Home heating forced hot air problem, need HVAC expert..

demarpaint · May 30, 2021

TIA

It's still cold on L.I. and I popped on the heat only to find our old hot air heater is not working. The diagnostics were easy enough however after trying to clean the part [Limit Switch or Limit Control] to no avail I need a replacement part. I can't locate it because of the age of the unit and would rather not upgrade the heating system since we will be selling the house soon. In the meantime I'd like to get it working and source a replacement part, before pounding the pavement looking for one. The unit is a Bryant 100-393U. Numbers off the switch not pictured are as follows: Therm-o-Disc Patent #2645692.
Thanks.

KJSmith · May 30, 2021

That does not appear to be a limit (safety) switch.

That looks like a fan control.
Older furnaces used them to bring the fan on after the heat exchanger warmed up a bit.
Avoids blowing cold air into the room.

demarpaint · May 30, 2021

KJSmith said:
That does not appear to be a limit (safety) switch.

That looks like a fan control.
Older furnaces used them to bring the fan on after the heat exchanger warmed up a bit.
Avoids blowing cold air into the room.

Thanks for the reply. This device IIRC was replaced by a flame sensor, which my unit doesn't have. The pilot is working, and when I turn on the thermostat the flame kicks in then shuts off in about twenty seconds or so, then it tries again and fails. Then unit stops trying. That points to a dirty or bad flame sensor, which this unit doesn't have. If I lower the temperature on the thermostat, wait about half an hour and try again, it repeats the process.

In the meantime I cleaned it up again and the heat has been running for about half an hour, but not with the intensity it normally would.

hemitom · May 30, 2021

demarpaint said:
TIA

It's still cold on L.I. and I popped on the heat only to find our old hot air heater is not working. The diagnostics were easy enough however after trying to clean the part [Limit Switch or Limit Control] to no avail I need a replacement part. I can't locate it because of the age of the unit and would rather not upgrade the heating system since we will be selling the house soon. In the meantime I'd like to get it working and source a replacement part, before pounding the pavement looking for one. The unit is a Bryant 100-393U. Numbers off the switch not pictured are as follows: Therm-o-Disc Patent #2645692.
Thanks.

View attachment 58750 View attachment 58751

KJsmith is right that is an on /off switch for your fan

hemitom · May 30, 2021

hemitom said:
KJsmith is right that is an on /off switch for your fan

could be issue with gas valve , maybe thermostat ?

Dennywires · May 30, 2021

is that plenum mounted?

Dennywires · May 30, 2021

check supplyhouse.com not knowing what the other end looks like makes it difficult to find a match.

Warstud · May 30, 2021

While it's running...... stand next to it and listen for anything out of the ordinary.

Fitter30 · May 30, 2021

Take a pic of the wiring diagram

Oro_O · May 30, 2021

Frank, maybe this would help:

I'm not an HVAC guy but several years ago someone on here directed me to this diagnostic flowchart for furnaces. It saved me a lot of money by making my troubleshooting logical and accurate. In case you want to double check. Maybe you have it ID'd correctly already and if so, it's still worth re-posting this great resource in case it helps someone else:

https://arnoldservice.com/wp-content/uploads/2012/12/furnacetroubleshootingflowchart1.jpg

I can't recall who here helped with my furnace issue years ago but it was very valuable; which I could recall.

Bryan

demarpaint · May 30, 2021

Dennywires said:
is that plenum mounted?

Here's a picture I took earlier of where it is mounted.

demarpaint · May 30, 2021

Warstud said:
While it's running...... stand next to it and listen for anything out of the ordinary.

It sounds great.

Fitter30 said:
Take a pic of the wiring diagram

I will look for it tomorrow, and post a picture if I can find it. I put it together and showered up.

Oro_O said:
Frank, maybe this would help:

I'm not an HVAC guy but several years ago someone on here directed me to this diagnostic flowchart for furnaces. It saved me a lot of money by making my troubleshooting logical and accurate. In case you want to double check. Maybe you have it ID'd correctly already and if so, it's still worth re-posting this great resource in case it helps someone else:

https://arnoldservice.com/wp-content/uploads/2012/12/furnacetroubleshootingflowchart1.jpg

I can't recall who here helped with my furnace issue years ago but it was very valuable; which I could recall.

Bryan

Thanks buddy I'll check it out.

It seems to be working now, the heat kicked on and off on its own via the thermostat setting. My problem is I really have no way of telling if it is putting out as much heat as it did. It seems to be running good, but my gut is telling me it could be hotter.

Again thanks for all the replies.

dbias · May 30, 2021

Yeah that a temp switch that turns the fan on at 130 degrees as the label indicates . This prevents a lot of cold air being blown into the house while the heat exchanger warms up. If that part fails, the fan fails to blow and if it fails to kick on the limit switch on the heat exchanger will get too warm and kill the flame.

Oro_O · May 30, 2021

demarpaint said:
It sounds great.

I will look for it tomorrow, and post a picture if I can find it. I put it together and showered up.

Thanks buddy I'll check it out.

It seems to be working now, the heat kicked on and off on its own via the thermostat setting. My problem is I really have no way of telling if it is putting out as much heat as it did. It seems to be running good, but my gut is telling me it could be hotter.

Again thanks for all the replies.

If it's the switch, maybe this related experience might help.

Last week my dad finally fixed a weird motorcycle running problem by disassembling and totally polishing the contacts in the kill switch.

'80 Honda CB400 Hawk GT, finished restoring it but then a weird running problem. Idled great, died under any acceleration or load. We've been going through coils, plugs, wires, resistors trying to figure it out. All the normal suspects made no difference. Dad noticed a slight anomaly with the kill switch. Stripped it, cleaned/sanded polished all the metal, and now it's great. I'm not entirely sure why the increased resistance in the switch was affecting the running at higher rpms, but it was. Maybe strip that switch and polish all the contacts?

demarpaint · May 30, 2021

dbias said:
Yeah that a temp switch that turns the fan on at 130 degrees as the label indicates . This prevents a lot of cold air being blown into the house while the heat exchanger warms up. If that part fails, the fan fails to blow and if it fails to kick on the limit switch on the heat exchanger will get to warm and kill the flame.

That's what I found out as well. If that switch is bad it kills the the flame in short order, then no heat, no blower.
I stumbled upon this and was wondering if it would work? I have to take measurements to see if it will fit in the rectangular opening, and set it at 130F. Its range is 110F-140F.

Or as @Oro_O suggested take it out again and really give it a good cleaning. I've been running the heat for the past two hours and the cleaning job I did earlier has the machine cycling on and off the way is is supposed to.

Dennywires · May 30, 2021

demarpaint said:
Here's a picture I took earlier of where it is mounted. View attachment 58766

from here it looks like a "pot switch" to dial in temp for the pop disc thermostat. you may find on supplyhouse.com a pop disc thermostat with a "pot switch all-in-one".

Dennywires · May 30, 2021

3F05-1 - White Rodgers 3F05-1 - 3/4" Snap Disc Fan Control, Temperature Range 90 to 130 Degrees F, Includes Tab-to-Screw Terminals

White Rodgers 3F05-1 - 3/4" Snap Disc Fan Control, Temperature Range 90 to 130 Degrees F, Includes Tab-to-Screw Terminals - Adjustable Snap Disc Thermostat Allows You to Set the Temperature Set Point to Match Your Specific Needs Which Simplifies Inventory Features <ul><li>1/4"...

www.supplyhouse.com

demarpaint · May 30, 2021

Dennywires said:
3F05-1 - White Rodgers 3F05-1 - 3/4" Snap Disc Fan Control, Temperature Range 90 to 130 Degrees F, Includes Tab-to-Screw Terminals

White Rodgers 3F05-1 - 3/4" Snap Disc Fan Control, Temperature Range 90 to 130 Degrees F, Includes Tab-to-Screw Terminals - Adjustable Snap Disc Thermostat Allows You to Set the Temperature Set Point to Match Your Specific Needs Which Simplifies Inventory Features <ul><li>1/4"...

www.supplyhouse.com

Thanks. I might have to make a few modifications to the sheet metal. I'll look into it.

KJSmith · May 30, 2021

If you have it reinstalled, set it to 100 and see if that helps.
Not sure what you mean by lower intensity.

You are correct, if the fan does not come on, the flames will shut down.
The device right above it is likely the safety doing that.

demarpaint · May 31, 2021

KJSmith said:
If you have it reinstalled, set it to 100 and see if that helps.
Not sure what you mean by lower intensity.

You are correct, if the fan does not come on, the flames will shut down.
The device right above it is likely the safety doing that.

Thanks for your reply. I was thinking of trying setting it lower, but left it the way it was originally set after cleaning the part. I might give that a try as well.

I gave the heat a try this morning, and it fired up the way it is supposed to. What I mean by "lower intensity" is, the "feel" of the heat coming out of the vents is not as hot as it was. The problem is I have no actual vent temps to compare it too. It seems to be taking quite a while to warm the house, especially in warmer temps than winter.

The next thing I am going to do is to check and clean the burners, and make sure I have a good strong flame.

FTR the bulk of my heating is via a wood burning insert. I cleaned that out and decided to run the heat to take the chill out of the house instead of starting a fire.

Home heating forced hot air problem, need HVAC expert..

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