Worcester 24i hot & leaky

[Meldrew's mate]

Here's one I haven't seen before.

Customer has a Worcester 24i.
When the heating is on the DHW cold feed gets very hot all the way back past the flowswitch, almost to the cold inlet isolator. This happens within 30 seconds of the heating being switched on, then the joints around the flowswitch start to leak steam & hot water (proper steam engine hissing sound). Running the hot tap even as low as a drip - drip - drip will prevent this symptom, and hot water production is fine.

Any ideas on the cause and solution?

BTW this model has the DHW heat exchanger up in the combustion box (A fat mate has suggested it needs Welsh coal, he doesn't know much about boilers but he is a fan of the Rev Aldrey)

 

[ChrisR]

Steam how hot's the heat exchanger getting?

The expansion has to go back up the supply pipe (to a wc or whatever) because the outlet side is closed. If there's a dcv on the mains the pressure would go way up, if the cistern float valve is one with hysteresis like a Torbeck or Fluidmaster.

 

[Meldrew's mate]

Thanks Chris, I've checked for a check valve, none is obvious, and there's about 13m of 15mm mains pipe from the stopcock to the boiler inlet.
Is there a DHW temperature pot on the control board? Don't expect that to be the cause as the DHW thermistor is 8" downstream of the combustion box.

Think I'll have to re-visit on a freebie to get to the bottom of this one....

MM

 

[bengasman]

This model seems to be more sensitive to limescale and corrosion problems than other worcesters. Take the pumphead off and see if it spins smoothly without too much play. Should also tell you if the system is soiled