Heating two indirect hot water cylinders

[Geoff Pearson]

I have a hot water cylinder upstairs which is heated from my boiler, controlled by a thermostat on the tank and a fully pumped central heating system.

I have another bathroom downstairs, with its own hot water cylinder, cold tank etc, entirely independent of the rest of the house - its water is heated by an immersion heater, although there is an unused indirect coil in the cylinder.

I know want to add this cylinder to the main system. I plan to use a thermostat on it, similar to the first one, connect it with 22 mm to the same circuit as the first one, in parallel. Both might take a hot feed at the same time or either one call for heat. Twin port valve to each controlled by the thermostats?

Any problems with this plan?

Geoff

[John Rumm]

The normal solution would be to make it a S+ plan system with a two port valve for each zone. So in your case, one heating zone and two hot water.

So similar to:

with two cylinders and one set of rads.

[meow2222]

I'd plumb the heating coils in parallel and connect the cylinder stats in p= arallel, so if either calls for heat, both get it. This means one will over= shoot the target temp, but only upto at worst the temp of the primary circ= uit. The cooler cyl will be heated more than the hotter one, since the delt= a T across the exchanger is greater.

NT

[John Rumm]

The risks of that could include one cylinder getting upto 85 degrees depending on the flow temp. This is risks to the users of the water if there are no blending valves, and can also make scaling of the cylinder more rapid and pronounced in hard water areas.

[harry]

You will need valves to balance the heating water flow between the two tanks in exactly the same way as radiators.

It will have quite a long recovery period. (ie warming up from cold).

[Richard Conway]

How about using the closed live output from one of the valves to trigger the second valve (via the thermostat) - that way you could give one tank priority over the other, so from cold you could use one bathroom first.

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[meow2222]

I guess it depends whether the OP wants water that hot or not. I had it hotter at one place to get enough capacity

NT

[Dave Liquorice]

That is what I would do with the addition of another channel of time control for the second cylinder. Keeping a cylinder of hot water hot "just in case" is a good way to burn more gas than you have too. The heat time of a single cylinder from cool shouldn't be more than 30 minutes, it's not difficult to "pop the water on" 30 plus mins before you want to use it. If if both cylinders are cold the reheat time might be an issue but again a simply manual priority switch (set the relevant programmer to OFF) solves that.