Adding thermostat to Gravity Fed Water / CH

19 Feb 2011
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United Kingdom

I have a gravity fed water / CH system powered by an oil fired boiler.
The CH is pumped. I am replacing very out of date controls (that are on their way out). There is currently no room stat, just a simple programmer feeding the boiler (W) and Pump (CH mode).

I have purchased a CM927 Honeywell RF stat and have it hooked up.

The question is do I use this to switch the boiler AND pump, or just the PUMP, or in fact just the boiler? The boiler documentation and reading other posts suggest that I switch just the pump, but I am concerned if the boiler is in mid-flow boiling away and then the stat calls to switch off the PUMP, say at the moment the boiler starts to fire, where does the heat go?

I thought about fitting an overrun type timer or stat so if the boiler is firing, it would at least keep the pump going and for maybe 1 minute afterwards too.

Am I overcomplicating this?

Thanks in advance.


p.s. The boiler is a Jetstreme 4+ 35kW

I don't want to damage it.
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I am not familiar with your particular boiler, but since no one else has replied...

Usually with gravity hot water systems, when you select hot water only, just the boiler is switched on and the water circulates by ..erm gravity around the hot water cylinder.

When you select the central heating the pump then starts to circulate water around the radiators.

You can quite easily check to see if this is how your system operates. Switch on the hot water only and check the pump isn't running. Then turn on the central heating and check the pump starts up. If this is the case you can happily connect the thermostat to the pump only, knowing that when it switches it off, your boiler will be operating exactly as it would when you have only the hot water selected.

Don't connect the thermostat to the boiler, otherwise in the summer when the room is warm, the boiler won't come on and you won't get any hot water either.

If you add a cylinder thermostat as in the above diagram, this will turn off the boiler completely when the stored hot water is at the required temperature and the central heating isn't on. Without it the boiler will keep firing occasionally just to keep the water in it and the pipework connecting it to the cylinder hot.

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