The UK has set in law a target to bring all greenhouse gas emissions to net zero by 2050.
New homes being built now will still be in use in 2050, so their energy efficiency standards must ensure that they contribute to the 2050 target. In fact, the Government expects the average new-build home to have 75-80% less carbon emissions than one built to current energy efficiency requirements. This will be achieved through very high fabric standards and a low carbon heating system.
So how will these new homes be heated?
The Future Homes Standard – the Government’s vision for low-carbon new build homes – says: “We anticipate that the installation of heat pumps, particularly air-to-water and air-to air heat pumps, will play a major role in delivering low carbon heat”.
With this in mind, there is an Interesting article in this month’s CIBSE Journal by building physics and sustainability guru Chris Twinn, in which he reports on a research programme he’s been involved with which analysed 11 potential heating systems for the zero-carbon home in terms of capital cost and energy supply capability.
The study’s findings give an interesting glimpse into the challenge of selecting an efficient and economical heating system for a new generation of energy efficient homes.
Interestingly, the system that emerged as “most favoured” is not a standalone air source heat pump but a heat pump combined with a mechanical ventilation system – what Twinn terms an autonomous Two-Stage MVHR + EAHP.
This combination of mechanical ventilation and heat recovery with an exhaust air heat pump was originally developed for super-insulated Passivhaus homes in Germany and Scandinavia. Currently these units are not widely available in the UK, but that will probably change as homes become better insulated and more airtight.
The MVHR+EAHP Concept
The MVHR+EAHP concept is simple: stale air, warmed by the occupants, is extracted from a home’s kitchen and bathrooms. The extracted air is ducted to an MHVR unit where waste heat is recovered. The captured heat is then upgraded using an exhaust air heat pump which enables it to be used to heat the home or to heat domestic hot water in the unit’s in-built water tank.
This combination ventilation and heat pump unit is a neat solution: rather than taking heat from outside air and using a heat pump to upgrade it for use in the home, the MVHR+EAHP solution uses a home’s waste internal heat as the heat source, which it then upgrades to enable it to deliver heating and hot water.
While these units do not have a huge heat output, if homes are well insulated and airtight they will not need much energy to heat them in any case. The units will, however, need to provide sufficient heat to meet the hot water demand. In fact, according to the Energy Saving Trust, heating water for use in our homes makes up about 4% of the UK’s total carbon dioxide emissions. Which is why, in addition to insulating a home Twinn says the key to bringing energy demand down is to reduce domestic hot water consumption by using low-flow, high performance shower heads and other fittings.