Performance of my heat pump

Last year we installed a heat pump at home and we now have almost an entire heating season of data for it. Here I present some results. Our overall mean COP (efficiency) is a little better than we were told to expect. During the heating season we have made a number of changes: increasing the night time setback temperature and reducing the configured flow temperature, with impacts on heat demand, electricity consumption or both. It is tempting to focus on efficiency alone but, being practical, it is energy use that we want to minimise. For example you can expect increasing setback temperature to increase efficiency but also heat demand - which wins? 

SMETER – measuring the energy performance of a house

I think we all realise that home EPCs are not always accurate in predicting energy bills for a house. This is a shame for a lot of reasons. It makes it hard for the government to form housing policy: how do we assess potential energy savings from energy efficiency measures on the housing stock, if we don’t know how bad it is already? It is a shame for people looking to rent or buy a home who want to know what they are letting themselves in for. So why don’t we just measure the energy efficiency instead? Would that it were that simple. The government is well aware of the problem and has provided financial support for some potential solutions making use of smart meter data. The project was called SMETER (Smart Meter Enabled Thermal Efficiency Ratings). The results were promising but not brilliant. Here I explain why it is so hard, and some of the solution strategies.

Whether you are modelling or measuring, the core of the problem is the balance of heating coming in and heat going out. This chart captures the balance from a model of a typical semi detached house with filled cavity walls, double glazed windows and 100mm loft insulation, in the East of England. The model is similar to that used for calculating EPCs. The chart shows month by month heat losses (blue and purple, downwards) through conduction through walls etc and air leakage, balanced by heat gains including solar gains through windows, use of electrical appliances, heat losses from the hot water system (mainly from hot water pipes distributing heat around the house) and of course the heating. 

Estimating carbon savings for switching to EV or heat pump

Suppose you want to invest some cash in reducing your personal carbon emissions - can you save more carbon by replacing your car with an EV or by swapping your boiler with a heat pump (and how much would this cost?). Unfortunately, the answer depends on lots of variables such as how far you drive and how much is your heating bill, do you have solar panels – if so how much power are you currently exporting? So I have prepared a tool to help you estimate your savings and in this post I use it to illustrate some examples. You probably know that energy costs for an EV are lower than for a diesel or petrol car. You may be surprised that the heat pump can reduce your energy bills too, using prices from April after the cap changes.

Click here for the EV tool and here for the heat pump one. These are only estimates! If you find them useful let me know.

Energy price increases - and the impacts for heat pumps.

Most reporting on the changes to energy price caps that come into force from April are about dual fuel bills and just report the total bill, not separating gas and electricity. However, if you are considering switching to a heat pump you are also interested in the ratio between gas and electricity costs - how much will it cost you to make that switch? Currently, heat pumps typically cost a little more to run than a gas boiler, but from April the gap narrows - plausibly to nothing in many cases.

The data behind this blog comes from OFGEM.

Firstly, the typical bill - there are slightly different costs for payment methods and I have assumed a standard credit payment. I am also using Eastern prices. There are regional differences but these too are small. 

Price cap for gas and electricity, whole bill £/year. This is based on 12000 kWh/year for gas and 3100 kWh/year for electricity.

Will heating your house constantly use more energy?

We are advised when we get a heat pump to change the heating schedule to be constant, or nearly so. This is because heat pumps are efficient when supplying gentle heat but not good at heating a home from cold quickly. This is completely the opposite of what we have learned about keeping our bills low when using a gas boiler. So how much are we currently saving, and how is this different with a heat pump? I have investigated this with a model of a semi detached house (using similar models to my work for BEIS [1]). In the boiler case, savings from intermittent heating are substantial - up to 21%. In the heat pump case, the difference is much less - at most 4%. 

Scaling up heat pump installation – counting the benefits

It is generally recognised that reducing carbon emissions is going to mean lots of heat pumps (or other electric heating systems) installed in homes to replace gas and oil boilers. However, heat pumps are (currently) more expensive to run and to install and not many households have been persuaded so far. The government is running a public consultation now on a market mechanism to increase the install rate [1]. By their own assessment this policy has a net social cost of £0.6 billion over 4 years. So how can this be sensible? Or is there a fundamental problem with the cost benefit analysis? - I think there is.

This chart illustrates the balance in costs and benefits – it is quite finely balanced.

Data from the consultation 'A market based mechanism for low carbon heat' cost benefit analysis [4]

Are buses doomed?

Bus use has been in decline since well before Covid - in fact since deregulation in 1986 and more recently - since about 2012 - even in London. Cycling on the other hand is on the increase. Many schemes have tried to persuade people to switch from cars to buses with limited success. More home working is likely to increase commuting distances making buses less practical. More and more people are discovering the joy (and health benefits) of electric bikes. Purchase price is a barrier for some but you can now buy an E-bike on a subscription basis for less than the price of a bus season ticket. Is there still a role for buses? - probably, but less for commuting and not the same sort of buses. (This is an opinion - tell me if you agree.)

The main driver for shifting is to save time. Low cost has a lesser role.

Environmental awareness has an impact but the main driver of mode choice is to save time - even in the context of free bus fares. This diagram illustrates the mode shifts observed in the various studies described below assuming very low cost or free bus travel. Cheap fares do increase use of buses, but a lot of this is due new travel demand or shifting from walking. On-demand buses are preferred because they are quicker to use than normal buses and more accessible to people with mobility needs. On-demand buses can also be quicker than walking or cycling in some circumstances. E-scooters and E-cycle are often quicker than the bus and sometimes quicker than a car.