Flow rates/Pressure/combi


29 May 2006
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United Kingdom
Would appreciate some advice, i have a conventional boiler that's over 20 years old now, and am planning to get a combi. I live in a 1 bed flat on the 4th floor, the current system has F/E tank, CWS about 1 metre off the 'ground' and the hot water cylinder that sits underneath that (so, there are no tanks 'in the loft' its all on my 4th floor level). The incoming water pressure isn't THAT strong. It is being tested in the next few days, to determine the cold water mains pressure and flow rate. My question is, what kind of pressure/flow rates should i be looking for, in order to get decent rates out of a combi? I see that, for example, the Worcester Greenstar 24i Junior combi has a DHW Flow Rate @ 35c of 9.8 l/min, ok but does that assume that my INCOMING cold water mains is as LEAST 9.8 l/m??? I would imagine some of the flow rate gets lost/reduced inside the boiler/pipe bends etc. So, what cold mains flow rates/pressures sould be acceptable?
Many thanks
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You really need to get it measured at peak times 7-9am. Its common for the top floor flat to suffer from poor water flow. especially in a converted house.

Ideally a minimum of 1bar just to fill the thing up but really its flow rate that counts. I beleive 9 is the minimum you should recieve.

You shouldnt lose to much through the boiler aslong as flow restrictor is removed.
No, combis require about 0.6 bar to work properly, minimum. (Vary, see instrs)
I went to a posh (well, posh to me, £2300 pcm rent, one bedroom) recently refurbed 3rd floor flat the other day which had flow problems. The boiler's flow switch is one which gradually drifts higher in switch-on point over the years, but the problem was really the supply.
I told the landlord the only real answer was to install a cistern at ceiling level, probably above the stairs, and pump it. It would fill up when no water was being used and the pressure was higher.
Landlord rather deliberately asked what it might cost. Builders, plumber, decorators to hide pipes - so I estimated maybe £2000, maybe more.

Yes, she said, I think it may be rather more, because we had to pay £3000 to have one taken out in the refurb, including replastering the walls and ceiling.
Who advised taking it out?
The architect. :rolleyes:

Boxbasher will confirm if it's 0.7bar and 7 litres/minute the Water companies all have to supply, but remember that's at ground level, not 4 floors up.
0.7bar = 7metres, 4th floor = 12 metres = no water.

Only other way is a break tank and pump set in the basement.
OK i've tested the water at 7am on Monday morning... I get 1.3 Bar static pressure, which drops to 0.2 bar when i open the kitchen tap, and the flow rate is approx 6 ltr/min. Do you think this is too low for a combi? I know the shower at 6 ltr/min would be quite weak, but at the moment we only get 7 ltr/min anyway via the current conventional tank system so that's no great loss... My only conern is 0.2bar 6ltr/min would be too low for a combi to even operate and the thing would shut off!!
Any comments would be much appreciated, i'd really love to be able to get rid of the cisterns and cylinders... By the way, there's no option to put pumped systems in the basement/roof, we live in a 1985 built 5 level apartment building, 32 flats, each flat has its own CWS + HW cylinder.
Many thanks
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For example, i'm looking at the specs of the Biasi Riva Compact 28HE (M90E.28S) and it says

for DHW: Minimum working pressure: 0.3 Bar
Minimum flow rate: 2.5 ltr/min

and i get 0.2 Bar, 6 ltr/min, so would this extra 'allowable' 3.5 ltr.min flow that i get above the min spec compensate for the 0.1bar loss in pressure?

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