Do I need a hot water tank if installing a heat pump?

Not everyone will have an appropriate place to install an air source heat pump. Also having to install a large domestic hot water tank may reduce the number of people even more.

The normal answer to this question of whether or not a dhw tank is necessary is “Yes you will” and if you are replacing say a gas combi-boiler with a ground source heat pump or air source heat pump (ashp) as many people are now starting to do then you will  not likely  have a hot water tank already and you will now need space to accommodate this.

However I will put the case for an instantaneous water heater that will be suitable for some people and I believe will become a growing number of people. With a new attitude to domestic hot water (dhw) they can reduce their carbon foot considerably and very easily by adopting this alternative to the hot water tank.

First I will describe why normally people are advised to get a hot water tank when they are considering replacing their existing heating system with heat pumps:-

Heat pumps in most cases are not generally capable of supplying enough power to heat water instantly and so you if you are using your heat pump for hot water you will need a hot water tank. I would argue that it may be better in many cases to have separate systems for your space heating and hot water system that will be cheaper to install, take up less room, provide more stable water temperature, be more economical to run and have a smaller carbon footprint. A win win win win scenario!

It is understandable that advice has been to assume that the heat pumps should do both tasks because heat pumps will deliver more heat than the electricity they consume by a factor known as the coefficient of performance (COP) which will vary with the outside temperature and on the temperature demand on the inside. If you have a household with a large hot water need then clearly the hot water tank is the way to go.

 However there are assumptions and other factors to take into account.

The most economical route to drive your car is not the one which gives you the best mileage but the one that uses the least fuel.

The COP for your dhw will be lower than COP for your space heating because you will have to raise the temperature of your hot water higher than that used for your space heating system. You will also periodically have to raise the temperature of your hot water further (because of the risk of legionella) with say an electrical immersion heater(COP about 1) to boost the temperature. All this may reduce your COP for your dhw effectively to somewhere less than 2 (compared to a COP of somewhere between 3 and 4 for space heating). This is still advantageous to having say an instantaneous electric heater with a COP of about 1 ( that is no extra heat gained of course)only IF certain other factors are ignored. So what are these other factors?

With an instantaneous electric heater you can:-

1.  Get the heat where you want it.

2. Get the heat when you want it.

3. Get just the right volume of hot water needed.

4. Get the precise temperature of hot water needed if your instantaneous water heater is “smart” enough with no need to have higher temperatures that need mixing down with cold water.

I will consider each of these points in turn.

1. It is common for washing machines and dish washers to have cold water supplies only and washing powders can often wash with little or no heat and so the main place where hot water is likely needed is in the shower room, perhaps accounting for well in excess of 95% of usage. We have a shower room and not a bathroom (lifestyle change). Having the instantaneous water heater (upmarket electric shower that can also supply to sink taps on occasions) in the shower room means my daily shower will require only a minute of hot water followed by a cool shower particularly after a 15 min run or jog  (lifestyle change). My hot water use will be about the same time as it took for hot water to actually reach the shower from the boiler, meaning all that hot water was wasted regardless of how efficiently it was heated.

2. Getting the heat when you want it means not waiting for the tank to heat up but more importantly no tank losing heat to the room particularly in summer when you are less likely to take advantage of the waste heat from the hot tank but rather the reverse...you have unwanted heat. Again this is true no matter how efficiently that heat was generated.

3.An energy performance certificate, EPC, assumes a certain lifestyle in water usage which I believe is dated. It assumes we like long baths topped up with hot water or long standing in the shower watching hot water run down the drain. I can see this in the estimated dhw usage stated in my EPC and on the size of tanks recommended when looking for advice on installing ashps with hot water cylinders or state of the art heat batteries.

Instead of having a 200 litre hot water tank with pipes coming from perhaps a buffer tank and pipes all the way back to the heat pump I will heat only the water I use.

4.With a modern electric heater the water temperature can be heated from the cold mains tap with no mixer needed to a precise and stabilised temperature which can be digitally stored and recalled. No fluctuations of temperature if other taps turned on whether hot or cold. (If another hot tap is used then a temporary loss of flow rate will occur but not really a problem as the water temperature is fixed). 

So in summary, adopting a lifestyle change that does not compromise on health and comfort can reduce your carbon footprint considerably. This would involve replacing a bathtub with a shower and if your demand is low enough an instantaneous electric hot water tank could further reduce your consumption.

If these ideas were adopted by enough people then this could  result in less fossil fuel power stations being required.

An extra thought here:- I find washing my hands in cold water usually adequate. In the past I remember using hot water but very often by the time the hot water arrived I had finished washing. Maybe up to 10 litres of piping hot water lying in the pipework was entirely wasted. That's about 2kwh wasted if that happens about 10 times.

Link page for further or future related posts.
If  thinking of making changes to your home heating it is best to get impartial advice.
In Scotland I would strongly recommend Home Energy Scotland.