
    DHW is more than the floor pump?? could you please clarify.. you have an internal circlation pump in the heat pump which either feeds the UFH (at which point you then have 2 circulation pumps in series) or it feeds the DHW tank on its own.. so am unsure what you mean by 65kWh for DHW.??

  5. The floor pump... there is a heating circuit pump inside the heat pump cabinet that is sized for circulating to radiators. But it cannot push around the hundreds of metres of small bore zig zag piping of an underfloor system, so the external pump is a necessary overhead if we have underfloor heating. It runs for as long as the external temperature is below 15.5º. This is settable by the user to any figure, for example, if going on holiday, one can reduce that temporarily to 8º or 10º.

  6. DHW during the summer months with no heating was about 66-70kWh/month, except for July when it was off altogether while we had 10 days holiday - that month was 45 kwh.
    Since then, I have fitted an extra mini-thermostat, to the cylinder wall, that stops the GSHP overheating the water, which it was doing far too often last year. So we expect the DHW fraction to be reduced next summer.


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