On 07/04/2018 19:44, Ciaran flanagan wrote:

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 cant heat the water up fast enough?

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)





--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/