Instantaneous hot water, combi boilers and thermal stores

Heating water as it flows

An instantaneous appliance heats mains water while it passes through, instead of heating a stored volume. Single-point heaters serve one outlet and are fitted at it. Multipoint heaters and combination boilers are mains-fed and serve several outlets through pipework. Electric showers are instantaneous heaters built into the shower itself.

Each has a fixed maximum output, so the temperature rise depends on the flow:

output (kW) = flow (litres per second) × 4.19 × temperature rise (°C)

A 24 kW output raising a 5 °C winter main to 40 °C, a 35 °C rise, gives 24 ÷ (4.19 × 35) = 0.164 litres a second, about 9.8 litres per minute. If the main warms to 15 °C in summer, the rise is only 25 °C and the same output gives about 13.7 litres per minute. So:

• colder winter mains water means less flow at the same temperature;
• a tap opened fully runs cooler, and turning it down makes the water hotter;
• two outlets at once share the same output, so both run cooler.

Makers therefore quote flow at a stated rise: one 30 kW combi gives 11 litres per minute at a 40 °C rise.

Inside a combi

1. A flow sensor, often a small turbine in the cold inlet, detects water being drawn. Below the minimum flow (1.9 litres per minute on one current range) the burner stays off, so a tap barely cracked open can run cold.
2. The diverter valve sends the boiler's primary water to the plate heat exchanger instead of the radiators. This is hot water priority: the heating pauses during a draw-off and recovers afterwards.
3. In the plate heat exchanger, primary water flows on one side of thin plates and mains water on the other. Heat passes across, but the two never mix, because primary water can contain inhibitor and drinking water must be kept separate from it.

In comfort mode some combis keep the hot water side warm, so the boiler may fire with no tap running; eco mode heats only while water is drawn.

Electric showers

An electric shower heats cold mains water with an element of fixed power at each setting, and its temperature control works by changing the flow. It must be fed from the cold supply at the running pressure the maker asks for; one common range limits the inlet water to 30 °C, so it cannot take a hot feed. Its shower head and spray nozzles should be cleaned and descaled regularly.

Thermal stores

A thermal store is a store of heat. Its cylinder holds primary water heated by a boiler or another heat source. Mains water passes through a coil or heat exchanger inside the store on its way to the taps, so they get mains-pressure hot water while the stored water never reaches them. One range charges its store to about 75 °C and fits a thermostatic mixing valve on the hot outlet, set to about 55 °C. Its hot water still depends on the mains flow and pressure.

Choosing stored or instantaneous hot water

At the survey, weigh up the household's hot water use, how many outlets run together, how far they are from the heat source, the space available and the incoming main.

• A combi suits a smaller household with modest demand, one bathroom, a good main and little space for a cylinder.
• A stored system (a boiler with a cylinder) suits larger families and several bathrooms used at once, because the store can feed several outlets together.
• Where the main is poor, an open vented cylinder fed from a cold water storage cistern depends least on it, because the taps draw on stored water. Combis, unvented cylinders and thermal stores all deliver mains water, so they are only as good as the main, however large the appliance.
• A remote outlet a long way from the heat source can have its own single-point heater, so the hot pipe to the tap stays short and less water runs to waste.

Checking the incoming main

Static (standing) pressure, read with nothing flowing, can look healthy on a small or furred main. Measure the flow rate and the working pressure with a tap running, and compare them with the maker's figures: one 30 kW combi needs 2.3 bar working pressure for full flow and 0.2 bar just to operate. Check that the main can feed the boiler while toilets, a washing machine or cold taps are also drawing water.

Hard water

In England, Approved Document L treats water above 200 ppm of calcium carbonate as hard. In hard water areas the feed water to water heaters and to a combi's hot water circuit should be treated, for example with an in-line scale reducer on the cold supply. Scale forms faster at higher temperatures, which is one reason makers limit the hot water temperature. Inhibitor protects only the heating side; it never reaches the mains side of the plate heat exchanger.

Common complaints

• Radiators warming whenever a tap runs, with lukewarm hot water: a diverter valve sticking part-way, often noticed in summer.
• Hot water flow or temperature falling off in a hard water area: a scaled plate heat exchanger.
• Outlets starving each other: an undersized main or unbalanced outlets.