Gravity fed Central Heating System Woes

I recently moved into a house with a gravity fed CH system, the Baxi back boiler is 4-5 years old but the heating system looks much older, each rad (except the one in the bathroom) has a thermostatic valve and there is one room stat in the hall.
As everyone knows, with this type of system it is not possible to run the heating without the hot water.
My problem is that I have been running the heating on "constant" over the weekend and as a result the hot water is scalding hot to the point where steam and water have been jetting into the header tank - my loft sounds like the local branch of Starbucks and the wooshing and banging is not only getting on my wick but I'm sure it's not doing the system any good.
I recently replaced the ballcock on the header tank as this kept overflowing, it still overflows so I guess it's the hot water cylinder that's up the chute! and I will have to replace this.
At the same time I am considering modifying the system so that the heating can be run independant of the hot water and maybe zoning the upstairs and downstairs.
I would like to ask if anyone has anyone undertaken this (or a similar)job and if they have any tips/tricks/do's dont's/information on parts/suppliers etc that may help me decide the best way to do this.
Thanks in advance,
Kev
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The simplest way of controlling the hot water - short of converting to a fully pumped system - is to put a zone valve in the gravity circuit. Control this with a cylinder stat, and use the voltage-free change-over contacts on the zone valve to contol the boiler. If you have a look at the C-plan details in http://content.honeywell.com/uk/homes/systems.htm you'll get the idea.
[It took me a few minutes to suss out the wiring - but I believe that it works like this: Hot water only: The zone valve is open, the boiler runs but the pump doesn't. As soon as the hot water demand is satisfied, the valve closes and the boiler stops. Heating only: The zone valve is closed. The boiler and pump run whenever the room stat is calling for heat. Both together: The zone valve is open. The boiler and pump both run. When one of the stats is satisfied, it operates as per HW or CH only (as appropriate) and when both are satisfied, it shuts down completely.]
This would certainly allow you to have CH all day without red-hot HW.
If you go go this way, be careful where you position the zone valve - to make sure that there is always a clear path between the boiler and the expansion tank.
Roger
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Perhaps it's not the correct thing to do, but on my gravity (convection!) system, I use the 'stat on the boiler to keep the water temperature under control when the heating isn't on. I normally set it to 'minimum' for the summer months and raise it a notch or two when I start using the heating regularly. Seems to work.
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being too hot in the winter when you turn the boiler up for the benefit of the heating.
This system also has the disadvantage that the boiler will cycle just to keep itself warm even when all demands are satisifed.
Far better to have proper controls on water and room temperatures - connected in such a way that the boiler (and pump) only run when required.
Roger
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