gas furnace/AC controls - Honeywell universal fan controller?

[Nate Nagel]

Have a friend with an old Rheem gas furnace and also central A/C... was messing with it today because his air handler wasn't running on a cooling call and therefore his A/C was all iced up and the house was nice and warm. Thermostat checks out OK, currently the furnace has a Honeywell S8910U ignition module and a Honeywell 47-22481-81 fan control board. Neither of these appear to have been original to the furnace which is a Rheem RGEB-06EC-FS which apparently originally came with a Honeywell S-890G ignition module and I neglected to copy down the part number of the original fan controller board (which was on the schematic on the box cover) but I believe that the one that is in there is actually the recommended service replacement.

Here's my question. I believe that the fault is in the fan controller board; I am getting control voltage on "G" when the fan is manually turned on at the thermostat and also when the thermostat is calling for cooling but the fan does not run. I made a kludgy little harness to force the fan to run 24/7 so he can have cooling. Rather than directly replacing the fan controller, I found this online:

Looks like a universal replacement for the same function, and has the benefit of providing "HUM" and "EAC" terminals as well which is nice (the old control does not have that, and he does have a humidifier which I started to get working for him but never wired up because I meant to bodge something together to interlock it properly and never got around to it - the house did not really seem to need extra humidification even over the winter. But it's there and the unit itself functions so he'd like it to be working...) The description says "The ST9120U can replace any ST9101, ST9120, ST9141 or ST9160 listed in Table 3 below" but I'll be damned if I can find Table 3.

Could someone more HVAC literate than I tell me if I should recommend to my friend that he order the part that I suggest above rather than the direct replacement? It's $50-100 cheaper depending on source and will make getting the humidifier working again a hell of a lot easier.

thanks,

Nate

[Stormin Mormon]

Sounds reasonable to me. I've not worked on this kind of thing, but it sounds reasonable.

Christopher A. Young Learn more about Jesus

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Here's my question. I believe that the fault is in the fan controller board; I am getting control voltage on "G" when the fan is manually turned on at the thermostat and also when the thermostat is calling for cooling but the fan does not run. I made a kludgy little harness to force the fan to run 24/7 so he can have cooling. Rather than directly replacing the fan controller, I found this online:

Looks like a universal replacement for the same function, and has the benefit of providing "HUM" and "EAC" terminals as well which is nice (the old control does not have that, and he does have a humidifier which I started to get working for him but never wired up because I meant to bodge something together to interlock it properly and never got around to it - the house did not really seem to need extra humidification even over the winter. But it's there and the unit itself functions so he'd like it to be working...) The description says "The ST9120U can replace any ST9101, ST9120, ST9141 or ST9160 listed in Table 3 below" but I'll be damned if I can find Table 3.

Could someone more HVAC literate than I tell me if I should recommend to my friend that he order the part that I suggest above rather than the direct replacement? It's $50-100 cheaper depending on source and will make getting the humidifier working again a hell of a lot easier.

thanks,

Nate

[Nate Nagel]

FWIW the original fan controller board was Honeywell ST788C as best I can figure. (original diagram is smudged, I guessed ST780C or ST788C. ST780C yields no hits and ST788C was apparently discontinued and replaced by the 47-22481-81 number that is in there now. That may be a Rheem/Ruud part number not a Honeywell part number however.)

I did drop Honeywell a line as I couldn't get anywhere with their online parts cross reference - they don't recognize 47-22481-81 and they say there is "no replacement" for the ST788C?

Am wondering if I could just replace both the fan control and hot surface ignitor module with this guy

I don't see why it wouldn't work, but it's not directly listed, and with something like a furnace that makes me a little nervous...

nate

[Nate Nagel]

It does, but the instructions for the ST9120U do not specifically show it being used with a hot surface ignitor control module, and Honeywell doesn't specifically list it to replace either the original fan control or the one that's in there now. I would really hate to have the furnace cause some kind of issue (what that would be, I don't know) and not have documentation to support my choice of replacement parts. My friend seems less concerned about this, but then again he's the guy with the 30 year old furnace (outdoor A/C unit looks newer; A-coil is definitely vintage) and he seemed surprised that I even suggested he have a pro come do an annual service/checkout on it since I don't have a gauge set or the ability to detect leaks...

I guess when it comes to stuff that I'm not super comfortable with, and especially other people's stuff, I like to have my ducks in a row.

nate

[N8N]

Ugh... well after finally talking to honeywell tech support (apparently they will actually talk to you if you say you are an engineer) I still didn't get the warm fuzzies (the answer was basically something like "eh, it should work") but the price diff was so dramatic that I went ahead and ordered the universal board anyway... showed up yesterday along with a new run cap, tore apart the spaghetti in the old controls box in preparation to get this working... now it appears that I have to also retrofit a flame sensor to this POS because the old ignition control somehow used the ignitor itself as a flame sensor and this new one doesn't... wish they'd have told me that over the phone... even though they're both Honeywell parts and I read off everything that I had to the guy.

...or is there something easy and obvious that I'm overlooking here?

nate

[Vic Smith]

Yeah. Calling in a HVAC repair pro. I know this a DIY group. But sometimes just using a pro is faster, safer, AND less expensive. That's what I did when I got stumped on my furnace. If you're really into it, stay on your path. Just remember 2 engineers talking got you nowhere. One "good" HVAC repair guy who's fixed that model furnace 30 times is probably better than 30 engineers in this case - with one hand tied behind his back. I saw that in action on my furnace.

[Nate Nagel]

I hear that... but the owner of this mess is, um, frugal (he makes me look extravagant) and at this point it's a personal challenge.

I've already ordered a flame sensor retrofit kit... to date the total parts bill is less than $100 and I've already got the A/C back on and working. I know the heat won't work until I get the flame sensor in and I also still have to figure out the stopped drain valve wiring as it was a bit odd in the original installation. But I seriously doubt heat will be required any time in the next six months.

Also, I am curious about something... this is a Honeywell control wired to a Honeywell thermostat, so you'd think that everything would work swimmingly, but when I originally tore into this I found "C" not landed on the thermostat and the thermostat running off batteries. I landed "C" at the thermostat and found that the thermostat would still not power off the 24V xfmr. only batteries (if you took the batteries out it would power down.) Now I have the new control in, same thing. Only thing I can think is some weird interaction with the A/C contactor? (am online from there now and don't want to power down A/C at the moment it is actively cooling the house back down to the temp it should be.)

nate

[Nate Nagel]

Never mind the latter... I found the point where the thermostat and contactor wires were spliced together (not inside the furnace) and the blue wire going up to the thermo was never connected to the common of the fan control/contactor. One problem solved (hopefully) I went ahead and powered down to make that connection... waiting for A/C compressor timeout to see if it kicks back on OK. Guess whoever wired it up originally didn't land that wire because house originally had one of those round thermostats, and whoever installed the new thermostat didn't put much time into troubleshooting it.

nate

[Nate Nagel]

Should have mentioned I was using the Fluke meter and not the Simpson up at the thermostat and I found the voltage between red and blue to vary but not to be zero... bet if I'd used the Simpson I would have suspected an open in the first place. C'est la vie. So used to low voltage that I work on being DC that I forgot that this was AC. Lessons...

nate

[Nate Nagel]

...aaand in the same place as the splice previously mentioned, the yellow from thermostat was never connected to fan control, so when A/C was commanded, only the green wire was turning the fan on, so in A/C mode the fan was running in heat speed... waiting for it to cycle back on to see if there's a noticeable improvement (I hope so, I was actually thinking that the fan didn't seem noticeably faster in A/C, guess I was right!)

I can't imagine that that was good for the A-coil, but it's ancient and still kickin'

nate

[Vic Smith]

Sounds like you're having fun. Wasn't for me. The fan only worked on slow speed. No question all wiring was good. I did the tapping on the MB relays with a screwdriver handle to no effect. The guy I called in - $75 for the call, deducted if he did more work - had it running right in about 20 seconds. He just powered it off, stuck his hands in there, turned it back on, and it worked. I told him you're a f**king genius, what did you do? He said he twisted the MB, and that freed the relay. I had him replace the MB, since it could happen again at a bad time. Cost $450 total, board was $320. MB came with new flame sensor. As a bonus, I haven't had to clean the flame sensor or touch the unit since then, about 4 years.. Either the original flame sensor or the MB was flaky. At least twice a year it would get cold in the house, starting the year I had it put in. Drove my wife crazy, and she worked that on me. Usually cleaning the flame sensor would fix it, but I also replaced the purge valve, which is part of the start cycle. Cheap, but I spent a lot of time horsing around with the thing. Good luck on getting the new flame sensor configuration working. Those IC's can be tricky. Hope it's not a case where a good HVAC guy would say "Won't work."

[Nate Nagel]

A good HVAC guy would run screaming from this setup. Furnace passed its "best before" date about 15 years ago, although it is a "90 plus" unit so as long as the heat exchanger holds together it's not costing him a lot of money in fuel vs. a new unit. A-coil doesn't look much newer; may even pre-date furnace. Outside A/C unit actually looks fairly good and quasi-recent. I cleaned everything and went over it with a fin rake, and of course blew out all the drains so the electronics don't take a crap again. I hope to get this stuff working 100% without spending any more $ and have already advised owner to start socking money away for eventual replacement next time something breaks. I suspect next failure will either be heat exchanger, air handler fan, or a-coil, and in my mind at least failure of any would prompt replacement of the entire containing unit of the affected component as everything is, um, antique.

Owner also wants humidifier to work correctly which it hasn't in ages (I found it valved off) I'll get that working although honestly I doubt it will ever kick in. I put some little humidor hygrometers around the place over the winter and the Rh never dropped below 50%.0 (that's what I did at the last place that I owned; used them to dial in the humidifier on the furnace and the dehumidifier that sat next to it that was required in the summertime. I had that place tuned so that if you set the thermostat on "auto" you could be comfortable 24/7 without ever having to touch anything.) But it'll look nice if he ever goes to sell the joint.

nate