Two hot water tanks with stats and 3 port valves

[Pezza]

Can anyone help, I need to wire into my heating system a second hot water tank with a tank stat and a 3 port valve Can anyone help with wiring. The problem I have is that when the original tank stat switches off this sends a live to the original 3 port valve, as this has a wire from the programmer which is live when the HW is switched off, when the stat on the new hot water tank calls for heat it starts the boiler and pump although the HW is in the off position on the programmer, I think the live from the HW off is feeding back through the first tank stat (SAT) and (COM). I have tried to fit diode between first tank stat (SAT) and (COM) wires as I thought it would not let the current flow backwards, but that didn't work. not sure if I used the correct diode. any help would appreciated.
Regards
Perry

 

[electronicsuk]

I don't really follow your description, but give up with the idea of a diode, it shouldn't be necessary and will not help on an AC circuit anyway, other than acting as a half wave rectifier.

Where are you picking up the supply for the tank stats? I would have thought a better solution would be 3x two port valves, one on each tank and then one for the CH. The two tank stats could then pick up their supply from the hot water demand output of the programmer, and the output of each stat would then go to its respective valve. The valve for CH would want to go through a room stat fed from the CH demand on the programmer.

The important bit is that the switching for the pump and boiler would need to be via the microswitch inside the valves. Otherwise, you will get the problem you mention where you have simply connected the output of the two thermostats together, and the valves can no longer operate independently. As long as you use the switches inside the valves to control the boiler and pump, you can link as many switches as you want in parallel without a problem.

 

[Pezza]

Hi Mathew
Thanks for your help, but the 3 port valves are already plumbed in (one original and one new). The problem is, if I only wire up one tank stat the system works OK but the tank with out the wired stat will obviously not be able to call for heat. I think I need to stop the current flowing back through the fist tank stat when the HW is in the off position on the programmer.

 

[Lectrician]

If you add any extra valves to a system to control additional zones, the original 3 port will cause you issues.

The 3 port will need removing, and two 2 ports used in its place.

The reason for this is that a standard 3 port valve can stay open when all demands are satisfied, and if you then have an additional valve calling the boiler and pump, water will still be able to flow through the open port of the 3 port valve.

3 port valves can only be used in isolation, never with more valves.

 

[toasty]

Lectrician is right, they are 3 port and 3 position valves, the positions are:
1) port A open
2) port B open
3) both ports open.

There isn't a both ports closed option.

You could of course buy an additional 2 port valve and use it infront of the inlet to the 3port - that would save you some money...
-Dan

 

[gas4you]

From what you are saying do as already instructed, remove the 3 port mid position valve, replace it with 2 x 2 port valves then add a third 2 port valve for the new HW cylinder.

Horstmann do a very good 3 channel programmer that would control all this perfectly.