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.

Government sees advantages - and the hydrogen lobby benefits.

Our government’s Heat and Buildings Strategy [2] is not definite about hybrids but sees advantages for having them in the mix of technologies for several reasons. Hybrids reduce overall peak electricity demand because in the coldest weather some of the heat demand will be met by gas. This sounds like a long-term requirement. Also, hybrids "provide an opportunity for consumers to become familiar with using a heat pump".

The hydrogen lobby would benefit hugely from hydrogen hybrids because they create demand for hydrogen. Widespread use could be used to justify converting the gas grid to hydrogen. The Heating and Hot water Industry Council says that the hybrid route reduces the upfront costs for consumers because there is no need for fabric upgrades or a hot water cylinder. However they also reduce emissions immediately and are a step on the way to, for example, an HP/hydrogen hybrid or a standalone HP [3]. If a hybrid is installed instead of improving the fabric it is hard to imagine those upgrades every happening - and the household will stick with hybrids forever.

A hybrid solution for the long term is not good for the climate or for our bills.

Why would you not want a hybrid heat pump? 

• The goal is zero carbon or 100% renewable heating but hybrids using natural gas are never going to achieve this.
• 100% renewable biogas is going to be in short supply and high demand for industry – hence probably expensive.
• 100% renewable hydrogen ditto.
• Rather than using natural gas or hydrogen in your boiler you would get more heat by using it to fuel a power station that supplies your heat pump [4].
• If most other people get a heat pump, demand for gas from the grid decreases drastically and that has consequences for cost and convenience.

As heat pumps become the norm, and fewer households use gas, the cost of maintaining the gas grid is spread over fewer users so more expensive. As of 2021, the network cost is about a quarter of the gas bill [5].

Perhaps the gas die-hards will end up using bottled gas which is not nearly as convenient as mains gas. 

We installed a hybrid solution due to initial doubt.

I mentioned above that I currently have a hybrid solution. This is intended to be temporary and we are already executing the next stage in our plan to get off gas completely. The main reason we went for the hybrid first was my doubting husband. Now that we have had a couple of satisfactory winters he has agreed to moving on. It will save us the cost of servicing the boiler each year – and eventually when we are completely off gas we will save the standing charge too, which is another £100/year.

BetaTalk recently featured Christoph Grossbaier of Econic talking about their hybrid solution. In followup communications with I was encouraged to learn that, at least in the Netherlands, many customers do move on from the hybrid solution to a pure HP system. This happens naturally as their boiler dies and they decide not to get a new one. 

Selecting the switchover temperature for gas/HP.

In our hybrid setup the HP provides all the space heating. The boiler is mainly there for backup (which has not been needed) and for hot water. Since it has been doing all the heating we know the HP is up to the job. However that is not how Econic normally does it.

Econic state that the HP typically provides space heat 90% of the time and the boiler does the rest, plus hot water. They have a switchover temperature of 3°C – above this the HP does the space heating. This  results in circa 59% reduction in emissions [6] – obviously this is weather dependent.

With a typical weather file for Cambridge, I checked these figures. The outside temperature was at or above 3°C for 91% of the time. This works out at 77% of the space heat demand – when it is colder heat demand is higher. Add a bit of gas for the hot water and the 59% reduction in emissions sounds plausible.

Setting a switchover temperature may sow doubt the HP is adequate. 

However with this configuration the HP does not get a chance to prove it is capable of doing the whole job. So how can this relieve doubt? 

Interviews on participants of the FREEDOM trial of hybrid heat pumps show that some came away more doubting of heat pumps than before. The hybrid control system was set up to minimise bills. They were told that the gas was used at low temperatures because the heat pump is then less efficient – and some took this to mean that the heat pump would not function adequately at the low temperatures, so the gas boiler was essential [7].

Optimum switchover temperature for reducing bills.

I did some rather simplistic analysis [8] and found that the optimum switchover temperature for saving money was very sensitive to the ratio of gas and electricity prices, and that this affected the overall emissions drastically too.

This analysis is only indicative as the assumptions are rather gross. However it demonstrates the sensitivity. I based prices on the October price cap which is (roughly) 27p/kWh for electricity and 7p/kWh for gas. I varied the gas price to see what would happen. 

Gas price (p/kWh)	Switch temperature (°C)	Bill savings (%)	Space heat provided by the HP (%)
5	11.5	0.3%	8%
6	9.0	1.6%	22%
7	6.5	4.1%	42%
8	4.0	8.2%	68%
9	1.5	13.6%	88%

With the same crude assumptions and the expected 7p/kWh for gas, the optimum switch temperature is 6.5°C, considerably higher than the 3°C recommended by Econic. The lower switch temperature gives less bill savings (only 1% instead of 4%) but considerably more work for the heat pump and hence higher carbon savings.

Conclusions.

Hybrid heat pumps are probably a good tactic to allow the doubtful public to change over to heat pumps in stages. However to make this work, you need to test the HP properly which means keeping the boiler as a backup, not as a cost minimiser.

You can also use the hybrid system as a bill minimiser and hedge against fluctuating gas/electricity prices: If electricity goes up relative to gas you use more gas and vice versa. However, the bills savings are often marginal and considering the cost of servicing the boiler, and having to pay the gas standing charge, they will be even more marginal. 

If bill minimising is the goal, the resulting GHG emissions are very sensitive to the relative price of electricity versus gas. This means that government policy on taxing energy bills becomes a direct and rapid acting driver for carbon emissions up or down.

If you intend to stay with a hybrid heat pump in the long term, you should consider that renewable gas prices are likely to be higher than now and also networks costs are likely to rise as more and more households go fully electric. You may end up using bottled gas!

In my view, including hybrids in the Clean Heat Market Mechanism would lead to suppliers pushing small heat pumps as part of an easy hybrid solution that actually achieves very little. Perhaps there will be some regulation to prevent this. We do not have the results of the consultation yet.

 

[1] Clean Heat Market Mechanism (www.gov.uk) March 2023

[2] Heat and Buildings Strategy (www.gov.uk) 2021

[3] Hybrid Heat Pumps: A Flexible Route to Decarbonise Heat (Heating and Hot Water Industry Council) 2021 or later

[4] Assume the boiler is 85% efficient, the power station 50% efficient and 10% losses in transmission. As long as the HP efficiency is at least 190% - which is certainly should be – then you will get more heat via the HP route than the direct boiler route. Hydrogen boilers are likely to be worse than 85% efficiency because they produce nothing but water vapour. You need to run the radiators cool to ensure all the water vapour condenses – the same as a natural gas boiler only more so.

[5] OFGEM all available charts (gas) (OFGEM) - and look for the 'Breakdown of a gas bill'

[6] The perfect partner for your new boiler (Econic)

[7] Consumers or users? The impact of user learning about smart hybrid heat pumps on policy trajectories for heat decarbonisation (Bryony Parrish, Sabine Hielscher, Timothy J. Foxon in Energy Policy) 2021

[8] For this analysis I took the hourly temperatures from the ‘typical’ Cambridge weather file. I used degree days with a reference temperature of 15.5°C to calculate heat demand. I assumed the efficiency of the heat pump was 40% of the Carnot efficiency, after applying weather compensation. I assumed the initial bill was 12,000 kWh of gas of which ¼ was for hot water and the boiler efficiency was 85%. The gas boiler provides all the hot water, as in the Econic normal configuration. Heating demand for the heat pump was asssumed to be constant, which is a considerable simplification – in fact most people reduce the thermostat overnight so that heat pump demand should be lower overnight and in the middle of the day but with a bit of a peak at the start of each heating period.