This is a page I intend to use to develop ideas for moving our heating from gas onto renewables. So that’s why it might read like my notes!
Since buying a “fixer upper” of a house in 2011 we’ve been … well, fixing it up. We added an extension and replaced all the windows and doors. Insulation of the older part of the house is not yet finished. We’ve ended up with a pretty well insulated home, but one that still relies on gas central heating. In the UK over 80% of homes enjoy a mains natural gas connection so the default heating method is gas boilers connected to a wet central heating system and “radiators” that work as convector heaters. It also happens that the electricity network is largely powered by gas fired turbines. Much of this gas comes from LNG tankers from Qatar and other far-off places.
We also have 4kW of solar PV; no battery storage, but store spare solar as hot water in a 190 litre tank. This can store up to ~10kWh with our typical water use.
Now we’ve enjoyed electric cars for almost 5 years, the focus naturally comes to other carbon intensive activities; and home heating is a big one. How much energy do we use? How much heating power is needed on the coldest days? Can we use renewables to heat our home? What will it cost? What changes might be required?
Fortunately we have a smart meter for both electricity and gas, so now have an detailed picture of how much energy we need and what the use per day, and use per half hour period is.
We have used 52kWh of gas on the coldest day, and winter 20-21 was colder in the UK than average. In total we used 6000kWh over the year. How does this compare with other houses? By normalising by floor area we can compare houses of any size. In terms of energy use per square metre per year – 38kWh/m2/yr. Passivhaus in the UK averages 10.8 and the average UK house is 145. The average new-build is 50. (Mitchell and Natarajan, 2020) So we seem to be better than a new build and nearly four times Passivhaus standards.
Another way to characterise energy use is to measure the energy consumed per day at a range of outside temperatures, to enable correct sizing of a heating system. We can then take the coldest temperature that is likely, and get an accurate idea of the power that would be required. A smart meter provides the power consumption and a local weather station provides an accurate temperature.
In terms of cost, we are spending £180 per year, or on a monthly bill, £15 per month.
£15 a month isn’t high in terms of money. In terms of carbon however, we’re emitting 1.1 tonnes per year.
Alternative – heat pump – depends on carbon content of UK electricity and COP. In 2020 UK electricity was 180gCo2/kWh, exactly the same as gas. However a heat pump achieves a COP of 3 or more (and gas burning isn’t 100% efficient) so a move to HP could cut the carbon content to a third of its previous.
How much solar PV can we use? A lot of our solar PV is used to heat water resistively (with an electric “immersion heater”), on average 6kWh per day or 2190kWh per year. Using the heat pump this could be reduced by roughly half. However during the heating season, solar PV is at its lowest so a far smaller amount is available.
Nov-Feb generated 450kWh. Most of that was used to resistively heat water. A move to HP would multiply that by 3 and so represent about a quarter of the electricity required by the HP. Maybe we could use some resistive heating to boost the water tank temperature?
Overall 6000kWh heat needs (2000-450) kWh of imported electricity = 1550kWh and 279kg carbon. Cost ~£170, similar to today’s gas bills. So, help is needed to overcome the investment hurdle that heat pumps represent. The RHI scheme’s replacement is still not published, and is due to become available from April 2022.
Sadly the tax regime on home heating energy in the UK favours gas, fossil fuels. This is totally at odds with the targets set by this government and I hope will change soon.