Old Honeywell Chronochrome II t-stat Existing zone controllerI am changing thermostat but I see that I have both an orange and a blue wire in use coming from the zone board. I found the c wire hooked up at the zone board but not on the old t-stat. I assume I will use that wire now but I don’t have single posts for the orange and blue wires on the new thermostat but I do have one that is labeled O/B. Any thoughts or advice? The new thermostat is a Honeywell T5 and the zone board is an EWC ST 2B

  • Can you post photos of the wiring at the zone controller and at the thermostat location? – ThreePhaseEel Nov 16 '19 at 21:11
  • Trying to figure out how to upload photos – user109167 Nov 18 '19 at 13:10
  • I take it you only want the first (master) zone to control system changeover, right? – ThreePhaseEel Nov 19 '19 at 2:56
  • I am only changing the t stat on zone 1 now. Zone 2 will be changed later if I get through zone 1 without any issues. The new t stat directions say if you have an O and a B to put the B wire in the C post. I don’t want to burn out anything though so I am reluctant – user109167 Nov 20 '19 at 10:44

This is going to require a bit of...help.

Your problem is that your zone board uses its O and B inputs to determine how to run the underlying HVAC system, while the Y and W inputs are used to control the damper for the zone. (Residential zone boards are basically very crude versions of what is called a Variable Air Volume, or VAV, zoning system in the commercial HVAC world.)

Your old (electromechanical) thermostat was able to provide these outputs independently of Y and W as it was...rather dumb, simply providing them through extra contacts on the thermostat's changeover switch. However, while the T5 is a much more sophisticated thermostat overall, it makes some assumptions about the way HVAC systems work that aren't true for you. In particular, it assumes that any O or B signal present is used to control a heat pump's reversing valve (some systems use O, others B, the difference being whether you need to energize the signal to get the thing to heat, or energize the signal to get the thing to cool), and thus you only need one or the other, not both at the same time, such as in your case.

So, we'll need to add some parts to adapt your new thermostat to work with your zone board. In particular, we'll need:

  • 4 DPDT HVAC (fan) relays (24VAC coils, quick-connect terminals, flange mount)
  • Some 18/8 thermostat cable you can shuck for 18AWG solid jumpers
  • Some wirenuts suitable for 18AWG wire, and some red (22-16AWG) female quick connects
  • Self-drilling sheet metal screws suitable for mounting the relays into the zone board's housing
  • A crimper (preferably of the ratcheting type) for attaching the quick connects to the wires, and a screwdriver and wire cutter/stripper (of course)

All these relays get wired as in the diagram below, where W, Y, R, C, R1, O, and B designate terminals on the zone board and corresponding relays in the circuit, and the wires are color-coded largely according to standard HVAC color codes, with green and black being used for internal functions. What you are doing here is using a pair of stick circuits to create a memory of sorts that stores the last function (heating or cooling) the system was using until a call for the opposite function is made, which causes the system to change over between heating and cooling at that time. Of course, if you aren't comfortable wiring this, this job is well within the domain of a skilled HVAC technician or controls electrician.

circuit wiring diagram

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  • Thanks so much. I really appreciate the extensive effort you have put into this. If you were close by I would by you a beer or two. I am always up for a challenge. I will give it a try. 5 stars for you – user109167 Nov 25 '19 at 0:24
  • @user109167 -- we thank folks around here by upvoting and/or accepting their answers :) – ThreePhaseEel Nov 25 '19 at 2:02

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