Many people give it little or no thought to their hot water and will just leave it to the manufacturer defaults on their programmer (and incidentally this is true for the heating as well). This might well work ok, however it’s worth considering your particular needs so as not to waste energy unnecessarily.
Ok, so let’s initially assume that you have a boiler with a hot water cylinder. The boiler and cylinder are connected by a primary circuit of water – this is usually a closed circuit that indirectly heats the cylinder using a heat exchanger near the bottom. In many houses this circuit is uninsulated. Every time the boiler pump circulates water round at (say) 60degC, the pipe will start radiating around 200W of heat along every meter in the circuit.
Things get worse close to the cylinder – even when the pump is not running, heat will be continually drawn out of the cylinder. You’ll also be losing heat through the walls of the cylinder itself. The cylinder and immediately adjacent uninsulated pipework would typically lose around 5kWh a day, working out at around £130 a year at current rates. And this is before you calculate the losses from the rest of the primary circuit.
Standing losses
It’s not only pipework that loses an inordinate amount of energy. The pumps that circulate the water also use a fair amount of power, maybe 25-75W depending on the length of the circuit. Therefore it makes sense not to run these any longer than necessary.
What to do then? Putting aside insulation of the pipework, it’s generally straightforward. You need to ensure you minimise standing losses by avoiding having the cylinder sitting full of hot water for long periods. This means checking your hot water programme and ensure it meets your needs and no more. It takes around an hour for a boiler to heat a 200-litre cylinder from stone cold. It would be sensible to double this if and when you are running your heating and hot water at the same time.
An efficient arrangement on a standard system would heat the cylinder an hour or so before the time you get up. If you find you are running out of hot water in the morning (damn those teenage kids and their 20 min showers….) then you may need to extend the runtime. A quick 30 minute top-up at lunchtime might be necessary, then perhaps another 30 mins early evening. As ever, some trial and error is often involved so tweak as you go.
Hot water and solar generation
Things can get a bit more complicated if you have solar thermal or a solar diverter from a PV system to your immersion element. In this case you will ideally need a cold cylinder at the end of the morning to maximise the capacity for the solar generation. An early evening top-up and an hour or so early morning should help to ensure that hot water remains available at all times. If, however, the sun is shining there should be no need to heat the cylinder up in the middle of the day.
Electric immersion heating
If you rely on off-peak electric immersion then it obviously makes sense to heat the cylinder during the off-peak period. Immersion elements will typically take 3-4 hours to heat the cylinder from cold. There is often a second immersion element that you can put on a separate boost countdown timer if necessary, i.e. if more hot water is needed through the day.
Hot water from a combi boiler
What about if you have a combi boiler? There’s much less to worry about as there is no cylinder or long leg of primary circuit pipework to worry about. It is, however, worth considering whether you have a ‘keep hot’ mode which maintains a small amount of hot water in an insulated cylinder contained within the boiler – this reduces the amount of cold water you have to run off before hot water reaches the tap, but inevitably uses more energy over a longer period of time. Therefore, setting this on a sensible schedule will help to reduce the standing losses overnight and during the day when you might have little draw-off.
Hot water timing for heat pump owners
For those owners with heat pumps, it is usually the case that they do not heat the cylinder and hot water at the same time. Doing so would mean making the heat pump much more powerful, expensive and – for much of the operating time – inefficient. It is more sensible to heat the cylinder outside of the times when heating is most necessary, i.e. early morning or evening. Alternatively you can intersperse hot water generation at short intervals throughout the day. Avoid ‘comfort’ modes that often result in a separate electric heater helping the heat pump out, usually at great expense.
If you found this useful you may be interested in our comprehensive Home Energy Survey which helps to identify many more ways in which you can save money on your energy use.
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