connecting a humidifier to Rheem furnace?

[Nate Nagel]

Given this wiring diagram

there's no connection for a humidifier, is there...?

My brain is working slowly this morning and I'm annoyed at having to search all over the place for part nos. etc.

this is for a Rheem RGEB-06EC-FS furnace which apparently uses a

47-22481-81 fan control board. The "Honeywell S-890G" has been replaced at some point with a S8910U. The schematic on the actual cover is slightly different from the PDF and labels the blower control as a "Honeywell ST780C" (or maybe 788? I took a photo of it and reinstalled; photo is not as clear as it could be) Also "IDM board" is not present on schematic on cover.

thanks

nate

[Nate Nagel]

Followup: looking at this diagram

it seems that I could just pull some thermostat wires from the furnace control board and piggyback off the "W" and "C" terminals (I'm assuming W is White e.g. heat and C is common) to only energize the humidistat when the thermostat is calling for heat... yes? No problem with that plan? (I could get rid of the sail switch and separate transformer if I did it that way, too...)

Yeah, I don't really know what I'm doing, but I do know enough to see that the way this installation that I'm messing with was done is kind of Rube Goldbergian and also does not lock out the humidifier when the A/C is on which is the main reason why I'm trying to redo it.

nate

[dpb]

On 10/23/2011 11:29 AM, Nate Nagel wrote: ...

Assuming you replaced it w/ an appropriate humidstat, yes.

...

...

Nothing but a comparative story follows, but--

Just discovered during a new furnace installation here the humidifier had been wired that way for 30+ yrs. (I hadn't used it since we moved back after dad died owing to a leak in the line and thinking were going to replace anyway, never got the necessary round tuit until this year...)

I suppose Dad simply closed the supply valve in summer. BTW, old units are not at all unlikely to not have the extra terminals on the control and the 120V sail switch was also pretty much the standard back then; same setup here. Also, the humidstat portion had failed so ended up replacing it and solenoid w/ the low voltage ones and the new furnace (Carrier) being new does have the terminal so all is well...

--

[Tony Hwang]

Hi, No big deal as long as you remember humidifier comes on when blower runs and if humidistat calls for humidity.(so called AND logic) And is depending on what type of humidifier.

[Tony Hwang]

Nat

You mean energize water valve solenoid thru humidistat? Just hope thermostat lead(W) has enough juice to do this. I used the 24V AC step down tranny which came with the humidifier. I don't use ODP.

yes? No problem with that

[dpb]

On 10/24/2011 8:45 AM, [e-mail] wrote: ...

...

'Cuz it also is the humidstat/humidity control.

If you're in an area dry enough it doesn't matter, ok, otherwise it might be too much at times.

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[N8N]

There is a mechanical humidistat, but it seems to me that the flow rate of the water is such that it might overpower the drain if the water isn't actively evaporating off the panel. That's my main concern. That, and the sail switch is already there and (now) functional, so it doesn't cost me anything to leave it in the circuit. Also, the issue that prompted me to immediately dig into this was the furnace not lighting, so it's possible (although not normal operation) that there might be a heat call but no actual air handler activation, and in such a case you wouldn't want the humidifier to operate.

nate

[bud--]

As you note the 24V water solenoid current may be an issue if you connect the solenoid to W-C at the furnace. I think it is unlikely the current is too high.

In the good old days the thermostat had a "heat anticipator" that caused the stat to shut down heat a little early. The anticipator at the stat was set to the gas valve current. The current would be higher if you add the solenoid. This is probably not an issue with recent stats. The round Honeywell T87s have an anticipator adjustment.

If the anticipator setting is a problem it is even more of a problem when the water solenoid valve is switched on and off by the humidistat. Connecting W-C to the coil of a control relay would eliminate that problem.

I used to replace the SPDT fan relay with a DPDT relay. There is a point to connect a transformer for the humidifier where the humidifier is only powered when the plenum gets hot and turns on the fan (like HUM in your other post). (The circuit you posted has a SPDT fan relay.)

A sail switch control is OK. I would add a switch to shutdown the humidifier in cooling season. I have a vague recollection that some water solenoid valves were water cooled. You might check if the valve gets a lot hotter if the water is turned off.

[dpb]

On 10/24/2011 11:20 AM, N8N wrote: ...

The solenoid should shut off the water when there's no demand...tied w/ the sail switch since you have one or from the HUM contact on a furnace w/ that logic on the board.

Generally, the old Honeywell sail switches I'm familiar with were dual-function humidistat/flow switch using a humidity-sensitive lever-connection that stretched/shrunk as humidity went up/down that controlled the contacts.

As noted, replaced both w/ the solid-state humidistat and the new furnace has the HUM logic onboard so all is much simpler that way.

I thought you said you do have a solenoid; doesn't it shut the water off when not demanded? If not, what _does_ it do? :)

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[N8N]

Right, I wanted to interlock the valve to something other than *just* the sail switch and humidistat - because all the sail switch does is detect airflow (this one appears to be a simple sail switch) to prevent the humidifier from running when someone manually turns on the fan just to circulate air and/or the A/C is running.

I think after thinking this over I will go ahead and get a MR-101 or similar relay unit to provide me a set of dry contacts that change state on a heat call and leave everything else substantially the way it was (re-running the 120V feed to the sail switch though so that it a) doesn't lay on top of the supply plenum and b) will be cut off by the furnace cut off switch, so that it doesn't require shutting off the breaker for maintenance.) I suspect that the reason that this wasn't done is that like my last house, the humidifier was installed prior to the installation of central air and/or a thermostat with a switch that could manually force the fan on without a call for heating or cooling, so no thought was given to interlocking it (or in other words, if air was flowing in the return plenum, it was a pretty safe bet that the furnace was operating at that time.)

nate