Hot water systems

Hot water often uses more energy than heating in a Passivhaus so it pays to concentrate on designing an efficient system. Use the DHW sheet in PHPP at an early stage in design, since the losses here are included in the summer overheating calculation.

Hot water heat losses fall into three categories: cylinder; continuous (secondary) circulation; and draw-off dead leg. In addition there is the primary heat loss between the boiler or heat pump and the cylinder. To convert the manufacturer’s hot water cylinder heat loss data in kWh/24hrs into Watts, multiply by 1000 and divide by 24. 

Circulation loss only applies if you have a pumped hot water loop – not normally necessary in houses. If you do have one, then insulation levels and pipe length are the important factors to optimise.

Draw-off losses represent the cooling down of the hot water pipe and its contents after each period of use. Here insulation is not significant – the pipe will cool down anyway: the important factors are pipe diameter and length. Shorter pipes have the advantage of lower pressure loss, which means it may be possible to use smaller diameter pipework. See AECB water standards guidance for details.

Cylinder location is the key to efficient hot water distribution. It needs to be as central to the various hot taps as possible in order to minimise the total draw-off pipe length and avoid the need for secondary circulation. A house layout which groups kitchen, utility and bathrooms in one area is also very beneficial. To minimise the primary pipework heat loss the boiler or heat pump also needs to be as near to the cylinder as practical.

Combi boilers are an alternative in smaller dwellings. Hot water output rate is limited so only use these in single bathroom dwellings. The advantage is that you get rid of the bulky hot water cylinder, connecting pipework and associated heat losses. The only combi boilers which completely avoid hot water storage are gas (or LPG) powered. Oil combi and heat pump combi boilers actually have a hot water tank built in.

Thermal stores are an alternative approach to hot water storage where hot water is generated on demand via a heat exchanger. Don’t use these unless you have to – say for a wood burning boiler. The store has to be kept at a significantly higher temperature than a normal hot water tank, leading to increased heat loss and reduced boiler efficiency (never use with a heat pump). And in the end the hot water performance in terms of flow rate and temperature is generally worse than the standard hot water cylinder. 

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