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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I want to replace my honeywell interface thm5421c with a

Customer Question

I want to replace my honeywell interface thm5421c with a honeywell thm5421r there are three wires off of lugs one, two, and three from the honeywell thmn5421c that I am unsure where to attatch to the thm5421r....also the instructions give a warning about hooking a sensor up to the S is the outdoor temp sensor on the 5421c board. Can you tell me if this is safe and where do I hoo?k the 5421c 123 wires up to one the 5421 r board
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: couple of weeks...the part I ordered online is supposed to be compatible with the VisionPRO IAQ and a replacement for 5421c interface
JA: Do you plan on doing the work yourself?
Customer: if I can
JA: Anything else we should know to help you best?
Customer: I feel comfortable with all the wired after stydying it ....the wires from hub s 123 are labeled communications and appear to go to the heat pum[
JA: Did you double-check the thermostat's settings? And what about the filters?
Customer: Yes...a tech came and looked at it and said the interface board was wet....he dried it off and it worked temporairly. his boss wanted to replace the whole system for a large sum of money but the tech suggested I try to find a replacement interface to save some money. I found a interface that is supposed to work but I am unsure of the the three wires and the outside temp sensor
Submitted: 6 months ago.
Category: HVAC
Customer: replied 6 months ago.
The honeywell thm5421c is the interface for the honeywell VisionPRO IAQ. This interface is discontinued but is supposed to be replaceable with the thm5421r. They are operating a high efficient gas furnace and a heat pump. If it is above thirty degrees the heat pump is activated. If below thirty degrees the furnace is activated. There are eleven wires to the 5421c. from top to bottom left to right the hubs (or lugs) are numbered 1,2,3,c,r,w1 o/b,w2 aux, y,g, outdoor temp sensor1, outdoortempsensor2. The 5421r board has similar labeling for the hubs except for the wires in hubs 1, 2, and 3. I am unsure where to hook these on 5421r. The installation instructions also says sensors can be hooked on hubs s1 to s4 but gives a warning do not connect a temp sensor to s1 to s4 if set up for a dry contact devise. Is the outside temp safe to hook up? both instructions and schematics for the thm5421c and thm5421r are available to view online.
Expert:  gotoman for ac replied 6 months ago.

Russ here. On the old C model 1 2 3 went to thermostat. On the new R model only 2 wires are needed at thermostat. Connect two of those wires to R and C at top right of EIM to R and C on thermostat.

Customer: replied 6 months ago.
Ok...does it matter which two of the three wires I pick?...When I checked them with a volt meter they had different voltages. Also do I hook up the outside temp sensor to one of the S1 through S4 hubs? The manual gives a dry contact ALERT ON these hubs and says DO NOT APPLY POWER TO S1-S4 TERMINALS. DO NOT CONNECT A TEMPERATURE SENSOR TO THE S1-S4 TERMINALS IF SET FOR A DRY CONTACT DEVISE. The outside temp sensor would be the only devise I would be using on these terminals.
Expert:  gotoman for ac replied 6 months ago.

Red to R the other White to W.

Expert:  gotoman for ac replied 6 months ago.

S terminals are for a dry contact on outside sensor. Normally open. Russ

Customer: replied 6 months ago.
Can I hook my outside temp sensor to the S terminals? The wires one two and three do go to the thermostat on the wall. They are not labeled at that point. they are colored respectively, 1 is purple, 2 is blue and 3 is grey. Which two wires do I hook to R and C terminals on the upper right corner of my EIM board?
Customer: replied 6 months ago.
both schematics for the old and new board are online.
Expert:  gotoman for ac replied 6 months ago.

My mistake you only have to use two wires. One to R and One to C. Use Purple and Grey . Disregard Blue or tape off.

Customer: replied 6 months ago.
Ok got it....What terminals do i hook the two wires from my outside temp sensor to?
Expert:  gotoman for ac replied 6 months ago.

S1 terminals

Customer: replied 6 months ago.
ok did exactly what you 27 volts going to thermostat but it will not light up or respond...what do I check next?
Expert:  gotoman for ac replied 6 months ago.

What model thermostat do you have. Russ

Expert:  gotoman for ac replied 6 months ago.

If you have 24 volts to thermostat at R and C it should light up unless you have a bad connection or bad thermostat. Russ

Customer: replied 6 months ago.
I have the model VisionPRO IAQ. There are three wires to the thermostat labeled one=purple two = blue three = grey. I have no idea which terminals are R or C. I have looked online for this info. I eliminated two, the blue or middle terminal as per your instruction. It has 27 volts to one (purple) and three (grey). The thermostat lit up with the old board but would not activate any thing which it displayed it was doing. Can you tell me of the one two and three terminals which ones are R and C?
Customer: replied 6 months ago.
The voltage was 27 volts on the old system as well.,
Expert:  gotoman for ac replied 6 months ago.

I have been working on your problem off and on. The thermostat you have in not compatible with the new interface. It uses a two wire control on R and C. Where the old used a 3 wire where 1 was a data wire. 2 terminal was power wire and 3 was the common wire.

Expert:  gotoman for ac replied 6 months ago.


Customer: replied 6 months ago.
The people that sold it to me listed it as the compatable replacement for that thermostat (the old board was discontinued) and I spoke with a tech that took my order said it was compatable. So I think that you are saying that 2 is R (power) and 3 is C (common) ? Will ha ving had hooked a power wire up to the data pin fried the thermostat?
Customer: replied 6 months ago.
I wired it exactly as you said. You said to not use the blue wire which was hooked to 2 but use the grey and purple wire which was hooked to one and three. you were not wire specific so the power from the board (R) got sent to pin one. Would this have fried my thermostat?
Expert:  gotoman for ac replied 6 months ago.

These thermostats are connected wrong all the time. All it will do if anything is blow fuse. Even if you connect the Power to 2 and 3 you will still need data terminal. Russ pi