topic Re: Replace older Honeywell chronotherm in Nest Thermostats

Replace older Honeywell chronotherm

jasonpowell — Sat, 18 Mar 2023 13:32:30 GMT

My antique chronotherms have separate wires going to clock terminals for the clock in the thermostat. These are in addition to the wires for y, w, & r furnace terminals.
my presumption is that these clock wires should be capped, taped and left in the wall. Anyone have a different conclusion?

Re: Replace older Honeywell chronotherm

CoolingWizard — Sun, 19 Mar 2023 02:22:33 GMT

I have replaced many of these types of thermostats and you can just tape off in tuck away those wires that go to the clock. All you need is the R,Y,G,W, and C

Ken, The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Sun, 19 Mar 2023 02:40:25 GMT

The Chronotherm I have (actually, I have one on first and second floors) controls both heat and ac. It only has the following wires, black wire to Y terminal, white wire to W terminal, and red wire to R terminal. As I said earlier, there are separate wires (white and red) going to the clock terminals.

I've installed the Nest learning thermostat with the black wire to its Y1 terminal, the white wire to its W1 terminal and the red wire to the Rh terminal. Unfortunately, the Nest doesn't respond, I hooked it up to a charger as recommended and tried reinstalling it once the Nest indicated to do so. No difference.

Your thoughts?

Re: Replace older Honeywell chronotherm

CoolingWizard — Sun, 19 Mar 2023 03:17:28 GMT

Please allow me to provide a little more details on the Chronotherm. There are a couple approaches to power the timer clock. One has a jumper from R terminal to one of the clock C terminals. The second clock C terminal is wired to the HVAC transformer Common. The second way is to have a separate wire connection from a dedicated transformer. Yours appear to be the second approach. Now, get a multimeter and let us check the power source.

Set the meter to AC Voltage, put one lead on the R terminal and the second to the wires, one at a time, that were attached to the clock C terminals. You are looking for 24-Volts AC +/- 4 volts. It is more than likely the White wire at the Click. When you find it, that wire is the Common that can be placed in the C on the Nest. You old R wire should be placed on the Rc of the Nest backplate. The Black wire goes on Y1, the old W wire goes to W1 on the Nest backplate.
It does not appear you have a G wire for your Blower Fan. The Chronotherm did not have a G terminal since it relied on the HVAC equipment to control the Blower Fan. That should tell how old this thermostat is.

Ken, The AC Cooling Wizard is

Re: Replace older Honeywell chronotherm

jasonpowell — Sun, 19 Mar 2023 14:15:07 GMT

fyi. The main body indicates it’s a T882A 1047 if this makes any difference.

Re: Replace older Honeywell chronotherm

jasonpowell — Sun, 19 Mar 2023 15:26:14 GMT

The voltage between the red and white clock wires is 24 volts. Between the Red wire for the R terminal and the white clock wire is about 6 volts, and between the Red wire for terminal R and the red clock wire is about 19 volts. Assuming that I connect the red clock wire to the C on the nest and the old red wire to Rc, what would I do with the remaining white clock wire - if anything?

Re: Replace older Honeywell chronotherm

CoolingWizard — Sun, 19 Mar 2023 15:26:45 GMT

It makes no difference. Just use the multimeter and find the common.

Re: Replace older Honeywell chronotherm

CoolingWizard — Sun, 19 Mar 2023 15:30:10 GMT

Just tape it off and stick it back into the hole. Or, repurpose it as the Blower control wire. This will require some rewiring in the Air Handler.

Ken, The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Sun, 19 Mar 2023 23:00:29 GMT

I just tried with with the wiring as you suggested: black to Y1, white to W1, red to Rc, red clock wire to C. That didn't work, so I tried putting the white clock wire to C. same response.

Question: Since the only time I had a reading of 24 volts was when I put the meter on both the red and white clock wires, and the unit doesn't power up when I plug one of them into the C terminal, should both of these be plugged in somewhere on the Nest backplate? While it seems logical to my electrically non-educated mind, I didn't want to take the risk and try it on my own.

Re: Replace older Honeywell chronotherm

CoolingWizard — Mon, 20 Mar 2023 14:22:04 GMT

Your system likely has two separate transformers. It sounds like the transformer from your air handler system is not probably working correctly. What you need to do is attempt a simple test.

What we can do is take the RC wire out of the nest back plate and touch the bare copper wire to the bare copper part of W1 wire. After about three or four minutes, your heater blower should turn on. And warm air should be flowing. If that does not work, then chances are we need to check to make sure that the transformer is receiving power in your air handler. The same would be true if you took the RC wire and touch it to the white one wire that would cause your air conditioner to come on.

please let me know how the test turned out.

Ken, The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Mon, 20 Mar 2023 18:34:37 GMT

I did as you suggested. As soon as I touched the Rc (red) wire to the W1 (white wire) the furnace started.

Re: Replace older Honeywell chronotherm

CoolingWizard — Mon, 20 Mar 2023 20:26:13 GMT

Jason, this proves to us that there is incoming power from the air handler‘s HVAC transformer on the red wire, coming to the thermostat and attached to the R terminal. Furthermore, we know that the white wire at the thermostat attached to the W does indeed connect to the furnace control board. As for the blower fan that is controlled by the integrated furnace control board since you have no wire attached to the G terminal of your thermostat.

for the best possible charging arrangement, it is best to have a common wire attached to the C terminal of the nest thermostat back plate. If you’d like to rewire your air handler to bring a common wire using one of the clock wires, let me know, and I’ll help you to proceed.

Ken, The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Mon, 20 Mar 2023 22:02:10 GMT

Yes, let's proceed.

Re: Replace older Honeywell chronotherm

CoolingWizard — Mon, 20 Mar 2023 23:59:55 GMT

Do you have access to the air handler?

The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Tue, 21 Mar 2023 01:03:38 GMT

Yes, one way or another

Re: Replace older Honeywell chronotherm

CoolingWizard — Tue, 21 Mar 2023 01:55:19 GMT

On the front of your Furnace/Air Handler, you will see where the power and thermostat cable got into the cabinet. That panel in front that entry point is the one that you need to remove. Sometimes there are held on with screws and sometimes they are twist knobs. When you open that panel, you will locate the two thermostat cables that are coming into there. One will be the one with only your red and a white wire, that will be your clock cables, the other one will be the one that has the red black and white wires inside. Before we make any wiring changes, let’s locate things. Look closely and do you see a control board with terminal screws R, Y, G, W, C? Look for the second cable with just a red and white wire? Does go to a separate transformer by itself? BTW generally on the side will be the make and model number. Get that to me I can pull out the service manual for it.

The AC Cooling Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Tue, 21 Mar 2023 15:21:49 GMT

Attached is a photo of the model # label.

Regarding the wires, there is a control board of sorts with the terminals/wires as follows:

R is in the upper left corner (red wire), note white wire capped, C is upper right with black wire, W is in lower left - nothing attached, G is lower middle with a blue wire, Y is lower right - nothing attached.

Re: Replace older Honeywell chronotherm

jasonpowell — Tue, 21 Mar 2023 15:47:50 GMT

FYI, the wires from the furnace appear to go to these transformers labeled for the zone 1 (upstairs) clock, downstairs clock, the AC, and the various dampers - which don't seem to work well (when the downstairs thermostat starts the furnace it heats the upstairs as well (old age?)

Re: Replace older Honeywell chronotherm

CoolingWizard — Tue, 21 Mar 2023 19:15:51 GMT

Your control system is fairly old. The Chronotherm thermostats were the first thermostats that could do both Heating and Cooling at the same time. The clock would be set to cool during the day and heat during the night. Prior to the Chronotherm a home owner would have to go to the basement and light the furnace manually at night.
I am not familiar with how they have wired up your system. What I see is the Chronotherm clock had its own dedicated Transformers and the AC transformer is used only to power the outdoor compressor/condenser unit. They have that Heat/Fan/Cool switch downstairs that frankly is wired to make the blower operate and lower speed for heating and max speed for cooling. However, the temperature set point is done at the Chronotherm. This makes me think the W and Y wires go from the Chronotherm to the Heat/Cool switch in the basement. When in heat mode, the switch controls the burner and blower fan. When the switch is in cool mode, it controls the blower fan and the outdoor compressor.

I need your help to see where the two wires on the AC transformer go. I think that the red wire gets cross connected and sent to the Chronotherms. Can you confirm that?

The AC Coolimg Wizard

Re: Replace older Honeywell chronotherm

jasonpowell — Tue, 21 Mar 2023 23:25:07 GMT

You are correct regarding the Heat/Cool switch. The two wires from the AC transformer go into the Mastertrol Mark V box as follows: At the transformer there is a gray wire at the top terminal and a red one at the bottom, as well as an unattached green wire wrapped around the cable.

At the Mastertroll, on the left side, the gray wire from the transformer goes into the Yellow wire nut. and the red wire to the "Common R" terminal. Also there are two gray cables in the area that resemble the cables that enter the furnace, each with both red and white wires. The white wires are joined with the gray wire from the AC transformer in the yellow wire nut.

One red wire attaches to the "Fan G" terminal, while the other red one attaches to the "Comp Y" terminal (above G).