Shower hot water flow rates and heating system types – why combi boilers can sometimes disappoint

Many people enjoy a “nice hot shower”, meaning a shower that’s hot enough with an adequate hot water flow rate (in some cases, almost powerful enough to strip paint). This short article looks at the various types of heating and hot water systems and their pros and cons for showers.


The worst option for a shower is one that’s gravity fed with an inadequate head of water. “Gravity fed” means that the water pressure is provided by a cold water cistern (traditionally in the loft) above the shower, which feeds the shower with cold water directly, and also forces hot water from a storage cylinder. (The hot water is heated by an immersion heater or “regular” or “heat-only” boiler or indeed solar or other heat source.) The pressure or “head” is 0.1 bar per metre, measured vertically from the shower head to the surface of the water in the cistern. For an upstairs bathroom it can be impractical to obtain as much as 0.2 bar, and flow rates can be less than exciting.


A solution to this is to add a pump to the system. A good pedigree pump, installed correctly, with adequate supply pipework, can deliver very high flow rates – 20 litres/minute or more.


Note that hot water is heated steadily to the temperature set by the cylinder thermostat, and the flow rate and temperature are independent. The size of the hot water cylinder just has to be adequate for the water drawn off.


Mains water pressure is generally in excess of 1 bar – equivalent to having a cold water storage cistern more than 10 metres above the shower, so this looks promising. Enter the combi (combination) boiler, which is fed from the cold water main, so has plenty of pressure behind it. Unlike the other systems discussed here, all of which have stored hot water, the combi heats the water “instantaneously” as it passes through. A remarkable piece of technology, the plate-to-plate heat exchanger, transfers virtually all the heat available from the boiler to the water. The limit on hot water flow rate is set by the laws of physics.


Here’s the thing: for a combi, flow rate and water temperature are completely inter-dependent. To have a greater flow at a given temperature, or hotter water at a given flow rate, requires gas to be burned more quickly – a more powerful boiler will deliver more than a less powerful one. An article on my Tech Notes page shows how to do the simple calculation. If a combi is running at full power for the shower, there’s nothing in reserve for hot water to be drawn elsewhere, from a basin tap for example.


The important message is that where shower performance is very important, the power rating of a combi boiler will be determined practically entirely by the required hot water flow rate for the shower, so look very carefully at this parameter (and it doesn’t matter which manufacturer you look at, it’s set by physics). Under-size the boiler, and disappointment will follow. Incidentally, the resulting boiler will almost certainly be very much more powerful than needed for heating the rooms (“space heating”), where it will run at much less than full power.


As an example, to obtain a flow rate of 15 litres/minute with a 35 celsius temperature rise (incoming cold water at 10, hot water 45 celsius), requires a boiler with 37 kilowatt net input – pretty large – and can still be outdone by a well-installed shower pump.


The final option considered here is the un-vented hot water system. This has a hot water storage cylinder, but instead of being fed from a storage cistern with limited pressure, it is fed from the cold water main via pressure regulator which sets the pressure at perhaps 2.5 bar. The hot water is heated by a regular boiler (or other heat source as discussed above), which can be relatively modest in size unless the hot water used is required to be replaced quickly. As before, flow rate and temperature are independent. Provided the mains pressure is adequate (not always the case, particularly in multi-storey dwellings) and pipework is adequately sized, an unvented hot water system will deliver an excellent flow rate at a temperature set by the cylinder thermostat.


Un-vented hot water systems incorporate a number of safety devices, and though it seems to be little known or observed, should be serviced regularly, ideally annually.