Replacing an old round honeywell T-stat (T87F1917?)with a VisionPro 8000 model TH8321U1006.
The unit it runs is a 20 ye old GE gas furnace with A/C.
Old wires are B,G,O,R,W,Y. For what it's worth, all wire colors match the marking.
I assume R,W,Y,G just swap over, but am not sure what the O,B go to. Only match I see in the honeywell lit is B to C and nothing for the O on a non heatpump unit.
I have replaced thermostats before, but all the wires lined up. Really don't want to smoke the new thermostat as I'm sure honeywell won't cover it. :o)
TIA,

Kevin R

Confused a little. You may have a heat pump outside. i.e. a dual fuel system because of the "O" terminal. "B" could be common.

It would take some troubleshooting to figure out what's going on.

See: Thermostat wiring code

Quote:

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Originally Posted by KevinR225

Replacing an old round honeywell T-stat (T87F1917?)with a VisionPro 8000 model TH8321U1006.
The unit it runs is a 20 ye old GE gas furnace with A/C.
Old wires are B,G,O,R,W,Y. For what it's worth, all wire colors match the marking.
I assume R,W,Y,G just swap over, but am not sure what the O,B go to. Only match I see in the honeywell lit is B to C and nothing for the O on a non heatpump unit.
I have replaced thermostats before, but all the wires lined up. Really don't want to smoke the new thermostat as I'm sure honeywell won't cover it. :o)
TIA,

Kevin R

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OK, haven't been up in the rafters yet, but if it's wired like the basement unit, the B & O aren't used, wired at the T-stat, but just tied off at the furnace.
That said, I want to run a common to power the VisionPro. If I don't connect the O and switch the B to C, what do I need to connect it to at the furnace?

Thanks again.

Kevin R

You'll notice from the schematic that (R) connects to the secondary of the 24V transformer. The (C) terminal will go to the other side of the secondary.

If you get stumped, we can probably help with a picture. If you have a voltmeter, you should have 24 VAC between (R) and (C) at all times.

The VP will work without the C terminal.

Will dig out the furnace papers, but if you have a photo, I'll take it. :o)
Every little bit helps.

Kevin R

You take the photo. You can use "Go advanced" to post it.

Thanks for the help!
As far as the adding the common, I will snap a photo if all doesn't click once I get eyes on.
As far as the T-stat change. Since we know which wire was connected to which place(letter designation) on the 20+ year old round honeywell, AND the original installer was nice enough to use the colored wires that matched said designations on the stat. Is there any reason just taking the wires of the old t-stat (G,R,W,Y) and connecting them to the same (G,R,W,Y) on the VisionPro would not work? I know this is an easy deal, but since this is not a cheap piece and I have read horror stories of some of the snafus on the site, it doesn't heart to be reassured by someone with more frequent experience than I.

Thanks again,

Kevin R

Yep, that should work. Just have batteries handy in the middle of winter. Dead batteries, no heat or AC.

Make sure you look over the documentation and program it properly. That should be possible without it connected since it is "Armchair programable".

OK, after making my way into the rafters, I found the following. Some what different than basement setup.
R,W,G go from the T-stat to the furnace. Y,O,B transfer from the wires coming from the
Stat to the wires that head outside to the condenser. Next stop I guess is open the condenser and see what's connected there. The wiring schematic I have shows the yellow connection form stat to condenser, but nothing about B or O. Now O is not connected and B is either common or not connected we're good to go.
Wouldn't the B if it were the common be connected to the furnace, not the condenser?
Maybe the installer just wired everything and the O and B aren't used, but would be if it were a heatpump?

OK, update 2. Checked outside, and all wires clipped off except for yellow from T-stat and red that connected to furnace. All seems good.
Back to adding a common then to power the new visionpro. Furnace has 5 screw terminals.
From L to R they have the following connections R to condenser, R to T-stat, another (R to gas valve and W to condenser same terminal), G to stat, and W to stat.
If the R from furnace to stat is one side of the secondary, which terminal would be the other?
i.e. what should already be connected to it?

This point: (R to gas valve and W to condenser same terminal) should be Common or (C). There will now be three wires here instead of two, thus making it common to gas valve, condenser and thermostat. Funny with names, Huh?

You will have 24 VAC from (R-to T-stat) and the above terminal (C) at all times.

Note: (R to gas valve and W to condenser same terminal) should be (W to gas valve and R to condenser same terminal)

So basically, when I go back up there ,(probably won't get a chance for a couple days)if I put a meter between the R (to the stat) terminal, and the one with the R, W (to gas/condenser) I should have 24v. Using the R(to the stat) as a constant, will any of the other terminals other than common return a 24v reading (or any reading)?

From what I'm getting, the R from the furnace to the stat is from the + side of the transformer.
The other R wire from the furnace to the condenser is from the - side, and when the stat needs to it closes the circuit and starts that unit.
Soooo, Y from stat going to condenser is really a switched continuation of the R going into the stat. The R(ignoring the similar color) from the furnace to the condenser is the other side of the transformer.
Stat closes circuit, + and - at condenser activate some sort of relay and condenser comes on.
Close, right/wrong?

Just Checking.
Is the above correct?

Thanks
PS, other than the running of a common, everything is in and working fine.

Kevin, you r making my head hurt.

Try this for an explanation from the t-stat point of view.

Your thermostat has three contact closures:
R to G, this turns on the fan
R to Y, this turns on the AC unit
R to W, this turns on the furnace

In the AC case, the thermostat controls the fan

There are safeties involved in the heat and cool case.

In the heater case, the thermostat can manually turn on the fan, but cannot turn it off. You normally think of the contact closure as a "call for heat". In simple terms the heater eventually gets around to producing a flame, but the fan stays off until the heat is sufficient so you don't get a blast of cold air, then then fan comes on and it will turn off some time later or temperature later when the plenum has cooled so as to extract the most heat.

In the AC mode an anti-short cycle delay must be satisfied (thermostat), otherwise breakers trip and fuseholders self destruct. In general, though the fan and outside unit turn on at the same time.

The bottom line, you can short G, W, R and Y togeter for a short time and nothing bad happens. You can short two at a time and determine which wire is which.

Don't throw common in the mix, though, because common abd R results in letting out the magic smoke and the control transfomer or control fuse dies.
It's wise to fuse the secondary with a 3A automotive fuse.

That kind of out of the way, lets look at at the outside unit for a moment.
There is a relay that turns on the 240 V power and a few pressure sensors. A high pressure switch that shuts the unit down if refrigerent pressures get too high and one that won't allow the unit to turn on if there is insufficient charge.

That relay needs a return line to complete the circuit and that return line is common. The gas valve also needs a return line and so does the fan relay.

Taking the condenser circuit.

You have the transformer connected to the R terminal of the stat. The stat has a relay contact between R and Y that is closed when there is a call for cooling. Then the Y terminal is connected to the relay coil in the outside unit and back to the other side of the transformer, the C terminal to complete the circuit. Somewher in this series circuit, there are the high and low pressure safeties.

To make your voltage measurements, you must ensure that the AC, FAN and heater are not running. Only then will you get 24 V between R of the stat and the common terminal of the transformer.

Some voltage measurements will indicate incorrectly if any contacts are closed in the stat, because the relay coils look like a wire when 1 uA, actually 24 VAC/10 MOhms, or so might be flowing through them. They just won't deliver any appreciable power.

Common by definition must have more than one wire attached to it.

As I said before, it is wise to protect the transformer, if not already with a fuse on the secondary, but more and more there is cost trimming. There are three choices: 1) An energy limiting transfomer; 2) No fuse or 3) a fuse.

Energy limiting transformers are not common. They have loose magnetics and this just heat up when too much current is drawn.

Fuses pop.

No fuses on a conventional transformer can fry the contacts in the stat or the secondary winding on the transfomer.

If you want to be "super careful", you can protect the stat only, by using a much smaller fuse to the (C) terminal or as a better option, insert a fuse in series with the R terminal of the stat. This is HIGHLY RECOMMENDED you do this. You can initially do your test with a much smaller fuse like 1 AMP and just use the fan as a test. 3 AMP will be better overall. Comes from 40 VA/24 * about a 1.5 "rule of thumb" factor.