A whole street of heat pumps - noise levels

Heat pumps make noises, of course they do. The question is, will their noise be a nuisance if every house on the street has one? The short answer is - not usually. Here I explain why we are not disturbed by ours, and why a whole street of heat pumps does not sound much louder than a couple. In urban areas, other noises are much more noticeable. In rural areas, you might be disturbed by yours but not so much your neighbour's.

Shifting policy costs on energy bills - revisited

As I said in a previous blog post, the current price ratio between electricity and gas means that switching to heat pumps often means bills increase - a major deterrent to many households considering taking on low carbon heating. Part of the problem is because of the way 'policy costs' are loaded mainly onto electricity bills instead of gas so when you switch to a heat pump you end up paying far more policy costs. However, if we were to change this so that policy costs were evenly distributed between gas and electricity, then heat pumps become cheaper. I have updated my previous calculation with a slightly revised method and based on the current price cap as shown in the following chart.

Chart shows the annual bill split into components: policy costs and other for each fuel. Current uses components as computed by OGGEM, 50% split means the variable policy costs are split between gas and electricity. HP costs are with the gas boiler replaced with a heat pump. Assumptions are explained below. Click on this for a larger image

Should we shift energy taxes to make heat pumps cheaper to run?

Tariffs vary but averaged across the country, under the current OFGEM price caps, electricity costs four times gas per kWh. This means that by my estimate [1] unless you go off gas completely (hence avoiding the fixed daily charge) you need a heat pump efficiency (SCOP) of 360% to get similar costs. This is not unknown but considerably better than average. If you do go off gas completely, you need 320% which is still better than average. If you only got 300% you would be paying 6% more with the heat pump than gas. This is discouraging for people wanting to switch to low carbon heating.

However, a significant part of the electricity bill is due to environmental and social policies or 'taxes'. If these were removed or shifted, the ratio of electricity to gas price would be smaller, making heat pumps relatively cheaper. Hitherto, policy has been to keep gas cheap because so many of us rely on it for heating which is essential for health. However, doing so penalises households that make the transition to low carbon heating. What are these taxes and how much difference would this make? Here is a graph showing policy costs on gas and electricity as of September. There are more policy costs on the electricity bill than the gas bill and the home with a heat pump uses more electricity, so pays even more policy costs.

 

Policy costs for a typical bill of 12000 kWh gas, 3100 kWh electricity or, with a heat pump, 6650 kWh annually. The policy costs are from OFGEM [3] and the heat pump kWh used is from [1] Costs are taken from the period July/Sep 2023, the latest available. These costs vary little by region. The acronyms are explained below. 

Are hybrid heat pumps a good thing?

A hybrid heat pump is normally a combination of a gas boiler and an air source heat pump (HP). The simplest arrangement runs either one or the other at any one time. For example, you can use the boiler (possibly your existing boiler) for space heating when it is cheaper to run, thus reducing your bills. This typically means using the boiler when the outside temperature is low. You might also use the boiler to provide hot water, if you currently have a combi and do not want to find the space for a hot water cylinder, or if you do have a cylinder but it is not suitable for the heat pump. The hybrid approach avoids some of the disruption and expense of switching to a standalone heat pump.

Government subsidy for hybrids – off and on.

Hybrids were supported under the RHI subsidy but only with meters to see how much you use the HP side. They are not supported by the Boiler Upgrade Scheme which gives grants for heat pumps now. The consultation for the Clean Heat Market mechanism proposed including them [1]. This mechanism aims to increase the rate of heat pump installs by requiring heating system suppliers to install an increasing proportion of heat pumps rather than boilers (a bit like the old Renewable Energy Obligation scheme). It will be interesting to see what different stakeholders think about allowing hybrids to count – the responses are not yet published.

A hybrid HP could be a temporary or a long term solution.

You might opt for a hybrid heat pump as a temporary solution:

• To see if it the HP is adequate, because you have doubts.
• Because your house has high heat loss at the moment and you intend to do some more work on it, so you will get a heat pump sized for when you are finished and use the boiler as a backup in the meantime. 

You might opt for a hybrid as a long term solution:

• To minimise your energy bill – from hour to hour you can use whichever is cheaper (see below for more analysis)
• Or to minimise your carbon emissions, using predicted emissions intensity instead of price
• Because your house has high heat loss and you do not believe it is possible to make it ‘heat pump ready’. You can have a hybrid solution with no radiator upgrades and it will reduce your gas demand but not completely.

Why my heat pump performed worse this winter

Over the first winter my heat pump (space heating only) gave me an average COP (efficiency) of 3.1 which was higher than I had been told to expect. I was moderately pleased. This winter it was only 2.9 which was a great disappointment.

The difference is due to the weather. The mean COP for one year is not necessarily a guide to the next. The mean temperature is part of it but colder days require proportionately more heat, which biases the average COP. So I have done my best to work out what the average COP should be over time. I was surprised at the year to year variation.

Air source heat pumps heat the outside air even when heating your home

Standing in front of a heat pump when it is working hard you will feel an arctic blast of cold air. So you would think that if every house has one the outside air will get colder. After all, we know that cooling buildings with air conditioning (A/C) adds to the urban heat island effect. Since cooling our homes makes the outside air warm, you would think that heating our homes would make the outside air colder. In fact, using an air source heat pump to heat our home heats the outside air as well as our home. Read on to find out what happens to the air outside when we use air source or ground source heat pumps in heating or cooling mode, compared with a gas boiler for heating.

1 Simplified heating scenario.

Why do heat pumps get less efficient in warm weather?

This post has been modified 5/April/23 after discussion on twitter with @AR_Clark convinced me that the reason for the continuing decline in efficiency in warm weather is due to increasing use of the immersion heater for hot water heating. This is hopefully less common now, as heat pumps can provide all the hot water necessary except for sterilisation cycles, and sometimes even then.

We know that air source heat pumps (ASHP) are less efficient in cold weather. However this chart shows decreasing efficiency in warm weather too. The chart comes from an article recently published in Energy and Buildings [1]. It was generated using field data from UK heat pumps installed in 2012. These are quite old installations and performance has improved quite a bit since then. However, this data is still being used to predict national electricity demand in the future. In the chart each dot represents the average COP (efficiency) of all the ASHPs included in the analysis over one day. Efficiency is low in cold weather, and increases rapidly but only up to about 8°C. After that things go downhill rapidly. This is less critical than it looks as the heat demand also decreases rapidly - performance in cold weather is much more important - but it is still surprising and not a good sign. What is going on?

 

Figure from [1]

The article has an equivalent chart for ground source heating pumps (GSHP). At low temperatures, the GSHP efficiency remains high because the ground is still warm even though the air is cool. However in warmer weather the efficiency declines, much the same as with the ASHPs.

How big should your heat pump be?

Most people have combi-boilers and the size of the boiler required is often dictated by the need to heat hot water for baths and showers quickly rather than space heating. With heat pumps, you normally have a cylinder for hot water and it is the space heating requirement that dictates the size you need. So how big should it be? Hearsay tells me that even with boilers, sales-people love to sell you systems that are bigger than you need. My heat pump is oversized by at least 50%.

My heat pump is much bigger than it needs to be. I have tried to make it less intrusive with some stick-on leaf patterns. Still, at least I have renewable heating :-)

Oversizing does matter, both for boilers and heat pumps.

• Large systems cost more - not as much as you might expect, but still more.
• Running at small fractions of full capacity reduces efficiency (usually).
• Large systems are physically large and, in the case of heat pumps, have a greater impact on the landscape (see picture above).

In this post I discuss a very simple rule of thumb for heat pump size - thanks to Michael de Podesta - and some reasons it gives an underestimate in my case. Also I show you how to cross check your installer's estimate using the EPC certificate for your home. I wish I had done this!

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]

Review of the Heat and Buildings Strategy

Last month the government announced the long awaited Heat and Buildings Strategy [1] which sets out plans to convert this sector to net zero GHG emissions. This came out almost at the same time as the wider Net Zero strategy [2]. One aspect I was particularly interested to see was the plan for hydrogen in heating – there is still indecision in this area we will have to wait until 2026 for more certainty. Another aspect is the balance between electricity and gas energy costs, also support for retrofitting heat pumps into homes and building up the supply chain for retrofit. There are definitely some good things in this policy but some serious gaps.

Scenarios: high hydrogen, high electric or in-between? 

The future role of hydrogen is still uncertain and the Net Zero Strategy refers to three scenarios for heating in the future. In all cases, heat pumps have a very large role. As well as the net zero in 2050 target there is an interim target to reduce GHG emissions from this sector by about two thirds by 2035.

By 2035, the target is for 13 million homes to be on low carbon heating of which two million will be on heat networks. For the rest there are different scenarios. If hydrogen does not work out, almost all the rest will be on heat pumps. If hydrogen does work out, then we can expect up to 4 million on hydrogen gas by 2035 and 7 million on heat pumps. Or, it could be something in-between.

Whatever happens, at least a third of homes in the UK need to be heated by heat pumps by 2035 – a lot more than on hydrogen.

How much does it cost to install a heat pump?

The recently announced Net Zero Strategy includes a boiler upgrade scheme with a £5000 grant towards installing a heat pump. How does this compare with actual heat pump costs? To be fair, you should bear in mind that when you upgrade your boiler you only need a new boiler slotted into the same space as the old, whereas when you convert to a heat pump there are additional one-off costs for plumbing and other work. So subsequent heat pump upgrades will cost less. Still, there is no doubt that heat pumps cost more than just replacing a boiler, which is usually £2,000 to £3,000. Here are some top level estimates for a heat pump installation – as you can see they vary greatly We will break this down in a minute.

Energy Savings Trust (2021)	£7,000 - £13,000
Renewable Energy Hub (2019)	£5,000 - £8,000
EDF Energy (allows 20% for installation costs)	£5,000 to £10,000
Heat Pump Retrofit in London (Carbon Trust, 2020)	£7,000 (3.5 kW) - £11,000 (11 kW), mean £8,800
Cost of domestic heating measures (Delta EE, 2018)	£9,000 (8 kW) - £15,000 (16 kW)
Development of trajectories for residential heat decarbonisation to inform the Sixth Carbon Budget (Element Energy , 2020) – excluding the fabric upgrades.	£10,000 (mean).

Fabric first is not the cheapest path – is it the best?

Greater Manchester has declared a climate emergency and set itself a target of delivering net zero housing stock by 2038. Now they have published a report detailing what is needed to get there. This shows that the fabric first approach is not necessarily the cheapest or even the fastest strategy to decarbonise houses in Manchester. However the authors still recommend it for a variety of reasons. They make some very good points, although there are going to have to be some trade-offs made in practice.

The report has been prepared by a consortium of very respectable consultants: Parity Projects, Energy Systems Catapult, ADE research and Bays Consulting. Their findings are in line with the results of work I have been involved in for BEIS (Cost Optimal Domestic Electrification - CODE) but frustratingly this is still not published - and nor is the government’s Heat and Buildings Strategy, probably delayed due to concerns over cost [2]. In any case, the Manchester report covers policy as well as costs.

The nub of the problem is illustrated by this one chart, representing an ‘average’ house. The height of the coloured bars shows carbon emissions, the yellow coins at the top show the capital costs and the black diamonds show annual energy bills.

Chart from the Manchester report [1]

Decarbonising your home or car - which saves more?

The global carbon budget is extremely tight and we all need to do what we can, as soon as we can, to reduce the carbon emissions from our home and other life choices. I was recently asked – will I save more by replacing my gas boiler with a heat pump or replacing my car with an EV? I was surprised to find the results were fairly well balanced. Which saves most depends quite a lot on how much gas you use and how much fuel you typically use in your car. Here are some typical figures and example calculations. 

Also there is at least one other factor to consider: lock-in. If you buy a new gas boiler today you probably will not want to replace it for 15 years. On the other hand you might buy a second hand car and expect to replace it in 5 years. So the decision on a new gas boiler might have more impact over the lifetime of the boiler even if the annual carbon savings are higher for the car.

Energy Ratings for heat pumps - what do they mean?

Example energy label from [1]

Do you check the energy labels when you buy appliances such as washing machines or refrigerators? I hope you do. To make comparisons easy each product has  a rating, which is generally between A and G except for some appliances it goes up to A+++. This is helpful only if you know what the best or typical rating is. In any case, the rating does not actually tell you what the efficiency is or typical energy use would be - hence my advice has always been to look for these on the energy label. I would love to be able to say the same for heat pumps but there is nothing like that on the energy label for heating appliances. The rating is the best we have to go on. Here is a picture of the design of the energy label for a heat pump with integrated hot water cylinder, so it supplies hot water for the taps as well as for the central heating system. To understand what this means I have been reading the legislation behind it [2].

Upgrading our electricity distribution networks.

Electricity North West plans to reduce the voltage of their customers' electricity supply which will reduce bills by up to £60/year - and be hardly noticeable except it will take a little longer to boil the kettle [1]. In this post I take a look at the Smart Street trial they have run to test out their concept [2]. As well as energy savings a key purpose of the trial was to evaluate new voltage regulation equipment that could delay the need to upgrade the distribution network. Reducing the voltage reduces the delivered energy and peak load, so the existing infrastructure can cope for longer.

Savings on customers bills are welcome but even more importantly (for climate change) we need to be able to install more distributed generation and run bigger loads due to heat pumps and electric vehicles. How much will that cost? A case study on low carbon heating in a town in Scotland has some answers to that [3]. The cost of the network was much less than I expected - much less than the heat pumps anyway.

Are hybrid heat pumps the solution for low carbon heating?

The Committee on Climate Change has just published their view on the role of hydrogen in a low carbon economy [1]. Their recommendation for people currently on gas is to insulate your home as much as possible and then install a hybrid heat pump/boiler system. You can get hybrid systems now, only burning methane gas rather than hydrogen. The idea with a hybrid is that the heat pump supplies all the heat most of the time, but with a boiler for backup when demand is very high or electricity supply is expensive. In theory, a hybrid system should supply around 90% of the heat from the heat pump - however, this is only the case when the system is configured correctly. In field trials the proportion of heat from the heat pump has ranged from 96% down to 30% [2]. No wonder the RHI subsidy requires that hybrid heat pumps have metering to actually measure how much renewable heat is supplied through the heat pump [3]. If hybrid heating systems are to play their part in a sustainable future we have to learn how to manage them properly.

Why I haven't installed a heat pump (yet)

If you read this blog you will know I am keen to promote heat pumps as a low carbon heating technology, so I feel a bit of a hypocrite not having one at home. Since we have insulated our house as much as we can it is a logical next step to a low carbon house. So I made enquiries with two heat pump installers - but no joy. This blog post is a personal story. Every house is different and doubtless you won't meet exactly the same issues, perhaps even none of them.

Update: In 2021 I did get a heat pump installed and I am very happy with it. For more details see 'Performance of my heat pump'.