What mains water pressure do I need for a combi boiler ?


Postby jardine » Tue Aug 03, 2010 5:32 pm

WE are plnning to re new our very old gas boiler. What water pressure is required for a combi boiler to work efficiently? How can we measure this water pressure?. I have measured the water flow , but that does not give me the water pressure. We have been given conflicting advice from different companies. 2 companies measured the FLOW RATE . One company turned on the tap and judged "by eye" that the WATER PRESSURE was not suffieient. None of the plumbers MEASURED the water pressure or told us what the water pressure needed to be.
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Postby plumbbob » Wed Aug 04, 2010 8:21 pm

Measuring static water pressure is not going to give you a complete picture of whether a combi will work properly or not. What is important of course is flow. As flow rates increase the pressure falls. Now if your incoming main is scaled up, then the restriction will restrict the flow and the pressure will be practically zero.

The average house main will have a static pressure of 3 bar or so. Some can be as high as 5 or 6 bar.

To test the pressure, you need one of these which connects to a washing machine or outside tap.

http://www.screwfix.com/app/sfd/cat/pro ... 2&ts=53166
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Postby eggers » Mon Aug 09, 2010 8:12 pm

weir guage will also measure flow rate, but there is no minimum flow rate as such, because each brand of boiler will have different requirements.
If you find a boiler you would like, then the fitting instructions will say how much water flow is required, for example a greenstar 42cdi will produce up to 18 litres of hot water a minute, where as an alpha HE25 will only produce 9.9 litres a minute. It largely depends on the size of the house, and the minimum requirements of your chosen boiler.
Personally i wouldn't fit a combi boiler, unless you really need the space, as a conventional system is alot more flexible, for example, you could have solar hot water fitted at a later date, have shower pumps fitted, have an immersion to heat the water if the boiler breaksdown, as if u have a combi, you will have none of the above.
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