Home Thermostat Wiring

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RMO87
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Location: Kansas City, MO

Home Thermostat Wiring

Post by RMO87 »

I am attempting to install a programmable thermostat (Honeywell RTH7500d) to replace my old non-programmable Lennox 51M33), but have a question as to which wires go where on the new one.

The old unit uses wires O, R, G, Y1, W1, C, E, T2, and T1, with a jumper between W1 and E, as well.

The quick start guide of the new unit mentions O, G, Y, Y2, W, W2, R, Rc, and AUX, so am a little confused by some of the 'extra' wires that may or may not be needed.

Thanks in advance for any guidance!
Ryan
tbradnc
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Re: Home Thermostat Wiring

Post by tbradnc »

A year or so ago I tried a Nest and then switched to an Ecobee. I found this forum very helpful:

http://hvac-talk.com/vbb/
lululu
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Re: Home Thermostat Wiring

Post by lululu »

Even the installer who takes care of my AC system couldn't figure out how to wire up my new Honeywell thermostat with the directions. He finally got it working, but gave up on the programming part.
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walkabout
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Re: Home Thermostat Wiring

Post by walkabout »

tbradnc wrote:A year or so ago I tried a Nest and then switched to an Ecobee. I found this forum very helpful:

http://hvac-talk.com/vbb/
I would also recommend this site:

http://ths.gardenweb.com/forums/hvac/

When we first moved into this house in 2001, I bought a programmable thermostat (maybe a Honeywell), thinking I would just label the old wires and connect to the new unit. I found, as you have, that the directions didn't seem to cover the wires that my old thermostat had. In the end, I called our HVAC company and had them install it. It took some time, including calling back to the office, to figure out the right combination, but they were eventually successful.

Good luck!
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Epsilon Delta
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Re: Home Thermostat Wiring

Post by Epsilon Delta »

At risk of being obvious, the manual for the old thermostat is on-line and might be useful because it describes how it should be connected. Look at table 2 on page 6. I'd copy it but it does not copy well.

http://www.lennox.com/pdfs/manuals/Lenn ... Manual.pdf
Khanmots
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Re: Home Thermostat Wiring

Post by Khanmots »

You may also want to look at your central air system; mine has a wiring diagram on it that helped me make sure that I had things connected right.
Topic Author
RMO87
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Location: Kansas City, MO

Re: Home Thermostat Wiring

Post by RMO87 »

I registered at HVAC-talk; unfortunately, not being a professional member there, I was told they couldn't offer do it yourself info there. I posted on Garden Web, as well.

Also, thanks for the table link, Epsilon!
tbradnc
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Re: Home Thermostat Wiring

Post by tbradnc »

RMO87 wrote:I registered at HVAC-talk; unfortunately, not being a professional member there, I was told they couldn't offer do it yourself info there. I posted on Garden Web, as well.

Also, thanks for the table link, Epsilon!
Interesting - I registered and posted several questions and received answers in this sub-forum: http://hvac-talk.com/vbb/forumdisplay.p ... ntial-HVAC. I am the epitome of DYI... :)

Same username as here.
Mudpuppy
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Re: Home Thermostat Wiring

Post by Mudpuppy »

Page 6 of the Honeywell manual shows how to connect conventional wiring, page 21 tells you how to connect alternate wiring, and pages 22-23 show you how to connect a heat pump:

https://customer.honeywell.com/resource ... 2487ES.pdf

Hopefully between the Honeywell manual and the Lennox manual, you can work it out.
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walkabout
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Re: Home Thermostat Wiring

Post by walkabout »

Epsilon Delta wrote:At risk of being obvious, the manual for the old thermostat is on-line and might be useful because it describes how it should be connected. Look at table 2 on page 6. I'd copy it but it does not copy well.

http://www.lennox.com/pdfs/manuals/Lenn ... Manual.pdf
Note that you'll have to scroll about halfway through that PDF because it contains manuals for both the Lennox 51M32 (non-heat pump compatible) and the Lennox 51M33 (heat pump compatible) thermostats.
Topic Author
RMO87
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Re: Home Thermostat Wiring

Post by RMO87 »

On the doityourself.com forums, I was told it is not recommended to use a setback stat on a heatpump system (which mine is). If your temperature is more than 2-3 degrees below the set point.... the electric heat will come on, causing higher electric bills.

So my new Honeywell programmable stat looks to be beneficial during the cooling season (summer), but the opposite during the heating season. In my case, does anyone think I should potentially switch back and forth (current stat in winter / new programmable stat in summer) in order to save on energy usage? I was thinking a programmable was the way to go, but from what I've learned about how heat pumps work, I'm having doubts.

Ryan
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Epsilon Delta
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Re: Home Thermostat Wiring

Post by Epsilon Delta »

RMO87 wrote:On the doityourself.com forums, I was told it is not recommended to use a setback stat on a heatpump system (which mine is). If your temperature is more than 2-3 degrees below the set point.... the electric heat will come on, causing higher electric bills.

So my new Honeywell programmable stat looks to be beneficial during the cooling season (summer), but the opposite during the heating season. In my case, does anyone think I should potentially switch back and forth (current stat in winter / new programmable stat in summer) in order to save on energy usage? I was thinking a programmable was the way to go, but from what I've learned about how heat pumps work, I'm having doubts.

Ryan
Ideally you get a thermostat designed to run with your heat pump. This should have the needed control rules built in.

But you don't have to switch thermostats with the seasons. At worst just program all the time zones to the same temperature (or say a 2 degree difference) for the heating rules.
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walkabout
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Re: Home Thermostat Wiring

Post by walkabout »

Epsilon Delta is right. If your thermostat is compatible with a heat pump, then it is smart enough to recover after a setback while minimizing the use of the heat strips.
Topic Author
RMO87
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Re: Home Thermostat Wiring

Post by RMO87 »

Hello again. I have finally found time to install my new Honeywell thermostat. Today, I had a question, so I called the Honeywell hotline phone number, and I was told that I need to have a HVAC contractor install a 'fossil fuel kit', otherwise the compressor may get damaged.

Is this correct? I'm really getting bummed, as I thought I was nearing the end of getting the new one installed myself.

I hadn't come across this during my research over the winter. We were talking about the T2 and T1 wires (Outdoor Temperature Sensor connections, per the old thermostat's manual). I was thinking that the T1 and T2 were simply not used with this new thermostat and could be taped off.

Thanks,
Ryan
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walkabout
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Re: Home Thermostat Wiring

Post by walkabout »

If it were me, and it was me in 2001 as noted above, I would call an HVAC contractor if I couldn't figure it out. You do run the risk of messing something up if it is not installed correctly.

Good luck.
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Meaty
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Re: Home Thermostat Wiring

Post by Meaty »

In addition to the above I wouldn't loop you compressor fan like you described. That just means the fan will always run (even if the AC/heat pump isn't)
"Discipline equals Freedom" - Jocko Willink
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