General Programmer control Question

[bon98uk]

Hi All,

A simple question (I hope).

I have a Drayton Tempus 7 programmer/timer. This has a switch on the back labelled 'Pumped --- Gravity'. Idea is with a Gravity fed DHW the HW circuit is supplied if the CH is on.

However I have a C-plan control system set up with a 2-port MV in the HW Flow controlled by a CylStat.

So although the system is not fully pumped it does have independant circuits.

(i.e. MV off - HW cct Off, MV - Open HW cct On. With the Pump running then the CH will be on etc.)

So although not fully pumped the two circuits should be able to be independently controlled? If so should the programmer switch be moved to 'Pumped'.

I believe the difference is an internal wiring shift that energises the HTG and HW terminals of the programmer seperately, whereas if set to Gravity then the HW terminal is energised with the HTG terminal.

Can anyone confirm this?

 

[mogget]

OK, here's a scenario that probably won't match your current setup, but should help explain. Imaginary wiring to follow

Let's assume that the HW output of the programmer controls the motorised valve through the cylinder stat.

Let's also assume that the CH output of the programmer independently energises the pump for the heating circuit.

That leaves a problem - how do we make sure that the boiler fires up, regardless of which circuit needs the heat?

For the hot water circuit it's simple - you use the microswitch in the zone valve to switch a live to the boiler.

For the CH you can't just connect the live of the pump to the boiler though - AC current flows either way, and the pump would run whenever the hot water circuit called for heat from the boiler.

If you want independent control, a way has to be found that when CH (only) is called for, both the boiler and pump would be energised, but that no connection remains between the two when the CH is turned off.

One way of achieving this would be for the programmer to control a double pole relay via the room stat - but probably a better way would be to convert to a fully pumped system.