Furnace is overheating....

[morgjj]

Okay, so after a very long day at a new house, I have run across a problem that I am not sure how to best tackle. Any advice would be helpful.

Walked in today and the temp was 53. Turns out the igniter was out, so picked up a new one and installed it. Things looked good. However, after two hours, the temp had only gone up 2 degrees. Went down to the furnace and noticed that it would run for 2 minutes of flame, kick off, wait 30 seconds and kick back on. Was thinking that the high light switch was to blame, but it turns our that it is working correctly.

So we begin to look for air obstructions in the airflow system to see why it is not getting enough air to cool down the unit. After removing the A coil from the system, we doubled the output of the furnace at the ducts and was able to get the furnace to run for a longer period, but it was still turning off at a fairly regular interval.

We removed the filter from the system, it was a 3m filtrete filter that is supposed to catch a bunch of stuff. With the Acoil out and the filter out all together, the system is able to run indefinitely. However you put either back in, the system will over heat and start to cycle.

Guess the problem that I am having is it seems like a design problem or something with the blower motor. Not sure that it is the motor, as it runs, just not enough air moving through. If I put the A coil back in, the system will have the same problem. There has been someone out to repair the system every year, it and seems like all the problems are related to overheating and constant cycling.

The furnace is an amana Air Command 90, that was installed about 98 or so I am guessing by the stickers and history of repair.

Is there anything I can do short of just replacing the system??

Thanks bunches.

Jeff

[Hube]

Welcome to the forum; A 1998 unit should give many years of sevice with the usual maintenance work. With all the removing of various features of this unit ,i believe that it is now completely out of any functional working design. A properly designed DUCT SYSTEM will work quite well with resistance from coils filters, etc. .Make sure the FAN (blower) is not obstructed ,or at fault ..Check that all supply and return ducts and outlets are open.Then you would be best to make sure the filters are CLEAN and then put everything back the way it was originally. Are you sure the HIGH LIMIT was not the culprit? Also check for a SHORTED or loose SEAT at the GAS VALVE . Check for any loose 24v power connections to this valve. Let us know how you make out, Hube

[morgjj]

Thanks for the ideas. Meant to type high limit instead of high light earlier. Sorry about that. Anyway, the furnance is getting that hot and I believe the high limit switch to be working correctly. As for the duct work, it appears to have enough volume and we made sure that all the vents were open, including the divert values in the duct work.

I was surprised that the system would be in this state, but it appears to have never worked right at all from the start. My friend is a furnace installer and repair tech and he ran through it from an electronic stand point as well. He believes that the issues is airflow as well, but can't figure out why, as the duct work is probably twice the size that it needs to be.

All and all it is very strange.

Thanks

[Hube]

You say it "appears" to be the right size. Only proper calculations will prove if its OK. If you want to send me a brief description of your home's layout,sq. footage,etc, size of unit(btu output) Plenun, and the existing supply and r/a duct sizes, I will gladly calculate what the problem could be. I have been in HVAC 50 years and definetly have a "little" knowledge as to sizing etc. Hube

[morgjj]

I appreciate it that!

It is a split level home built in the 70's. Approximately 2200 sq feet. The furnacne is a 70,000 btu furnance. As for the duct work, it has a furnance filter that is 16X25X1 so, the I would say that the return unit that is attached to the furnace measures about 25 inches deep and a 15 or so across. It runs up the legth of wall, so about 7 feet high, where it connects the returns. if you need more specific measurements, I can get them. I am not there at the moment. There is also a vent cut in the return air next to the filter with a standard register type cover on it it. It appears to get more air into the system. I haven't seen that before myself. Each room has a return air duct that has an opening of at least 12 X 2 or greater. In the living room it is 30X6

As for the output side of things, the plenum measures about 20X30. The A coil had a shelf in it that had about 3 inches of extra sheet metal that wasn't needed to hold it in. We trimmed that back in order to increase the airflow. Again, happy to get exact measurements if that would help.

As for the fan and speed, I am not sure where to get that off the motor or blower unit. I grabbed the model number off of it, but that didn't tell me a whole lot. The fan has been hard wired so that it runs on high all the time. There is not speed difference between heating and cooling.

Again, I appreciate your help with this.

[Hube]

Is this 70,000 btu the OUTPUT btu or the INPUT? LOOK inside the burner compartment at the specifications label. HOPEFULLY ITS THE OUTPUT. That would make the total input around 90,000 or less. If its a 70,000 input it is a way too small. , We assume the unit is approx 6 years old, and an MID efficiency (80 %) From your description this is a "HIGH BOY" type unit with a single plenum (HOT) and a dropped leg type of Return air main. Filter size of 16 x 25 is OK . You say each room has a R/A of 12x2 ?? How MANY of these Returns are there? (A size of 12x6 would be more suitable ,anything less would be too small ,even with the 30x6 Livingroom return. How many supply runs come off what size supply trunk.?What size are these supply runs? 4"-5" or 6" and what size are the supply registers? What size is the R/A trunk coming into the drop leg to the blower area? I,m sorry for all these questions, but thats the ONLY way to determine what the problem is, other than for you to call in a PRO to anylyse the same as i'm doing. When you get time post back with info and we will do some calculations.. BTW, a fan that runs on HIGH all ALL the time is another indication of a poorly designed system. Hube