Hello,
I have an old (really old, I think) Carrier furnace, Model# 5GS0___ (can't even read the rest of the model number). The blower fan stays on even when the thermostat temp is achieved, when the thermostat is set to "off" and "auto", even when the thermostat is unplugged. This has only started happening recently.

After some research, it seemed like the limit switch was the likely culprit so I bought one online and replaced it.

Old limit switch: L250-40, model# HH12ZA251
New limit switch: L250-40, model# HH12ZB250

The new one is supposed to replace HH12ZA250--not exactly my model number, but close. Purchased from this site.

However, even after replacing the limit switch and the rusty-looking connectors on the old wires, my blower fan still stays on. There is an old diode in series with one of the wires that connects to the limit switch. I'm not sure why that's there, but maybe that's the problem? Otherwise, is there something wrong with my control board?

Does anyone have any ideas on how to fix this? This is very frustrating and any help would be appreciated. Thanks in advance.

 

there is a good chance that the relay in the control board has stuck closed.

 

I was afraid of that. How easy is it to change this relay? Or do I have to change the whole control board?

 

The "diode" is probably a fusible link that burns out easily if flames back out into the burner compartment.

The limit switch has nothing to do with operating the fan.

I doubt you have the model number correct. Look on the rating plate in the burner compartment of the furnace. Look more carefully at the model number --- I suspect you aren't reading it accurately.

 

if it were the relay you would have to replace the whole control board. Do you have a control board? Can you post pictures?

 

I have something that looks like a big circuit board with a bunch of wires attached. I'm assuming that's a control board. Without opening the housing I can't tell if there's an on-board relay. I'll try to take pictures later this evening when I get home.

SeattlePioneer--the limit switch certainly does control the fan. When the flue pipe gets hot enough (because the heat exchanger is switched on), it trips the lower limit and triggers the fan on. When the exchanger switches off, the fan stays on. When the temp drops in the flue, the upper limit is tripped which switches the fan off. Apparently when the limit switch goes bad, it is stuck on the upper limit. Read this:
Furnace Fan Limit Switch Control: a guide to the fan limit switch, settings, manual fan override, temperature settings for warm air heating furnaces - Honeywell L4064B Combination Fan and Limit Switch

 

You have now supplied enough information to have a reasonable understanding of which furnace you are talking about.

Does your furnace look like this one?

http://www.xpedio.carrier.com/idc/gr...i394g-25-1.pdf


In particular, does it have the pictured opening (draft diverter) above the slotted burner compartment cover? If you have a counterflow furnace it would look somewhat different than this picture.


This type of furnace will run the fan if the 24 VAC power is interrupted to the "R" terminal. This can happen if the transformer is burned out, the limit switch is open, the vent safety switch is open or the fusible link (the "diode" ) has burned out. There might be other parts in the circuit as well.

Do you have a multimeter and understand how to use it?

You need to start by checking for 24 VAC at the "R" terminal, and work your way back until you identify which part of that circuit is open.

 

Yes, it looks just like that one.
Here is all the info I have on my furnace:
Model 5GS0_(9,5)0-_(2) [I couldn't read the full # so values in () are guesses]
Series 201
Serial D9C25_(c)39 [I couldn't read the full # so values in () are guesses]
In 50,000 BTU
Cap 40,000 BTU
58ES900410

I'll check it with my multimeter later today. Thanks so much for the help.

I do have a couple of follow-up questions before I start, though:
1. Which "R" terminal are you talking about? The only one I see on the diagrams goes to the thermostat.
2. Since I can't read any specs on the fusible link, where do I get a one and what rating do I get? Can I operate without one?

 

The R terminal on the circuit board in the fan compartment. Measure between C and R or Chassis ground and R. If You don;t have 24 VAC with the power on and the fan compartment door switch pushed in, you need to check further up the circuit until you identify what is blocking the voltage.

The fusible link protects the furnace wiring from being burned up if the flame rolls out the front of the burners into the burner compartment.

If it's bad, you can get a replacement at a Carrier, Bryant, Day and Night or Payne dealer or distributor or on line.

 

Okay, so I used my multimeter to test some of the components with the power switched on.

For the two wires going to the limit switch:
I'm able to get about 24 V AC from the red wire (the one with the fusible link in series), on either side of the fusible link. I'm also able to get 10 mA when I connect my multimeter in series with the two leads (bypassing the switch altogether).

On the control board:
When I give it a tap with my flashlight (while power is on), I see a small spark from one of the relays on the board but not from the other. I'm suspecting that the non-sparky relay is the bad one.

Based on this, I think the problem is the relay on the control board. I took some pictures, but I'm having some trouble uploading them. I'll try again later.

Control board model number:
302075-302
Again, it's a Carrier furnace (so this is probably a Carrier number).

I found the following replacement boards:
From Patriot Supply, $45 (click for link)
From Shortys, $75 (click for link)
From myhvacparts, $100 (click for link)
From allparts, $95 (click for link)



I'm assuming all of these are adequate replacement parts and are the correct part. Am I right? Is there any reason not to go with the $45 board?

Thanks again for the help--it's much appreciated.

 

If you don't want to follow directions, why ask for help?


You can replace the circuit board if you wish. When you get done with that you can follow the testing procedures you've already been given if you want to identify the problem.

It's up to you.

 

I agree with SP you need to go through the checks as he has described so that you don't replace the control for no reason other than to through parts at your furnace to fix it.

 

Sorry, but I did check that too--I guess that wasn't obvious.

I am definitely getting 24 V AC (actually, more like 27) between R and C. After I checked that, I then proceeded to do the other things I listed in my previous post.

So where does this leave me? Is it the control panel? Something else?

 

without being there I can't tell you for 100% but I would replace your board.

 

I'd measure the voltage between "G" and "C". If the voltage there is zero or thereabouts, then I'd replace the circuit board.

 

0.64 V between Gc and C

Any problem getting the cheapest circuit board I posted above? $45 is a lot better than $100, but not if it's made from cheap parts and will fail again in a year.

Thanks again.

 

typically you'll find something is a lot cheaper for a reason. not that it is a bad board. I don't know, but i do know from experience you get what you pay for.