@Mike21 or anyone, did you find out if CH relay still operates in Opentherm mode

[speedyuk]

Hi, I found a post by @Mike21 who pretty much asked the same question that I have. I'm about to install an Ideal Logic Max Heat2 (Heat only) boiler. This obviously uses external pump but has no pump live terminal :(. I will be using Openterm. I think I have a wiring solution for DHW to tell the boiler to go into 80degree C mode. But I need a method to switch the pump for CH operation. I'm hoping the NestLink CH relay still operates in sympathy with 'call for heat' being controlled by Opentherm to the boiler. If someone has this setup and can confirm that would be great.

[Mike21]

Fairly sure both relays still operate when Nest is in OT mode, but didn't test it as my pump is housed in the boiler and directly controlled by the boiler logic; the HW relay definitely operates as that feeds my DHW tank thermostat which goes back to the boiler DHW input. If you've got a separate pump then you'll obviously need to power it from both CH and HW relay outputs.  You just need to get the boiler to fire in high temperature mode for DHW; have a look at Urban plumber videos- IIRC he got an Ideal boiler to work with OT.

[speedyuk]

Hi @Mike21 . Thanks for reply. My boiler is heat only with external pump. It's a real pain that the Ideal Heat boilers do not have a Pump Live feed so you need separate control. For the DHW it's OK you have the orange OP from the NC 2 way. For CH the 2 Way is changed to NO so no Orange 'signal' wire to use for the pump :(. Hence the question of if the CH relay still operated in the Heatlink. I've watch the Urban Plumber video several times. He was working on a system boiler like yours so doesn't need to worry about the pump control. He created schematics for both which I've found via internet archive, but they are wrong as the operation of the SL2 is back to front to what logic would think. I believe it actually needs 0V when Cylinder heat temp is required and 240v when either Cylinder stat temp is satisfied or DHW off. My boiler has just been fitted and I've had it commissioned leaving it in S-plan with the intention of changing to PDHW / Opentherm once I have all the necessary info and time to tinker. If you are able to put a meter across the CH relay contacts and adjust CH timer / temp to see what happens that would be fantastic. I won't be beaten by this! 

[Mike21]

Don't have to dig the multimeter out!

Turned the Nest thermostat up & down and can hear the relay clicking on & off!

[speedyuk]

You are a star @Mike21  thank you very much. 

[speedyuk]

Hi @Mike21  and anyone else following this thread, I'm happy to report that I successfully made the wiring / control switch over to this system this morning. CH is being controlled by Opentherm and PDHW is being separately controlled so the boiler fires at full output. It was quite straightforward in the end. The whole 0V thing on SL2 was a red herring, that is not needed certainly for the Opentherm controlled setup. The crucial things are. 1) Make no connection to SL1 (control for CH is done by Opentherm alone). 2. Use the CH NO control from Heatlink direct to Pump for CH. 3. Use Relay to isolate Orange from NC 2 port valve in DHW pipework to  control Live to pump when DHW calling for heat. 4. Use same Orange control to Close NO 2 port Valve fitted in CH pipework when DHW calling for heat (so 80degree water doesn't flow round Radiators when cylinder recharging).  5. Wire same Orange control to SL2 IN on Boiler. I might even try to do a YouTube video and diagram. I'm not sure why @Urbanplumber used a 3 port valve in his heat only example, I used the 2 separate 2 port valves as I would think the most common when switching from S plan - you just need to change the CH one to NO head. And his diagram had a wire going from Heatlink CH control to SL1  - you definitely don't want that, just Opentherm alone for CH control. Hope that helps, and thanks again Mike21.