OpenTherm + ATAG and Adding Hot Water

[jon798]

Hi,
I recently upgraded the CH controls on my system to Evohome, using an OpenTherm bridge to control my Atag A200 system boiler. As I wasn't sure the Opentherm would work (it's not offically suported by Atag on the A series), I have temporarily manually locked open the CH 2-port valve and disconnected the volt-free relay closure at the boiler that would normally trigger a CH demand. I will remove the valve completely later.

The OpenTherm control seems to work well and I am seeing the boiler adjust its modulation/flow temp in line with demand and I am finally seeing an end to the constant overshoots and underheats I had with more traditional controls. Hooray!

Hot water is currently separately controlled outside of Evohome and uses another 2-port valve which is wired to the DHW trigger contact on the Atag boiler when there is DHW demand. The atag boiler treats DHW demand differently when triggered by contact closures - it cranks up the flow temperature and controls a proprietary 3-port valve installed in the return pipework so that the hot water cylinder gets heated as quickly as possible. Now that I have the CH zoning controls handled by EvoHome in theory I should be able to remove all the existing 2-port valves and just let the ATAG boiler and 3-port valve handle things.

If I want to have EvoHome controlling the DHW, what's the interaction between OpenTherm and the contact closure, does anyone know? Will Evohome tell the boiler that it wants DHW as opposed to CH? Will the boiler then be clever enough to control the 3-port valve itself, or do I need to also have a BDR91 connected to the DHW trigger at the boiler?

How would this then look in my configuration? If I enable the Hot Water, it looks as though I will have options to bind:
Hot Water Sensor (obviously this will be the sensor that comes with the DHW kit)
Hot Water Valve (is this a BDR91 connected to the ATAG Boiler DHW trigger, a BDR91 connected to the existing 2-port DHW valve, or the existing opentherm bridge currently bound to boiler control)?
Heating Valve - what would this be in my scenario? Just leave it unbound?

Thanks in advance for any help here.

Jon.

[jon798]

Just a slight follow up: Atag support have confirmed that if the Honeywell OpenTherm bridge provides a DHW demand command (as opposed to a CH demand), then the boiler will fulfill that DHW command, raising the flow temperature and controlling the 3-port valve.

My only unknowns are:
1) Whether the Atag A200S support receiving the DHW command via OpenTherm.
2) Whether the Honeywell OpenTherm bridge sends a DHW command.

Then, assuming these two are both good answers (i.e. yes and yes), then I should be able to remove the existing DHW 2-port valve and just add the cylinder sensor and receiver to the Evohome system.

How do I configure the Hot Water Valve and the Heating Valve? Or don't I?

Thanks for any help...

Cheers,

Jon.