My air source HP comes with a nice diagram of where to put the volts. Some connections are volt free, hence extra relays but the main thrust of my query is the timer and immersion relay. In short, the heat pump is effectively controlled by some on off switches that activate it, toggle ufh or dhw, toggle heat/cool etc on terminals 3 6 7 13 and 15. The command unit just sets parameters of Heat performance and reports on sensors, it doesn't seem to issue and control commands unless it is functioning as sole thermostat (it doesn't have dhw orogramming)

In this diagram, a time clock activates it for dhw.. But unless the stat has a temperature below which it is open eg 40 degrees, and then it is closed up to 65 at which point it opens again, then it looks like a call for dhw means both HP and immersion activate? If the stat has a range then I can see how a tank at 0 would be first heated by the pump, which would then tail off as it peaked at eg 40, and the immersion take over til whatever temp is dialled in..

I just don't seem to find stats that work that way, so im wondering if this diagram is not really a situation one would create in practice. I think instead if it had a 2 channel progger, I could ask for dhw on channel 1, and really just do pasteurising on channel 2 using the immersion.. But if I did want a larger volume of very hot water regularly, configure channel 2 to coincide with channel 1 ending its call for dhw from the pump and use the immersion to top off? (Or fit an inline heater)

Edit: added a couple of other pics from a different manual that help explain the inputs 3,6,7,13,15 etc

The relay isn't a normal relay - it's time delay.

It works on the process of saying that if the tank hasn't got to X degrees and the relay is opened then the timer allows for another 15/30 mins before it flicks the HP onto the heating circuit and let's the immersion finish the job on the tank.

There is a graph in the book somewhere - I've also got some other info somewhere on what each of the connectors does that JSH sent to me.

If I find it I will send it to you

So its only used to drive the immersion to stop DHW hogging the Heat Pump at the detriment of the heating circuits.

I've looked at this as it depends how you want to run it but I've looked at wiring as a W-Plan circuit with DHW as priority and the heating - UFH - as the secondary.

The HP I've looked at can do multiple temperatures - I'm using a 3 way valve and a relay to switch the temperatures over so when its on DHW its 50c, and UFH its 35c.

The downside on W-Plan is that unless the tank is hot, then DHW isn't available. You could do something cunning with a secondary on your timer where if it kicks in - either by manual select or timer - then it overrides the cylinder stat and makes the HP assume that the tank is hot and allows the HP to transfer to the heating circuit and uses the immersion to finish off the tank.

If you want to keep it simple and top off the cyl to a set point higher than the HP can achieve, use a stat with a BOR and MOR terminal, the MOR when satisfied can then be used to close a relay and bring the immersion on to achieve a higher set point.

Turns out I wasn't being patient enough with the other control switches; pump takes a while to start/stop in response to the run command from 3/6

I haven't found a volt free programmer but I can use a couple of contactors to switch the loads (one is 16a) so all gravy! Finding an enclosure seems to be the biggest pain