Hybrid heat pumps

Your home’s heat demand is the amount of heat required to provide heating and hot water. A single heat pump on a domestic electricity supply might not be able to provide enough heat to keep properties with a high heat demand to a comfortable temperature.  

Well specified heat emitters (radiators or UFH) will help a heat pump to run at a high efficiency, keeping running costs to a minimum. You could also reduce your heat demand through improved insulation. We have more information on insulation and draught-proofing. 

However, if you live in a large home where insulating is either impractical or too  expensive, and a single heat pump would not cope with the high heat demand, your installer may suggest a more specialist solution. This might include installing either a hybrid heat pump, a high temperature heat pump or a twin heat pump installation (often referred to as a ‘cascaded’ heat pump system).

Generally, the largest size of heat pump you can install on a standard (single phase) domestic electricity supply is 14kW. However, using either a hybrid, high temperature or cascade system you can find a solution for heating most homes.

It’s always worth finding out what it would cost to upgrade your electricity supply to a three-phase supply. If this can be done at an affordable cost, it will greatly improve your electricity options for both heat pumps and electric vehicle charging. Your DNO will be responsible for upgrades to your supply – you can ask your installer to help you get a quote.

Replacing a boiler with a heat pump can result in lower heating bills, however this depends on the efficiency of the boiler and the fuel you are replacing. For example, a modern gas boiler could deliver heat at a cheaper cost than a heat pump when mains gas prices are low.

Some hybrid systems have controls that automate when and how the heat pump operates based on several inputs, including:

• Electricity costs.
• Fossil fuel supply costs (mains gas, LPG or oil).
• Time of day (if this has an impact on electricity prices).
• Whether a solar photovoltaic (PV) system at the same property is generating or exporting energy.

Configuring the hybrid system to react to fuel prices should mean that the boiler only operates when it’s cheaper to run than the heat pump, lowering the overall running costs compared to running the fossil fuel boiler or heat pump alone.

If you’re considering a hybrid system for this reason, it’s important to check the assumptions used by your installer in deciding when the heat pump will run versus the boiler. For example, you should review the gas and electricity tariffs and compare them to what you’re paying now, and what you expect to pay in the future. Speak to your installer about the configuration and ask them to explain their running cost calculations.

The two most likely configuration options would be:

• Running the boiler as a ‘top up’ system for periods when the heat pump cannot supply the required amount of heat in very cold weather. It could also assist with hot water production when the control system dictates this is the most sensible strategy based on the different tariff costs.
• Separating the function of hot water and space heating so that the boiler only supplies hot water, while the heat pump supplies space heating. If this was a combi boiler (or instantaneous water heater), it would remove the need to install a hot water cylinder alongside the heat pump.