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Old 10-24-2009, 06:54 PM   #31
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Re: Gustav

Hey Gang,

I was working on my control wiring today and wanted to get around to installing my thermostat. It came with the furnace, but there are some discrepencies that I can't quite figure out. I was hoping that this was an easy HVAC type thing with a simple solution.

The furnace is a two-stage furnace, 25K or 40K output, and the Thermostat is supposed to also be two-stage. Here are the wiring diagrams:







So on the furnace we have:
+12VDC
-12VDC (assuming ground)
+Thermostat
-Thermostat (labeled on actual wire as just Thermostat, no -)

On the thermostat we have:
+12VDC
-12VDC (assuming ground)
TH FROM FURNACE (from? shouldn't it be to?)

According to the furnace manual +12VDC coming from the battery is also connected to the thermostat, while -12VDC (ground) is not, while according to the thermostat manual, -12VDC is connected to the battery (in parallel with the furnace) and there is no connection from the positive terminal of the battery.

My best guess would be: [furnace <--> thermostat]
+Thermostat <--> +12VDC (on thermostat)
Thermostat <--> TH FROM FURNACE
-12VDC <--> -12VDC (common ground to battery)
and just plain ignore the +12VDC going to the thermostat?

Any wisdom you could pass on would be greatly appreciated.

John
Attached Images
File Type: jpg Thermostate Terminals.jpg (25.8 KB, 672 views)
File Type: jpg Furnace Wiring Diagram.jpg (81.1 KB, 672 views)
File Type: jpg Thermostate Wiring Diagram.jpg (65.3 KB, 672 views)
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Old 10-24-2009, 10:03 PM   #32
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Re: Gustav

Quote:
Originally Posted by Smitty
John, you need to power the t-stat with 12VDC, so it can be sent-on to the various controls once the t-stat is "made" (or calls for heat), look at yout last diagram, you'll see what I mean. All your t-stat is a switch, it's open or closed to make/break the circut. Generally the only time you need a "ground" at a thermostat location, is for a digital/programmable type thermostat, for the clock/memory.

Can you tell if this furnace has a 2-stage (hi/low flame) gas valve? I can't imagine how else it could be 2-stage otherwise (unless it uses an outdoor thermistor, so after a certain outdoor temp, it kicks the hi-flame on). Basically, in a 2-stage thermostat, there are 2 sets of contacts, on 2 seperate bi-metal coils. They are "set-apart" by just a few degrees. Say you're set at 70*...and the furnace comes on (low stage) and runs 5 minutes, but it's too cold out, and it "loses ground" with the temp inside, so after the temp continues to drop another couple degrees, it makes the 2nd-stage switch in the t-stat, kicking-on the 2nd-stage (hi-flame) on the gas valve for more heat.

So you need 4 wires at the t-stat, power (12VDC+), ground (12VDC-), 1st stage heat, and 2nd stage heat.

A house furnace is mostly 110VAC, but uses a 24VAC control circut through a transformer, and various controls, relays. Yours is straight 12VDC, including the blower motor, so the power-feed begins at the thermostat in your case, not at the furnace. No 12VDC supplied to your t-stat = no power through it to the furnace.

lol, not sure if this helps or not? Post-up if you need more, or I didn't explain what's in my head

Smitty
Smitty,

Thank you kindly for your response. Of the four wires connecting to the furnace the +12VDC and -12VDC are heavy 14-gauge wires, while the thermostat and "thermostat +" wires are smaller 18-gauge wires. Rather than acting as a switch, I believe the thermostat just sends a signal back along the circuit and then an internal relay is used within the furnace. Curiouser, on further inspection, is that the +12VDC, -12VDC and thermostat wires are all steel while the "thermostat + wire is copper. The thermostat is rated to only use 100mA.

The furnace is indeed a two-stage furnace and it says multiple times in the manual that it has to have a two-stage digital thermostat and quotes the model number, "Atwood 2H2C" is a "Two stage furnace digital thermostat." I'm guessing that if there is only one wire terminal to send back to the furnace that perhaps it either encodes a message, or encodes a voltage (+12VDC or +6VDC, etc...?).

There's a number on the thermostat manual, I'll give them a call on Monday.
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Old 10-25-2009, 09:33 AM   #33
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Re: Gustav

Quote:
Originally Posted by Smitty
Did this t-stat come with the furnace, meaning is it specifically compatible with the furnace? In the 2nd diagram you posted, what is it showing at the bottom of the pic, right in the center (directly below the limit & sail switches)?

Smitty
Yes, the thermostat came with the furnace. In the 2nd diagram everything to the right of the Bulk Head Connector is part / inside the furnace. As per the unit below the limit & sail switches, I have no idea. Clearly the -thermo wire attaches to it as well as -12VDC, so it may be some kind of tie to ground? The picture looks like a looped screw terminal. It's pretty hard to see that part inside the furnace. As for the +thermo, it clearly goes into a "dual control" which then feeds two separate wires to the ignition control.
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Old 10-25-2009, 10:14 PM   #34
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Re: Gustav - furnace wiring

The thermostat does match the Atwood 2H2C document stored at: http://www.fiberglassrv.com/files/At...ermostat-2.pdf

If you blow up the first picture you sent, you'll see that terminals 5, 6, and 7 are marked "AC" and that corresponds to the connector block with terminals Y, GH, GL, and B in the third diagram. These are air conditioning connections. They don't go to your furnace. Terminal 2 is marked "not used," but the Atwell sheet says that is for 7.5 volt furnaces. That leaves terminals 1, 3, 4, and 8.

1 - 12 volt ground
3 - 12 volt positive
4 - relay positive
8 - "TH from furnace"

The third diagram shows 1, 3, and 8 going to the furnace, and instructs you to connect 3 to 4 (12 volt positive to relay positive)
The outline diagram shows only 3 square blocks inside the thermostat on the right side, which must be drivers for the AC compressor and AC low and high fans. That, plus the requirement of using a particular thermostat, leaves me with the impression that "TH" is a serial data link between the processor in the thermostat and the processor in the furnace controller, not a traditional contact closure. The power ground would also serve as the minus side of the data link.

Looking at the furnace diagram, red is obviously the +12 volt lead. From Pin 1 of the bulkhead, it goes to the (circuit breaker??), then to the blower motor, the dual control module, and feeds the limit and sail switches sensed at inputs 6 and 5 of the dual control module.

Yellow is obviously the -12 volt lead. From Pin 4 of the bulkhead, it goes to the glob at the bottom of the diagram, and to the ignition control and dual control modules. The gas valve notes "chassis ground," so the red wire from the ignition control module and the brown wire from Pin 1 of the dual control module must be the positive commands for your two heat levels. The blower motor has positive power applied, so it must be controlled by the dual control board applying a ground to the black wire.

The electrode that runs from chassis ground and high voltage on the ignition control is the spark to light the gas. The orange and blue wires from Pins 2 and 3 of the dual control module to the ignition control module must provide +12 volts and ignition requests.

That leaves the blue wire called "+THERMO" running from Pin 4 of the dual control module to Pin 2 of the bulkhead connector to be the data link.

SO . . . . . .


1. Furnace bulkhead Pin 1, thermostat Pin 3, and thermostat Pin 4 all go to +12 volts, preferably at the same point (see #4).
2. Thermostat Pin 8 goes to furnace bulkhead Pin 2.
3. Furnace bulkhead Pin 4 goes to 12-volt ground, and so does the furnace chassis (unless the blob with the yellow and green wires turns out to be an internal ground tie point).

4. In spite of the third drawing showing a battery connection, I would then connect furnace bulkhead pin 5 "-THERMO" directly to thermostat pin 1 ground, and see if everything lights up. Running the negative wire directly from the thermostat to the furnace rather than using the vehicle frame as a conductor is desirable. This will minimize the amount of electrical noise pulses that might interfere with information in a data link. If the blob with the yellow and green wires turns out to be a ground tie point inside the furnace, you are done. If the thermostat doesn't light up without an internal battery, and the heat doesn't come on, then ground the wire running between these two points to the vehicle 12-volt ground, preferably at the same tie point as the yellow wire on bulkhead pin 4.

Does this make sense, looking at what you have there?
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Old 10-26-2009, 01:57 AM   #35
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Re: Gustav

Quote:
RB- How the hell did you enlarge that 1st pic?
I don't remember. I usually right click on the picture, select "Copy Image Location," (in Firefox), then open a new browser tab and paste the URL into the address box. That might have been enough to see it on a 19" monitor. In the past, I have also used "save image as" to put a copy on my hard drive, and then open the copy in an imaging or picture viewing program. I know I didn't do that tonight.

Quote:
Why would a heat/cool stat not be compatible?
According to John, the manufacturer says so, I didn't.

Quote:
Generally a stat controlling 2 independent systems uses a segregated (split) sub-base which divides the control circuitry of the two systems.
No argument here. Apparently, on this model stat a single microprocessor uses segregated output circuitry for the two systems. For the AC, contact closures using Pin 4 as common and Pins 5, 6, and 7 as switched outputs for compressor, low fan, and high fan. The block diagram of the thermostat board shown on this post shows three boxes which look like power transistors with heat sink tabs, but might be actual mechanical relays with mounting tabs used for each of the three A/C contact closures.

The furnace, on the other hand, has one wire on Pin 8.

The .pdf title for the link I posted is hydro flame Model 2H2C Two Stage Furnace Digital Thermostat FOR TWO STAGE FURNACE

HIS THERMOSTAT HAS BEEN DESIGNED TO OPERATE STANDARD AIR CONDITIONING UNITS IN CONJUNCTION WITH AN EXCALIBUR 2-STAGE FURNACE.

Quote:
Also, can you identify what I was asking John about?
From what I can tell, you are saying there are residential units that compare inside and outside temperatures and use the temperature differential to decide whether high heat or low heat are required to maintain interior comfort. I've never seen or read about one, but always thought it was a good idea and wondered, "Why not?"

Quote:
Or what initiates the call for 2nd stage heat?
The .pdf indicates it can be automatic, or manually selected. There is a 4-position slide switch that selects Cool/Off/Heat/Fan. There is a "mode" button, plus two up-down set-point buttons. According to a switch + mode truth table, the modes are "AU" automatic high/low, "HI" high only, "LO" low only, "HF" high only plus continuous fan, plus "LF" low only with continuous fan. In the cool position, these control the AC levels and AC fan. In the heat position, they control the heat levels and furnace fan. In the fan position, only "HI' and "LO" are available, and use both fans. There is also an indication of "OF" for system off.

The description of "AU" in the heat mode is "Furnace automatically switches between high and low BTU valve and high
and low speed fan when cycling to satisfy set point.
Since there are no other connections, I must ASSume that the thermostat decides based on the difference between measured room temperature and the set point. If it were really smart, it might judge the rate of temperature rise over time, or the length of time between "call for heat" and temperature rise. It might even use the length or frequency of "call for heat' cycles to decide. The one-page .pdf did not explain how this hapened.

The one-wire control of the dual heat was what got me thinking that the furnace and stat were designed to work together using digital information sharing. Almost all of the two-way radios manufactured in the last 15 years have started using RS-485 serial data to tell the units in the vehicle trunks what the operators have done with the control boxes on the console. There are no more individual wires telling a radio which channel is selected or whether the mike button was pressed. Lately, even some microphone and speaker audio is digitized, and with no audio wires this data is just added into the combined data stream between the front and back of the vehicle.

In the past, we would just solder jumper wires or change components to customize the way a radio system would work. For example, if we wanted to change the scan speed, we would just identify and change a capacitor. No more. Now, since everything is a function of the microprocessor code, you call the manufacturer's help desk and try to open a business case to make a modification or fix a bug. If the supervisor there thinks your problem is worthy of a response, research time is scheduled, the effect of that code change is (hopefully) compared to see what "side effects" changing the code will cause to other functions. What used to take 15 minutes now takes 30 months if approved, but more often it gets turned down in 4 weeks.

I don't want to knock an old tin knocker, but since my own experiences form my frame of reference, when I looked at the diagrams I saw what I have been seeing the past few years - computers talking to computers. Maybe I was over-thinking this. Whatever the case, there is only one wire to the outside world from the dual control board to the stat. It could be just a simple tri-state lead, for example 0 volts for off, 5-7 volts for low, and 10-14 volts for high. That would be much simpler, and my concerns about noise on the lines would be unfounded, but the hook-up wires between the units would be the same.
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Old 10-26-2009, 06:16 PM   #36
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Re: Gustav

Quote:
Bottom center, 2- wires....what is it? That's what I was asking John to identify.
I called that the "blob." Since the diagram is half schematic and half pictorial, I'm guessing it is a ground connection point with 4 spade lugs, 2 of which are used for the yellow furnace ground and the green thermostat ground. If I had the cover off of John's furnace, that's the first thing I would try to spot.

Quote:
Something here doesn't make sense.....the furnace has to have a 2 stage gas-valve to fire at 2 different inputs (btu), but what controls it? Are you saying it's electronically done through 1st stage/2nd stage "sensors" in the stat?
I had interpreted the red wire from the Ignition Control module and the brown wire from the Dual Control module are the two BTU levels at the valve, with chassis ground as per the note. Since the stat has manual mode control of the two levels using only the blue wire to Dual Control module Pin 4 for signals, the blue wire must either be a tri-state input or serial data exchange.

Fan speed control would be performed by limiting the power from the yellow system ground to the black motor ground onboard the Dual Control module.

If the stat has manual control if the modes, plus has an automatic mode described as choosing between the two levels, then it stands to reason that information developed either inside the stat or shared digitally together with information from the Dual Control board must contribute to the heat level decision, however that decision is made.
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Old 10-26-2009, 07:10 PM   #37
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Re: Gustav

Quote:
Originally Posted by Redbear
Quote:
Bottom center, 2- wires....what is it? That's what I was asking John to identify.
I called that the "blob." Since the diagram is half schematic and half pictorial, I'm guessing it is a ground connection point with 4 spade lugs, 2 of which are used for the yellow furnace ground and the green thermostat ground. If I had the cover off of John's furnace, that's the first thing I would try to spot.
I did have the cover off today and saw that it was indeed a four lug terminal grounded (mounted) directly to the chassis of the furnace. The green wire (thermostat -) goes there and terminates, while the yellow wire continues onward. Being that -terminal and -12VDC are both ground, it must be as we thought that the "thermostat +" wire carries information rather than power. I wonder if ensuring its ground to the chassis and semi-isolated helps to reduce ground-noise when the motors kick on and off. Whether that information is serial communication or analog set-level is yet to be seen. Back in the lab where I went to school we had a fancy signal analyzer, but these days I don't have access to anything like that. I could get a voltage reading on it in both manual-low and manual-high and see what those are, give us an idea. But, I should definitely be able to get it hooked up.

Unfortunately, I was supposed to get my propane tank filled today (the bus is under construction and temporary not easily drivable). We had a local propane company sign us up for a delivery schedule, run a credit check, send out a technician to do a pressure test (same thing we did ourselves), only to tell us that they weren't going to deliver because they didn't know what to do about rv propane regulations. We're going to give another company a shot, and then I'll just have to buy those 20# grill size tanks and run them back and forth myself.

Installing water plumbing, sink, faucet and drain tomorrow.
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Old 10-26-2009, 10:52 PM   #38
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Re: Gustav

Quote:
Originally Posted by John
The green wire (thermostat -) goes there and terminates, while the yellow wire continues onward.
Meant to say that the yellow wire also goes there, then continues onward.
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Old 10-27-2009, 10:27 AM   #39
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Re: Gustav

FYI, The Atwood thermostat sends a pulsing signal to the furnace. I'm not sure if this is Pulse width modulation (http://en.wikipedia.org/wiki/Pulse_width_modulation) or a simple count (one pulse for low, two pulse for high, etc...)

I didn't think analog levels (12VDC for high, 6VDC for low) would be appropriate on a long wire and I wasn't sure of the noise a serial communications signal would incur.
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Old 10-27-2009, 10:44 PM   #40
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Re: Gustav

Quote:
Originally Posted by Smitty
I hope for your sake this isn't similar to the old Lennox Pulse Furnaces. Those things were noisy as hell.....sounded like a tin can held over the exhaust of a dirt-bike, only lower-toned, riding through a tunnel.....seriously. Surely they'd never use the same "technology" for RV furnaces?!?!?!

Smitty
No, I should be ok. There's two valves in order to burn at two different flow rates. The signal from the thermostat is just read by a computer to determine which valve to open and what fan speed to use. The furnace itself shouldn't pulse.
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