Below is our short guide to Air Source Heat Pumps (ASHP), highlighting details you won’t get from the manufacturer’s sales material. In our experience, they are important factors to consider before taking the plunge.
Most manufacturers offer two types of system, either a ‘Monobloc’ or a separate external condenser and internal compressor unit, sometimes called a ‘split’ system.
- If internal space is tight the Monobloc accommodates some of the large elements of plant outside. These units heat water and circulate this into the building, and, because of this they must be located within 6m of the house to prevent excessive heat loss from the pipework between the unit and the house.
- If you can accommodate an internal compressor unit you will have around 30m to play with, because the connections between internal and external are cold refrigerant lines so don’t lose heat.
This is connected to the issues around location to a certain extent. The further away from the property, the less you will hear the unit. In the daytime, this should not be an issue but if you live somewhere rural and want your heating on overnight this may be something you need to get used to, and to a certain extent your neighbours will too. The noise from an ASHP is directional, so think about where you’re going to put it carefully, and before buying visit an installation if possible.
When installing an ASHP the best system to pair this up with is underfloor heating for reasons of efficiency which we touch on below. This doesn’t preclude radiators in a few rooms, first floor bedrooms for example. These radiators will be two to three times larger those typically connected to a fossil fuel boiler and will have a low surface temperature. If you want a house solely served by hot radiators we’d suggest steering clear of installing an ASHP.
- Underfloor Heating (UFH)
Design your house to be as low energy as possible, which will allow you to run the underfloor heating at the lowest possible flow temperature to maximise the ASHP efficiency. If you get a specialist contractor design, make sure proper heat loss calculations are conducted to ensure the flow temperatures are as low as possible. Typically, they estimate the heating requirement based on benchmark figures and this can lead to flow temperatures of 45oC – 50oC proposed when you’re really aiming for 35oC or lower.
- Hot Water
You may build a low energy home in terms of heating requirements but if you have a large family with a large daily hot water demand, requiring a minimum storage temperature of 50oC this will have an impact on the ASHP and its running efficiency and therefore your running costs. Additionally, by generating hot water at 50oC and not 70oC, your storage volume will have to be greater than a typical fossil fuel equivalent, so space planning is important to consider as well as the type of hot water cylinder. Ensure the heating coil is of a suitable surface area for operation at ASHP temperatures. Some manufacturers offer cylinders specifically paired to their ASHPs so you can be sure this has been engineered correctly.
Watch-it Note: Manufacturer’s literature frequently quote seasonal efficiencies known as the Seasonal Co-efficient of Performance (SCoP). Essentially these equate to an average performance/efficiency. Notionally using an external reference temperature of 7oC (UK) typical value are listed in the table.
Remember, the instantaneous ASHP performance varies with external temperature and heating flow temperature, the further apart these become the lower the performance of the ASHP will be.
You can’t control the weather, but do what you can to ensure the heating temperatures are as low as possible and hot water usage is as low as practical. By doing this you can see significant efficiency improvements.
A successful, efficient ASHP installation must carefully account for the thermal performance of the fabric, and incorporate a heat distribution system with the lowest possible water temperature. A better insulated house can use even lower water temperatures, creating a double whammy benefit of less demand and more efficient delivery.
Give us a call and find out how we can help you get the most out of an ASHP.
Have you thought about Ground Source Heat Pumps? Read Five Things To Consider
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