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Al, Professional HVAC Technician
Category: HVAC
Satisfied Customers: 34
Experience:  35 years experience HVAC field, licensed to install, repair, service all types of equipment
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Why would my Rheem furnace shut off before reaching the target?

Customer Question

Why would my rheem furnace shut off before reaching the target temp, then immediately start a new heat cycle? I've jumped the 2 switches for gas flame and heat exchanger temp - no change, I've replaced the fan limit switch - no change. But, with the door off the furnace, and the safety switch taped down, the furnace will run until the target temp is reached.
Submitted: 8 years ago.
Category: HVAC
Expert:  Electrifier replied 8 years ago.
What does taking the door off the furnace change regarding the availibility of air to move thru the furnace? Does it open up an air flow differently than when it is installed?
Customer: replied 8 years ago.
There is defenitly more air with the door off. I've tried taking out the filters and letting the furnace run - door on, will still shut off before the target temp. The ducting has never been changed since installed. I bought the house the way it is. Is there anything i could check, like temp at a sensor or voltage at the board?
Expert:  Al replied 8 years ago.
What kind of thermostat do you have? I believe your problem is with a setting within your thermostat. It is called a heat anticipator. If it set properly this will most likely fix your problem.
Customer: replied 8 years ago.
I have a digital t stat - I replaced it 3 weeks ago. Same problem now and before. I don't think it has a heat anticipator. I can't remember the efficiency rating, but i do know that the sticker on the front read towards the lower end of the scale. Something like 83.3% sound right.

I hooker up the wires to the t stat from the instructions and it matched the old t stat. Is it possible to have the wires wrong. Can i jump two wires to eliminate the t stat as a problem?
Expert:  Electrifier replied 8 years ago.

It has nothing to do with the thermostat. It has to do with air. When you remove the front panel you open up the furnace to receive more combustion air. When you put the panel back on you restrict this air. Without enough combustion air it won't run correctly. Check out the manufacture's installation instructions in the book provided with the furnace and read about the combustion air. Not being able to look at the furnace I can't tell you where your problem lies exactly. Is there filter material in the door? Is there an opening for combustion air? Something is restricting this air flow. What is the brand and the model of your new digital thermostat. I can get the manual on it and tell you how to check the anticipator setting.

Customer: replied 8 years ago.
It is a rite temp - I bought from Home Depot. Its either MN# ***** or 6022. I don't have it right in front of me. I hope this helps.
Expert:  Al replied 8 years ago.

Look in your manual and look under swing andthen look under calibration. Do what it says and let me know what reading you get

Customer: replied 8 years ago.

I checked the swing - .5. I changed it to 1.0, same result. I'm going to try removing the air filter again to see if the furnace will run without anything in he duct for restriction.

Expert:  Al replied 8 years ago.

Good Morning,

Check the temperature rise across the furnace. Put a thermometer in the return duct and take a reading, then put the same thermometer in the supply duct and take a reading. Subtract the return reading from the supply reading and you get the rise across the furnace. The rise should be between 30 to 70 degrees. Tell me what the rise is.

Also, check the calibration of the thermostat in your manual. Do you see a box on the back wall of the compartment where the gas valve and burners are. If you do, unplug the furnace and take the cover off. You will see a round dial face. There are 3 setpoints on that dial, the first one is "fan off", the second is "fan on", the third is "high limit". Tell me what each setpoint is.

Customer: replied 8 years ago.

I measured the temp rise across the furnace with and without the door.

With the door off intake air is 74 - 76 deg. The door outside in the garage is off and temp out there is 34 - 35 deg. The return duct is 115 - 117 deg. The furnace runs until temp is reached.

With the furnace door on, intake air is 74 - 76 deg. There is very little air coming in from the garage. The return duct is between 144 - 146 deg. The furnace has to stop and restart several times to get to target temp.

Could the furnace be overheating?

Just to be sure, I removed the air filer before measuring and testing again. The furnace can't get anymore air than its getting now.

The door for the furnace is above the burner where he blown motor is. The control board is there and a solenoid and capacitor. I think that is right.

Expert:  Electrifier replied 8 years ago.

When you remove the door you allow enough air to go thru the furnace so you don't open the high limit when you have your 115 degree temp. With the door on the air flow is reduced and the air temp goes to 144 degrees. Like I said a week ago you have an air flow problem somewhere. You have some type of restriction in the return air duct preventing it from allowing the air to flow freely. Check all of your return air ducts and registers to make sure none of them are blocked. Make sure no furniture is blocking any return air register. Someplace you will find something simple I'm sure. Let me know when you find the answer.

Customer: replied 8 years ago.
I couldn't find any return registers blocked. I don't know if this helps, but a register that was installed into a utility room after the original furnace install. I removed it because it was attached to the garage and did not have intake air to the furnace. Is it possible that the gas valve was calibrated differently for the added air register?

I adjusted the gas pressure into the gas valve - down haft a turn. Tested the system by installed doors and running the furnace to a target temp of 3 deg. Target temp was reached and furnace did not shut off until that point.

Does this sound right to u?
Expert:  Electrifier replied 8 years ago.
The gas valve shouldn't have anything to do with the amount of air circulating thru the ducts. Did you eliminate ventilation air to the return air? This is air from outside that dilutes the recirculated air with fresh air from outside? It is possible that this furnace moves much more air than the previous furnace and there is too much restriction in the return air. The key is air moving thru the furnace. Not enough air moving thru will trip the hi temp limit as you have been doing.
Customer: replied 8 years ago.

I checked the air supply tube from the ceiling to the top of the furnace - all clear. Checked the air conditioning coils down to the main outlet tube - all clear. All registers clear and all have about the same air - a little less to the far register. Now the vent temp is 15 deg cooler than before I adjusted the gas valve.

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