Low carbon heating in real houses

The quick way to get to low carbon heating is to replace our gas boilers with heat pumps. We don’t have to wait for low carbon gas or district heating infrastructure to be installed - we can do it tomorrow. However, it isn’t always that easy. The Energy Systems Catapult (ESC) have done a fantastic job of modelling how this could work in five real homes [1]. They envisage a staged conversion in each case, usually starting with fabric upgrades and moving on to a heat pump (or district heating) later on, over 10-15 years. Each house (two semi detached, two terraced and one detached) has its own story, but I am not going to worry about the paths, just about the end points. Interestingly, in two cases they envisage keeping a boiler in place as well as the heat pump, running the heating in a hybrid scenario.



All the homes got multi-zone heating controls.
All the homes got multi-zone heating controls. This gives you control of temperature setting and timing on every radiator individually so each room is its own zone. This usually brings savings because you can choose when to heat different rooms. It also brings much better control in each room. The cost depends on how many radiators you have – they estimated £600 with seven radiators.

Fabric only measures reduced gas use by 8-42%.
The potential savings from fabric measures were fairly modest. The ESC only considered conventional techniques: standard insulation, new double glazed windows and doors – there were no T-cosy style retrofits with very high air tightness and mechanical ventilation with heat recovery, not even underfloor heating. The biggest savings were from the homes that were worse to start with; the best savings were 42% for the terraced house with solid walls. This got external insulation at the back and internal insulation at the front which is quite typical. The 8% savings case was for a detached house with cavity walls that were already filled so there was not that much more to do.

All the homes got an ASHP – finding a place for this can be awkward.
All homes were modelled with an air source heat pump option (ASHP). For the detached house a ground source heat pump was also possible. ASHPs have an external unit that is a bit like an air conditioning system and finding a place for this can be difficult. Ideally it should be close to the house, with short pipe runs to the internal unit. For one of the semis they put it on top of a single storey extension at the side. For the other they put it at the front, which requires planning consent (see permitted development rules here).

Hot water cylinders are needed...
The homes started out with combi boilers but heat pumps are not sufficiently powerful to provide running hot water on demand so they also need a hot water tank. ESC modelled a 200 litre or 250 litre tank depending on the family size. This is a little larger than normal. I suspect this is to allow for running it at lower temperature to ease the strain on the heat pump.

... or run a hybrid Boiler/ASHP system using the boiler for hot water.
For the two semis, it proved so difficult to find a space for the cylinder that they modelled the house keeping the gas boiler too. This would be used for hot water and also as a boost for the ASHP in cold weather. This is a very different sort of hybrid system than envisaged by the Committee for Climate change. They intend the hybrid to use the ASHP most of the time and only switch to gas when the electricity grid cannot handle the demand (see Are hybrid heat pumps the solution for low carbon heating?).

In the hybrid scenario, the heat pump provides about half of the heat.
The CCC’s intention was for the ASHP to provide 90% of the heat. In the ESC modelling of these homes the ASHP provided about half. These graphs show the resulting carbon emissions for the two cases based on electricity grid intensity in 2030, at 93gCO2/kWh. Over time the carbon emissions from electricity should decrease much further and the all-electric case would be effectively zero emissions.

Carbon emissions for heating and hot water following progressive upgrades to two semi-detached homes [1]. The fabric only changes are (A) cavity wall insulation, new doors and windows (C) loft insulation and internal wall insulation. The hybrid system is with multi-zone controls, new radiators where necessary, and an ASHP but retaining the gas boiler for hot water and as a boost for the main heating. The full electric system is with ASHP only. Carbon emissions are from 2030, assuming 93gCO2e/kWh for electricity.

Costs outweigh savings – but there are other benefits.
The capital cost for upgrading the five houses ranged between £18,000 and £35,000. These costs far outweighed the financial savings from lower bills. In fact in most cases the ASHP was more expensive to run than the gas boiler, at least initially. By 2050 they expect gas to have increased in price more than electricity and so the heat pump is cheaper. However there are other benefits, such as better comfort.

Multi-zone controls alone made a huge difference to comfort in cold weather.
For example for house D (terraced), the heating system was totally inadequate to start with. During cold January weather, the living room was below the desired temperature 67% of the time. This was mostly corrected just with the multi-zone controls (down to 3.8%) and with full upgrades would be down to 0.6%. Also for house B, the multi-zone controls improved the time the living room was cold from 41.8% of the time down to 1.4%.

For energy storage, electrical batteries are more compact than heat batteries.
Heat pumps are most efficient if they are configured to run continuously. However to minimise electricity demand at peak times, it would help to reduce power demand for a few hours during the early winter evenings. To do this without loss of comfort you would need storage – either of heat or of power to drive the heat pump. ESC compared a Tesla Powerwall battery with a SunAmp heat battery. This is one of a very few heat batteries available for the domestic market. The SunAmp battery uses phase change materials so that it has a better energy density than just hot water. However, ESC found this was still only a third the energy density of the Powerwall (by volume). Assuming a heat pump COP of 2.5, the effective energy density of the Powerwall is 7 times better! Given the limited space in most homes this is a considerable advantage.

My view
ESC based their study on real houses and did very detailed modelling – but this is still only a model and there were only five houses. So you could be sceptical of their finding that 'Electric Heat pumps can provide good comfort in existing homes if sized and operated effectively in combination with targeted building fabric upgrades'. However their assumptions seem reasonable to me.

In their key findings they say that although the low carbon heating does not bring much in the way of financial savings there are other benefits from increased thermal comfort and controllability. However, most of the improved comfort is from the multi-zone control which is a relatively small part of the system and not particularly low carbon. They have made an effective case for multi-zone controls, but not heat pumps.

ESC have shown that heat pumps are viable technically and practically, if you allow for compromises like the hybrid systems where there was not enough space for a hot water tank. However there does not seem to be much of a case economically, at least from the householder viewpoint, without subsidy, or carbon pricing. The only clear advantage of heat pumps is in reducing carbon emissions – which is of course the critical issue.

[1] Integrated Electric Heat – Upgrade Analysis Final Report (Energy Systems Catapult) Oct 2018