It’s become clear from monitoring electricity use over the 3 months that the heat pump has been running that it is using far too much current. Today Dave Greenwood, who first introduced me to the idea of heat pumps, came out to find out what was wrong with the set-up.
The main issues turned out to be that there was an air lock in the heating system and water circulating was very low. Pressure should be around 1.5 bar. Secondly, he slowed the heat pump to increase the temperature difference in the heat exchanger. This should have the effect of stopping the heat pump running so much of the time. Over the past three months is has been running almost 80% of the time. These adjustments should mean that it cruises at a winter average of 12 hours a day.
In terms of cost this means it should be consuming around 9,000 kWh a year at a cost of around £720. This is much more in line with expectations and other case studies.
The calculation goes like this. The pump uses 2.8 kW, thus using 67 units a day or 2,016 a month if running continuously. The extra heating element uses 3.1 kW or 74 units a day or 2,232 a month. In the winter third of the year the heat pump should be on around 50% of the time and the extra heating element for 15%. This gives a winter total of 5,000 units. At an average unit cost of 8p that will come to £400. In the summer, with some heat to the pool, the heat hump should be on for around 25% of the time with no extra heating element required. So 8 months at 500 units a month should give us 4,000 units at a cost of around £320.
So we should be running at £720 a year for hot water, winter house heating and summer pool heating. If we can achieve that it will be well below even the old cost of oil, let alone what future oil costs could be.
To ensure the we are on target I can now monitor for the rest of the winter: heat pump 12 hours a day with extra heating element on 4 hours a day. And in summer just the heat pump on for 6 hours a day.
Other important lessons from what Dave explained:
- room temperature is controlled not by the room sensor alone but in combination with the outside temperature sensor and the slope and fine tuning settings which determine how the system reacts and when it switches the pump on.
- the bottom of the radiators are cool, which suggests that the flow around the circuit is either too slow or they are not balanced. This may need a more powerful circulating pump.
- there is more insulation to do on the outside doors and the bathroom skylight.
So let’s hope this gets us back on track and gives us the savings we were expecting. Dave has promised to come back next month to see how we are doing.