# Goodman furnace short cycling



## beenthere (Oct 11, 2008)

What kind of air filter are you using. Is it a 3M?

Your A/C coil could be dirty, if you have central A/C.


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## charlie902 (Jan 23, 2012)

Thanks for your reply. Filter is 3M brand new. Yes I do have A/C. Last night I went to basement and used by wet vacuum in the drain tube that is coming from furnace. It got a lot of dusty and dirty water out of it. What is the best way to check that it is totally clean? If so, what is next?


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## user_12345a (Nov 23, 2014)

Does sound like a limit opening, but to be sure check sight glass on the blower access panel for error code - there's should be a flashing led on the board, number of flashes, length is the code, check schematic for the codes.

The evaporator coil is unlikely to be dirty, but the secondary heat exchanger could since it's upstream of the coil. Check the blower wheel for dust and dirt buildup.

What size is the filter? 16x25x1, the common size may be too small for this 90 000 btu furnace with anything but fiberglass.


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## charlie902 (Jan 23, 2012)

Light is steady red. Filter 20x25x1. Where is the secondary heat exchange? How can I check?


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## Bitters (May 28, 2014)

If it runs normally with the blower door off then there is a restriction. Pull the filter out and put the cover back on the blower section. See if it runs normally.


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## user_12345a (Nov 23, 2014)

The blower doesn't "care" which side the restriction is on, it could be before the blower or after.

Open the blower compartment and check the fan blades for dust buildup. 

If the blower wheel is very dirty, the secondary is too, you have to pull the blower to get to the secondary.

The light will only flash when the limit is open.


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## charlie902 (Jan 23, 2012)

How can I take the blower down? Do I need to remove the circuit in front of it?


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## user_12345a (Nov 23, 2014)

If the blower is clean you don't need to pull it. A clean blower wheel means a clean secondary.

Power off first. You can pull it if needed by unscrewing the panel that the board is on and pushing it out of the way; in your case, down. Blower motor wires get disconnected, some other wires may need to be disconnected.

There's also a aux limit that need to be removed.

From there, you unscrew the bolts holding the blower wheel and pull it out.

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I think the filter is the problem.

The type of filter in the picture - high merv 1" is horrible for all but the smallest systems.

The highest grade 1" filter I can recommend is a merv 7 or 8, and on your system you may need to use fiberglass on the 90 000 btu furnace to keep it from cycling on limit. Depends on your ductwork.


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## charlie902 (Jan 23, 2012)

Thanks. I'm confused of what this works with no issues when the cover is off... I also try removing the filter and when I close the cover same issue happens.... I can get a different filter as your recommendation but I'm confused as mentioned before.


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## user_12345a (Nov 23, 2014)

Which cover?

The blower access panel while holding the door switch closed?

The filter cover?

With the filter cover off and a restrictive filter, just enough air leaks around the filter into the furnace so it gets enough air to stay off limit. The allergen filters can actually make for a dirtier furnace if the filter holder isn't well sealed to the furnace.


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## charlie902 (Jan 23, 2012)

with the blower access panel off while holding the door switch closed works fine with no issues. No short cycle. Once I put that access panel back on whether with filter or not... short cycle starts, 3 mins flames go off, blower stays on another 3 mins and cycle repeats over and over


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## ben's plumbing (Oct 6, 2011)

make sure all the cold air returns are open...not blocked by something...


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## user_12345a (Nov 23, 2014)

charlie902 said:


> with the blower access panel off while holding the door switch closed works fine with no issues. No short cycle. Once I put that access panel back on whether with filter or not... short cycle starts, 3 mins flames go off, blower stays on another 3 mins and cycle repeats over and over


Running with access panel off is like disconnecting the return duct and filter; of course it won't trip the limit like that.

My question is, has it always done this or not? Did it start with a filter change, furniture placement, etc.

Limit tripping you should verify 100% b4 going further; check the code. 3 Minutes is a very short run for overheating, so something else could be up.

For overheating there are a couple of tests you could do to really see what's going but, but make sure the limit is actually tripping.


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## charlie902 (Jan 23, 2012)

It has been like that the last couple of weeks. No changes in furniture, all vents are open and same brand of filter.

How can I check the limit is actually tripping?


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## beenthere (Oct 11, 2008)

So is it possible its been operating like this for a long time now. And you just noticed it? Remove the air filter, put the cover back on, and run the furnace. See if it keeps on running without the filter in.


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## user_12345a (Nov 23, 2014)

Check the flash code.

If i'm looking at correct schematic, should be four flashes in a row, repeat when the burners cut out.

If it stays steady when this happens, bypass the t-stat by jumpering R and W at the board, repeat the test.


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## BayouRunner (Feb 5, 2016)

Almost sounds like a heat anticipator setting. But the part where you say it runs fine with the door open is throwing me off. Is your thermostat one of the old Mercury types? If it's a newer thermostat disregard.


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## charlie902 (Jan 23, 2012)

It is a new thermostat.... 

No short cycle when cover is open 

When cover is closed... work but goes to those cycles of 3 - 5 mins


What else can I check?


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## user_12345a (Nov 23, 2014)

Did you check for flash code?

Run it with the filter pulled out, but just blocking the opening where you insert it.

For limit trips, you need to check the temperature increase between return and supply and clock the gas meter. (do a search to know how) This is a timer controlled two-stage furnace, if yours is set up to stage, disable that feature before doing the tests; you want it just running on high to really show an airflow or overfired condition.

This furnace if set up to do so will run at 75% capacity for the first few minutes of every cycle. Runs at a lower fan speed too. Heats maybe a little more evenly when staged but not as efficiently -> single speed venter motor messes up the fuel to air ratio on low.


Check schematic; there are dip switches on the board to check.


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## charlie902 (Jan 23, 2012)

Still same issue. No flashing codes but flames turn off after 3-5 mins, then blower stays on 3-5 mins and then cycle starts.

Got a technician at home yesterday, spent an hour and left saying maybe to change the limit switch but he was not sure. I swapped that from the furnace I have in my basement (same model, brand, size, etc but working fine) and still same issue. 

Any other help or guidance will be greatly appreciated


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## roughneck (Nov 28, 2014)

Is the limit actually opening? If it is then what temp is it tripping at vs the rated temp? How many CFM is the furnace moving? What's the return duct look like? If there's a bunch of flex then make sure nothing's crushed.
If it is the limit, and the furnace is in fact overheating, then it's reducing the lifespan of the unit. The heat exchanger isn't going to take that kind of abuse without eventually failing.


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## user_12345a (Nov 23, 2014)

> Got a technician at home yesterday, spent an hour and left saying maybe to change the limit switch but he was not sure.


 Then you got a clown. This is a very simple problem to fix and if the limit is tripping, there's a reason why. If the limit is tripping prematurely, even that can be verified.

Jumper R and W at the board to bypass the t-stat and see if the problem recurs. Low voltage could be dropping out.

If the limit is tripping you could easily identify it with a multi-meter. Assuming something isn't interrupting the call for heat, voltage drop across the switch will be 0v when closed and around 24v when the limit opens.

In fact right after the limit trips you could cut power, disconnect the leads and check resistance.


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## vicbt (Dec 13, 2017)

So Charlie, did you ever figure out what was happening with the furnace? What was the problem? I got the same exact symptoms as you. I've been researching for quite some time and run into your post. I know the post is bit old.. but any respond would be appreciated!
Thanks!


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## NitrNate (May 27, 2010)

for limit trip you should be able to leave a voltmeter on it while its running. what i noticed on mine, is you get 0 volts across the terminals when its running (continuity) and then right before the furnace shuts off you get 24 volts (limit tripped open). you have to literally be staring at it or you will miss it because it will drop back to 0 when the furnace shuts off.

also, if you have a temperature probe of any sort (oven thermometer for turkey, etc.) you can unscrew one of the screws that hold the limit switch on and poke the probe through there during operation. this will give you a pretty good idea if the furnace is reaching the temp of the limit probably causing it to trip. the limit temp should be printed on the switch itself somewhere.


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## dmark1867 (Dec 20, 2021)

Charlie or vicbt any updates on this??


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## roughneck (Nov 28, 2014)

dmark1867 said:


> Charlie or vicbt any updates on this??


Its been 5 years since either have even logged on to the site. 
If your having an issue much better to start a thread. Then reference a dead one.


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