How to read an HVAC electronic schema

brad_lub used Ask the Experts™
I have an 6-zone HVAC panel (non-heat pump). One of the zones (let’s say zone-6) is connected to a thermostat in a small sun room. I have (2) other zones (let’s say zones 2&4) that have their own thermostats. The issue is that when zone 6 calls my 5-ton AC/Heater is not conditioning any other zones; in similar fashion, when zones 2 or 4 call, zone 6 does not call.

Intended functionality:
If either zone 2 or 4 calls, I would like to have zone 6 call as well (i.e. vents open at zone-6). If zone 6 calls, then zone 6 vents open as well as zone 4.

I have attached an electronic schema of a possible solution.

(1) does anyone see any issues with the attached solution?
(2) any suggestions of where I can purchase the relay switches?
(3) what gauge wire should I use to connect all this?



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Start with the basics:

R = 24VAC
W1 = Heat stage1
W2 = Heat stage2
Y1 = AC compressor stage1
Y2 = AC compressor stage2
G = Fan
O = Reversing valve, cooling
C = 24VAC common

I assume the "AirBoss Unit" controls a set of dampers that direct air from the HVAC unit to various zones. The only reason it has terminal strips that mimic thermostat -> HVAC unit communications is that it makes it easier for a HVAC trained tech to connect.

With that out of the way:

(1) does anyone see any issues with the attached solution?
Yes, zones 4&6 are now paralleled. Without more detail on the total system I can't say much else. Specifically how are the thermostats and the HVAC unit(s) connected tot he AirBoss unit. Is this your drawing? If not where did ic come from?

(2) any suggestions of where I can purchase the relay switches?
Mouser, Newark, eBay. I'd use socketed relays to make troubleshooting easier.

(3) what gauge wire should I use to connect all this?
18 AWG

Is this your AirBoss system?

There are some priority zone methods here.

Or give them a call at (800) 611-8312. If this is your system the relays are needlessly complex since they have another method using the heat and air stage 2 connections.


Lewisq: Thanks for the comments... excuse the delay in my comments.

Yes, this is my AirBoss unit. I got the schema from an Arzel technician. Admittedly I am very weak in HVAC wiring and relay setup but I do have some background in working with detailed procedures and wiring; I built a television when I was in my teens (Heath-kit) and I routinely build PC's from their components.

I am posting here in that I do not want to pester the tech with basic questions... he was kind enough to send me the schema.
You said:
<<Yes, zones 4&6 are now paralleled. Without more detail on the total system I can't say much else. Specifically how are the thermostats and the HVAC unit(s) connected to the AirBoss unit.>>
-- I'm not clear how they are connected...
Q: What do you mean by 'this is now paralleled'?  

<<Is this your drawing? If not where did it come from?>>
-- this is answered above...

Q: what does the black wiring connecting 4 and 6 mean (to the right of zone 4 and 6) ?

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I assume zone 6 has no thermostat.

So what you have is:
** Zones 4 and 6 paralleled, that is the "black wiring".
** Relays controlling zone 6 from zone 2.

Since you got the diagram from the manufacturer I assume it is correct. It does make sense and I see no reason it wouldn't work.


Thermostats are essentially three simple (SPST) switches:
** A heat switch that comes on when the temp drops below a setpoint.
** A cool switch that comes on then the temp rises above a setpoint.
** A fan switch that comes on manually OR automatically when either of the other two switches comes on.
** These switches, when closed carry 24VAC from the R (24VAC) terminal to W1(heat), Y1(cool) and G(fan) terminals.

So when zone 4 is in control the current is simply paralleled to zone 6.

Zone 2 is a little trickier since you don't want zone 4 to turn on zone 2. Relays are used. A relay is a electromechanical switch. When sufficient current is applied to it's coil it closes (or opens) a switch. Terminals that are marked NO (normal open) are closed when current is applied to the coil.

You have 3 relays. One each for heat, cool and fan. For example, when zone 2 calls for heat the top relay (black wire) coil is energized and completes the circuit from R to W1 on zone 6. If the thermostat in zone 2 is set to Fan Auto then relay below the heat relay (green wire) will also pull in and 24VAC will go from R to G in zone 6. If instead the fan setting on the zone 2 thermostat was set to On then the relay would already be pulled in and the fan would already be on.

Remember, the idea is to turn ON zone 6. None of this will result in zone 6 being turned off unless both zones 2 & 4 are off. It does seem like zone 2 would turn on zone 4 BUT I am assuming the AirBoss unit takes care of that. After all they said this would work. But I would check once it is connected and be SURE zone 2 doesn't control zone 4. If it did another set of relays would clear it up. You would do the same thing for zone 4 and was done for zone 2. But, it should work as diagrammed.

Parts: Relays, Sockets and Wire

Don't worry that the relay I linked has 4 unneeded terminals. If you can find a better deal on a 24VAC coil SPST socketed relay then go for it! If you are puzzled by SPST, SPDT, DPST and DPDT then go here.


Lewisq: I appreciate your detailed response... very helpful.

(1) what are the benefits of having a socketed relay? It seems much less expensive to go without the socket?

(1.1) does the socket allow me to attached the wire with screw driver and not soldering the wire?

(2) how would you recommend I mount these socketed relays? I'm thinking maybe a small cutout plywood section?

Thanks again!
The advantage of socketed relays are ease of installation and troubleshooting. In this case simply removing the relays will disconnect zone 2 from zone 6.

Most relay sockets have screw terminals.

Any time I have stuff to mount I count the screws. I can mount a board with 2 or 4 screws. In this case you can mount the relays with 3 or 6. Another possibility with these sockets is DIN rail. Using that you could mount all three relays with 2 screws


Great help.... thanks!

Good luck with your project. If you have problems post back in this thread so I will get a notification.

I built a good amount of Heathkit, Dyanco and SWTPC stuff years ago. Still have a few of them and they still work!

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