Hi, We've been without heat all day now. This Bryant is 10 years old, it's an upflow 395CAV042091 model. Blower motor runs but there's no gas ignition. Filter is new a few days ago. The DSS (draft safeguard switch) contacts are closed, the PRS (pressure switch) contacts are open like they're supposed to be when de-energized. BUT, the IDM (Induced Draft Motor) is not turning; there doesn't appear to be any voltage to it. LED flashes 3 short, then 3 long repeatedly. I don't know what that means. We've occasionally had to power cycle this furnace to get it running the past couple weeks. Today, it's completely failed. Help determining cause would be greatly appreciated.

RDL

Oh, I see from the schematic that the IDM is controlled by the IDR (induced draft relay). The relay is controlled by the CPU and switches the line side of the transformer, 115VAC, to the IDM. Relay could be bad ? If not, what would cause the CPU not to activate the IDR?

RDL

LED flashes 3 short, then 3 long repeatedly

Look on the inside of the blower access door for the meaning.

Unable to find a decode anywhere for the 3 short, 3 long blinking LED. The IDR is definitely not pulling up. I got good continuity across the IDM motor winding. If you look at the attached image, it shows item 12 to be the Manual Reset Limit Switch. I can't find anything like that on the furnace.

RDL

Look on the inside of the bottom panel for the service label.

It is a code 33 not long and short. Look on the diagram.

Ok, I found it. Yes 33 - Limit or flame roll-out switch is open. Check for

- Inadequate combustion air supply flame roll-out switch or fuse link
- Open flame roll-out switch or fuse link

Manually reset or replace.

Also says something about if open for longer than 3 minutes, code changes to 13, which means flame roll-out switch lockout. Also tells you to check for inop blower motor or starter cap. loose blower wheel, dirty filter or restricted duct system. I didn't notice a change to 13.

I know where the flame roll-out switch terminals are. Can't find a manual reset, though.

RDL

Bypass rollouts and high limit one at a time. Or you could check for continuity at each switch with power off, and wires disconnected.

Ok, this furnace doesn't appear to have the flame roll-out switch. It's called a Limit Switch and there's no manual reset. And it's open allright. I guess I'll place a quick jumper across the terminals and see if the furnace starts up.

Im betting it will. Make sure to replace switch ASAP. Do not use the furnace with the limit bypassed. Get some electric heaters and keep comfy(and safe) until you can get a new limit. Good luck and please let me know if the limit solves the problem.

Success ! Yep, it's a limit switch, 1ea with an inline fuse link in one of the leads that looks kind of like a diode. I'll look around for a part number for the switch this evening and try to buy one tomorrow. We'll start up the gas logs downstairs which should warm the house up a little overnight.

Thanks for the help!

RDL

Glad you found it. If defective replace right away with original part since the temp setting and allowable variances must be maintained has to be maintained. Many times there is a reason the safety failed so if the new one has a problem you will need to dig deeper into the problem.

Success ! Yes, it's a Limit Switch with an inline fuse in one of the leads that looks kind of like a diode. I'll look around on the Internet for a part number for the switch this evening and try to buy one tomorrow. We have gas logs downstairs so the house won't cool down too much overnight. Thanks !

RDL

Some additional info for folks who have the same problem as me. In this furnace, the part is HH12ZB170. 170 for 170 degree F. I couldn't find anyplace open today (Saturday), so I looked around the Internet for one. Price varies a lot. As high as $35 with $30 shipping (2nd day). But, I ended up ordering from Patriot Supply. $16.72 for the part and about $16 shipping 2nd day. Patriot Supply - (http://www.patriot-supply.com).

Not to rain on your parade, replacing the limit treated your symptom (no heat)... but why did it fail? You may have a dark underlying problem... or maybe you're just not changing the air filter often enough... but you may not be out of the woods yet.

It is snowing now not raining. LOL

Well, at this point with the furnace down, I can't really continue shooting trouble. I need the new limit switch installed and the unit up and running to see if there's actually a problem with over temp. The switch was wide open but I couldn't see any evidence of over-heating. Possible and I hope this is the case, the part just failed. Furnace went in mid-1997.

RDL

These parts do fail. I hope this is the case. You should know soon enough.

This will be my last posting since my furnace is up now. We had a real cold night for this part of Virginia. Last night it got to 12F. I ordered the limit switch Saturday but it won't arrive until Wed. I tried again today to find it locally and did. $7.36 from a supply house here. To recap and try to understand what happened, I realize I probably didn't keep the filter changed out often enough. Over the past two weeks the furnace has been failing sometime during the night. We would have to power cycle it in the morning to restart. A dirty filter might have reduced air flow through the chamber causing temp to climb and open the limit switch. It's been running a lot today to catch up and so far no problems. Here's a photo of the limit switch.

RDL

If you are right, that limit was cycling for some time and finally wore out (stuck open) - a valuable, but hopefully not costly, lesson on the importance of maintaining the air filter.