Heater blows hot AND cold.

[oldpops]

Hi all, The last couple of weeks we have noticed that our heater was blowing warm air the first time we turned it on, and then it would blow cold air. At first I thought it was because the attic was cold and that the air moving through the duct-work was getting cooled down by the cold attic. So we kept the heater off. A couple of times it was so cold that I would turn the heat back on (with the thermostat set to auto--not set to 'on'). On those occasions, the heater would kick on and the first time the heater would send out warm air out of the duct-work/vent. But after that, the next time the heater cycled on, only cold air would come on. I have read online that it could be cause by the filter so we changed it. The old one was a bit dusty but I could see a lot of light coming through if I help it up to the window. However, even after changing the filter the cold air blowing is still coming out. We keep the heater turned off unless it gets really cold inside the house, and then we turn on the heat (thermostat set to 68 degrees). After the heater has been off for a day or two, then warm air will usually come out the first time it comes on. But then the next time it cycles on, cold air comes out of the vents. Then again, sometimes cold air comes out the first time the heater comes on so I don't know what to check on this intermittent problem. Anyone have any suggestions? Let me thank everyone in advance for all advice and help, I do appreciate it.

 

[Done That]

Give us a brand & model number or equipment type to get started....I mean, are we talking about a natural gas furnace, or an electric air handler, or ??

Your average gas furnace has a control board in it, and most have a diagnostic led on them that blinks, with a decoder document stuck to the inside of the sheet metal door you open up. That would be a good place to start.

 

[Richard Cranium]

with out knowing more info, Not much we can do to help you.

 

[zmaxmotorsports]

Is the fan switch on stat turned to to On?
Otherwise you might have tripped the limit switch,hard telling without more info or at least picture of furnace.

 

[oldpops]

Thanks all. I was hoping the symptoms would be something common to all furnaces and hopefully easy to remedy. Oh well, I will have to get my ladder back from my brother tomorrow so I can get back up in the attic to see the make and model. If it helps any now I can tell you it is a gas furnace for a home in southern California that was built in 2002, but i will get the make and model number for you and get right back to you.

 

[LS6 Tommy]

Faulty limit switch. It's locking out the heat and turning the blower on like it would for a high limit trip condition.

Tommy

 

[hichamaziz]

you might have tripped the limit switch

 

[oldpops]

Hi all! Finally got my (tall) ladder and was able to get the make and model # of our furnace. We have the Goodman furnace model GMT070-4. Hope this helps diagnose this problem. ** Also, does anyone know where I can download a manual on this heater. I went to the Goodman website but they don't seem to have it. As always, any and all help and advice is welcome.

 

[SALIV8]

try this link for some parts and tips.

https://www.repairclinic.com/Repair.../Goodman-Furnace-GMT0704-ModelTroubleshooting

 

[zmaxmotorsports]

Do you have central air attached to the unit?
I know some parts of California don't use ac,but if you do it may be worth checking to make sure the coil isn't plugged up.

 

[oldpops]

Actually we do have an air conditioner. Out in the Inland Empire where I live it can get over a 110 degrees! But we haven't had the ac on since last summer. last summer our ac condenser [outside] start/run capacitor exploded so we thoroughly it (got all the fins clean, the shroud, everything) and it ran great. Is there something on the Goodman unit upstairs in the attic involving the AC that would cause the furnace to blow warm and then cold air (thermostat set to heatand on AUTO)? I never even thought about that. (see attached link: https://imgur.com/a/R4hc2 )

 

[48RON54]

a couple guys said limit switch. some searches on youtube should turn up all the info you need to to troubleshoot the limit switch and to ultimately replace it if need be.

 

[48RON54]

Also, welcome to the forum from another fella out here in the IE.

 

[zmaxmotorsports]

Actually we do have an air conditioner. Out in the Inland Empire where I live it can get over a 110 degrees! But we haven't had the ac on since last summer. last summer our ac condenser [outside] start/run capacitor exploded so we thoroughly it (got all the fins clean, the shroud, everything) and it ran great. Is there something on the Goodman unit upstairs in the attic involving the AC that would cause the furnace to blow warm and then cold air (thermostat set to heatand on AUTO)? I never even thought about that. (see attached link: https://imgur.com/a/R4hc2 )

A plugged up A coil could cause the limit switch to kick in and try to cool furnace down.
It's worth checking anyway.

 

[Done That]

That unit has an integrated control with LED diagnostics. Open her up and see what you got on the display....

 

[oldpops]

Where is the integrated LED display? I posted a link to the inside of my attic unit. Where would the LED readout be? (see attached link: https://imgur.com/a/R4hc2 )

 

[zmaxmotorsports]

Lower part of furnace on board,fault codes should be on wiring diagram.

 

[Done That]

Yeah, what zmax said. Most furnace have two removable panels, one is the burner section and the other has the blower section. PCB control board is attached to the blower on a sheet metal bracket. That board has a single red led on it that blinks codes....you have to count the number of flashes and decode it with the label that will be attached to the metal access panel you just removed.