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.
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!).
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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