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Old 09-21-2013, 10:22 AM   #1
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Questions about installing a new thermostat


I have a Carrier furnace and a/c. The Furnace is model 58CTA090-14. The cover of the owners manual says
58CTA/CTX
2---Stage Deluxe, Induced Combustion
4---Way Multipoise Gas Furnace
Series 140/E
I'm planning on getting a new Honeywell programmable thermostat to replace the current Honeywell stat which is about 15 years old. It only has terminals for W,Y,G and Rc.
In reading through the new tstat manual online it says you can identify a 2-stage furnace because your old tstat will have wires on W and W2. My old tstat didn't have a W2 screw. Looking at the control board on the furnace there is a terminal for W2, which is unused. The one used is identified as W/W1. The new tstat (Honeywell RTH7600D) does have a W2 terminal. The tstat wiring has a couple extra wires that were snipped, so I could easily connect a wire from W2 on the furnace to W2 on the tstat. Should I do this? Will I gain anything by doing it?
Similarly, the installation instructions say a 2 stage cooling system will have wires on Y and Y2. My old tstat only has a Y, but the control board on the furnace has a Y/Y2, which is connected to Y on the tstat, and a Y1, which is unused. The new tstat has a Y and a Y1. Should I run a wire from Y1 on the board to Y1 on the tstat?
Lastly, the tstat can run on batteries, but also on 24V from the furnace. There is a Com 24V terminal on the control board. Should I run a wire from there to the C terminal on the tstat? (I'm also considering another Honeywell tstat, RTH9580WF, which apparently requires 24V from the furnace. I would use the Com 24V on the board for this, correct?)
Honeywell has a customer service line to answer questions like this, but I'm a lot more comfortable getting answers from experts here, rather than someone I can't understand in some foreign country, so thanks for your help!

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Old 09-21-2013, 10:42 AM   #2
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Questions about installing a new thermostat


1) U have a 2 stage furnace but it is using a timed on feature on the board 2 switch stages. after 7 mins it goes 2 high fire. you can use a 2 stage stat but you have to flip a dip switch on the board to activate the W2 and we can find it 4 U and it is in the install manual.
2) If you don't want 2 use batteries then you can run a C wire. personally I would use batteries as they last 1-2 yrs if U use good Energizers
3) Unless U have a 2 stage AC you won't use a 2 stage cooling stat.
4) Comm or C on the board is C.
5) not sure about Y or Y1 on the new stat. need to read the manual for it. they should have a picture diagram in the manual and Honeywell manuals are very good and easy to follow.
6) a 2 stage heating stat will kick into high fire if the house temp drops more than 1 deg F and may save a few $$ as it may run longer on low fire than the timed on feature. very few people notice the difference in comfort.

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Old 09-21-2013, 11:01 AM   #3
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Questions about installing a new thermostat


Thanks, Yuri! Sounds like I'm better off if I just use the 4 wires I have now and just hook up the new tstat same as the old one, connecting it as a single stage system. IF I decide to go with the second tstat I mentioned then I'll connect one of the wires I'm not using from the Comm on the board to the C on the tstat. This second tstat allows you to make changes via WiFi, and has a color screen that stays on, so that's why it needs 24V power - otherwise I guess you'd be changing batteries all the time. Depends on what kind of a deal I can get on it on eBay.

I really appreciate you taking the time to answer all my questions here. How do you manage to get any work done?
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Old 09-21-2013, 11:07 AM   #4
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My house is all stucco and vinyl windows and has no painting and is fairly new so no work needs to get done. I grew up fixin cars and carbeurators with my old man on a 4 acre hobby farm so I learned to work at 6 yrs old. learned about technology and how things work from a very young age so it comes naturally and I like helping people (not sure why ).
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Old 09-21-2013, 11:11 AM   #5
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I would only buy a tstat from a proper heating supplier. Z bay etc you may get used or freight damaged or who knows what.
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Old 09-21-2013, 11:18 AM   #6
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Very true, but about half the price there. If it doesn't work I just return it and try another.
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Old 09-21-2013, 11:23 AM   #7
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Yeah, but if it blows your new board or causes intermittent problems later then what? I have seen where the major manufacturers get so many tstats and controls returned by techs and people mis-diagnosing problems ( parts changers) that they now test them and re-stamp them with a new date code and resell them probably to Z bay sellers at a discount price. looks new in a bag etc but not what I would want. you get what you pay for.
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Old 09-21-2013, 04:32 PM   #8
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By using the furnaces timer to control heat staging. You aren't getting the comfort you paid for when you got the furnace.

You have a single stage A/C. So the wire from the stat to the furnace board should connect to Y/Y2.

I'd run the stat off the furnace's 24 volts.
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Old 09-21-2013, 05:55 PM   #9
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Thanks for the reply, beenthere! So you're saying I should run a wire from W2 on the furnace control board to the W2 terminal on the tstat? And keep the cooling wiring the way it is? What's the benefit of having the tstat control things instead of the furnace?
I think I'll stick with the batteries (assuming I don't go with the more expensive tstat that needs 24V to keep the screen on). The batteries should last a couple years, and the tstat tells you when they're getting low, so shouldn't be a problem. Not sure what would happen if I hardwired it and the power went out - I wouldn't want to have to reprogram the schedule, and I'd probably have to reset the clock. Also, with the batteries I can take the tstat off the wall to sit and relax and program it, instead of standing facing the wall to do it.
Thanks again!
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Old 09-21-2013, 06:55 PM   #10
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If the power went out. The stat would still retain the program. You can have it use the furnace 24 volts, with the batteries as back up.

Using the stat to control staging. Keeps the furnace in first stage when second stage isn't needed. As it is now. there are times when it goes to second stage needlessly for a minute before the stat stops calling for heat.
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Old 09-21-2013, 07:49 PM   #11
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Thanks, beenthere. Yuri said I would have to flip a dip switch to use W2. I did a quick look through the installation manual to find out about this but didn't see anything.
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Old 09-21-2013, 08:11 PM   #12
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It is in the electrical section. there is a dip switch for low heat only and then high heat only and then a combo used to give you 2 stage.

a true 2 stage setup will run the inducer motor longer/more wear and tear and wear it out sooner. you don't get something for nothing. IMO very few people can tell the difference between timed on. technically your furnace (unique to Carrier) uses an adaptive algorithm to determine when to time on to high stage. it samples 15-30 cycles and the time it took to satisfy a cycle and then extends the low fire 1 minute longer at a time until it starts losing ground and dropping 2 deg F. complicated but if you understand an adaptive algorithm then you are OK. more than likely you will get a low heat cycle around 11 minutes and it can go as long as 15.
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Old 09-21-2013, 10:14 PM   #13
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Longer run time is better for a motor then turning on and off is. Just like a long run time is better for an A/C compressor.

I know of techs that had 2 stage units and left the furnace control staging. They thought their heat was great compared to the single stage they replaced. Then when they went to a 2 stage stat, they were amazed at how much more comfort they got leaving the thermostat control staging.
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Old 09-22-2013, 08:52 AM   #14
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Depends on the individual I guess. If it is easy to hookup and the wires are there go 4 it. If it is a huge hassle to run new wires because of a plaster ceiling etc then I would stay with the timed on system.

Those fans spin at over 3000 rpm probably 2000 on low fire and bearings wear over time. Plus the bearings are cheap as we know we can buy them for $100-150 so you don't get industrial lifetime quality bearings at that price point.

AC compressors are not so bad as they are sealed in oil.
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Old 09-22-2013, 01:12 PM   #15
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Questions about installing a new thermostat


If its hard to run new wires then I would go with a wireless or IAQ thermostat which requires less wires at the thermostat location. IMO, you should utilize the furnace to get what you paid for out of it by getting the most comfort afforded to you by utilizing a 2 stage thermostat.

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