Bathroom Hot Taps Playing Up

[scottpetrie]

Hi
We recently had our gravity fed system replaced with a combi in the garage. We are in a bungalow. I have noticed that the bath and bathroom sink hot tap, if you fully open it, doesnt get fully hot. It seems to go hot ish then cool then hot ish. If I turn it down to about half way, it gets boiling hot but only half pressure. Could this be the taps that are the issue as the shower and kitchen tap is fine? They are the waterfall mixer type

 

[jj4091]

I think that is the way most combi's work, the greater the flow rate then the cooler the water and vice versa. I'm not a plumber but ours have always operated like that. Perhaps if it is a long run from the garage that might make it worse. Are your bath and bathroom sink fed by a larger diameter pipe than the shower and kitchen tap.

 

[scottpetrie]

Hi
Ive never noticed this before in any other place Ive been with a combi. As said, the shower is fine but just the taps is the issue so the distance thing is not the issue. Bath is 22mm sink is 15mm. Kitchen tap also 15mm. Seems a bit weird. If I turn it full blast it goes warm, then cool , then warm, then cool. Turn it down a touch, hot as normal. Not ideal when trying to fill a bath if I have to turn it down. Could it be the actual taps maybe no use for a combi boiler?

 

[oldbuffer]

1. Filling a bath from a combi is nearly always a slow process.
2. A combi takes in cold water and heats is as it flows through a secondary heat exchanger. Each boiler is designed to raise the temperature of the cold water by a certain temperature (typically 35 or 40 degrees Celsius). Clearly this is impossible unless the flow rate through the heat exchanger is limited. Depending on the power (kilowatt) rating of the boiler, it can only manage somewhere between 11 and 16 litres per minute.
3. Bath taps on a stored hot water system (non-combi) typically have 3/4" BSP connections and use 22 mm pipe. This provides a good flow rate with relatively low pressure, but the flow rate will almost certainly exceed the rate at which a combi boiler can heat the incoming cold. Hence the water runs hot, then cold then hot again.
4. The only real way round this problem, if the flow rate is unacceptable, is to install a hot water cylinder, preferably near the point of use. A combi boiler can heat a hot water cylinder. Often people use the combi's direct hot water feed for the kitchen and stored hot water for the bathroom.