On Tue, 09 Mar 2004 15:32:46 +0000, Christian McArdle wrote:

water to the shower. It seems to heat the water well initially and
provides an adequate flow of water to have a comfortable shower but
then after a minute or two of running the water will go cold for 20
seconds or so before getting hot again.

This can happen if your combi isn't modulating properly, (or is a
non-modulating type). What happens is that the temperature rises and rises
until the main boiler thermostat kicks in and turns off the burner until it
reduces again. It takes a good 20 seconds to relight the boiler when the
temperature recovers (which will be almost immediately).

If the boiler is non-modulating, then your only hope is to increase the
water flow. This might be achieved by installing a wider bore hose on the
shower or a power shower head with larger holes. You could experiment by
leaving a basin hot tap on slightly to see if it cures the problem.

IME this problem most likely (there are several possible causes) is caused
by limescale build up on the DHW side of the secondary heat exchanger.
The boiler simply can't transfer enough heat without over heating so it
shuts down temporarily.

Other causes a
Out of calibration temperature sensors.
Gas valve modulation control faulty.
Too little flow of HW (but enough to start the boiler).

A work around which may help is to turn down the HW thermostat (assuming
this is distinct from the CH thermostat). This may help to bring the
boiler into a steadier state, until the real cause can be tackled.











--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html