combi boiler loosers hot water presure when you run the cold tap?

Question

hot water runs ok as soon as cold water turned on hot water just dribbles,cold water presure runs at 4bar flow rate is 12lpm

im not messing with boiler im asking a seriouse question

Answer 1

Unfortunately this is an inherent problem with combi boilers (by the way, the heating operates at between 1 and 1.5 bar NOT the hot water, you don't need a pressure reducing valve!).

A combi boiler is a central heating boiler and an instantaneous hot water heater all in one. Because of the heat input any instantaneous water heater will heat cold water by a certain number of degrees, it varies between boilers but lets say for arguments sake that it's 40 degrees.

In the summer, the temperature of your incoming main might be as high as 20 degrees, so when it's heated it comes out of the hot taps at 60 degrees, in the winter it could be as low as 5 degrees so when it comes out of the hot tap it's only 45 degrees.

To overcome this problem of cooler hot water in the winter, most installers will cut down the flow rate through the boiler, the slower the water passes through the boiler, the hotter it will be coming out - imagine passing your finger through a candle flame, do it quickly and you don't get burnt, do it slowly and you burn your finger.

Because the flow rate has been cut down, the water going through your boiler is no longer at mains pressure, therefore when you turn on a cold tap which is at mains pressure, the flow rate through the boiler is reduced even more (and it's likely that due to the low flow rate the boiler will cut out and stop heating the water).

You can have the flow rate adjusted, but you will have cooler hot water in the winter.

The solutions are -
1/ Don't turn on a cold tap when you're running hot water!
2/ Have a conventional system installed (hot water cylinder, tanks in loft etc)
3/ Have an unvented (mains pressure) hot water cylinder installed, your existing boiler would be able to heat it and you get mains pressure hot water at your taps
4/ Have a high flow rate output combi installed (i.e a Worcester Highflow)

Probably not the answer you wanted, but hope it helps anyway.

Answer 2

Most combi boilers operate at between 1 and 1.5 bar. However, whatever your cold water supply pressure, when you run the cold taps you will reduce the pressure of the available supply to the boiler. This is a common problem with combis. You may need your boiler serviced to check the pressure sensors or have the operating pressure threshhold adjusted. It is wise to have a Corgi registered engineer service your boiler each year. Another indication of need for service is when the boiler does not ignite when the hot taps are run. I had this problem which was again due to a pressure sensor requiring replacement. An engineer may also advise you whether you may need a pressure reducer on your cold water supply which can ease the problem.

Answer 3

Your Combi is supplied by the same cold mains inlet pipe into your house that supplies all your cold services..you really need 3 bar of pressure comming in but no more than 6 bar.. If your mains is quite low in pressure, the more taps you open, the lower the flow will be through the boiler. Also combi's suffer from scale (older ones quite badly). My advice...make sure all stopcocks are fully open,and check you incomming cold mains pressure,either with a "flow cup" or gaugue..If ok then..goto furthest hot tap from boiler,open it about 50% and watch the flow of water.If it starts to slow up and look cloudy after a few seconds, then your boiler is scaled. This can be removed depending on bolier condition.. Any probs email me [e-mail]

Answer 4

if its a coil in boiler then it probably needs to be cleaned. if thats the case you should have a licensed plumber take care of this as it involves running acid thru it.

Answer 5

has to be a water pressure problem.Either a restriction in the hot water side, or supply side.