Water flow rate problem and Combi boiler

[Andrew Beck]

I think my house has a water flow rate problem and I'm looking for a solution.

The house has a combi boiler and everything is run off the mains water.

The water flow rate from a single cold tap upstairs is about 10 l/min The water flow rate from a single hot tap upstairs is about 8 l/min

The difference is probably due to the resistance of the combi boiler.

The problem is that if you turn on a cold tap the hot water flow rate reduces. If you turn on a hot and cold tap the flow rate is about 9.5 l/min from the cold and about 0.5 l/min from the hot. The low flow rate of the hot causes the combi boiler to stop heating the water due to inefficient flow rate.

Solution Ideas, will the work?

1) A pressure equalisation valve on the hot and cold water supplies?

2) replace the cold water supply pipe to the main pipe in the road as the input in to the house is only 15mm Not really an option.

3) run the cold of a tank in the loft that is pumped?

4) run the cold and combi of a tank in the loft that is pumped? Before the combi was installed by the previous owner there was a cold water tank in the loft.

5) Any others?

[Set Square]

As I see it, there are only 2 options:

A) Sort out the cold supply to the house, or

B) Throw out the combi, and revert to a stored hot water system

[There are some rules of thumb for cold flow rates needed to sustain mains pressure hot water systems. I can't remember exactly what they are - but I'm sure that they're closer to 30 litres/min than to 10! Whoever fitted a combi clearly shouldn't have done so without first sorting out the cold supply].

Cheers, Set Square

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[IMM]

Check the flowrate at the kitchen tap. Replace the stop c*ck with full bore version.

Check again.

If still bad, replace the water main back to the street.

[Andy Hall]

See (2).

.andy

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[IMM]

It must be made an option.

[Andy Hall]

Sigh....

.andy

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[Ian Middleton]

A friend of mine had to have your option 2 done (22mm new supply pipe fitted) when he got a combi boiler fitted as the flow rate into the house was not high enough. House previously had no cerntral heating, just a tank in the loft feeding cold taps and immersion heated hot water tank.

Also had to have quite a bit of internal plumbing re-arranged so that no matter what was happening with the cold taps sufficient flow was left for hot water.

Anyway the lodgers he had hated it as it took ages to fill the bath compared to the tank and immersion heater. Mind you they now had heating !!!

[mike ring]

"Andrew Beck" wrote in news:41363feb$0$8468$ [e-mail]:

I think option 3 and 4 have a chance.

I may be talking rubbish, but I think a pumped supply from a reservoir tank would help.

Perhaps if you ran the cold off the main, and the combi off the tank with a pump, you would get a result.

My only experience(!!) is only running a pumped electric shower off the cold tank because of crosstalk between the shower, bog cistern, cold taps etc.

This has provided a perfectly buffered feed, and on a larger scale might suit you.

You would need a suffieciently beefy pump (not a problem, I think) and be sure the cold tank is big enough to take up the slack while the combi is in use, perhaps lots of baths would require a big tank.

mike

[IMM]

Fit an accumulator, then no pumps and high pressure showers all around.

[Ed Sirett]

Before deciding what to do I would want to work out what the problem with the mains is.

If the standing pressure is low then some sort of storage tank/pump will be needed, or even reverting to a conventional system (no need to throw the boiler out - just add the HW cisterns and cylinders).

If the standing pressure is 2bar+ then the problem really is the supply and/or internal pipework.

You just might find that there is a stack of crap stuck in the mains pipe work somewhere (on the stop c*ck most likely) whose removal will fix a lot of things.

[IMM]

On a poor performing old mains pipe I once removed the house stop c*ck and replaced it with a full bore one. I thought there might be a bock age in the line, so inserted a large bore hose pipe on the new full bore stop c*ck. Lead the hose to the garden and tuned on the water at the street. The crap that came out !! I never measured it, however the water pressure was excellent after.

[Andrew Beck]

It was British Gas that fitted the Combi for the previous owner.

The boilers manual says minimum flow rate for the boiler is 2.7 l/min The mains water pressure for maximum flow rate is 0.8 bar

I know the pressure of the mains above 2.5 bar (as it can fill the closed heating circuit to this pressure)

Andrew

[Set Square]

Yes, but I fear that you're confusing static pressure with pressure when there's a significant flow. From what you said earlier, the flow to the combi more or less stops when you turn cold taps on. This indicates that, when water is flowing elsewhere, the pressure drops to a level which is insufficient for the combi - presumably to below the required 0.8 bar. This is almost certainly because of restrictions in the supply line - which will cause a pressure drop which is dependent on flow rate.

To illustrate with a guesstimate example: Attach pressure gauge to pipework just above the stop-c*ck. Static pressure, with all taps off: 2.6 bar. Turn kitcken tap full on - giving 10 litres per minute, or whatever. Pressure now

0.6 bar - because a flow of 10 litres per minute causes a pressure drop of 2.0 bar down your 15mm supply pipe. [These aren't the actual figures, of course - but I have a shrewd suspicion that they ain't that far out].

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[Andrew Beck]

You are right, this is static pressure.

The boiler spec says it requires a pressure of 0.3 bar for minimum flow rate (2.7 l/min)

Interesting I have found the following quote about water flow rate on my water company's web site

" A simple check: provided the cold tap in your kitchen (whilst running at full bore) can almost fill a 2 gallon bucket within about 1min, your flow and pressure is satisfactory. "

is about what I'm getting. My problem is an unbalanced flow, all cold and no hot.

What I'm looking for is a balanced flow i.e. the cold water flow is no more than 50 % of the total water flow.

Andrew

[Andy Hall]

They would say that.

This works out to slightly over 9 litres per minute.

The statutory minimum for water companies to meet is.. you'll never guess.... 9 litres per minute,

Of course it isn't satisfactory - a more reasonable figure is twice that. The trouble is that in some places the water companies would have to spend a fortune to meet 20 lpm.

You could try a simple solution of introducing a restricting valve on the cold path to force more flow through the hot path; or perhaps a pressure equalising valve. The trouble is that you haven't got much to work with.

The best solution would be to look into getting the supply from the street upgraded.

Failing that, another solution would be to install a conventional hot water cylinder with a storage tank in the roof and fill that from the mains. Heat this cylinder from the heating circuit by adding a divertor valve and cylinder thermostat. Use the stored hot water for baths and showers and the combi for less critical applicatiions like the kitchen.

.andy

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[Set Square]

Presumably the combi works ok on just hot - but stops working when you turn on a cold tap?

Water will always take the line of least resistance. It is fair to assume that the path through the boiler presents more resistance than the path to an open cold tap. What happens if you partially close the cold tap and reduce it to a trickle, does the hot pick up again? If so, you can probably get a *balance* by adjusting the cold tap - but you still might not get an acceptable *total* (i.e. hot + cold) flow rate if the basic cold supply is inadequate.