Slow Hot Water Flow Rate

11 Jun 2005
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United Kingdom
I have just fitted a new bathroom with thermostatic shower and the hot water flow rate is awful.

The system is cold from rising main (no header tank), hot water from Combi boiler.

The hot pipe runs from combi boiler in 22mm, under the floor through the seperate toilet where a branch of 15mm supplies the kitchen and then into the bathroom where a 22-22-15 T supplies the basin and a 22-22-15 T supplies the shower then to the bath taps. The flow is more of a trickle, although the kitchen doesn't seem too bad.

If I turn the shower on as hot as it will go (i.e. no cold flowing) then turn the bath tap on the shower stops!.

Any ideas?



The boiler is a Sime Freedom (I think). Could it be an airlock, and if so, how do I clear it?
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PI know it's a combi and obviously the pressure/flow will be lower because of this.

But as I said previously this is only affecting the bathroom not the kitchen.
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The problem is possibly the mains pressure and/or flow which is available to you. Measure the pressure or ask the water supplier what it should be. If kitchen cold + garden taps on together is less than about 20 litres a min you should expect less than great flows.
The combi is quite old though so could be scaled up as well. Hard water area?
Measure the HW flow rate - what is it and is it constant? ANd how hot?
Thanks for the reply.

I don't think it'll be scaled up, we're in a soft water area, The flow rate from cold tap near the branch pipe is just under 10 Ltrs/min at a pressure of 4 bar with a flowing pressure of 2.8 bar. Water company have tested this. I am on a common supply pipe with 5 other houses at the moment, (replacing this is my next task after the bathroom!).

I'll measure the flow of hot water properly tonight on all the taps in the house and see if there is a difference..


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