Valve on gravity HW pipe

[Dadwood]

Mine is quite an old heating system. It comprises a cast iron boiler, indirect HW storage cylinder, pumped HW to rads, and gravity heated domestic HW. At present there is no valve on the gravity pipework between the boiler outlet and the cylinder, so in winter when the heating system is on for prolonged periods I can't control the domestic water temp. I want to therefore fit a two port motorised valve on the gravity pipework. Does it matter where I position the new valve? Can it go on either the flow or the return between the boiler and cylinder? Or is it best put on the flow into the cylinder? Thanks for any advice you can offer.

[plumbbob]

Firstly, the important thing is, if you have a solid fuel boiler, then you cannot fit anything into the pipework to the cylinder that will interfere with the flow. Doing so will cause the water in the circuit to boil.

For boilers where the water temperature is controlled by a thermostat, a valve can be fitted provided there is always a clear route for steam and boiling water to escape up the expansion pipe to the header tank in the event of a boiler fault. Similarly, any valve should not be placed in such a position as to cause the system to be starved of water.

The M/V will normally be installed very close to the DHW cylinder and after any connections to the F&E tank.

[Dadwood]

Very many thanks for your advice Plumbob, the boiler is gas fired. I'll install the valve very close to the coil inlet.