new furnace - this isn't right, is it?


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Old 01-19-10, 05:05 PM
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new furnace - this isn't right, is it?

Hi there. Kind of a screwy deal here. I'll try to be as detailed, yet as brief as possible. 7 days ago, I had a new furnace installed in my house. The old one was from 1958 and the motor locked up. Gas fired units. The new one is a Gibson model# KG7SA 126C 45D. Approximately 1200 square feet of space. The new unit is cycling several times before the thermostat shutoff temperature is reached. Not just burners, but the whole system. Inducer fan comes on, burner lights, blower comes on, burner goes off and blower throttles back, blower goes off when the unit has cooled. This is followed by an IMMEDIATE restart of the whole process. The contractor that installed it said it was because some of my vents were partially closed and the filter was dirty. Opened them all fully, and put in a new filter - no change. It goes through about 4 cycles of this every time to raise the house temperature 2 degrees. I have a digital thermostat, and don't think there is an anticipator on it - Ritetemp 8022C. Cold air return trunk at the furnace is 8x24. Warm air trunk lines are two 8x12s. This isn't normal operation for a new unit is it? Is this something that will damage the new furnace, and how quickly? I'm tired of dealing with these guys, and would like to be armed with information the next time I talk with them. A friend of mine mentioned adjusting the high limit setting. Could this solve the problem? Sorry for all the questions - I'm reaching for a solution here. Let me know what other info you need, and I'll get it to you asap. I'm a pretty handy guy, with the exception of HVAC stuff, but with an explanation, I'll do what I can. Appreciate the input.
 
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Old 01-19-10, 05:44 PM
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I am willing to bet your problem is due to your furnace being 3X bigger than it needs to be. Your furnace is kicking out on the high limit due to overheating. A 126,000 btu furnace could heat a 1200 sq ft house if it was uninsulated and you had the windows open in the middle of winter. Obviously the heat load was not calculated properly. Your ductwork is also waaaaay too small for that furnace, hence the overheating. Has all the makings of a low ball hack install. I have a feeling you won't get any satisfaction from your contractor. My first call would be to the local mechanical inspector.
 

Last edited by Skip4661; 01-19-10 at 06:03 PM.
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Old 01-19-10, 06:12 PM
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Limit

I doubt it is overheating. The way most furnaces operate if they overheat is to shut down the burners, keep the vent fan running, & kick the circulating fan into high speed. If this furnace is stopping completely, something else is wrong. Try removing the thermostat & jumping the wires which went to terminals Rh & W. If the short cycling stops, the problem is with the stat. If the problem persists, call your installer.
 
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Old 01-20-10, 07:04 AM
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I have to agree: the furnace is oversized and the ducts too small.

Check the LED on the circuit board. It should indicate if the furnace is overheating and the limit switch has tripped.
 
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Old 01-20-10, 11:32 AM
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I have to agree: the furnace is oversized and the ducts too small.
................................
 
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Old 01-20-10, 12:23 PM
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If the 'installer' didn't modify the filter housing or drop, I think he could have checked the condition of the filter.
 
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Old 01-20-10, 03:44 PM
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Hey guys. Thanks for all the input. Sorry to be so long getting back to you, but been at work all day. I was thinking that hitting the high limit was probably the case, but never considered the unit actually being TOO big. The guy told me the old one was 175,000 BTU, and because of all the advances they've made in burner technology, etc. since 1958 he was going to downsize the new one a little. I was actually worried he may have gone too small, but you're thinking the opposite? Just to clarify the square footage - 1200 feet living space upstairs, with the same in the basement. But the basement only has two vents. One at each end of the house. If the thinking is the furnace is actually "choking" because the ductwork is too small, what is the minimum square inch area that should be there as far as the main trunk lines in the ductwork? Also, where is the LED that would tell me if the limit switch has tripped? I see a cluster of three lights. One red and one green that are on every time I look in there, and one yellow for the flame when it's on. Skip, I would call the mechanical inspector, but I'm thinking it could backfire on me as the homeowner for not going throught the proper channels. It was 11:00 at night and I needed to get heat the next morning so I didn't end up with frozen plumbing, etc.
 
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Old 01-20-10, 03:48 PM
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Originally Posted by delta l View Post
If the 'installer' didn't modify the filter housing or drop, I think he could have checked the condition of the filter.
They actually reused the old filter because at the time it had just been changed about a week before. But you should've seen the huge clumps of stuff that were stuck to it after the new furnace had run a while. Apparently there is a LOT more velocity now than before!
 
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Old 01-20-10, 05:10 PM
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Overheating

I just looked up the manual for this furnace &, as I thought, if the furnace overheats, the circulation fan & burner fan will continue to run. The furnace may be overheating but that's not what is causing the short cycling.

Have you tried what I suggested with the thermostat?
 
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Old 01-20-10, 05:24 PM
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Grady - you're saying to tie the Rh and W wires together after removing them from the thermostat terminals? And then bump it up a few degrees to trigger the furnace to start, I'm assuming. Just wanted to clarify so I don't end up damaging something else. The furnace isn't totally shutting down, by the way. The burner does shut down, but the blower continues to run until the furnace is cooled down. After the "cooled off" temperature is reached (which is where the furnace would normally shut off), there is a split second stop, followed by a restart of the entire process - the vent fan comes on, followed by the burner, followed by the circulation, etc. It's a normal cycle for the furnace start to finish in terms of order of events. It's just being triggered to shut down the heat phase before the set thermostat temp is reached.
 
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Old 01-20-10, 05:29 PM
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Blower Runs

OK, more clear now. Does the exhaust blower shut off while the furnace is cooling down?
If it does not, there then is a strong possibility the furnace is operating as it should when it overheats.
 
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Old 01-20-10, 05:49 PM
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That's one thing I haven't checked. Will watch the next time it comes on. Just sat through an entire cycle in front of it with the upper door off, and it ran through once and that was it. Figures, right?!?! One other thing I forgot about was after the guys left and it ran for a day or two, I started checking things out and noticed the fan switch on the thermostat was set to "on" instead of "auto", however the blower was only running when the furnace was on. Does this add any new diagnosis info? After moving it back to auto, it didn't change anything.
 
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Old 01-20-10, 05:55 PM
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fan on/auto

With the fan switch set to "on" the fan should run all the time.
On "auto" it should only run during a heating cycle.
 
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Old 01-20-10, 06:03 PM
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Originally Posted by Grady View Post
With the fan switch set to "on" the fan should run all the time.
On "auto" it should only run during a heating cycle.
So why, then, was it only running during the cycles even thought the switch was "on"????? Back to thermostat possibilities?
 
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Old 01-20-10, 06:08 PM
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Fan "on"

If you only have two wires connected to the stat & furnace, the fan would not operate in the "on" position. Beyond that, I'd have to dig into the manual.
 
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Old 01-20-10, 06:45 PM
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Just sat through another cycle, and it ended up operating normally again. It gets pretty warm sitting there staring with the cover off, which has me wondering... Is it possible to overheat something on the control board that would make it shut down early? Will try again in a little while with the cover back on and see what happens. Meanwhile, the blower not running in the "on" position has me bothered. It worked like that before.
 
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Old 01-20-10, 07:05 PM
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control board

Usually the control board is in the blower compartment but yours seems to be up near the exhaust fan. It is possible (how probable?) the contained heat with the upper panel in place is causing something in the board to break down.

How many wires run between the furnace & thermostat? What colors are they & to what terminals (furnace and thermostat) is each connected?
 
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Old 01-20-10, 07:23 PM
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Four wires connecting the therm to furnace...

thermostat wire color furnace
G green G
Y blue Y/Y2
W red R
RC white W

there is a jumper from RH to RC in the thermostat.

Y/Y2 in the furnace also has one of the two air conditioner wires on it. I noticed the red wire has bare copper showing on part of it. If I can make this a visual - they ran the thermostat wires through the right side of the furnace, looped up to the top, then came back down to the board. Where the top loop is, the red insulation on the compressor wire looks almost like it has heated up enough to pull loose from the copper. The copper still loops up and back down, but the insulation itself goes straight across. And yes, the control board is just to the right of the exhaust fan, which puts a steady blast of hot air right at it.

put the upper panel back on, kicked the thermostat up, and lo and behold, it's doing it again. Looking like the control board overheating is getting closer to being very probable in my case. How do I check this other than keeping track of when it does it with the cover off as opposed to on? Is there a place I can check to see if Nordyne (Gibson) is having other problems with this?

went through four complete cycles to get the thermostat from 70 to 71! Gotta be hard on it, right?
 

Last edited by sbrown580; 01-20-10 at 07:56 PM. Reason: additional info
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Old 01-21-10, 06:27 AM
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Wow, a 126k furnace for a 1,200 sq ft home in MO!?

60k would of been more than enough for you in MO.
 
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Old 01-21-10, 02:12 PM
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Wiring

At the thermostat switch the red & white wires so the white will be on W & the red on either Rh or Rc (since the jumper is in there it makes no difference).

It certainly sounds like the board is giving you trouble. I suggest you call Gibson to find an authorized dealer in your area who could check things out.
I do agree with the others that the furnace is way oversized.
 
 

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