DIY Doctor

Thermostat not turning off the boiler when hot water reaches tenperature

Postby hoppyposh » Thu May 21, 2020 6:19 pm

Hi, we've just had a baxi 824 system boiler installed. When we turn the hot water on set to either 50 or 60 degrees, the thermostat is failing to turn the boiler off when it reaches temp. I know that it's reached temp as when I put water back on again the thermostat knows its reached temperature and doesn't fire the boiler back up again but it doesn't seem able to turn the boiler off when it is fired up. Any ideas? It's an old honeywell thermostat which was on the house since 2004. Could it be incompatible?
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Postby ericmark » Tue May 26, 2020 3:02 am

The boiler can have a host of external controls, from an opentherm control, with outdoor weather sensor to simple 230 volt on/off the most likely is simple on/off.

So the most likely is there are motorised valves, and the thermostats work the motorised valve, and the valve has micro switches which in turn work the boiler.

We normally have plans, C, Y, S, most common although there are others like W, and the C plan has three levels, unlikely you have C plan, but could be S or Y the S plan has two or more two port motorised valves the Y plan normally has a single three port valve.

Most the valves have micro switches which are pushed with a cam to switch on, and can stick on, this is most likely cause. Normally one would swap the head on the valve.

The boiler is designed to modulate (turn down) so rather than simply switching off as the return water temperature increases the boiler output reduces, once it reaches around 4.9 kW the boiler can't turn down any more so it starts to cycle, (turn off and on) it may be the warm water in the boiler is holding the boiler off, but unlikely.

But as to exactly what the fault is I can only guess, but I would be looking at the motorised valve first.
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