January 8, 2018 dga.energy Admin

Sizing Heat Pumps

In order to ensure that the heat pumps you specify will perform adequately all year round it is vital that they are accurately specified in terms of their CoP Coefficient of Performance and also their kW power capability.

The performance and effectiveness of heat pumps varies considerably between the seasons and in the various climate zones around South Africa. They are ideally suited to warmer climates as their CoP is substantially higher when the inlet water temperature and ambient air temperature is higher. When the ambient air temperature and/or the inlet water temperature is low they have to work very hard in order to heat water up effectively and their CoP can in reality drop back down closer to just over 1.0 which makes them no more efficient than a traditional electrically fuelled hot water tank under these conditions.

Heat pumps should obviously always be located outdoors and NOT in enclosed spaces (i.e. garages) – they exhaust cold air as a by-product which would slowly reduce the ambient air temperature in any enclosed space and thus reduce their CoP efficiency over time.

The advice for sizing heat pumps is generally accepted as: use a 5kW heat pump on a 200 litre hot water tank, a 7.5kW heat pump on a 300 litre hot water tank and use a 9kW heat pump on a 400 litre hot water storage tank.

The type of gas that any heat pump uses is also an important consideration to review and understand. R22 gas was phased out due to the Montreal Protocol in 2010 so we shouldn’t be using or seeing that in the industry. R407C (the replacement gas for R22) and R410A are the most common gasses now used whilst R134A gas type is commonly used in commercial and high-end pumps.

R407C however doesn’t heat water very well above 45 degrees without requiring electrical resistance backup in the hot water tank and can reportedly be quite problematic with high humidity summer temperatures (especially in Durban) causing the units to trip out. The solution to this seems to be to drop the heating capacity of the heat pump to just 50 degrees C meaning the electrical element has to pick up the 10 degree C shortfall required to get the storage water to the required 60 degree C level. This means that in reality the average efficiency on a R407C gas type heat pump is heavily reduced and these ‘cheaper’ units will simply cost the client more money to run as the electrical back-up element in the hot water storage tank is still doing quite a lot of work in heating their hot water and pushing up their electricity bills.

R410A gas can heat water comfortably to around 50-55 degrees C and thus relies less on the electrical element in the hot water tank to heat the water. The ‘little bit’ extra a client thus pays for these units will result in a comparatively reduced electrical bill and the units thus pay for themselves quite quickly within a year or so dependant on use.

It is vital that an appreciation of how heat pumps perform and what factors influence their performance are considered when specifying units for your clients. Don’t be mislead with a cheapest price approach as clients will pay more for the cheaper unit in the long run with reduced efficiencies and higher electrical bills.

Heat pumps in South Africa should be SABS tested & approved (although most of them aren’t) so the actual CoP performance values of any unit should be available from the supplier – Insist that they be transparent with those reports and make them available for you to review and understand! One of the biggest challenges we currently have is that manufacturers only typically reveal their units maximum performance CoP values achieved under ideal test conditions (in a lab overseas somewhere) which bear little to no relevance to location-specific or climate-specific local performance!


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