I have a Myson boiler with hot water tank - tank has two pumps one for rads and one for hot water - both working.
Radiators are working fine and tank is getting hot.
When the hot water tap is turned on a small amount of warm water comes out then flows cold - pressure is good. pipes at the side of the tank go from hot to cold when tap used.when mains water is turned off and hot tap opened only small amount off water comes out then stops. Can anyone help please

Nev, February 2007

Thanks for the advice - found the fault. mains water enters the tank and forces hot out however there is a bypass pipe which is opened by a thermostatic valve to add cold water if hot water to hot. Seems a complicated system. Blocked off bypass and now use cold water tap to adjust temp like we have been doing for years.
Thanks again for your time

Nev, February 2007

dont listen to alan, he is wrong. sorry al! you have two pumps on your system. when you open the hot tap the second pump (not the heating one) should start. needs a new dhw pump

mike baxi, February 2007

Check air flow sensor if boiler is working with fan at slow speed and minimum flame level is observed when hot water is being drawn.The sensor is a box containing 2 thermistors, one of which has air drawn past it by the flue fan. This results in a voltage being produced proportional to the fan speed. If the voltage is low the boiler thinks the fan is too slow and will not allow full gas status.For some reason the fault is more critical with hot water than with central heating.This is a personal experience with my own boiler Midas SI. Hope this helps.

alan senior, February 2007