Best option for two thermostat C-Wire placement on dual-zoned boiler [Crown AWR Aruba]

FIshermanPete

I have a Crown AWR boiler with the DHW/Zone2 setup as a secondary zone with it's own thermostat. I am upgrading our current thermostats from two older circular dials (only Rh and W) to two Honeywell T6 z-wave. Each of the old thermostat wiring runs have 5-wire cable so no need to run a new line.

Looking at the schematics it looks like I have a few different options for a COM wire but I'm unsure if one is better than the others.

• Option 1: Share a C-Wire between both thermostats terminating at either P9-3/P3-3 (EnviraCOM terminals preferred as it's simply a screw post). I do not know if sharing a C-wire in this use case is acceptable.
• Option 2: Similar to above but sharing a C-Wire to the J-Box Ground Screw.
• Option 3: Wiring directly to the 24v 40VA transformer and splicing that to Rc+C on the thermostats. Might I need to buy a new ~50VA transformer?
• Option 4: Buying a 24vac power supply and splicing that to Rc+C on the thermostats.

All these seem like easy solutions but I'm hesitant to short the boiler's electronics in December. I appreciate any recommendations!

EBEBRATT-Ed

P4-2 is where I would go. I would terminate both Common wires from the stats there. There are other place you could connect to like Envirocom terminal #3

HVACNUT

EnviraCOM P9-3 looks like a set screw you can land the Common right there.

Also use P5-7 as Rh at the thermostat and P5-9 as W.

EdTheHeaterMan

Before you do anything I need to completely understand your definition of two zones. Do you have two central heating (CH) zones and a DHW zone (that would be 3 zones in my book). or do you have one central heating zone and one DHW zone? ( that would be 2 zones in my book).

That will determine where you get the C wire from.

If you have only one CH zone that needs a C wire, then that is easy. First You must connect R from the thermostat to the wire coming from the 9 pin plug P5 yellow wire from the #7 pin and the W on the thermostat to the 9 pin plug P5 yellow wire from the #9 pin. That is important in order to get the R from the transformer and the C from the transformer in phase with the thermostat. Then any ground connection on the control system will be connected to the C on the thermostat. See diagram Below

So basically your option #2 is the best option. and if it does not work the first time then swap the yellow wires.

HOWEVER, IF YOU HAVE 2 CH THERMOSTATS THEN THIS IS WRONG

With two thermostats you need to use the transformer that the thermostat is connected to not the transformer from the heater.

EdTheHeaterMan

After thought. You may have a DHW indirect that has one of those electronic aquastats that also requires a C wire. you can do the same thing as long as you use the P* #2 pin for the R and the P8 #1 pin for the W. then keep your fingers crossed that your transformer has enough VA for those two extra loads.

FIshermanPete

Thank you for the advice.

The boiler has a secondary input that can either be used for DHW or a second CH zone. It is currently setup as a secondary CH zone (P8-Rh/W) for an addition added to the house in the 80's running baseboard radiators (primary zone (P5-Rh/W) used to supply main structure built in late 1920s with cast iron radiators). Each zone is controlled by a thermostat in different locations in the house.

p.24 of the manual:

Low Voltage Connections –

Low voltage field connections are located as shown in Figure 9.3 and are as follows:

a) Heating (CH) Thermostat - Connect to a 24 volt thermostat or other “dry contacts” (such as a zone panel end switch) that close upon a call for heat. Provide Class II circuit between thermostat (or zone controls) and boiler.

b) DHW Thermostat - If used, connect to a 24VAC domestic hot water thermostat or 2nd space heating thermostat.

I was thinking last night that the J-Box ground was the best bet and I opened it up to inspect the wiring. I'm now guessing that Crown actually designed it to be used as the COM as there is an extra/unused 8" ground wire spliced into the main ground ring terminal on the ground screw.

I was hoping it was as simple as putting a wire nut on the 3 wires (GND+C+C) but I'm now questioning that approach. I find it interesting that the main zone (P5) calls for the thermostat to be Class 2 dry contact, and the DHW zone (P5) does not. Does the main call for a relay or going with option 3 from first post?

Thanks again!

Edit: The old thermostats are millivolt but I've been going forward under the assumption that the newer boiler was designed to power 24vac thermostats. I do not see anything in the manual that explicitly details one way or the other (unless "note a" above is that designation)

EdTheHeaterMan

It sounds like you have a grasp on the situation. Your common wire connection sounds correct. I believe that you can use the same Class 2 dry contact thermostat on both the DHW and the CH leads. Crown boilers are very user friendly in most cases. Also their Tech Support is top notch.