Can somebody help me. I have a combi boiler, when I run a bath the water is only hot if I keep the tap turned low, (minimal pressure) If I open the tap fully the water is tepid. However the kitchen sink does not have this problem. My bathroom and kitchen are on the same level (next to each other) I am wanting to fit a mixer tap shower but need to resolve this problem first. Is this a blockage? In the pipe, however no pressure is lost,. Or is it because the taps on the bath have a wider outlet flow than the kitchen sink, therefore the boiler can?t heat the water up fast enough? Please can someone advise. Thank you
Can somebody help me. I have a combi boiler, when I run a bath the water is only hot if I keep the tap turned low, (minimal pressure) If I open the tap fully the water is tepid. However the kitchen sink does not have this problem. My bathroom and kitchen are on the same level (next to each other) I am wanting to fit a mixer tap shower but need to resolve this problem first. Is this a blockage? In the pipe, however no pressure is lost,. Or is it because the taps on the bath have a wider outlet flow than the kitchen sink, therefore the boiler can?t heat the water up fast enough? Please can someone advise. Thank you
Most likely your last theory is right, what happens if you adjust the bath tap to about the same flow as the kitchen tap? Test by timing filling a bucket if not sure. Should still be enough for a reasonable shower. What power is the combi?
Another possibility if the amount of hot water seems less than it used to be is a problem with the valve switching off the central heating when hot water is demanded, but there may well not be anything wrong.
Yup, that is likely the case - you can't change the basic laws of physics. The boiler only has enough power to heat a certain amount of water per minute to a usable temperature. If you pull water through it faster than that, the temperature will have to fall. If the water starts out colder (say in winter) then again you will have accept a lower maximum flow rate if you want the water as the same temperature as in summer.
Now if you are saying that it used to be better, then its possible that the boiler is not working as well as it did before. It might be for example that the secondary heat exchanger that heats the hot water is partially blocked (lime-scale etc). That could reduce the maximum rate of heat transfer.
To work out what is actually happening you will need to do some tests and some sums. Firstly measure the temperature of the cold water, and also of what you consider to be "hot enough" on the hot water. Then work out the flow rates you can get from the various taps in litres per min. After that all you need know is the specific heat capacity of water (about 4200 J/kg/c) and the power of the boiler.
So for example, say the mains cold is coming in a 7 degrees C, and you want it heated to 45 degrees. That's a 38 deg change in temperature. Say you have 24kW boiler (and lets assume that is the actual heat output to the water and not its input gas rate (which will be a bit higher)). 24kW is 24,000 Joules per second. So in a minute that gives 60 x 24,000 =
1,440,000 J of energy available.
Now just divide that by the temperature rise, and the SHC of water:
1,440,000 / 37 / 4200 = 9.25 kg
So that means the maximum flow rate is 9.25L (since 1L of water is 1kg) per minute. Plenty for a good shower, but could take 10 to 15 mins to fill a bath.
If you turn the tap on hard and take 15 lpm, it won't be 45 degrees any more. (not some boilers actually include a flow regulator to keep the output water temp as the requested level - however many don't. You can achieve much the same thing by throttling the flow rate at the bath tap with its service valve if it has one)
Wot he sed. Make sure you're comparing apples with apples. If you can really get a much higher flow rate of properly hot water from the kitchen sink than from the bath, there's something very odd going on.
Don't forget that the incoming cold water is much colder this time of year than in the summer, so the boiler has a lot more work to do to make it hot - limiting the rate at which it can produce hot water.
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