Will my water be hot enough with a heat pump?

Isoenergy has been installing heat pump systems for clients for over 10 years. We have had countless conversations with homeowners who have heard that the temperature of the hot water out of a heat pump is lower than a fossil fuel boiler. We are most commonly asked the question &#;What about the dishes? Will the hot water be hot enough for us?&#;

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Conventional plumbing and heating engineers are used to installing fossil fuel boilers which generally output temperatures in the region of 60 &#; 70°C. In their view, anything that outputs a lower temperature is inadequate for producing domestic hot water. This is simply not the case.

Gone are the days when heat pumps were limited to an output temperature of 45°C. Most quality heat pumps will provide 55°C with ease and some higher temperature units can produce as high as 70°C (at a reduced efficiency of course).  55°C is certainly hot enough for domestic use and in fact most hotels and care homes limit the temperature of the hot water at the tap by adding cold water to the mix. The resulting temperature is usually just below 48°C as specified in the Building Regulations.

We have heard plenty of other arguments claiming why a fossil fuel boiler is better for hot water than a heat pump. Here are some of the most common:

Can I still have a steaming hot bath?

When running a bath we would normally add both hot and cold water to the mix to get the right temperature. Bathing in 60°Cwater straight out of the tap will cause severe scalding. The Department of Health recommends temperatures no higher than 43°C for bathing and 41°C for showering.  It is true that with a heat pump, the water coming out the tap is cooler than the water you can get out of a fossil fuel boiler, but by adding more hot water and mixing in less cold, the same desired bath temperature can be obtained. This does mean that you will need a larger store of hot water and isoenergy will normally replace your existing water tank with a bigger one for this reason.

The water in the tank may be hot, but it is cold by the time it reaches the bathroom at the other end of the house.

To address this, isoenergy recommends insulating any hot water distribution pipes where possible. Of course, sometimes these pipes are not accessible and so we can specify a heat pump with a higher output temperature. It is worth stressing, that this has not been necessary in the vast majority of systems we have installed.

But what about Legionella?

In the UK there is regulation in place saying that hot water storage must be protected against the growth of harmful bacteria, such as Legionella. All well-designed systems will have an immersion heater in the hot water tank that can be used once a week to pasteurise the water. For isoenergy installations this is set on a timer, usually once a week at 2am when the immersion switches on and the tank is held at over 60°C for more than an hour. This pasteurisation keeps the water safe for you and your family.

But what about the dishes?

And yes, the hot water that comes out of the kitchen tap is hot enough to clean the grease and grime off those pots and pans, so there goes your excuse for not doing the dishes.

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For more information on how a ground source heat pump or air source heat pump can replace your current fossil fuel boiler, get in touch or call us on 821 345.

Posted by: @editor

I received this interesting from Andy this evening, and it's a good question and would be interested to hear what your strategy is regarding hot water as part of your heat pump setup?

We love your YouTube videos watched after moving into our new build home a few weeks ago. We are trying to work out the best way to manage our ASHP and wondered if it was best to heat the DHW twice/day as we do at - & - or leave it on all day for it to reheat as required e.g. after showering? We've also lowered our flow rate temp from 60 to 45 degrees.

My response to Andy was that there are a couple of variables to consider. If you have a time of use of tariff, reheating the water at 3-4am makes sense. If you need a second reheat, then 12:00-13:00 would be better as it&#;ll be warmest at that time of day.

We reheat our water on demand as we&#;ve not seen much difference in efficiency and that way we&#;re always guaranteed to have hot water.

The bigger question is water temperature. In my opinion is that what Andy has done is sensible by dropping the water from a scalding 60C to 45C. We heat our HW to 44C. Anything hotter for us means we&#;ll use cold water to make the hot water cooler, so you&#;re wasting heat and energy.

Naturally, we make sure that our legionella cycle kicks in once a week. We also have an iBoost that heats our water with surplus PV production.

Having studied in detail how ASHP's work, and how to use them most efficiently, I would not advise heating your hot water during nighttime, which is often the coldest part of the day. In the first place, many ASHP's may struggle to get water up to 60C, and would have a COP of little over 1 to 2. Even heating the hot water to 45C, with an ambient temperature of say 0C, would give a COP value of approximately 2.5, whereas heating the same water at an ambient temperature of 7C, would be at a COP of approximately 3.5.

A well insulated hot water tank losses approximately 2 kWh of energy per day, provided no hot water is used, obviously, the hotter the water, the greater the heat loss, but the difference is not that great. So the more hot water that is actually used, the greater the heat demand will be.

From the point of view of usage, the hotter the water, then the less hot water, you will need to mix with cold water, to produce the required temperature. From a heat energy usage point of view, there should be very little difference if the water is piping hot, or slightly cooler.

If you have solar panels and a diverter unit, it would make sense to allow the water to be heated by solar energy first, and then if necessary use the ASHP to bring it up to the desired temperature. Most days, the warmest part of the day is mid to late afternoon, so this may be the most efficient time to use the ASHP for this purpose. It is also after the Sun has peaked, so most of the available solar energy will have been used.

It is recommended that the hot water should be heated to between 60C and 65C for a period of a minimum 10 minutes for Legionella protection, at a maximum frequency of 7 days. Probably best overall efficiency could be achieved, by allowing the water to initially be heated by any available solar energy, then heated as high as possible using the ASHP, and finally being topped off by the immersion heater, if the ASHP cannot achieve the specified 60C.

Are you interested in learning more about Air Source Heat Pump Hot Water? Contact us today to secure an expert consultation!