Can an air source heat pump heat a whole house?

can an air source heat pump heat a whole house
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    The simple answer is yes an air source heat pump heat a whole house, but it doesn’t have to. There are various options when looking into what heat pump system is best for your property.

    The most important factor is determining the property’s heat requirement (or peak heat loss). Once this is known, we can best advise on what system will be most suited to your property.

    It is worth noting that in order to qualify for the BUS voucher, the heat pump needs to be capable of heating the whole property, and the fossil fuel heat source (i.e., your old gas or oil boiler) be decommissioned and removed.

    However, there are circumstances where heating the whole house via a heat pump isn’t feasible and/or financially viable. In these instances, you may opt for what is called a hybrid system.

    A hybrid system is a heat pump coupled up with an additional fuel source, such as a gas or oil boiler. In this scenario, the heat pump will be designed to perform a certain amount of the property’s heat demand, but when the external temperature drops too low, then the boiler will take over and the heat pump turn off.

    This may sound counter-intuitive; however, it can have a large impact on the carbon reduction and energy savings of a property. See the below example for a property with a 20kW peak heat loss.

    On a standard 100amp domestic electricity supply (single phase), the largest single air source heat pump can provide in the region of 16.5kW and is therefore not capable of heating this property alone. In this scenario, installing a 12kW heat pump will likely be more cost effective.
    The below figures are based on a Vaillant aroTHERM Plus 12kW heat pump, at a design flow temperature of 50c.

    • Property total peak heat loss @ -2c = 20kW
    • ASHP output at -2c = 12.6kW
    • ASHP can provide 63% of heat load at external design temp, and is therefore not large enough, it would therefore be couple with a gas boiler with an output of around 30kW which then adequately covers the property’s heat requirement
    • The ASHP can provide all the property’s heat requirement when the external temp of 5c or above (this is called the bivalent point). Below this temp, the heat pump will switch off, and the gas boiler provide the heating instead.
    • Despite the heat pump providing only 63% total coverage at -2c, it is not often that cold in the south of England, and therefore the heat pump will provide approximately 89% of the annual heat requirement for the property

    It is worth noting however, that hybrid systems do not qualify for the BUS voucher.

    Do air source heat pumps need radiators?

    There are two types of air source heat pump:

    • Air to water heat pump – these take the energy from the air outside and input it into your central heating system. These can be used with either radiators, underfloor heating, or fan convectors.
    • Air to air heat pump – like the above, these take the energy from the outside air, but instead blow it around the house as hot air. Essentially air to air heat pumps are reversible air conditioning units. Like air conditioning they can often provide both heating and cooling.

    Generally, most air source heat pumps that are installed in the UK are air to water as they can be relatively easily retrofitted onto an existing heating system, rather than needing new ductwork or refrigeration pipework installed throughout the property (as would be needed by an air to air system). It is also worth noting that air to air heat pumps do not qualify for the BUS voucher.

    Do my radiators need changing if I get a heat pump?

    It may be necessary to change all, some, or none of your radiators as part of your air source heat pump installation. One of the first steps in the process is for Cinergi to carry out room by room heat loss calculations on your property.

    We do this by measuring all the walls, windows, floor, and insulation values of every room. This tells us how much heat energy each room needs to maintain design temperature which leads onto two key aspects of your heat pump design:

    • Sizing the correct heat pump –  the total of your room-by-room heat loss calculations allows us to size the correct heat pump for your needs. It is important that your heat pump is large enough that you are warm but is not too big as it will be less efficient to run.
    • Sizing of your radiators – it is likely that your currently radiators are sized for a boiler system running at a flow temperature of 70c (this is typical across most retrofit properties). When installing a heat pump, we want to run your heat pump at as low a flow temperature as we can, to maximise its efficiency. By reducing the temperature of the heating system water, we are reducing the amount of heat energy that it can carry. Simply put, we need more water (increased flow rates), and a larger surface area (bigger radiators) to get the same amount of energy into the room

    This all comes down to differential temperature.

    • Your boiler system, with a flow temperature of 70c, is sized with a differential temperature (DT) of 50 degrees. This is because flow temp=70, room temp=20, DT=50. Most radiator manufacturers advertise their radiators at DT50.
    • With the heat pump system running at 50 degrees flow temp and a room temp of 20, the DT=30. Where the differential temperature between the radiator and the room is less, the energy transfers from the heating system to the room less, therefore meaning that the output of your existing radiators drops.

    Is underfloor heating more efficient than radiators?

    Heat pumps can run underfloor heating as well as radiators. Underfloor heating can be much more efficient as it covers a large surface area and uses a lower flow temperature (around 35°C) than radiators (70°C -90°C).

    Can I cool my house with my air source heat pump?

    Simple answer yes you can cool your house with an air source heat pump, but not easily. As the internal components of an air source heat pump are very similar to that of air conditioning, most units can be run in reverse to provide cool water rather than hot. However, the tricky bit comes as to how this cold water energy is transferred to your property. There are several things to consider:

    • Conventional radiators cannot be used for cooling as they will create condensation
    • Underfloor heating (UFH) can be used for cooling, however, typically can only provide a 2c drop in temperature of the room
    • A property’s cooling load is typically double its heating load (i.e., if your heating load is 10kW, then you will need 20kW to cool the property)
    • All pipework carrying cooling water needs to be fully lagged throughout its entire length, therefore cooling in this way is typically not feasible in a retrofit property – it is however more realistic on a new build project where the complete system can be installed correctly.

    Talk to Cinergi for all your air source heat pump requirements

    With over 40 years experience in heating homes and business, Cinergi are experts when it comes to advising you on your best heating options. Contact us today for a free discovery call, visit our FAQs page for more helpful advice, or read our ASHP reviews here.

    Dan Loveridge
    Dan Loveridge

    Dan is the managing director at Cinergi, a renewable energy company based in southern England specialising in air source heat pumps and EV charging solutions.

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