Need to fit a pump & thermostat switch to existing Gravity fed HW

Hi,
I now need to fit a pump and thermostat to an existing gravity fed HW system (gas cooker/boiler cuts out occasionally before the tank is really up to heat, so I suspect the gravity loop isn't that efficient at moving the water round - it was OK through last winter, but we've had a few cut-outs this summer) So... Can anyone suggest where to get the thermostat from? I've only found tank thermostats when looking online and really need something that I can fasten round the 28mm pipe that comes out of the unit, with normally open contacts, which will close (ie turn on the switch) when the pipe gets to a certian temperature (thus supplying power to the pump) and breaks the contact when it falls to a preset temperature (ie. some time after the boiler is turned off)
Also, the pumps I've seen (standard CH style pumps, any to avoid?) all seem to have 22m connections, but I've seen isolating ball-valves which are 28-22m conversions - I presume this won't really have any effect on the flow through the system, given that it'll now be pumped...
Thanks for any advice,
Gordon
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Have a go with one of these of these babies. I have had one installed on my system for over two years and it really works.
http://www.deeter.co.uk/boiler.htm
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<http://www.discountedheating.co.uk/shop/acatalog / Online_Catalogue_Pipe_Cylinder_Thermostat_by_Honeywell_1183.html> is one example, but not the one I remember finding last year. (I have an ancient one which is selectable for normally open or normally closed, which in the end I got working. I presume this is for hot water only, not radiators too).
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Maybe I should have made myself a bit more clearer. This is a gas fired range cooker which in addition to cooking, heats the hot water tank. It's got nothing to do with central heating. The range replaced a coal fired rayburn which was capable of boiling the water in the pipes. It was installed in the winter and worked fine, but this summer it's cut-out a few times because it's internal temperature sensor has gotten a bit too hot. The manufacturer (Stanley) suggests a pump and a pipe thermostat connected to the exit pipe, but when it was installed, both Stanley and the plumber thought it would be OK with gravity feed only, as there was an existing gravity feed system and pipe-work in-place. It was fine in the winter, but not so fine in the summer. If the tank is full of hot water and I run it off, it's also fine (but a waste of a tankfull of hot water) Actually, it's not that hot which is why I'm thinking the flow thorugh the gravity loop isn't that efficient.
The pipes go to the hot water tank upstairs through the tank coil and back again. There is a separate heat-sink radiator in paralel with the tank coil. There are a few more bends in the (28mm) pipe-work with the new cooker than were in the old one, so maybe thats a factor.
I do also have a separate gas CH unit which can heat the tank via a 2nd coil fitted in the tank, but thats fully programmable to work either CH or HW or both. No point using it to heat the water when the cooker can do it.
Thanks to others for the links so far, but best I have is possibly:
http://www.tlc-direct.co.uk/Main_Index/Water_Heating_Index/Central_Heating_Other_1/index.html
If that has a normally open set of contacts which close when it reaches the set-point I think I'll go for that.
Cheers,
Gordon
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Don't be so sure. It may be more efficient to use a modern condensing boiler to heat the hot water than use the cooker. It will depend on how the cooker's controls work.
Christian.
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