Convert Viessmann combi to PDHW with unvented cylinder

[sonicboom]

Hi All,

 

Has anyone got any pointers on this ? I was going through the docs for the 100-w B1KF, and there is plug 96 which can be configured to external demand, whereby a thermostat can be connected to it. There is also a cylinder thermostat but it has (system) in brackets so I don't know if that would even function in the combi variant).

 

The plan is to have a NO value on the CH side and a NC on the cylinder. Leave CH Thermostat as is with weather comp etc and hook up the themostat to plug 96 and cyl stat to X5 in the boiler.

[JohnMo]

Various way to do it. My Atag combi is configured like this.

 

Switching WC on and off no plumbing needed

Cannot run Opentherm, cannot run on off control via simple thermostat (can but a few fudges needed and has to think it's in WC mode). If you are already running WC you could try this by disconnecting the WC outside sensor to see what the boiler actually does.

 

Set boiler to weather compensation mode and max flow temp set to 60 to 70 degs.

 

So cylinder heating control. Cylinder thermostat controls diverter valve kor 2x 2 ports) and a relay. The relay has the WC sensor wires going though it. When you power the relay the Weather Sensor is circuit is broken. Boiler now thinks it's no longer WC compensation mode, so ramps to 60 degs (could anything you set it too).

 

Once DHW cycle is complete, relay makes WC sensor circuit and boiler goes back to WC mode.

 

Convert to 4 port boiler. Plumbing needed

Another way, is use the DHW circuit that is there. Add a pump, expansion vessel filter, relief valve etc. make a loop direct to cylinder coil. DHW call for heat, starts pump. Boiler assumes tap is open and sets to DHW temp. This maybe the easier route?

Edited December 30, 2024 by JohnMo