Let's look at the house and heatpump more closely.
Last night, I came up with a figure of approx 14,500 kWh for the year for our house, based on some very rule of thumb calcs for UValues and Airchanges, floor space, volume and average fenestration.
He ran the details of our house through the estimating web page provided by IVT (VPW2100) and I hope in future that I can have an access ID to use the same. I have found similar sites with 'VPW2100', eg the Bosch site, but it is entirely in Danish and much of it in Flash, so google can only translate the first page.
For our house size and occupancy and location, David's software came up with an annual heating demand of 14,590 kWh. I am quite pleased at how close that is to my first estimate.
14,590 kWh can be approximately divided into 11,600 kWh for space heating and 2990 kWh for domestic hot water (based on a predicted percentage split). That is still a long way lower than the average British gas heated medium size house of about 25,000 kWh.
Of this 14,590 kWh, the predicted heatpump overhead is likely to be 4,800 kWh for pumping, compressor etc, and the expended heat goes into the house. Therefore, the amount extracted from the ground is expected to be 9790 kWh. The VPW page prints out a Histogram with monthly estimates. The table below starts in September 2009 when I put a meter onto the GSHP, and compares the predicted workload and performance for the GSHP compared with the actual. If these figures are to be believed, it seems that a very pessimistic expectation is made for the COP. We also have an extra overhead that during the winter, the underfloor heating system pump burns 125W all the time, also working through the GSHP's meter. This pump adds 60 kWh per month to the monthly figures, which makes our March and April figures look even better, as the theoretical figures predict the GSHP performance without the auxiliary pump.
We shall see after a year how all this pans out. Here's the table.
|Monthly 'Expected' GSHP and PV Roof performance, against 'Actual', |
balanced by contribution from Sunboxes. PV harvest indicates 'sunniness'.
|Month||GSHP expect |
|kWh expect |
|PV Roof||PV |
|Feb 10||742||1558||750||690||98.6 ||122||Cold|
|Mar 10||652||1423||513||453||227||281.3||217||SB start 7 Mar|
|Jun 10||110||139||71||71||375||513.0||407||HW only|
|Jul 10||110||139||45||45||315||432.6||427||HW only|
|Aug 10||110||139||66||66||302||382.8||362||HW only|
|Sept 10||110||139||100||90||280||278||263||HW + Heat|
It is too early to be sure until more months elapse, but at first glance, the reduction in workload for the GSHP in April is quite stunning, it is less than half the expected amount. April was a sunny month. Then the first 2 weeks of May were surprisingly wintry, and the May GSHP consumption finished up as 155 kWh, about the same as the VPW prediction, because of the very cold start (and end) to the month.
I shall return to this article later when the monthly figures roll in.