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So just had a Brand new Goodman furnace and a/c installed before winter hit. As soon as heating season started i noticed it was short cycling and had a flash code of 4. I contacted the installer and he did a few things. 1. increased blower speed. 2. Said it was not getting enough return air and cut a large vent into ducting right before filter. #3. Turned gas pressure down. It was no longer overheating but the air coming out did not even get the ducting warm. It would take forever to warm the house enough for furnace to turn off. I verified all ducts are open supply and return. Filter is brand new. I checked gas pressure and he had it adjusted to 1.50 where manufacturers spec is 3.50. I adjusted it back up to 3.50 but again after 5 minutes it would turn off and flash code would turn on. I adjusted it back down to 2.50 and at that pressure it does not overheat anymore but it bothers me that the gas pressure would need to be turned down. It should operate at 3.50 for best efficiency as that is what manufacturer wants it set at. What do i need to look at to figure out why my furnace overheats when gas pressure is set to its optimal pressure? Any insight would be appreciated!!

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How many square feet is your house? When was it built? And where do you live? You should never alter the gas pressure from 3.5 inch water column. It has more to do with the flame speed then the capacity. If the pressure is too low then your flame will ride on the burner faces and destroy them. NG burns at about 12" per second and 3.5 inch NG comes out of the orifices at about 12" per second. Without the requested information above answering your question is essentially impossible. Duct size may be an issue, but it's more complicated then it's 8x24. Also if you have air conditioning make sure the evaporator is clean.

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Do you know what the gas input of the old furnace was and if it had A/C what was it's size or rating? If the new furnace is larger than the old unit did the contractor say he installed a larger unit? If the old furnace heated well, and you are not planning on a new addition in the near future, the old furnace would have been the correct size. List the model number and make of the new furnace and A/C unit, measure the discharge ducting and the return ducting, the number and location of all the supply and return registers. There a lot of guys well versed in heating systems, that monitor this site and can give you a lot of information. Post a few pictures of the installation if you can. (hope this helps)

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Your ducts are too small. Assuming it isn't a bad limit switch (that should be #4) all you can do is #s 1-3.

It should operate at 3.50 for best efficiency in a system designed to handle the output of your furnace. If the heat load calculation says you have the correct sized furnace, yours, just like every house built without a ~24" square chase for the HVAC to go in, doesn't have the correct sized ducts.

If you want efficiency, you have to tear apart your house. If you want comfort, cut the gas back some.

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Consolidating some of the answers, a limit switch protects the furnace from overheating. The cause of a trip is usually lack of adequate airflow. Could be a bad switch but not likely on a new furnace. Air travels through the return ducts, through the filter and blower and out the supply ducts. Two things come to mind. Sometimes when replacing a furnace there is no identical match so I need to decide between a larger or smaller capacity unit. If you installed a larger furnace you would have made ALL the ducts smaller relative to the original design not just the return. In this case a duct modification would be the answer. Second, did you upgrade to a higher efficiency air filter? Air Bear or Aprilaire type? These have a higher resistance to air flow than a standard 1" filter. The fix might be go back to a 1" filter. I assume you had a clean filter all this time.

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