Lennox furnace not hot enough?

Old 03-24-13, 07:51 PM
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Lennox furnace not hot enough?

Hi everyone,
Thanks in advance for reading, I feel like I've been beating my head against the wall with a question that doesn't really make sense. The summer of 2012 gifted me with a failed A/C, and the homeowner warranty which covered that unit also provided a new furnace and new coil unit at the same time. Instead of paying the warranty company several thousand to install the set a friend of a friend did the job on a weekend for cash (which may be the source of my problems?).
The units previously in the house when I moved in were builder's-grade Amana units from 1985 which both looked to have never been serviced; this would presumably make the Lennox units that replaced them exponentially more efficient, or so I thought.
The new unit models are: furnace - ML180UH070P36A, A/C - 13ACX-036-230, and coil - CX34-38B-6F. All of these units were installed into a 2-story, colonial house of about 2300 sqft. While I can say with certainty the house never feels uncomfortably warm, or cold, it seems like the units are always running (which I've confirmed via my utility bills). I presume that the unit isn't getting enough return air via the solitary 8" duct connected to a 12x20" return grille on the first floor, but I really don't think that's the whole picture.
The A/C is generating enough cooling, it just has trouble circulating thanks to poor return air previously mentioned. The heat, though, just doesn't feel warm. If you stand by one of the vents - even closest to the furnace - it feels luke-warm at best. I know that I need to address the return air issue and make sure that I am returning more air from both floors back to the furnace to condition, but I feel I am I missing something. Is the blower set too high for the furnace but fine for the A/C, did I just invent that solution, or am I just paranoid?

Thanks again for your help!
Old 03-24-13, 08:14 PM
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An eight inch round return air duct? Or is it a rectangular duct and if so what is the other dimension.

Both Amana and Lennox manufacture several different grades of equipment from "builder's grade" to "premium grade". Your Lennox is a lower grade unit just as was the Amana so no real difference there.

Please post back with the dimensions of that RA duct. Something is rotten in the installation of your furnace and I think it starts with the RA duct but probably extends further.

Pictures of the area showing the furnace and ductwork might also help.
Old 03-25-13, 07:23 PM
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Furd, you're awesome. I'm travelling this week but will post this weekend, if not sooner, when I return home. I apologize for the delay, but appreciate your help.
Old 03-26-13, 05:52 PM
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It sounds like the furnace was never set up correctly.

There are two things which could be wrong:

1. Furnace not burning enough gas when on, which messes up the fuel to air ratio and reduces efficiency
2. Furnace moving too much air. Keep in mind that the air from a newer furnace won't feel "hot" since they have strong blowers compared to the stuff manufactured in the 1980s.

How to tell:

1. Shut off all gas appliances, turn on the furnace and clock the gas meter. The input should be close to the input BTU rating of the furnace. Google how to do this.

If the input is too low or too high, you'll have to call a tech out to adjust the gas pressure.

If the input is correct, proceed to the next step...

2. Run the furnace for 15 minutes and measure the temperature difference between the return and supply air (at the furnace itself). The thermometer should be placed in the supply air trunk duct, not the plenum - otherwise, you'll get an inaccurate reading.

The temperature difference should be within the range printed on the furnace nameplate. Ideally, it should be 45-50 F degrees - or the middle of the range.

The a/c may not be setup correctly either, but that can only be taken care of by a tech. (refrigerant charge and blower speed could be set wrong)

I think that if there was a single 8" return, the furnace would be cycling on high limit for sure. Post some pictures of the installation

One thing to consider is that new equipment is not the cure-all solution for high energy consumption. The house may be poorly insulated, leaky, and/or have single pane windows.

"Running all the time" is fine in extremely cold or hot weather.

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