hot water only lasts 10 sec`s Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[griffin]

Help!
Saunier Duval 23e, 12 yr old.
Direct hot water runs cold after 10 sec`s then repeats hot/cold
Changed thermister, no change in problem!
any advise?

[htg engineer]

Check - when the water runs cold, is the main burner off ?

Especially with the age of the boiler there could be a problem/blockage in the heat exchanger, if the heat is not getting from the boiler to the water fast enough the water will keep running cold when the burner cuts out - as the water in the boiler will be at temperature.

htg

[griffin]

Hi htg engineer
Yes, main burner is off when water runs cold, I can turn on tap and watch as burner comes on, hot water, then burner cuts off, cold water!
we live in a soft water area, no problems with limescale in kettle etc

[htg engineer]

Remove the thermistor from the boiler pocket, does it work?
If it's sensor - connect the old one to the PCB but do not fit to HW section.

If the boiler still cuts out, it could be the central heating sensor at fault, try same again with the CH one.

Don't run for a long time with the boiler thermistor is removed, just long enough to know it's working correctly.

It's not uncommon for the central heating sensor when they're old to maintain it's temperature and cut the boiler off.

If the boiler still cuts out with the new sensors, check the flow and retrun through the heat-ex if the flow is and stays hot it's a blockage in the heat-ex.

It could also be a PCB fault. just trial and error unless you want to get an engineer out.

htg