Feedback | Where should heat pumps be sited?

Feedback heat pumps
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Luke Osborne’s article on heat pumps generated some animated responses from CM readers, including the question of where heat pumps should be positioned.

CM’s article on heat pumps last month (Heat pumps: dispelling the myths) came as the Competition and Markets Authority (CMA) is examining whether it needs to protect consumers from poor practices in the green heating and retrofit sectors.

The CMA said it wanted views from consumers, businesses and other interested groups as more people switch to solutions like heat pumps amid the energy crisis.

D Pearson MCIAT

I have been thinking about air source heat pumps and in particular their position next to the building they are meant to be heating. There is a theoretical chance that when an air source heat pump is placed next to a wall of the building it is meant to be heating it will actually suck out as much from that external wall as it puts in.

I liken it to the rising damp scenario where you put an air dehumidifier in a room to reduce the moisture content in the atmosphere, the high level of moisture being due to the damp rising up through the wall and then evaporating into the room.

When you put the air dehumidifier on, it removes the moisture from the air which in turn allows the raising damp to immediately replace the removed moisture, so increasing the rising damp flow and making the situation worse.

Steve Frizell (responding to D Pearson)

The scenario presented appears to be two different effects: that of dehumidifying to deliberately draw moisture out of an internal room space and, by extension, out of the enclosing structure with some heating and a reduced relative humidity. This should be self-limiting if effective damp proof membranes (DPMs) and damp-proof courses (DPCs) are in place.

This is different to creating a greater temperature gradient across a structure due to reduced external temperature, perhaps inadvertently causing a dew point to arise within the structure, in the absence of a DPM and DPC.

An air-source heat exchanger should be sited and orientated to blow cooled air away from adjacent walls. The examples I’ve seen (in Scotland), are set away from the wall. If the orientation and opportunity for free circulation of air is poor, then certainly cooled air could recirculate and effectively create a self-reinforcing ‘cold trap’. This will lead to higher heat loss through the chilled walls, and also reduce the efficiency of the exchanger performance.

Other considerations include prevailing wind direction and solar orientation to take advantage of warmer ambient air conditions.

The positioning of heat pumps is crucial to effective operation

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  1. One comment above mentioned ‘orientation’. I met someone who heat pump had been destroyed during a long storm as the wind blew from the wrong and unexpected direction and forced the fan backwards. Of the three we had in the spring in close succession, No 3 blew down a difference group of trees. So orientation is an important issue.

  2. In the spirit of dispelling myths, there is no chance at all that a heat pump could suck out as much heat as it puts in, just by being sited close to a wall. The photo in the article above shows a typical installation for small-scale heat pumps and those units would be blowing away from the wall. It looks to me like the space behind them for the air to enter the unit exceeds the minimum by quite some margin, as the minimum is usually surprisingly small. There would be no noticable “sucking effect” of any heat from the wall as the air into the back of the unit is the ambient air, so no different than any other surrounding air. If a unit blows towards a wall, there is a different minimum clearance and in any case, there has to be reasonable airflow around the unit for it to function correctly and to its specified capacity. It’s usually fairly straightforward to find a reasonable location around a domestic property, but even in a less than desirable restricted location, the chances of a heat pump causing as much heat to be sucked out of a building as it puts in is as ludicrous as saying a boiler flue pointed at a wall can put as much heat into that wall as the boiler rating. Talk about scare-mongering.

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