Wiring AQ475A as 2 stage

Flash_2

I would like to set up the AQ475A outdoor reset controller as a 2 stage system with the T8624D thermostat. The first stage would call for lower temperature (140-190*) water, as regulated by the AQ475, and the second stage would call for max temperature (180*) hot water. See diagram below.

Setting things up this way would permit the AQ475 to be set to a lower design temperature, thus using lower temperature water and keeping the circulator running more constantly. This is because the T8624 will automatically engage the second stage (ie full temperature hot water)when it comes out of overnight setback, as well as when the thermostat is manually increased in the evening when people get home, giving a reasonable recovery time. I expect the second stage may also engage if the front door is left open a bit, but that it might take a while.

I do have indirect fired DHW, but it does not need priority. So, I would like to use the AQ475 DHW demand terminals to sense the call for the second stage, and wire the DHW aquastat in parallel with the AQ475 boiler output. I think the AQ475 should be fine with this, as the
last page of the instructions states "Note: Factory supplied boiler controls can also energize the boiler independently from the AQ475A." The thermostat may require a resistor in parallel with the HW inputs if they do not present the thermostat enough of a load.

Doesn't a 2 stage outdoor reset scheme make sense? I haven't seen any discussion about it.

Also, does anyone see a problem with the control wiring I describe?

Thanks!