Current Sensing Relay Failure?

[SAS]

When we moved into this house there was a non working Honeywell HE300 humidifier attached to a Trane XV80 furnace. The humidifier was disconnected, and when I connected it I discovered that the solenoid valve would no longer open. I replaced the solenoid valve, but the humidifier runs whenever the humidistat calls for it, whether the furnace is running or not. Since the humidistat connects directly to the humidifier, I figured the problem must be a relay providing power to the humidistat. Opening the cabinet to the furnace, there is indeed a current sensing relay, Research Products Model 50, installed there. Do these just fail? If so, would I be better off using a terminal on the furnace to control the humidifier, assuming I can figure out which one? Or should I just spend $30 or so and replace the relay?

 

[SAS]

I have taken a closer look at the wiring and I'm more confused. I had expected the relay to be connected to a transformer, but it is simply connected into the humidistat loop. There are two low voltage wires connected to a "HUM" terminal on the humidifier which loop through two switches, the humidistat and the 24 volt relay. When I test with my multimeter, I never see 24 volts in that circuit, so I'm wondering if the relay will work properly wired this way. There are separate terminals on the humidifier for what they call an "Air Proving Device". What is that? I have seen sail switches. Is that what they mean?

 

[Sponsor]

 

[SAS]

I'm still trying to figure this out. Here's the wiring diagram for the humidifier:

Does this mean that I can splice into the thermostat wires in the furnace and just connect the G and C wires to the terminals in the humidifier and forget about using an additional control module (like the relay or a sail switch)?

 

[WorthFlorida]

I'm still trying to figure this out. Here's the wiring diagram for the humidifier:
Does this mean that I can splice into the thermostat wires in the furnace and just connect the G and C wires to the terminals in the humidifier and forget about using an additional control module (like the relay or a sail switch)?

You want the W lead. You want the W lead which will have 24 v from the furnace via the thermostat when the thermostat calls for heat. This will turn on the humidistat and the voltage from the W lead should open the solenoid valve.

The C lead is the common, one side of the transformer, the G lead is the fan and there will be 24 volts when active but he voltage will come from the furnace when the fan is switched to "on". Usually this is not what you want.

With a voltmeter, turn up the heat and see if there is a voltage on the G lead when the fan kicks on. At the thermostat the fan setting needs to be in "auto" mode. If there is voltage you can use the G lead. Some digital thermostat will put voltage on the G lead to run the fan. My Honeywell WiFI has a "Circulate" fan mode. At least 35 minutes of ever hour the blower will run to circulate air movement. This keeps my second floor cooler by sucking up the excess heat that builds up there. Using the W lead will activate the humidistat only when heat is called for and if the humidistat setting is set to operate for the current humidity.

 

[SAS]

You want the W lead. You want the W lead which will have 24 v from the furnace via the thermostat when the thermostat calls for heat. This will turn on the humidistat and the voltage from the W lead should open the solenoid valve.

The C lead is the common, one side of the transformer, the G lead is the fan and there will be 24 volts when active but he voltage will come from the furnace when the fan is switched to "on". Usually this is not what you want.

With a voltmeter, turn up the heat and see if there is a voltage on the G lead when the fan kicks on. At the thermostat the fan setting needs to be in "auto" mode. If there is voltage you can use the G lead. Some digital thermostat will put voltage on the G lead to run the fan. My Honeywell WiFI has a "Circulate" fan mode. At least 35 minutes of ever hour the blower will run to circulate air movement. This keeps my second floor cooler by sucking up the excess heat that builds up there. Using the W lead will activate the humidistat only when heat is called for and if the humidistat setting is set to operate for the current humidity.

Thanks for the help. My confusion around the W lead is that I have both a W and a W2, and I'm not sure how that works. I know how it worked with the heat pump in my previous house, but I'm not sure how it works with a gas furnace. Will the W lead be hot even if the W2 is hot? In that case I could use the W as you suggested. If not, it just seems simpler to use the G lead.

 

[John Gayewski]

An air proving switch is a pressure switch that proves thre fan is running.

 

[LLigetfa]

You want the W lead.

I think the W lead is only energized when there is a call-for-heat and so provides a shorter cycle. It does give time for the humidifier to dry at the end of the heat cycle as the fan continues to run for a while after the call-for-heat ends. This might be important to prevent conditions for legionella bacteria.

Some folks consider a humidifier to be a Band-Aid solution to a problem that ideally should be fixed at the source.

 

[SAS]

Thanks for the help. My confusion around the W lead is that I have both a W and a W2, and I'm not sure how that works. I know how it worked with the heat pump in my previous house, but I'm not sure how it works with a gas furnace. Will the W lead be hot even if the W2 is hot? In that case I could use the W as you suggested. If not, it just seems simpler to use the G lead.

As a final follow-up, I removed the relay from the humidistat circuit, and connected the G wire and the C wire to the humidifier. So far it appears to be working as it should. One thing I don't understand is why I have two terminals on the humidifier for the G wire: a Gt and a Gf. Apparently one is for the furnace and the other for the thermostat, but since the G terminal on the furnace is connected to the G terminal on the thermostat, I don't understand the difference. I used Gf and it worked, so I'll leave it there.

Thanks again.

Note: I corrected a typo. Where you see the bold "thermostat" above, I had typed "humidifier", which would not make any sense. My point was that the G wire was directly connected to both the furnace and the thermostat, so how could there be a Gt and a Gf. They would always be the same. It turns out that in some installations you connect the G from the furnace to one terminal on the humidifier and the G from the thermostat to the other.

 

[LLigetfa]

One thing I don't understand is why I have two terminals on the humidifier for the G wire: a Gt and a Gf.

You could try each one and see what the difference is. My guess is Gt is tied to the call-for-heat while Gf is to run whenever the fan runs. If the shorter Gt works, that is probably the safer bet.

 

[SAS]

You could try each one and see what the difference is. My guess is Gt is tied to the call-for-heat while Gf is to run whenever the fan runs. If the shorter Gt works, that is probably the safer bet.

Thanks, I think I've figured it out. In my case, it doesn't make a difference which one I use. In other installations G wires from both the thermostat and the furnace are connected to the humidifier.

 

[Fitter30]

G wire is only energized with electric furnace need to read the notes on the wiring diagram inside of blower door. Makes humidifier wiring easy to common of blower motor to switch 24 vac.

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