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How do I add a cylinder thermostat to existing system

Postby petejigsaw » Tue Nov 20, 2007 4:36 pm

I have conventional boiler with a pumped CH system and a gravity HW tank. At present, the CH and HW are on a simple timer clock, which allows me to have either of them OFF/TWICE/ONCE/ALWAYS.

I also have a room thermostat upstairs which I plan to move to the upstairs landing as its better suited than its present bedroom location!

However, while this will control the heating, I want to put a thermostat on the HW cylinder so that it can turn the boiler when the water is below a certain temperature. At present, the HW doesn't seem to get very hot when both the CH and HW are on - if its just the HW then its fine.

Anyone give me advice on the simplest way? To my knowledge I don't currently have any motorised valves in the system.

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Postby htg engineer » Tue Nov 20, 2007 6:56 pm

you don't need a cylinder stat on a gravity HW system, use the thermostat on the boiler to control the water temperature, or convert to a fully pumped system which will be a fair bit work and will cost a few hundred pound. You'd be better off fitting a combi boiler that way the heating and water temperatures can be set.
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Postby robertw63 » Tue Dec 18, 2007 12:47 pm

It's not quite as straightforwards as you'd like. USUALLY with these systems the boiler runs all the time the CH or HW is on and the CH is controled by the room thermostat which turns on the pump and diverter valve. This means that the HW is always as hot as the boiler can make it and fitting a thermostat to the cylinder won't help anything. If it isn't hot enough then probably with this cold weather most of the available boiler output is going to the radiators. This is probably made worse because the gravity system doesn't work as well when the CH pump is pulling water away from it.

You could do one of these (in order of ease and cost):

1. If the boiler isn't full up already turn it up a bit.

2. Set the timer if you can so that the HW comes on an hour or 2 before the CH so the water gets all of the boiler output first.

3. Fit a pump and valve in the return from the gravity system and control it from a tank thermostat. A small pump running at a low setting will likely be enough. This will help recover from the effects of the HW pump stealing all of the water. Be careful with the layout here so that you don't block the expansion or feed pipe!

4. Last resort - get it changed to a fully pumped Y-system. (This means that your pump pumps everything and you have a diverter which can choose the CH or the HW or both together).

5. Absolute last resort - get it updated to a new system.
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Postby marrtin » Tue Dec 18, 2007 10:15 pm

In theory, you could fit a tank stat and wire it in series with the contacts of the hot water timeswitch. You don't mention which timeswitch you have and some won't allow the heating to be on without the water so the tank stat would in that case control the heating.

Really the tank stat is there to switch the boiler off not on, so may not help your problem unless you want to leave the water timer on 24/7. If the hot water is cool, there are other aspects I would consider first before fitting a tank stat.
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