On Fri, 19 Nov 2004 19:18:27 +0000, "Ed Sirett"
wrote:

In a private e-mail to , on Thurs, 18 Nov 2004
20:43:28 -0000, Tania wrote:

HELLO

I have recently had a combi boiler put in, and whenever the hot tap is put
on there is a really hot burst of water (too painful to use). It then goes
cold for a while, then heats up to the temerature I set on the boiler
thermostat. I have two very young children and worry about them turning on
the hot tap in the loo or bathroom. Has anyone else found a solution to
this. I asked the engineer who came out to sort out a lighting problem
today, and he said 'other people don't moan about it' which left me feeling
fed up and no wiser. He did explain to me how it happens several times,
each more slowly than the last, he was a very rude man! he had no will to
try and reassure me, but when pushed did say it would be 64 degrees
maximum - so not scalding. Any ideas will be gratefully received.

Regards

Tania

On Fri, 19 Nov 2004 15:20:17 +0000, Phil Addison wrote:

Water temperature fluctuations seems a common problem with combis in
houses I have visited. Can anyone help with a solution to this problem
that I received via email from the FAQ website?


IME experience there are several reasons why (ordinary) combi boilers have poor
regulation of DHW temperature.

1) Drawing off water from elsewhere especially in the same dwelling.
2) Thermostatic mixing valves fighting the boiler.

but mainly:
3) When the DHW exchanger becomes scaled up the heat transfer rate becomes
limited, that's bad enough. Eventually the heat transfer rate drops below
the lowest modulated setting of the burner (typically around 10-12kW on a
popular 24kW unit), at this point the control system, in order to modulate
the burner further has to start switching it on an off. Given the
considerable thermal inertia of the components a fluctuation of the output
temperature begins.

1. Can be mitigated by good design and practice. Namely don't fit a combi
where the supply is inadequate. Take it's supply upstream of other
draw offs (even consider putting a slight restriction on the feed to
everything except the combi.

2. This is not a common problem and shoudl be mitigated by getting 1 and 3
right.

3. Descale or replace secondary heat exchanger.

Hi Ed, Thanks for getting us back on topic. Sorry if my post was
confusing, but I am passing on a problem I got from a reader of the FAQ
site, who does not seem able to access the ng.

However, she says the combi was recently installed, so scaling (3)
should not be the cause, and her description appears to rule out 1 & 2.

The problem seems to be that from when turned on the water initially
comes out very hot, then goes cold, then warms up to the correct stat
temperature. I wonder if it could be because she has the tap turned on
only a small amount initially, so it overheats and shuts down, then she
turns the tap on more in order to make it re-light. Could this last
action could produce cold until the over-heat resets, and then the
greater flow enables it to modulate properly and warm up to the right
temperature?

Phil
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