Now I have a Question.........

Old 06-14-02, 07:19 PM
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Now I have a Question.........

Lennox Pulse gas Furnace

M# G14Q3-80-7
S# 6385G60962

current evaporator

M# C16-31-1FF (has a small leak-I add 2 lbs every other summer)

Lennox Condensing Unit (Compressor shorted to ground and terminal block blew-out, losing all refrigerant)

M# HS18-411-6P
S# 5185F27561

PROBLEM: Condensing unit, lineset, and Coil WILL be replaced, my friend wants to up-grade the tonnage for future renovations what is the highest tonnage I can go with this model furnace?

if it matters the Blower is a 1/3 h.p.
front take-off is 20x8
rear take off is 8x12

I am thinking I can only go up to a 3.5 ton a/c unit with this furnace, cause I think the "Q3" in the model number means 3 tons cooling????. I dunno.
thanks for the Help.
Old 06-15-02, 09:29 AM
lynn comstock
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AC Size

The current coil seems to be 2.5-ton (31) and the OD unit is 3-ton (41).

This is not a good match. I cannot tell you the air capacity of the furnace, a good Lennox dealer should know. I can tell that most manufacturers go to 1/2 hp for 3.5 to 4-ton furnaces and 3/4 hp for 5/ton furnaces. The 1/3 hp motors are good for 2 to 3-ton.

Furnaces are size rated at .5 inches ESP. I think that is ridiculously, low because the clean, new AC coil typically has .25 ESP pressure drop. This leaves only .25 for the duct system. The result is that the airflow drops off and the rated capacities and efficiencies of the equipment are not achieved. As the coil ages and gets dirty, the bad problem gets worse.
Old 06-15-02, 10:03 AM
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so basically...........

in order to keep this furnace set-up I would have to stay with the 3-ton a/c unit? unless I also change out the furnace....
if I replace the blower motor to a 1/2 or 3/4 h.p. I don't think that would help cause the RPM's are the same.....I'll try to research this some more, thanks Lynn
Old 06-15-02, 01:03 PM
lynn comstock
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I think you got it.

Do a heat calculation on your home. Chances are that you have a big enough air conditioner.

The 3-ton OD unit is underperforming with a 2.5-ton coil. Upgrading to a 3 or 3.5 ton coil is a good idea. The manufacturers usually approve match-ups with larger coils. The larger coil will have increased area and lower pressure drop which will increase airflow and performance.
Old 06-16-02, 06:52 PM
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The furnace you have has a 3 ton drive aprox 1200 cfm and would probably not handle a 3 1/2 ton. I think lynn comstock was right put a three or 3 1/2 ton coil and 3 ton condensor and you will probably have enough cooling.
I would recommend a heat exchanger pressure test on that 1985 pulse furnace. These furnaces had a problem with weak welds and should be checked for safety.
Old 06-16-02, 07:07 PM
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actually the air volume range is 805 to 1585 cfm, and a 3.5 ton needs a minumum of 1600cfm, I found this in my lennox books, thanks anyways, I talked this over with my friend and he said that the 3 ton replacement would be ok. I t has allways cooled the house when it worked. and he did have the heat exchanger tested a few years back.
thanks for your help everyone, looks like my buddy and I are going to do this job next weekend

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