Radiators hot but no hot water?

Question

I have a Knightsbridge combi boiler
all radiators are working and are very hot, however the water starts off hot when the tap is running but within seconds runs warm then cold. I have tried turning the whole system off - nothing. I have tried opening the value on the boiler nothing. I have also bled all the radiators nothing.
I have a service manual which shows
GAIA

Models G.C appl No.

424S 47 970 08
428S 4797009
424 RS 41 970 04.

Can anybody please suggest some help

Answer 1

Sounds like the divertor valve is stuck.................. you will probbly need to change it. But where you get parts for that make of boiler I can't think!

Answer 2

First your boiler is a GAIA 424/8 S or RS don't know the model off hand but most combes are very much alike. Don't know were you got "Knightsbridge" from.
Your problem sounds like a diverter valve diaphragm at fault. With the central heating turned off, look at the valve unit (large brass block 3 large pipes and two small ) on one end is usually mounted a switch block (black in colour) that is actuated by a pin that is pushed out from the valve when the hot water tap is turned on, is this working if not then its the diaphragm. Using a small screwdriver gently operate the micro switch does the boiler fire up now. If yes then the electronics are OK.

Answer 3

If it had been a tanked system, I would have said it was the three-way valve in your cylinder cupboard. As its a combi, it sounds like the boiler is overheating and switching off. Combi boilers store a small amount of preheated water within themselves and then the boiler fires to heat the cold water entering. If it detects or thinks it detects an overheat situation, it shuts down for safety. Have the Thermostat and Thermocouple checked out by a Corgi Plumber.

Answer 4

the initial hot water that you get is from the heating pipes under the floor boards warming the hot water pipes , this is why it does not last long , there is a valve on the boiler controlled by a thermostat and the controller which shuts off the heating and transfers all of the boiler heat to the hot water heat exchanger when you open the tap. sounds like something has gone wrong with it

unless you are diy expert call a plumber do not touch the gas as you MUST be corgi registered to do this

Answer 5

Sounds very much as though your bypass valve has failed. Many combi boilers use this and basically what it does is senses when your hot tap is open and bypasses the heating system, sending hot water through the heat exchanger direct to the tap. You will unfortunately need to contact a heating engineer to confirm this as it is a part which, although can be replaced DIY, to make sure you get the right part, the engineer will be able to sort this. Hope this helps.

Answer 6

I'm no expert but am a longtime boiler sufferer. Do you have a hot water tank or is it a tankless system. We had a similar problem with our boiler system. We could never seem to syphon off enough hot water to take a shower or bath with the tankless system. In the summer forget about it...cold showers all the time. We had an "aqua booster" installed. Which is basicly a hot storage tank that is hooked up to the boiler system so we could finally have a hot shower. It costs about $1000 but I think it was worth it.

Answer 7

Don't know that particular model but all boilers have a flow valve this valve splits the water between the radiators and the hot water tank, but I don't know what it will look like in your model. Sorry it isn't much but hope it helps in some way.

Answer 8

couple of possibilities
1) the hot water flow switch has failed and the boiler doesn't know there is a demand for hot water
2)the temperature sensor has failed and the boiler thinks the water is hot enough already
3) the diverter valve diaphragm has failed and cannot make the change from CH to DHW

Answer 9

I've experienced this problem and can only say that I hope my answers make sense since I might not know what I'm talking about.
First time, the problem was the pump. It needed to be replaced (also told it wasn't large enough).
The second time, my tub faucet was taken out and maintenance replaced something behind it, put my tub faucet back on and it worked.

Answer 10

Not sure on that model but I have another model and it has a seperate tempreture setting for the water and the heating. It is possible that it is either turned down or has failed in the low position.