Minimum flow rate required for unvented domestic hot water system

John Aston · 2004-10-05T23:40:02+00:00

Many thanks to Andy Hall and others for their help earlier this year on thedesign of my new plumbing system ( etc.).Unfortunately, thanks to a few months illness, I'm still at it - with a fewissues to resolve.A question regarding an unvented domestic hot water system: Cambridge Watertested my incoming main supply and informed me that the pressure = 2.8 barand the flow rate = 25 litres per minute. Is this sufficiently high for anunvented supply feeding a family home containing two power showers, a bath,five sinks, three toilets, a washing machine and a dishwasher?My feeling is that it isn't. At worst, I would guess that only one showerand the bath would be running concurrently. Even so, a power shower consumes15 lpm and a bath consumes 20 lpm.Is there any way around this problem? A pump and hydraulic accumulatorsounds expensive. Due to a loft conversion, there is not any convenientspace for a cistern, although I'm thinking that I may have to create some...[For information, the hot water will be heated indirectly in a 250 litretank by a condensing boiler. The mains supply pressure will be reduced (to 2bar?) by a water softener, although a "high flow" softener, such as theKinetico 2020c HF, will be fitted to minimise this problem.]Presumably the central heating could operate as a sealed system, even if thehot water tank is vented?TIA

Oct 05, 2004 43 Replies

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Peter Clarke 10 years ago

replying to IMM, Peter Clarke wrote: interesting, I have a similar issue. But I would add that my concern is people downstairs at breakfast time flushing loos and using mains cold at the kitchen sink; this will reduce pressure and flow to the showers if everything depends on incoming mains. In my case I am going to be using 2 tanks in the loft to supply the 2 ensuites only. The main drawback will be the heat loss from the existing combi , on its long journey to the new DHW cyclinder that I am installing for the ensuites, and that in summer some of my central heating flow/return will be running hot just to get heat to the DHW cylinder.

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Graham. 10 years ago

Peter, please read this.

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The Natural Philosopher 10 years ago

Shouldn't do.

Not if you spilt 24mm cold from right after the stopcock to geed everything but the hot water tank. Roof mounted tanks are always less pressure than mains is. So as long as you have adequately sized pipework you will always do better with mains pressure than a header tank.

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