Wiring an Aprilaire to GMP075-3 Furnace

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Old 12-26-08, 03:12 PM
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Wiring an Aprilaire to GMP075-3 Furnace

I recently moved into a house with a pre-existing Aprilaire 550 humidifier (standard Aprilaire humidifier with manual humidistat), but it's currently not wired to anything but its humidistat. I understand that this model usually comes with a 120 VAC to 24 VAC transformer and a RP Model 50 current sensing relay which, when wrapped to the furnace blower's common lead, switches the humidifier solenoid on. However, I've found neither the transformer or the relay anywhere, nor does it look like either has ever been present. Is there a way to wire this humidifier directly to this furnace (Goodman GMP075-3 rev B)?

The blower has three wires going to it; one heavily insulated from a pressure switch (into which two wires go), and two from the wire bundle. The gas controller (?) has three terminals, labeled M1, P2 and C3, but only M1 and C2 are connected to anything. The bundle has wires leading to various sensors and the ignition, and they all stem from the lower part of the furnace which I can't figure out how to open. It is into this lower area that the power supply, thermostat and air conditioner wires feed in from the exterior.

Is there a HUM or accessory terminal somewhere I'm not seeing, or so I need to buy a transformer and relay and try to get it to work that way? If I do need to go the transformer/relay route, then onto which wire do I connect the relay?

Thanks, folks.
 
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Old 12-26-08, 10:41 PM
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Your furnace is pretty basic stripped down unit.

You could try tapping for low voltage on W and C on the board.... if the furnace acts up, then we need to do the relay on the fan wire, and transformer.
 
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Old 12-27-08, 07:25 AM
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Jay11J: is the board the gas controller in this case, or is there more beneath the lower panel? I don't see any terminals labeled W or C beneath the main access panel.
 
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Old 12-27-08, 08:16 AM
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The manual I am looking at, it's the board down by the blower where the t-stat wires hook up to. Unless they got up in the upper section on your furnace.
 
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Old 12-27-08, 08:27 AM
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The t-stat and a/c control wires all head into the mysterious lower section, but the blower is located at the top. This manual you have doesn't happen to mention how to open this area up, does it?

I'll give it a look when I get home from work and get some pictures uploaded.
 
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Old 12-27-08, 09:31 AM
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Sounds like you got a downflow set up.. (Air going down under the floor)

Here is the manual.

It didn't show much, I think you will have to remove the blower door with some screws to get that cover off.
 
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Old 12-27-08, 01:56 PM
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Thanks for the manual. I managed to get the lower door off and examined the circuit board. It's a B18099-13 board (the same as this guy's). Sure enough, it has a 24V HUM terminal on it, but where should I attach the other lead from the humidifier solenoid?
 
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Old 12-27-08, 07:23 PM
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Originally Posted by cmcclel3 View Post
Sure enough, it has a 24V HUM terminal on it, but where should I attach the other lead from the humidifier solenoid?
The other wire goes to "C"
 
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Old 12-27-08, 08:40 PM
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Originally Posted by Jay11J View Post
The other wire goes to "C"
There being no "C" on the aforementioned board, should I just ground it to where the transformer is (chassis ground)? There are a couple of wires labeled "neutral," and they all appear to be grounds, but there's definitely no post labeled "C."

Thanks for the help, Jay11J. We're almost there.
 
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Old 12-27-08, 09:10 PM
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There's no C? Do you have A/C? Where is that low voltage wire going to?

Follow the wire from the gas valve, that has to lead to C.

I am seeing C on the manual for the low voltage control going to it.
 
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Old 12-28-08, 08:08 AM
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Originally Posted by Jay11J View Post
There's no C? Do you have A/C? Where is that low voltage wire going to?

Follow the wire from the gas valve, that has to lead to C.

I am seeing C on the manual for the low voltage control going to it.
Page 27 of the manual may show a C lead, but the diagram on the inside cover does not. Once again, the circuit board looks just like this.

The low V from the exterior A/C unit leads to the neutral terminal of the 24 V side of the transformer, which is also tied to chassis ground. One of the wires from the gas valve also leads to chassis ground. Is it OK to use the same way for the remaining humidifier terminal?
 
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Old 12-28-08, 08:16 AM
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Originally Posted by cmcclel3 View Post
The low V from the exterior A/C unit leads to the neutral terminal of the 24 V side of the transformer, which is also tied to chassis ground. One of the wires from the gas valve also leads to chassis ground. Is it OK to use the same way for the remaining humidifier terminal?
yeah, you can put more than one wire on to another low voltage wire.

I have to admit, this is the first time I've not seen C on a furnace! I am not a brand basher, but I guess why some guys don't like Goodman.
 
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Old 12-29-08, 08:41 AM
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works like a charm; that's all it took. thanks for the help.
 
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Old 12-29-08, 10:09 AM
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Thanks for the update, and glad to hear it's running.

Have a happy and safe New Year!
 
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Old 12-31-08, 08:42 AM
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I have a similar problem and tried what it sounds like worked for you, but not for me. I have a GMP050-3 and Aprilaire 500A. The furnace board is almost identical to the one cmcclel3 linked to...as mine also has no "C". I tried wiring to the 24V chassis ground like you did but when I turned the furnace power back on the board fuse blew. I installed the separate 24V transformer to power the 500A so could I jump the two commons on the 500A controller board?
 
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Old 12-31-08, 12:10 PM
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Originally Posted by mikecsm View Post
I tried wiring to the 24V chassis ground like you did but when I turned the furnace power back on the board fuse blew.
sounds like something got shorted out some where. How did you have it wired up?

I installed the separate 24V transformer to power the 500A so could I jump the two commons on the 500A controller board?
This trasnformer wired up to be on all times or how?
 
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Old 01-01-09, 08:43 PM
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I wired the W on the furnace board to the W/G on the humidifier control and the blue wires from the furnace's 24V transformer (shared with chassis and gas valve) to the Cf on the humidifier control. I installed the other transformer for the humidifier control to be always on, before the furnace cut-off switch.
 
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Old 01-01-09, 08:46 PM
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Originally Posted by mikecsm View Post
I wired the W on the furnace board to the W/G on the humidifier control and the blue wires from the furnace's 24V transformer (shared with chassis and gas valve) to the Cf on the humidifier control. I installed the other transformer for the humidifier control to be always on, before the furnace cut-off switch.
It is wired up right. It should work out.
 
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Old 01-14-09, 06:55 PM
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Now that the holidays are over I have time to try and get this humidifier running. It's still not running so here's some more info. for troubleshooting. The humidifier controller gets power from the 24V transformer connected to the 'R' and 'C' terminals. When the furnace is not blowing heat, the 'R' reads about 28VAC but the 'C' reads virtually nothing. Then once the heat comes on the 'C' goes to 28VAC and the 'R' goes to nothing. Also while the heat is going, the 'W/G' and 'Cf' read 28VAC as well as the two 'H' terminals that go to the solenoid valve. But I never hear the solenoid open when the controller is in test mode, which is when it should open it. So I'm not sure where my problem is. Any ideas?
 
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Old 01-14-09, 09:44 PM
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You are getting voltage on H and H? then read at the valve. If you are reading power at the valve, then be sure you got water going to it.
 
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Old 01-17-09, 12:24 PM
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Originally Posted by Jay11J View Post
You are getting voltage on H and H? then read at the valve. If you are reading power at the valve, then be sure you got water going to it.
I'm getting voltage at the valve (or at least the two wires coming out of the valve) and am getting water to it, but water never comes out of it. Is there a way to check the solenoid is getting charged when voltage is applied, or maybe it's just bad? It's brand new so I would have thought everything would be working out-of-the-box.
 
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Old 01-17-09, 09:27 PM
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If you got voltage to the valve, then I'd say it's bad..

You can Ohm it out and see if you get a reading or not.
 
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