Furnace Short Cycling


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Old 08-17-07, 08:19 AM
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Furnace Short Cycling

Furnace recently installed. Payne model PG8J. Shows code 33 (Limit Switch Open) then changes to code 13 (Lock-out). Found dirty filter and changed. Now does not go into lock-out; however, still shows code 33. Furnace runs and heats for approx 3 minutes then flame shuts-off while blower continues to run for an additional 3 minutes; then flame reignites and furnace heats for 3 minutes then cycles off with blower on for 3 minutes, etc. This on/off cycle repeats continuously until temperature reaches set point on thermostat.

Removed Blower Box cover and furnace ran normally for 15 minutes with no cycling problem.

There are two return air ducts: one to the only downstairs room (wall) and one to the upstairs main hallway (ceiling) which do not appear to be restricted. Both ducts combine to one at the furnace box and enter into a hole beneath the upright furnace. The blower motor is perpendicular to the hole with the blower opening actually facing the sheet metal side of the furnace box (a few inches away from the metal). The blower does not face the bottom where the return duct enters.

1. My question is does the fact that the furnace appears to work fine with the blower cover removed conclusively point to a return air issue?
2. If there are in fact no restrictions in the return ducts, is duct sizing then a potential issue?
3. Is there any issue with the duct entering the bottom of the furnace rather than the side?
4. Any help with what may be causing this short cycling problem.

Thank you
 

Last edited by dnrepair; 08-17-07 at 08:22 AM. Reason: add model type
  #2  
Old 08-17-07, 07:40 PM
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Short Cycling

Your furnace is "cycling on limit". This means the furnace is getting too hot & shutting of the burner, allowing the fan to continue to run to cool the furnace. All furnaces have a heat rise specification. You are exceeding that spec. To contiue to operate the furnace in this manner will dramaticly shorten the furnace life & void the warranty. If the furnace was professionally installed call the installer back.

From what you describe of your return duct system, I can almost guarantee you do not have enough duct capacity for the size furnace you have.
 
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Old 08-17-07, 08:41 PM
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Short Cycling

Thank you for the response.

Also found that the return duct is installed directly into the bottom of the furnace at the blower box. There is no seperate Filter Box. In fact, there is almost no room to slide the filter between the bottom of the blower motor and the duct hole flange.

Is it appropriate to install the return duct directly into the furnace without a Filter Box or should there be a Filter Box?

Thanks again.
 
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Old 08-19-07, 06:17 PM
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Return Duct

Normally there is a plenum but it is not absolutely needed. If this furnace was professionally installed, I suggest you call the installer back & make them fix the heat rise problem. DO NOT let them just cut in a vent grill in the blower door. Either the furnace is too big for the house or the ducts are too small for the furnace (or both).
 
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Old 08-19-07, 08:31 PM
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Furnace Short Cycling

Thanks. We don't know who installed the Furnace; however, the indications are that it may not have been correctly installed. I believe this problem may have been in existence since installation.

I also received an email requesting more information regarding the furnace location. The furnace is located in the garage. There is one room adjacent to the garage. This room has a small return (same size grill as the supply grill in the same room - perhaps about 4-5" by 10-12"). The other return runs from the garage up into a closet on the floor above the garage (where the living space is located) and comes out into a larger grill (about 16 -20" square) in the ceiling outside the closet in the hallway. There are three bedrooms, a living room, dining and kitchen area all located on the floor above the garage. There are about 5-6 supply ducts on this floor. There is no additional floor. All the ducts are flex.
 
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Old 08-19-07, 09:44 PM
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Furnace Short Cycling

Correction to last post: return and supply grills are approx 6" by 14" each in room adjacent to the garage. Return grill in hall on floor above garage is 13" by 15". There are a total of only 4 supply ducts in the living space (one in each bedroom and one in living room). The supply registers are 5 1/2" by 13".

The house is a total of 1350 sq ft. 1080 upstairs and 270 for room downstairs.

The furnace spec indicates-Input: 66,000 BTU upflow, 63,000 BTU downflow; Output: 53,000 BTU upflow, 51,000 BTU downflow.
 

Last edited by dnrepair; 08-19-07 at 09:45 PM. Reason: add word
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Old 08-21-07, 05:38 PM
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Duct Sizes

The sizes of the registers/grilles is helpful but what I really need are duct sizes. On the surface, it sounds like you are really short on ductwork.
 
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Old 08-21-07, 09:13 PM
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Furnace Short Cycle

The return ducts: Both are 8" diameter. They join together at the ceiling in the garage and enter the furnace via one 8" duct. (There is a short 2" section that takes the 8" into a 9 3/4" cut hole at the bottom of the furnace.)

The supply ducts: All five are 6" diameter.
 
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Old 08-22-07, 04:18 PM
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Ducts

A single 8" is nowhere enough.

According to one of my books, to attain a 60 temperature rise thru the furnace with a 63,000 btu/hr output, you need to flow almost 1000 cu. ft. of air per minute.
Six inch ducts will carry about 100-125 CFM.
Eight inch ducts will carry around 200 CFM.
As you can see, you only have about 1/2 the amount of supply & 1/5-1/4 the amount of return you need, presuming the furnace's temperature rise spec is 60 & many gas furnaces are not that high.
 
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Old 02-19-13, 11:35 AM
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Same Issue Different Problem

Grady,

I started having this exact same short cycling issue today. I am rather certain that I do not have a duct work issue. Is there something else that might have recently caused my furnace to start short cycling??? Thanks!
 
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Old 02-19-13, 12:38 PM
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Please don't revive an old thread. It causes confusion and can lead to bad advice.


Start a new thread and describe in detail the issues you are having with your furnace.
 
 

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