Concord 90 Plus furnace issues


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Old 03-25-12, 04:21 PM
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Concord 90 Plus furnace issues

I recently purchased a foreclosed house. The furnace appears to be new, or almost new. I have a feeling that the previous owner sold the furnace that existed in the house (he sold everything else), and the bank replaced it with this unit. As a result, I have no information on the furnace's history.

I'm having two issues, which I think may be related:

Issue #1 is that the flame rollout switch is getting tripped. I reset it a few times (only while standing next to the furnace to make sure it doesn't catch fire). It trips within 10-15 minutes, each time. I've read that this could be due to intake restrictions, which leads me to #2.

Issue #2 is that the filter (16x25x1) is horribly bowed when I pull it out. It deflects 2-3" after it has been used in the furnace for a day or two (the blower runs constantly after the rollout switch has been tripped). This is a 125kbtu, 5 ton furnace. Is that an appropriately sized filter? All other large/high efficiency furnaces I have encountered have had much larger (3-5" thick) filters. I fear that the bank took the cheap way out when installing this furnace, and didn't do what was right for the situation.

Any input would be greatly appreciated. I haven't been able to find a manual for the furnace to verify the amount of filter area that is recommended.

Dave
 
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Old 03-27-12, 08:20 AM
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That furnace has an unusually large BTU input, especially for a high efficiency furnace. But it may be properly sized, especially if it's in a large house or you have a cold climate.

The air filter seems undersized for a large furnace. If you have or get the installation manual for the furnace it will describe the kind of filter needed.

There are two airflows through the furnace --- the path the combustion gasses take on the inside of the heat exchanger and the path the room air takes flowing over the outside of the heat exchanger, absorbing the heat from the combustion gasses.

There shouldn't be any mixing of those two air streams. So an undersized filter would not be causing the rollout swtich to open.


That would typically be caused by an impaired airflow of the combustion gasses. If everything is working properly, the combustion gasses flow into the furnace as fresh air, burn the fuel and flow out of the furnace as combustion gasses. An obstruction in that air flow will often cause the combustion gasses to start backing out of the burner openings, tripping a rollout switch.
 
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Old 03-27-12, 10:55 AM
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Thanks for your helpful reply. From what I understand, I should be investigating a possible obstruction in the vent?

I did find the manual (jammed under the cold air return). I believe it specifies 2x 20x25 filters for the size furnace I have (I read it over the weekend, and don't remember the specifics, but it was definitely more surface area than what it has). Clearly the filter is undersized. It also specifies that for 1600+ CFM installations that the cold air return should be in an upflow configuration (mine enters from the side).

I have noticed that if I have one of those cheap filters (4 for $2, the ones that you can pretty much see through), the rollout switch does not trip. However, if I use a decent filter, it trips almost every time it runs. What could explain this? Could the combustion area be in an overtemp situation due to less air flow with the more restrictive filter? It sounds like a lack of room air flow couldn't affect the flame pattern at all, by your description.

I want to make it clear that I'm not challenging your statements. You obviously know significantly more than I do on the subject, which I appreciate. I'm just trying to understand the root of the issue. This is my first high efficiency furnace, so many pieces of it are new to me.

Dave
 
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Old 03-27-12, 06:45 PM
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>


Based on these comments, I suspect that the limit switch which shuts off the burners if the furnace overheats is opening.

Flame rollout switches have to be reset manually, by hand.

Over temperature limit switches reset after the fan continues to run and cools off the heat exchanger.

Do you understand which switches are limit switches and which are flame rollout switches?

Describe in more detail what is happening when your furnace is operating.
 
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Old 03-27-12, 07:40 PM
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The code the furnace gives is flame rollout switch open. The switch is about the size of a dime, located near the combustion area. It has to be reset by hand (with a small button between the two wires). From everything I've read, it must be the rollout switch that's being tripped, but I don't understand why. The blower will run continuously until this is reset. The furnace power needs to be cycled after resetting the rollout switch in order for it to operate normally. Can an overtemp situation trip the rollout switch? Or can it only be tripped by the flame actually rolling out?
 
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Old 03-28-12, 12:13 AM
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You are correct, that is a flame rollout switch from your description ----another good theory shot to hell by reality!

Flame rollout switches are temperature sensitive, so they don't need to be tripped by flame. If the metal is heated enough by radiant heat from the burners they can be tripped. But that is still very likely caused by something plugged up and impairing the flow of combustion gasses through the furnace.
 
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Old 03-28-12, 03:31 AM
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Leave it to me to have a furnace that breaks all the rules.

I looked in the end of the vent as best I could yesterday, and I couldn't see anything inside of it. It's one of those vents where the two pipes combine just inside the exterior wall of the house, and it has a hat on the pipe outside, so I can only see one of the pipes.

If I'm understanding you correctly, the vent *should* be the only place that a clog could cause this behavior, right? If so, I'll see if I can figure out a way to verify that it's clear all the way back. It's about a 25' run, and it does not have a screen on the outside, so anything's possible.
 
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Old 03-28-12, 05:37 AM
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No, something can be plugged up pretty much anyplace on the air intake line, the combustion gas line or the heat exchangers inside the furnace. To add to the fun, an inducer motor running slow, deteriorated or plugged inducer motor fan blades or cracked heat exchanger can cause the problem. Water pooling in the vent pipes or obstructing drain lines can cause the problem.

While I wouldn't ordinarily expect a new furnace to have a cracked heat exchanger, the fan circulating air around the house should NOT be affecting the way the flames are burning, which you seem to be reporting.

In short, a wide variety of often hard to detect problems could be the cause of your difficulties.
 
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Old 04-11-12, 12:55 PM
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new furnace

I think that the key here is that the old furnace was taken out,and a new one installed by somebody in a hurry who did not size it properly.
all new 80 percent and 90 percent furnaces have a designed temperature rise on the furnace nameplate. you start the furnace,put a thermometer in the return and one in the supply duct per manufacturer and check the temp rise.....example 35 degrees to 60 degrees. if your return temp is 70 and the supply temp is say 45 degrees plus 70 you are within the temp rise design fo the furnace. if you are not within the design temp rise,you must adjust the blower speed. lets say you are only getting 20 degrees temp rise.you need to put the blower speed on a lower one to bring the temp rise up. this stpe was probably not taken whe the furnace was installed.
 
 

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