Honeywell AT175F Wiring Help

[GrizzlyAK]

Hi, I'm replacing an old AT88A with the AT175F for my hydronic heating, which is connected to my 2-wire mercury t-stats and Honeywell V8043F zone valve motors (yellow leads). Wiring diagrams always show the "hot" red wire from the transformer going first to the t-stat, then white wire from t-stat to zone valve, then from zone valve back to "common" on the transformer. The two secondary wires on this transformer are Blue and Yellow, and the "common" on the Primary side is Black. The footnotes on the instructions say: "Black is common with respect to the transformer winding only and not the external circuit".

I need some clarification on how this should be wired:
(1) On the Primary side, the Black lead should be connected to Line White (Neutral) and the Red lead should be connected to the Line Hot (Black) (120V). Is this correct, or does it matter?
(2) On the Secondary side, which of the Blue/Yellow leads should I consider as the "red/hot" wire shown in the diagrams for wiring up to t-stat? And, does it even matter?
(3) If if doesn't matter in #2 above, and I want to use this setup with a smart t-stat that requires a third C-wire, does it THEN matter which is connected to R/Rh on the t-stat?

You can find the link to the AT175F manual here.

Any advice would be appreciated.
Thank you.

 

[GrizzlyAK]

NOTE: I've moved this post to the DIY Forum.

 

[Avo Mk8]

I need some clarification on how this should be wired:
(1) On the Primary side, the Black lead should be connected to Line White (Neutral) and the Red lead should be connected to the Line Hot (Black) (120V). Is this correct, or does it matter?

You have the right idea, but the red transformer wire is the 208V tap according to the data sheet. The white transformer wire is the 120V tap. Use whichever corresponds to your supply voltage.
I don't believe it actually matters which way round.
The unused ones obviously need to be insulated as they will be live!

(2) On the Secondary side, which of the Blue/Yellow leads should I consider as the "red/hot" wire shown in the diagrams for wiring up to t-stat? And, does it even matter?

It doesn't matter. The secondary is 'floating'

(3) If if doesn't matter in #2 above, and I want to use this setup with a smart t-stat that requires a third C-wire, does it THEN matter which is connected to R/Rh on the t-stat?

I don't know, but if you have more information on the t-stat I might be able to comment.

Any advice would be appreciated.
Thank you.

 

[GrizzlyAK]

It doesn't matter. The secondary is 'floating'

That's what I would assume as well, but I keep seeing wiring diagrams like this that indicate that one lead is "hot" and one is "common", and the "hot" side always goes to the T-stat, white from T-stat to Zone motor, and then back to the "Common".



And, as for #3, what I am trying to do is install a smart thermostat that requires a C wire (for continuous power) to replace my old 2-wire mercury switch t-stats (which just acted as a 2 wire switch). I have a third wire, so I can use it as C, and I assume that I can connect it to the transformer where the return from the Zone valve is. This would basically give me a circuit from one side of the transformer to the R on the T-stat and a return from the T-stat via C to the OTHER side of the transformer, which would provide 24Vac for operation. That's my end goal.

I appreciate your previous response and help provided.

 

[Avo Mk8]

I'm afraid I'm not clear whether the smart stat requires continuous mains (120v) power, or if it requires 24v?
If mains, I would hope it has its switching circuit(s) isolated from mains, so the 24v secondary control side can remain not connected to mains voltage, which would seem safer.
Could you point me to technical details for the smart stat?

Update - I see the Honeywell valve manual has some example wiring diagrams that are vaguely similar to what you describe, eg pages 5 and 6 etc.

http://www.honeywellmanual.com/pdf/ea156978-8edb-4793-8014-c6a1af3e0932.pdf

So I think what you are proposing is OK. Just be careful to treat all your wiring as if it is at mains voltage!
Again from the info I see, it won't matter which way round the transformer secondary leads are connected.

 

[Dseelye]

That's what I would assume as well, but I keep seeing wiring diagrams like this that indicate that one lead is "hot" and one is "common", and the "hot" side always goes to the T-stat, white from T-stat to Zone motor, and then back to the "Common".

View attachment 91968

And, as for #3, what I am trying to do is install a smart thermostat that requires a C wire (for continuous power) to replace my old 2-wire mercury switch t-stats (which just acted as a 2 wire switch). I have a third wire, so I can use it as C, and I assume that I can connect it to the transformer where the return from the Zone valve is. This would basically give me a circuit from one side of the transformer to the R on the T-stat and a return from the T-stat via C to the OTHER side of the transformer, which would provide 24Vac for operation. That's my end goal.

I appreciate your previous response and help provided.

It is typical to see a low voltage ac transformer with a + and -. All this means is the - or neutral is terminated to ground at the transformer inside the unit. It sounds like your new thermostat requires a constant 24v, just like you mentioned this can easily be done with a 3 wire. One wire on the R in the unit. One wire on the heat(W). One wire on the common(c) or you can simply land this wire on the grounded side of the 24v transformer. You will notice that between common and ground you have continuity.

 

[Avo Mk8]

You will notice that between common and ground you have continuity.

That's interesting. There was no mention of that in the data sheet of the transformer, so I'd assumed the secondary was floating. Would be worth a check.

 

[Dseelye]

That's interesting. There was no mention of that in the data sheet of the transformer, so I'd assumed the secondary was floating. Would be worth a check.

It won't be mentioned in the data sheet for the transformer, and can be used without grounding one side. It usually only grounded inside of a unit like a furnace. This is were the terminology for "neutral" or - comes from inside of an AC circuit. In your scenario the secondary (24v)may not be grounded.

 

[GrizzlyAK]

In your scenario the secondary (24v)may not be grounded.

I just checked and did not get continuity between either secondary lead and the transformer mount frame.

...this can easily be done with a 3 wire. One wire on the R in the unit. One wire on the heat(W). One wire on the common(c) or you can simply land this wire on the grounded side of the 24v transformer.

Without knowing the grounded side... here where I am now.

I just completed doing some rewiring of my thermostats. Whomever wired this did the following:

(1) Upstairs Tstat - connected to system via a single 3-wire conductor
(2) Garage and Living Room - connected to system via a shared single 3-wire, where W was common to all and connected to one side of the TF, and Blue was used by one Tstat and Red by the other (i.e., the conductor ran through one thermostat and terminated on the other) both of which ran through the respective Zone Valve, terminating back on the other side of the transformer.
(3) Downstairs and Master BR - same as #2, running throught the downstairs tstat. Wire color scheme was consistent.

Result was that I only had one zone with a complete 3-wire run, the Upstairs. I noticed that I could easily reach the downstairs and garage thermostats from the crawlspace, so I ran two additional 3-wire conductors to each of those, and removed those tstats from the runs that terminated in the Living Room and Master BR, freeing up the 3rd wire in the other two. Now I have 3-wire conductors running to all 5 zones. I have one of the new smart tstats I'm testing for a company that I am going to hook up.

I think I now have what I need, but I'm waiting on a response from the company because I'm not sure if the tstat is looking for 24VAC power across R-C or W-C. I assume R-C, but they labeled 'R' in their manual as 24VAC RETURN. It probably doesn't matter, but one of the two (R or W) is connected directly to one side of the transformer, and the other is connected to the Zone Valve motor (which I believe is typically W).

Now, in the case of R being connected to the transformer, and W to the Zone Valve, my thinking is that I would connect my C from the tstat to the transformer side of the Zone Valve motor lead (the OTHER yellow lead). As I proposed before, that would give me a continuous flow of 24VAC whether the tstat was calling for heat or not. I'm just waiting to hear which tstat terminal needs to go directly to the transformer. If any of that sounds off, please let me know.

You guys have been very helpful. Thank you for your help and responses.

Cheers!

 

[Dseelye]

I just checked and did not get continuity between either secondary lead and the transformer mount frame.

Without knowing the grounded side... here where I am now.

I just completed doing some rewiring of my thermostats. Whomever wired this did the following:

(1) Upstairs Tstat - connected to system via a single 3-wire conductor
(2) Garage and Living Room - connected to system via a shared single 3-wire, where W was common to all and connected to one side of the TF, and Blue was used by one Tstat and Red by the other (i.e., the conductor ran through one thermostat and terminated on the other) both of which ran through the respective Zone Valve, terminating back on the other side of the transformer.
(3) Downstairs and Master BR - same as #2, running throught the downstairs tstat. Wire color scheme was consistent.

Result was that I only had one zone with a complete 3-wire run, the Upstairs. I noticed that I could easily reach the downstairs and garage thermostats from the crawlspace, so I ran two additional 3-wire conductors to each of those, and removed those tstats from the runs that terminated in the Living Room and Master BR, freeing up the 3rd wire in the other two. Now I have 3-wire conductors running to all 5 zones. I have one of the new smart tstats I'm testing for a company that I am going to hook up.

I think I now have what I need, but I'm waiting on a response from the company because I'm not sure if the tstat is looking for 24VAC power across R-C or W-C. I assume R-C, but they labeled 'R' in their manual as 24VAC RETURN. It probably doesn't matter, but one of the two (R or W) is connected directly to one side of the transformer, and the other is connected to the Zone Valve motor (which I believe is typically W).

Now, in the case of R being connected to the transformer, and W to the Zone Valve, my thinking is that I would connect my C from the tstat to the transformer side of the Zone Valve motor lead (the OTHER yellow lead). As I proposed before, that would give me a continuous flow of 24VAC whether the tstat was calling for heat or not. I'm just waiting to hear which tstat terminal needs to go directly to the transformer. If any of that sounds off, please let me know.

You guys have been very helpful. Thank you for your help and responses.

Cheers!

The polarity on the secondary of an AC transformer is irrelevant (There is no + or -) if it's not grounded. Typically we use Blue from the transformer directly to the R on the thermostat. The yellow from the transformer would go to the C on the thermostat as well as one of the wires going to zone valve. Your third wire would be your switch wire which will land on W on the thermostat, this wire lands with the second wire going to your zone valve. Now because you have multiple thermostat all need to be wired in the same manner.

 

[Dseelye]

I just checked and did not get continuity between either secondary lead and the transformer mount frame.

Without knowing the grounded side... here where I am now.

I just completed doing some rewiring of my thermostats. Whomever wired this did the following:

(1) Upstairs Tstat - connected to system via a single 3-wire conductor
(2) Garage and Living Room - connected to system via a shared single 3-wire, where W was common to all and connected to one side of the TF, and Blue was used by one Tstat and Red by the other (i.e., the conductor ran through one thermostat and terminated on the other) both of which ran through the respective Zone Valve, terminating back on the other side of the transformer.
(3) Downstairs and Master BR - same as #2, running throught the downstairs tstat. Wire color scheme was consistent.

Result was that I only had one zone with a complete 3-wire run, the Upstairs. I noticed that I could easily reach the downstairs and garage thermostats from the crawlspace, so I ran two additional 3-wire conductors to each of those, and removed those tstats from the runs that terminated in the Living Room and Master BR, freeing up the 3rd wire in the other two. Now I have 3-wire conductors running to all 5 zones. I have one of the new smart tstats I'm testing for a company that I am going to hook up.

I think I now have what I need, but I'm waiting on a response from the company because I'm not sure if the tstat is looking for 24VAC power across R-C or W-C. I assume R-C, but they labeled 'R' in their manual as 24VAC RETURN. It probably doesn't matter, but one of the two (R or W) is connected directly to one side of the transformer, and the other is connected to the Zone Valve motor (which I believe is typically W).

Now, in the case of R being connected to the transformer, and W to the Zone Valve, my thinking is that I would connect my C from the tstat to the transformer side of the Zone Valve motor lead (the OTHER yellow lead). As I proposed before, that would give me a continuous flow of 24VAC whether the tstat was calling for heat or not. I'm just waiting to hear which tstat terminal needs to go directly to the transformer. If any of that sounds off, please let me know.

You guys have been very helpful. Thank you for your help and responses.

Cheers!

Also just realized something you said. This transformer dose not have an internal ground, This would be a external grounding. One of the secondary wires to ground from line (typical for 24v AC transformer in a furnace). For this application it is not necessary to have a ground.

 

[GrizzlyAK]

Great! Thanks guys. Based upon your comments, insight, and feedback, I think I'm OK to move forward with my original plan. Notwithstanding the wiring color confusion in my installation, now that I understand HOW it's wired, I can move forward to installing the smart stat with the wiring for C that I described earlier, but which is much better illustrated in the modified image below. My smart stat comes with RH and RC jumpered by default (for single transformer systems, which mine is), and labeled the connectors as 24RH, 24C and 24RC (plus others), which indicates that I wire my transformer across the 24C and 24RH for 24Vac power - blue to 24C, the other two to 24RH and W1. Other wire colors notwithstanding, I will wire the blue C wire as shown below. Thank you for your help.

 

[Dseelye]

Great! Thanks guys. Based upon your comments, insight, and feedback, I think I'm OK to move forward with my original plan. Notwithstanding the wiring color confusion in my installation, now that I understand HOW it's wired, I can move forward to installing the smart stat with the wiring for C that I described earlier, but which is much better illustrated in the modified image below. My smart stat comes with RH and RC jumpered by default (for single transformer systems, which mine is), and labeled the connectors as 24RH, 24C and 24RC (plus others), which indicates that I wire my transformer across the 24C and 24RH for 24Vac power - blue to 24C, the other two to 24RH and W1. Other wire colors notwithstanding, I will wire the blue C wire as shown below. Thank you for your help.

View attachment 92020

That looks correct!!

 

[GrizzlyAK]

I've got it installed and integrated into my Home Automation system and it works perfectly! I can now control that thermostat from anywhere there is an internet or cellular connection! Thanks everyone. This forum rocks.