Hi, I am fairly competent DIYer, but the evoHome hot water kit slightly concerns me and I could do with a sanity check.

I have three heating circuits., When I updated the boiler a few years ago the plumber put in three BDR91s (in a row, 2cm apart) to control the circuits with remote thermostats. The BDR91s control the valve on each heating circuit. It took some fun and games before I worked out (RTM...) that my assumption that the controller just "needed to know" which BDR91 belonged to which zone was wrong. Eventually I put them all in binding mode and then they were all bound to the controller when setting up "Boiler Control". I expected them all to come on every time any room asks for heat, but weirdly they don't. But the ones coming on don't always match where the heat is required, although in the end the heat always arrives (e.g. circuit one is demanding heat now, and the BDR for circuit 1 and 3 are on, but circuit 2 is off). Thinking this was the nearness I have separated them by 30cm, but it has made no difference. I do wonder about binding them all to one BDR91, but, it works... so I will leave it as it is. The BDR91s have power all day now because I changed the timer controls to ON for those circuits. evoHome is in charge.

Anyhow... I just purchased a hot water tank kit to fit to my Megaflow tank. Mine is a CL210. They are very much sealed but my model has an immersion heater half way up with a pocket for a thermostat. I intend to remove the thermostat and isolate it, then stick the remote sensor in to the pocket. I can refit the immersion heater thermostat if I want, though have only needed it once in 20 years, and that was when the boiler was replaced.

I have talked through what to do about wiring with a friend who is a commercial heating engineer (not domestic), but wanted a sanity check, so if anyone sees this a metaphorical thumbs up would be appreciated. The guarantee on the Megaflow is 25 years and I don't want to interfere with it (though it is 20 years this year!).

This is the plan:

1. Isolate heating system from mains.
2. Tap L and N from the junction box where all the other BDR91 come from and take to the L and N in the new BDR91. BDR91 will now be ready to rock and roll.
3. Wire the A and B sockets in the BDR91 in SERIES with the ORANGE (or grey) wire to the inlet valve to the megaflow.
4. Tweak up the Megaflow stat to higher than the set point on the evoHome (I have not taken the cover off the control point on the megaflow, it is in an unbelievably awkward position, it will be possible but I am not going near it until the power is off. I only want to do it once).
5. Remove and isolate immersion thermostat, freeing up a void half way up the tank.
6. Push remote probe in to the void, seal lead in with something sensible, I have a vast array of rubber grommets, one should be right.
7. Connect probe to hot water unit, pair it all up with the controller...
8. Power on.

As I see this, the megaflow will always think the water is too cold, so will be powering the valve. The BDR91 will then have full control over whether the valve opens or closes. If the tank gets too hot, the megaflow will cut the power to the valve.

I have not touched anything on the megaflow to do this, so I think the warranty will be fine. (If anything goes wrong and the BDR freezes open, the wiring is identical to what it was before), I will just get hotter water.

Does this make sense??