Central heating: Boiler thermostat


Postby tonyhoward1308 » Sun Dec 28, 2008 12:48 pm

My boiler fires up and closes down every 15 seconds or so.

I noticed that the thermostat which reads the water temperature in the system and instructs the boiler to open or close is installed on the FLOW pipe from the boiler so is only a foot or so from the heat exchanger - Surely it must register increased temperature almost immediately after the boiler fires and then immediately instruct the boiler to close down? Within a few seconds, more cool water will have passed through the boiler and into the thermostat, prompting a further instruction to the boiler to fire up again.

A thermostat on the RETURN pipe however, would not register temperature increase until most of the water in the system has been heated - isn't this how it's supposed to work? Was my thermostat installed in the wrong location or don't I understand how it works?[/b]
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Postby plumbbob » Mon Dec 29, 2008 10:49 am

The thermostat has to be installed in the flow side because this is the hottest part of the circuit. The whole idea is to control the maximum water temperature within the system and if the stat was elsewhere, technically a situation could arise where the water was able to boil.

The cycling has nothing to do with the probe position. In actual fact, that would still happen wherever it was fitted.

Older style boilers that are oversized always show this symptom because as soon as the stat senses a drop it temp, the boiler has to fire up again and because the boiler is powerful, the water is reheated quickly. If there was to be a delay in the firing, the output of the radiators would be effected because of the fall in water temperature.

Modern condensing boilers unlike traditional boilers can avoid this problem by modulating their output to match the heat output of the radiators. The boiler fires continuously when the system is calling for heat and sensors monitor the incoming and outgoing water temperatures so as they get closer, the boiler output is steadily reduced until it is just ticking over maintaining the desired water temperature.
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Postby tonyhoward1308 » Tue Dec 30, 2008 2:32 pm

Thanks, Plumbob.

I though there was an additional thermostat which dealt with overheating. The one I was referring to was that controlled by the water temperature control knob on the front panel. If this is the same one, thanks for the explanation.
tonyhoward1308
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Joined: Sun Dec 28, 2008 12:28 pm


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