Converting from Honeywell PC8900 wiring to conventional


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Old 08-29-13, 05:38 PM
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Question Converting from Honeywell PC8900 wiring to conventional

Hey all,

I am currently trying to determine how to remove a Honeywell Perfect Climate thermostat system. The thermostat unit casing is cracked and I want to upgrade it to something with a wifi interface. My current goal is to install a Nest, but in general, I'm not a fan of the proprietary format that my control wiring is in. The Honeywell thermostat (PC8900) that I have does not have a conventional C, R, Y1/2, W1/2, G set of wires and instead communicates with a second unit in the attic using wires labeled "1", "2", "3", and "4". I've posted on the Nest forums without much luck and was hoping that someone here might have more experience with the wiring of these PC systems. Perhaps my HVAC system is just stuck with Honeywell?

Near my furnace is the Perfect Climate interface box that connects to my furnace. When I open it, it has a bunch of wires going all different ways. Teasing out what's going on in the picture, I have the wire from the furnace (a Carrier Weathermaker 9200), the wire that heads to the thermostat (top left), and one that ends just a few feet away dangling loose. When I bought the house, the inspector said this last wire probably went to a vent, but was taken offline (before my time). Here is an image of what I see and the box cover, below I've made a diagram of the wiring paths.





My thoughts are that I can bypass this box and just run these down the four wires that I have (the ones currently feeding 1 2 3 4). The problem is, I'm not entirely sure what they did here - there is a lot of jumping around going on. Here's a quick diagram: (F is the furnace, V is terminated vent line - goes nowhere, HUM1 is the top "HUM" port, and "HUM2" is the bottom "HUM" port). Obviously, there are four wires going to 1, 2, 3, and 4 and then down to the wall unit, but I'm ignoring those.

F -> W1
F -> C <- V
F -> G <- V
F -> HUM2 -> Y1 (Not sure why they are running to HUM)
F -> R -> RH -> HUM1 (Not sure why they tied all this together)

Has anyone seen a set-up like this before? What I'd like to do is get the box out of the loop, but I'm not 100% sure about what wires I should be pulling from the furnace to wire nut into the existing wire runs (1 2 3 4) and into the Nest. And I'm wondering if I should drop a fifth wire since there appear to be 5 heading out of my furnace (W1, Y1, G, R, and C).

Based on what I understand, I think I want to grab the W1 C Y1, and R wires and bring them down. Should I be working towards dropping a fifth line to account for G?

Thanks for any insight that you have. I'm a bit out of my element here.
 
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Old 08-30-13, 11:31 PM
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I've seen that setup before.
I would pull new 6 conductor 18 gauge wire from the furnace to the stat and connect the stat (and outdoor unit) control wire directly to the furnace.



 
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Old 08-31-13, 07:23 AM
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Houston,

That all makes sense to me. When I checked the wires coming down to the tstat, I noticed that they already had a 6-wire run, but had just truncated two of the wires. I fixed that and now have a 6-wire without having to pull a new one (yay).

I checked the furnace and everything checked out with conventional labels (not sure why I didn't think to do this earler). So I connected the furnace's G, Y, W, R, C to the 18/6 thermostat wire that goes to the tstat location. I used wire-nuts and made sure that all the connections between the lines coming out of the furnace (Weathermaker 9200) and the 18/6 run were tight.

I checked the voltage at the new tstat location (Nest) and they only read 2.3V between R/C. Shouldn't that be 24V?

Now the furnace is complaining about a short in the 24V wiring. I took all of my wiring apart and measured the lines coming directly from the furnace. They measure 23.6V coming out of the furnace (so that's good - right?). But the furnace is still complaining about a short (even when it's not plugged into anything). Is it stuck in an error mode and I have to do something to have it recheck?

Any help would be appreciated - I'm in Houston, too, and it's starting to warm up. I think that even putting the new tstat in place wouldn't solve whatever is wrong now since it will complain about the short.
 
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Old 08-31-13, 07:43 AM
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If you forgot to remove power to the furnace before performing this wire change you probably popped the low voltage 3 amp blade type automotive fuse on the furnace circuit board.

This is an example Carrier board showing the 3 amp fuse...
 
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Old 08-31-13, 07:46 AM
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The power was off when I did all my wiring. The only time it was on when when I checked with the voltimeter. I assume I can check the fuse if I open the front panel up? Is that something I can easily find/purchase/replace?
 
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Old 08-31-13, 07:49 AM
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Radioshack, Autozone or O'reilly should have a 3 amp fuse.

Are you measuring 24VAC at the transformer or at the stat wire from the furnace?
 
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Old 08-31-13, 07:59 AM
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My board was a little different - but I found the fuse. You're right, it was blown.

Is the problem that I turned on the power when I checked the voltage? If I try this again, I should just go through the whole change and not turn power on until the new unit is in place and everything is connected?

Thanks for all your help!
 
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Old 08-31-13, 08:06 AM
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Was the stat wiring connected to the thermostat, stripped and pulled apart to prevent a short, or cut and a possible short when you powered on for your test?

You can isolate and ohm your wiring if you think you have a short.
Considering the price of gas at the pumps I would get more than one fuse.
 
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Old 08-31-13, 08:49 AM
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I had the same thought - I picked up 5 of the fuses just in case I have a repeat performance.

The wires look good now, but I did restrip the ends and get clean connections since I first tried this. I must have shorted something at some point, because replacing the fuse seemed to have brought it all back in line. Since it's a new voltimeter, I probably shorted it when I was fumbling around with the electrodes. Having already put everything back the way it was (PC8900), I'm currently back to where I was this morning and everything appears to be working.

When it's cooler (tomorrow? next weekend?), I'm going to try it all again, but this time with new wire ends and with a complete change-over before turning the power back on. I'm confident that I must have been a little sloppy this morning and that a second go around will prove more successful.

Thanks again for all your help. I'll post again after I've made a second attempt.
 
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Old 09-08-13, 05:11 AM
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I wanted to do a quick follow-up. As expected, the second time around everything went fine. No shorts and the wires were reading 25V (a little high) at the tstat location. When I plugged the new Nest in, everything turned on and was responsive.

Thanks again for the sage advice. We've had everything back together now for a couple of days and have no issues to report!
 
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Old 09-08-13, 07:58 AM
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That's great news. I hope that you enjoy the operation of your new thermostat
 
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Old 01-13-14, 02:19 PM
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I am too tried to bypass Honeywell Perfect Climate (HPC) box and run a cable directly from a zone control board to a thermostat. But if I bypass a HPC box then I have to forfeit a cable going to a damper operator (my cable actually connected to something not just dangling loose few feet away like your). I am in debate to either leave everything as it or install a Nest and bypass damper control. I want to install a Nest to save energy, but if install a Nest will eliminate damper control then I may just leave everything as it. Is a damper control an importance element of HVAC system? I would really appreciate any feedback or idea. Thanks.

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Old 04-14-14, 11:54 AM
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My apologies for resurrecting an old thread, but it describes exactly the problem I am having.

I tried to install a Honeywell 9580 wifi thermostat. First time around I placed the jumper between the wrong terminals since I didn't have a C wire - jumped C to G and blew the 3 amp fuse on the IC board. Replaced the fuse and the thermostat came online, but the rooms are not cooling even though the thermostat says that the fan is on and that it is cooling. The AC unit outside is not coming on, even when I set the temp to 70F inside and the thermostat is measuring 76F.

I contacted Honeywell via online chat and the rep said that I had to remove the Perfect Climate unit out of my setup, which means I would lose humidity control. I don't know if that is a big deal living in Houston or not, since it gets very humid here. I spoke to a few friends and they do not have humidity control in their homes, so maybe it will be okay to eliminate the Perfect Climate unit.

Alternatively, I wondered if there was a way to mount the old Honeywell thermostat (with wires marked 1,2,3,4) somewhere in the attic next to the Perfect Climate unit so that I could still control humidity, without using it to control temp. So my question is which wire(s) would I connect from the old thermostat to the Perfect Climate unit to do that, and still be able to control the temp from my new wifi thermostat downstairs?

Thanks in advance! This thread has been tremendously helpful already.
 
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Old 04-14-14, 06:15 PM
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If you want to control your damper you will need the Honeywell Prestige IAQ.
You cannot tie the new thermostat to the old 1,2,3,4 terminals.
 
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Old 04-15-14, 12:35 PM
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Will just wiring the new thermostat directly to the furnace terminals work? i.e. I assume the damper will default to off so as not to interfere with the system. Or do I need to get someone to dismantle the damper?

Also, Is it possible to wire the old thermostat separately to the Perfect Climate box and use it to control humidity only, while using the new thermostat connected directly to the furnace for temperature control ?
 
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Old 04-15-14, 05:27 PM
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Anything is possible but I would consider installing a separate humidistat to control the damper if The Honeywell Prestige IAQ is not an option.
 
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Old 04-15-14, 08:35 PM
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Problem resolved!

Went up to the attic today to try some experimentation, which I found was unnecessary. Opened up the Perfect Climate panel and found that there was no humidistat or damper connected (or installed) i my system. Took the wires for the thermostat (1,2,3,4) stripped 2 more wires for W2 and C from the bundle and connected directly to the furnace terminals. Also discovered that W2 (second stage heater) had not been functional previously, but now came online once the wifi thermostat (Honeywell 9580wf) was connected. Everything has been running great so far tonight.

Thanks for the help! i'm a happy camper now.
 
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Old 07-06-14, 08:02 PM
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Hi newbie member - I came across this thread. I've also got a pc8900 and lately it will not shut off the a/c unless we physically turn off the system at the thermostat.

If I swapped the t-stat for another 8900 would I be good to go or do I have a bigger issue? Also, if swapping would work, do any of you all that pulled yours off have your t-stat still with you?
 
 

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