Penzance Town Centre

Penzance Town Centre

Friday 7 July 2023

Standalone air source heat pump hot water systems.

 The Government is promoting the replacement of gas boilers by heat pumps with mixed results and horror stories in the press.  Meanwhile other heat pump solutions that could contribute to the transition from fossil fuels are ignored..  I illustrate the point below with my project to upgrade my hot water system to a high pressure (unvented) system following the demise of my Aga and the desire to improve the performance of newly installed showers.  Dick Cliffe 


My gas powered Aga has been cold since Jun 22 with no prospect of it being turned back on again given the price of gas.  The Aga was pricking my conscience anyway even before the price of gas went through the roof because it was environmentally ‘unfriendly’ and wasteful.  I am currently using an  immersion heater to heat water – it uses about 2.5 units of electricity day as there are only two of us in the house (domestic average 4 units a day). 

High Pressure (unvented) Hot Water Systems.  

I have been planning to install a high pressure (mains pressure) tank to make two new showers work better and get rid of the header tank in the attic.  Logically the new cylinder would be connected to the central heating system boiler to allow the gas boiler to heat the hot water.  This would involve a lot of new plumbing and does nothing positive for the environment because about half of the electricity I use today for water heating is renewable or low carbon electricity so going for gas would be a backward step.


The solution, discovered by accident rather than design, will be the installation of a hot water tank with built in air-source heat pump.  The input air is sucked in via duct from the roof or soffit and the cold air expelled through a second duct to the roof or a grill in the soffit – there is no requirement for an external air source heat pump unit and all of the associated ‘pipery’ . 


This is an idealized installation but it illustrates the principle of a standalone hot water

 tank with built in air source heat pump connected to vents/inlets in the wall.


The system has a COP (excuse the jargon - coefficient of performance) of 3.36 meaning you get 3.36 units (Kwhrs) of heat for each unit of electricity consumed.   I am consuming 912 units/year with the immersion heater and the standalone air-source heat pump tank would take 304 units assuming a COP of 3.0 (manufacturers spec less 10%) giving a saving of  608 units which equates to about  £183/year at 30p per unit of electricity.


There are several manufacturers of these standalone tank systems (Dimplex Edel is one).  A 200 litre tank is approximately £2000  but they range from £1700 - £3000.   For someone like me who was planning to convert to a high pressure (unvented) system anyway, the additional cost of an Edel tank over a good quality high pressure tank is about £1300.  By going for the standalone solution, I avoid the cost of modifying the Central Heating pipe work and control system to heat water using the gas boiler – not something I wanted to do anyway for environmental reasons.   I am expecting to get my money back in 3-4 years after allowing for savings on central heating systems modifications and reduced operating costs. 

Table 1.  Alternative solutions and their running costs.

Hot Water





Eff %

Fuel Cost










Assumed 100% efficient

Air Source HP Tank





608 Kwhrs taken from from air.  COP 3.0 assumed

Gas (CH Boiler)





Boiler ~91% but less efficient when just heating water + pipe losses.

The table shows three alternative solutions considered.  All provide 912 Kwhrs of hot water.

Long Term. 

In the longer term the Government & National Grid can be expected to encourage diversion of electricity demand to off-peak times to reduce peak electricity consumption which is expensive to meet today and which will get more expensive in the future as renewables replace gas generation.  The incentive for customers to move demand (switch off in peak times) will be driven by price - this will suit owners of hot water tanks heated by electricity and by air source heat pump.  The good economics of heat pump powered hot water systems is likely to get even better compared to alternatives.  A second advantage is that installing an air source heat pump hot water tank does not compromise any future investment in air source heat pump central heating  - in many ways it simplifies the plumbing and reduces the total demand on the new system by ~15%. 

Government Policy.  

Standalone hot water systems with built-in heat pumps are common in the USA but are not common here in the UK.  The Government seems to be missing a trick in not encouraging these because heating hot water accounts for around 15-17% of all domestic energy consumption. There will be millions of households where air-source heat pump powered central heating installations are impractical or unaffordable (despite grants) but standalone hot water systems powered by air-source pumps are. 

Health Warning.  

This solution suits my particular circumstances and will not suit everybody.  I was planning to replace my vented system anyway to improve two new bathroom showers and my vented system was not already connected to the CH boiler – a substantial expense to connect to my new tank given the positions of the boiler and the tank.  Links to products are below (these are for information and are not recommendations)

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