Roger Russell

Mains hot-water Combination Boilers, condensing and standard. Compared to a conventional atmospheric hot water storage system.

The conventional Hot Water System in a 2-3 bed roomed house with 1 bath room, would be a 36” x 18” vented copper indirect cylinder with 25.7 galls/117 L of stored hot water @ 65C. (2 bath rooms 48” x 18” with 35.6 galls/162 L).

The typical Combi boiler installed in a 2-3 bed roomed house with 1 bathroom would be a 100,000 Btu/hr/29.3kW per/ hr output. The central heating requirements would only be 40,000 Btu/hr/11.7 kW per/ hr. Because the Combi boiler is heating hot water drawn from the tap instantaneously, it needs full heat output into water. In winter the incoming mains water could be 9C. Temperature rise for a 100,000 Btu/hr/29.3kW Combi giving 2.62 galls/min/11.9 L/min of domestic hot water at a 35C temperature rise would give a 44C showering temperature. This is single point of use. If another hot tap was opened the flow from both taps would be reduced, if not the temperature would be reduced. This does not normally happen because a flow restrictor is fitted in the Combi. If the full 11.8 L/min were not drawn the hot water would rise to a maximum of 60C - 65C, which on most Combi’s is adjustable by the hot water thermostat or the summer/winter switch on the Combi control panel. Showers on Combi’s without thermal stores should be thermostatic or the compensate type to control fluctuations in temperature. If any other tap including cold taps, some cases next door, is used lower pressure = hotter water. Some cases when next doors cold taps are turned off, higher pressure = cooler water.

To compare this with the conventional Hot Water System above, a tap will normally be designed to pass 7.5 L/min basin, 15 L/min bath, 12 L/min shower and sink at 1bar. So when more taps are used at the same time on a conventional system the reduced flow, if any, would be due to incorrect pipe sizing or insufficient atmospheric head pressure. The temperature would always be constant regardless of how many taps are use or not. Combi’s fitted with a thermal store can be used to give the same or better hot water flow-rates than a conventional system. A thermal store with 13L at 65C will give stable temperatures with one or more taps in use. 55L at 65C much higher flow rates are available with 18 L/min/3.9 galls/min at 45C more than two taps can be used without loss of flow providing a new 25mm mains water supply is installed.

More than this... UNVENTED cylinder, heated by remote system boiler, at 3bar - 6bar is used with flow rates higher than conventional Hot Water Systems supplying 30L/min/6.9 galls/min at 45C. In new build large vented thermal stores are used, 205cm height x 66cm width x 60cm depth, with 200 litres at 85C of primary stored water. The DHW is supplied directly from a 25mm mains supply, passing through a high performance plate heat exchanger, giving 35 L/min at 45C, based on an incoming supply temperature of 10C.

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+44(0)1223 843230
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