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How to use an air source heat pump to heat your hot water

Posted by John Lightfoot on 20 July 2011 at 11:32 am

Q: Can you use air source heat pumps for heating domestic hot water?

A: If you are reading this and looking for a quick answer, it's yes!

For those of you who would like a little more information here we go...

Just because you can use a heat pump, should you?

Well, of course, while the sun is shinning a solar thermal  panel will produce domestic hot water far more efficiently than a heat pump, but obviously in the mid winter months you will probably need some additional heat source and using your heat pump will be far more efficient than using many other heat sources such as electric immersion heaters or oil boilers.

Required temperature

We normally bathe or shower using water around 38C to 42C and most air source heat pumps will have no problem in supplying this sort of temperature.

Word of caution

There is however, as always, a slight snag!

Let me try to explain the reason for the snag (if you’re not interested in the reason miss the next few paragraphs and go straight to the paragraph headed safety precaution).

The domestic hot water (DHW) is normally heated through some kind of heat exchanger allowing heat to be transferred from the heating water into the DHW. One way of doing this is to use a “tank in tank” – see illustration. Here the DHW tank is surrounded by the space heating water and therefore absorbs heat from the space heating water through the walls of the inner tank.

Given that many heat pumps (although there are of course exceptions) are only be able to supply water at a maximum temperature of around 50C the domestic hot water will have to be stored at below 60C.

What’s the problem?

Well what’s the problem I hear you saying?

You have already told us that we bathe around 40C so why does it matter that the water is stored at a temperature lower than 60C?

Well none ... it it wasn’t for a bug or bacteria called Legionella. This bacteria breeds in warm water and if inhaled (in the tiny mist droplets you get in a shower for example), can cause a potentially fatal disease called Legionella disease. So to ensure every one is safe, we like to kill off this bug, which is possible by increasing the temperature of the water to over 60C. (At this temperature the bacteria dies within two minutes and at a even faster rate at higher temperatures!).

Safety precaution

Therefore, to keep every one absolutely safe, most installers will arrange for the DHW to be heated once a day to above 60C, usually by an immersion heater on a timer.

Given that the heat pump should take the temperature to over 45C, the DHW only has to be heated by a temperature rise of 15C.

This means that the majority of the heat energy is supplied by the heat pump for that one hour, and all of the energy, assuming it is not being used for heating, for your DHW requirements can be supplied for the rest of the day by the heat pump.


So by a very simple control scheme we can ensure that your DHW is perfectly safe.

If you have read the previous blogs you will know that the higher the water temperature the heat pump is required to produce, the lower its efficiency.

With this in mind I recommend that you make sure your installer includes controls that only require the heat pump to supply high flow temperatures when there is a DHW demand.

For the rest of the time, when only space heating is required, the controls should then allow your heat pump to run at a lower temperature and thus at increased efficiency levels.


With a purpose made storage vessel, or cylinder, and the right controls it makes perfect sense to heat your domestic hot water with a Heat Pump especially if you link in a complimentary solar thermal panel.

Photo by Kevin Spencer

About the author: John Lightfoot is director at Thermal Energy Ventures Ltd.

If you have a question about anything in the above blog, please ask it in the comments section below.

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8 comments - read them below or add one


alfapatComment left on: 13 November 2018 at 7:03 pm

I am at present re installing my air source system due to bad workmanship , unfortunately 2days in , I need dhw without the pump work being unfinished , can I switch to use the immersion or forced HW  on its own?

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John W Lightfoot

John W Lightfoot from TEV Ltd.Comment left on: 30 August 2016 at 8:39 am

Hi Rmsummerell,


The recommended temperature for killing any legionella bacteria is a minimum of 60 deg C.  It might be worth having an immersion heater fitted ( if not already there) and boosting the temperature to 60 deg C once a day to be safe. 



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NewtothisComment left on: 28 August 2016 at 4:38 pm

Any help with the setting of my ASHP and DHW gratefully received. Since the heating isn't required at present, my settings are DHW via heat pump, 10am to 11am and 15.00 to 14.30 pm daily. The immersion heater is set for 60 c to come on at 11.30 daily for 30 mins "

it this ok ???

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John W Lightfoot

John W Lightfoot from TEV Ltd.Comment left on: 6 November 2013 at 11:34 am

Hi Stewart,


Glad youfound it of interest!





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stewartyardleyComment left on: 4 November 2013 at 4:49 am

Great article! Prior to reading this, I have no clue whatsoever that such a bacteria even exist on the face of this earth. I thought that all residential water supply is safe for usage/consumption with no risks in any form that might be invisible to mankind. After reading this, I am now going to be more wary about the water heater system because sometimes friends and family stay over and they might have young children who use that particular water system during showers and I want everyone to be safe.

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Be Green

Be GreenComment left on: 21 July 2011 at 12:53 pm

Interesting to read your comments I can't completely agree...most half decent ASHP can heat DHW well above 50 and still be more efficent than an immerstion heater. Also if the start point for heating is set to say 46 and you have a good tank the heat pump should not heat the DHW for well over 12 hours if the hot water is not used, this will take out the need for a timer.  I do agree on raising the temp to over 60 once a week, but no more, only in the UK do we insist on such over kill engineering, we must of course ensure everyone's safety but were also trying to save energy. The most likely chance of getting legionella in a domestic environment is a shower that is not regularly used and the warm water in the pipe is at the perfect temperature for legionalla, turn the shower on and its airborne. 

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JohnWLComment left on: 20 July 2011 at 7:03 pm

Hi - thanks for the comment.

These are the legionella figues being quoted on Wikipedia which I hope are of some assistance :-

According to the paper "Legionella and the prevention of legionellosis," [15] found at the World Health Organization website, temperature affects the survival of Legionella as follows:

Above 70 °C (158 °F) - Legionella dies almost instantly At 60 °C (140 °F) - 90% die in 2 minutes (Decimal reduction time (D) = 2) At 50 °C (122 °F) - 90% die in 80-124 minutes, depending on strain (Decimal reduction time (D) = 80-124) 48 to 50 °C (118 to 122 °F) - Can survive but do not multiply 32 to 42 °C (90 to 108 °F) - Ideal growth range 25 to 45 °C (77 to 113 °F) - Growth range Below 20 °C (68 °F) - Can survive but are dormant, even below freezing

Other sources [16][17][18] claim alternate temperature ranges:

60 to 70 °C (140 to 158 °F) to 80 °C (176 °F) - Disinfection range 66 °C (151 °F) - Legionella die within 2 minutes 60 °C (140 °F) - Legionella die within 32 minutes 55 °C (131 °F) - Legionella die within 5 to 6 hours 20 °C (68 °F) to 45 °C (113 °F) - Legionella multiply 20 °C (68 °F) & below - Legionella are dormant

The point you make with regard to Heat Pumps using electicity is well made, as our power supply is cleaned up, as is planned over the next decade, the CO2 savings will increase during the life of the heat pump.

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RmsummerellComment left on: 20 July 2011 at 12:01 pm

thanks. We installed our air source heatpump along with a wood burning stove and have the gas boiler as a final back-up. The idea is that in the winter when it is really cold, the wood burner is the first call for the hot water, then the heatpump, and if needed the gas boiler. We installed it just after the worst of the weather last December so don't have official experience yet, but it definitely seemed to work at single figure temperatures. When we installed our air source heatpump we were told that Legionnairesis killed at around 40 deg c so it isn't an issue to have the water around 50 deg c. I hope that is correct! the other thing to be aware of is that it does use electricity, so isn't totally renewable unless your electricity supply is renewable. We use Good Energy for that.

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