[charlie902]

Hi All,

I have a goodman GMH950904CXAC 5 years old in my attic. Recently I noticed that it has been short cycling. When calling for heat, flames go on for 4 mins and then shut off but blower continues for another 3 mins. Then shuts off completely but in a few seconds starts again (since room is still cold and calling for heat), it continue doing the same short cycles until room reaches the desire temperature. I noticed that if I open the cover (bottom one but in my case the one of the left since it is not standing up) where the blower and filter is, it works fine with no issue, doing a normal cycle, meaning until room reaches temperature.

I cleaned the flame sensor, then decided to replace for a new one. I also changed filter and same short cycles. But as mentioned before, it works well if I leave the cover off.

Please let me know what other information I need to provide.

Thanks in advance,

Carlos

 

[beenthere]

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.

 

[charlie902]

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?

 

[user_12345a]

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.

 

[charlie902]

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

 

[Bitters]

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.

 

[user_12345a]

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.

 

[charlie902]

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

 

[user_12345a]

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.

------------------------------

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.

 

[charlie902]

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.

 

[user_12345a]

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.

 

[charlie902]

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

 

[ben's plumbing]

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

 

[user_12345a]

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.

 

[charlie902]

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?

 

[beenthere]

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.

 

[user_12345a]

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.

 

[BayouRunner]

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.

 

[charlie902]

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?

 

[user_12345a]

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.