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A new intelligent method of operation for air source heat pumps

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Heat pump manufacturers are researching and developing their models to be more efficient by improving components, refrigerant and using inverter driven compressors. Weather compensation has proved to be an excellent way to control your heat pump and improve running costs and performance.


As I'm not employed by a manufacturer and therefore am not involved with internal product improvement, I started to think about other parameters which could influence an air source heat pump's efficiency.


The  EN 14511 standard of presenting air source heat pump's performance is based on 45oC flow temperature and 7oC outside. Heat pump manufacturers like publishing this, because the unit output and COP are then extremely impressive, however professionals who are familiar with the technology know how important it is to review the performance at low external temperatures and against design flow temperatures; COP’s will fall and therefore running costs will increase.


 What if we could eliminate the air source heat pump working at these cold external temperatures of say -3oC or -5oC. simply by turning off the heat pump when the property requires heating? Using a method to predict what the external temperature is likely to be in the evening / early morning (10pm through to 5am)


Newly built homes don’t have much thermal mass so turning off the heating in the evening isn’t often recommended. As most heat pump installers will recommend, the heat pump should remain on at a low steady operation, trickling out a 35 to 45oC flow temperature into the distribution system radiators, underfloor, fan convectors etc.


Using weather forecasting live data which communicates to the heat pump program via internet connection could well provide a new intelligent method of operation.


Since air source heat pump performances are improved with warmer external air conditions, and winter day time temperatures are warmer than twilight hours, it would make sense (not in all cases) to turn the heat pump on in the day time and store the heated water in a thermal store. This saves the energy within the thermal store for twilight demand. There will of course be days when heating is required in the day time and therefore the weather forecasting heating operation could not be activated. But during day times (6am to 8pm) when heating demand is low and the weather is predicating low temperatures at twilight hours this new approach of operation could work well.


The size of the thermal store will be calculated from the daily energy demand for the property, hence a room by room heat loss calculation is required.

What size thermal store would be required?

A typically well insulated new build 2 to 3 bedroom property would potential only need between 5000 to 9000kWh of energy per annual for heating. Therefore if we assume on a cold winters day 10kWh of energy is required to heat the home (24 hours period) and the maximum flow temperature to the underfloor heating is 45oC. The delta T is (45oC – 20oC) = 25oC

We can assume if the thermal store energy is completely used each time there is a demand, then towards the end of the usage on the return pipe temperatures could be 20oC (feeding into the bottom of the thermal store). The thermal store would then be ready for another forecasted heating period.

We can calculate the estimated size of the buffer.

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Therefore litres of water required for thermal store to be heated on a daily basis (winter days) = 344 litres

A 3.5kW air source heat pump would be able to heat up the 344 litre store within 3 hours.

A good location for this thermal store in a 2 to 3 bedroom house could be challenging, but the architect could design a suitable location for it, possibly bury it in the garden using a anti-corrosion cylinder. This of course can lead on to more ideas to discuss.

Written by Rich Cartwright BEng MCIPHE, Director of RDC Energy Ltd

Mobile: 07989603246


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RDC Energy Ltd, 32 Oxlease, Witney, Oxfordshire, OX28 3QX              Company registration number 07336992