Adding Control to Gravity Heating System

Postby eddjohn » Sun Aug 10, 2008 12:32 am

Hi, wondering if anyone can help me.

We have a semi-gravity heating system (ie the CH is pumped, but the DHW is just gravity fed) and i'm looking at upgrading so we have some control over the cylinder water temperature. Typically the water is far too hot and you have to add large amounts of cold, which seems a waste. I know I could upgrade to a fully pumped system, but i'll be too expensive to have the plumbing changed.

I done some research and found I can use a "C Plan". Basically add in a 28mm 2 port zone valve, Room and Cylinder thermostats and i should have more control than I have already. Currently I have TRV's, programmer and just the Boiler stat.

What I need to know is where can I put the motorised valve. I understand it goes on the return from the cylinder??? Can I place it near the boiler itself, or does it need to be near the tank? The tank is about 8meters from the boiler. Having it near the boiler means the wiring is so much easier then.

If anyone knows any issues/problems I may have with doing any of this please let me know. I don't want to start cutting the 28mm pipe and find it wont work after :(

Many Thanks
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Simply Build It

Postby htg engineer » Sun Aug 10, 2008 12:34 pm

htg engineer
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Postby eddjohn » Sun Aug 10, 2008 2:57 pm

Thanks for your reply.
I did come across that webpage, and it does show the valve nearer to the cylinder, but do you think it woud matter being next to the boiler? I can't see it would make any difference, as it's still stopping the convection flow.
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Postby Steve the gas » Sun Aug 10, 2008 4:08 pm

Don't take this the wrong way, but, cos of the cost these days I for one, only use what hot water I need and cool it minimally, e.g. bath
Steve the gas
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Postby htg engineer » Mon Aug 11, 2008 3:27 pm

I cannot see the position making any difference, fit near the boiler. If it doesn't work then it's only a case of renewing a section of pipe and extending the cable.

Not sure what steve the gas means, on older systems you have to heat a full tank, so unless you reduce the amount of water the tank holds or convert to a fully pumped or even better a combi system. You cannot save that much - the most you could do is turn the boiler stat down and the water shouldn't get as hot - or as you say, try to adapt the system so that the temperature of the water in the cylinder is controlled not just the boiler temperature.

htg engineer
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Postby eddjohn » Tue Aug 12, 2008 1:57 am

Seems it does have to go next to the cylinder after all, otherwise it blocks the cold feed to the boilder. I had a look and theres just about enough pipe before the cold feed T's into the return. Thanks for your help.
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Postby plumbbob » Tue Aug 12, 2008 9:35 pm

Wherever you decide to install the valve, ensure that the supply of cold water from the F & E tank is not restricted, and there is always a passage for steam and boiling water to escape from the boiler up the expansion pipe should the system ever boil.
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Postby ericmark » Wed Aug 13, 2008 10:41 am

I have done same job for my father using a radio controlled tank stat and a motorized valve about 10 inches from boiler and it works fine. I was talking about this to someone else and put this on my web site maybe it will help?
Main thing there are different motorized valves some with and some without aux switches and you will need one with the aux switches.

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