Tap flow rates

[fonzy]

Just had a new combi boiler fitted from a previous hot water tank installation.We have a Ferroli Maxima 35c boiler which I think is high flow.

Problem is when toilet is flushed or water is needed by the washing machine, it stops cold water coming out of the bathroom taps. I've sort of read that this type of behaviour (reduced flow) is expected with combi systems but is it normally that bad (ie. flush toilet and no cold water flow to sink in bathroom even if nothing else on).

Secondly, is there a way to restrict the flow rate into toilets/appliances so at least there's some pressure into the taps which is where you would want the instant access?

[IMM]

Combi's do not reduce flow. If the mains pipe is fine then usually all is OK. When you have problems It is best to:

1. Replace the mains stop c*ck with a full bore c*ck.
2. Run 22m to the combi only.
3. After the stop c*ck tee off and run a cold pipe to the cold only taps.
4. The cold pipe for the shower take from the cold supply just before the combi.
5. In-line isolation taps on the cold taps and hot taps to dishwasher & w/machine. Throttle these back to suit.

What you have done is have the combi take the line of least restistance is the cold water flow from the street.

If still problems, then consider having the mains pipe replaced back to the street with large bore plastic pipe.

[John Rumm]

Is this only when the combi is suppling hot water, or at any time?

The reduced flow that you often hear about is typically a reduction in flow of *hot* water when compared to a stored water system. With no hot water being used, the cold flow rate to the rest of the house should be the same.

With the hot running, your combi will be able to heat the hot water to a usable bath or shower temperature at a rate of approx 10 - 14 L/min (the colder the ground water coming into the house the lower the rate). There ought to be enough mains cold water flow rate to supply this and at least another cold tap at the same time.

If not then your rising main is not adequate for the setup you currently have. (Plus the person who fitted the combi without checking the flowrate first was a cowboy!)

A quick thing to check, make sure the main stopcock is turned full on.

You can fit a service valve before each of them and then partially turn it off to limit the flow rate.

You may also also improve matters by improving the supply from the main. This can be as simple as fitting a full bore valve in place of the more typical stopcock, or as extream as replacing the whole main back to the street!)

[Andy R]

What was the flow rate and pressure before the combi was installed? If you used stored hot & cold water beforehand this can mask a very poor flow rate from the mains. If you've got 13mm mains into the house then IME you simply can't get enough water through that pipe to reliably meet demand in anything other than a small house/flat with no more than two people living there. Your case does sound extreme though, if you're lucky it could just be something simple like the mains stopcock isn't turned on fully. If you're unlucky it could cost a small fortune putting larger bore pipe out to the mains in the street.

Go back to your plumber, ask him what the flow rate was before the boiler was fitted, on what basis did he feel it was adequate and why a combi was the best solution for your needs. When you get up off the floor and after the black eye's faded you'll probably find he fitted the combi cos he could then justify charging for a pressure flush and additional pipework yet the wiring's so simple it didn't tax his limited electrical skills too much.

Rgds

Andy R