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 instantaneous combis are mains-fed and serve several outlets through pipework. Storage combis can include an internal hot-water store. 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. These figures assume the stated heat output is being transferred to the water. Colder incoming water reduces the maximum flow available at the same outlet temperature. A fixed-output heater runs cooler if the flow increases, while a modulating appliance can adjust its output to hold the set temperature until it reaches a limit. Two outlets share the output; neither need run cooler as long as the appliance can still meet the demand.

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

Inside an instantaneous plate-exchanger combi

1. On this design, a turbine at the incoming cold-water connection registers a tap draw-off. The control uses its signal to request hot water. A trickle below the model’s threshold will not start the burner for that draw-off; one current range specifies 1.9 litres per minute.
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.

Some models preheat the hot-water path in comfort mode and may fire without a tap running. The flow threshold and diverter sequence above are for that instantaneous design; a storage combi has its own charging and delivery controls.

Electric showers

A conventional electric shower such as the Mira Sport uses fixed heating power at each selected setting and changes temperature by adjusting water flow. Other models can use electronic control, so check the actual design. The Mira Sport instructions require a cold supply within the specified running-pressure range and limit the inlet to 30 °C; do not connect that model to a hot feed. Its shower head and spray nozzles should be cleaned and descaled regularly.

Because the power is fixed, the outlet temperature follows the incoming water. In summer the mains arrives warmer, so the same setting runs hotter, and once the temperature control is fully to cold the flow cannot rise any further. If the shower still cannot be made cool enough, Mira's fault guide says to turn the power control down to Eco. Slowing the flow, with a reducing valve or by closing a valve part way, makes it hotter instead.

The Mira Sport has a pressure relief valve inside its case to limit damage if the outlet is blocked or the water in the shower freezes. When it operates, water runs out of the bottom of the case instead of the shower head, and the heater tank has to be replaced once the cause is put right. Mira also rules out anything on the inlet that would hold pressure in the shower: a double check valve on the inlet causes a pressure build-up, so it must not be fitted, and nor may a servicing valve with a loose jumper. The servicing valve that is fitted must be full bore. Where backflow protection is needed beyond the hose retaining ring, the double check valve goes on the outlet.

Thermal stores

A thermal store is a store of heat. Its cylinder holds primary water heated by a boiler or another heat source. In a mains-fed thermal-store arrangement, the domestic water takes heat from the stored primary water through an exchanger. Depending on the design, this may be an internal coil or an external plate exchanger. The primary water is not delivered to the taps. One range charges its store to about 75 °C and fits a thermostatic mixing valve on the hot outlet, set to about 55 °C. In that range, Gledhill's Torrent, the store is open vented and the mains water is heated only as it passes through the exchangers, which carry their own small self-replenishing expansion chamber. So, unlike an unvented cylinder, it needs no expansion vessel and no temperature and pressure relief valve, and Gledhill says Section G3 of the Building Regulations does not cover it. 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. The usual combi, unvented cylinder and thermal store all depend on the incoming main. Assess any other supply arrangement on its own merits; a bigger appliance cannot make up for a poor main.
• 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.

Open outlet water heaters

A small vented (open outlet) water heater under or over a sink is fed through its tapset, which controls the cold inlet; the outlet spout is never closed. Heatrae Sadia's tapset instructions explain that the tapset acts as the heater's open vent, so it is normal for the spout to drip while the heater warms, as the water expands. That is not a fault, and the user should be told. Where the heater is mounted oversink, above the tapset, a check (non-return) valve must be fitted between the tapset and the heater's inlet; without it the heater can partly drain down and eventually trip its thermal cut-out.

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 while hot water is drawn, with lukewarm taps: check whether the diverter valve is letting primary water through to the heating. Check the rest too: the valve may not be the only fault.
• Falling hot-water flow or temperature in a hard-water area: consider scale in the plate exchanger, then check the supply, heat transfer and controls before deciding the cause.
• Outlets starving each other: measure simultaneous demand against available flow and working pressure, and check the distribution. The size or condition of the main, and other draw-offs at the same time, can play a part.