Looking to replace this old furnace...


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Old 10-05-19, 02:55 PM
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Looking to replace this old furnace...

I have a Carrier 58gs075 furnace that is 30 yrs old. It is rated at 75,000 btu input. I'm hoping to replace just the furnace and keep the current AC setup for now. Honestly, I'm just looking at inexpensive 80% efficiency units. Preferably a 14" wide so it'll be easier for me to connect to current ductwork and plenum with the AC coil.

Any recommendations on a unit I can purchase myself, and install? We don't plan on keeping this house for more than 2 yrs, but don't want an obsolete piece of junk either. I'm not seeing a lot of 14" wide units once you get above 60000 btu? My house is a 1000 sq ft ranch with a full basement, poorly insulated.

Also, is it acceptable to set the new furnace on top of a metal box to raise it up to the existing plenum?
 
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Old 10-05-19, 03:01 PM
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Current furnace photo...

 
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Old 10-05-19, 03:08 PM
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Lets try this image thing again...

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Last edited by PJmax; 10-05-19 at 04:58 PM. Reason: resized picture
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Old 10-05-19, 04:50 PM
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Even if the insulation is poor, that is a gigantic unit for 1,000 square feet.
I’d start with a load calculation to see specifically what size furnace is needed.
 
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Old 10-05-19, 06:46 PM
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Is there a good load calculator available online? The current unit doesn't seem to be too much. It runs for long periods or long heat cycle in the winter.

I can't find what the efficiency might be for the old one. What was the norm for a furnace installed in 1990?
 
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Old 10-05-19, 07:23 PM
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That’s just a basic 80% furnace.

www.loadcalc.net

Make sure the information you put in is true and accurate.
 
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Old 10-05-19, 11:07 PM
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Your existing furnace being natural draft is 60 to 70% efficient seasonal. the draft hood at the top makes it very inefficient. It likely has a standing pilot too which wastes gas constantly.

Heat output of these old units is 70 to 80% of input. This does not reflect typical efficiency, only steady state after it's been running for a good 10 to 15 minutes. You have to check the rating plate to know the actual output btu/hr.

I would not get a mid efficiency in any climate that experiences real winters. The price difference isn't great.

Even if you're selling the house, it's a bad idea -> condensing is a selling point and the next owners will be stuck with a crappy obsolete furnace.

Mids are gone in canada, you can still get them in the states due to weak energy regulations. Still legal but obsolete!

Very few furnaces these days are 14" wide - the ones that are have weak blowers which may not be able deliver proper airflow if you have undersized ducts. (airflow requirements to get the same number of bTU/hr have increased) Few are more than 40k btu/hr which may not be enough.

A 60k 96% unit would roughly match the heat output of what you have. It may not be the right size.

You need to have a load calculation done and the ducts need to be checked to see how much airflow they can handle.

Square footage is a poor gauge of heat loss. If the house is long and narrow and has zero insulation in the walls, very good chance you need 50-55k+ output. (60k condensing furnace needed)

There are 2000-2500 sq ft houses that can heat on the same number of BTUs. They have insulation and vapor barrier.

Every house is different.

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I do not recommend you install this yourself.

If you're good with sheet metal and electrical, you can those parts yourself. The gas connections, venting and commissioning should be left to a pro.

In some places it's illegal for you to install your own gas unit.

It's better just to get it all done through a contractor.
 
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Old 10-06-19, 07:23 AM
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Thank you for that info. Can a new wider unit be used with my existing A.C. coil? Wouldn't that choke the airflow by adapting the sheet metal to fit the current plenum?
 
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Old 10-06-19, 02:03 PM
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Yes, it's done all the time. The transition might slightly reduce airflow but a 17" wide furnace will have a better blower to begin with.

The small 14" wide furnaces even if the blowers move enough air for heating are bad for a/c if the house is more than one story. The blower wheel narrower and there's also less free available area for air to flow through the furnace. The cabinets have insulation and baffles on the inside, reducing free area.

With the bigger blower you can always slow it down to match a small a/c and still get good torque/force to push air to the second floor.

A lot of manufacturers don't make 40k furnaces with strong blower.

*

The question in this is - why are you want to replace the furnace when selling the soon house?

Wouldn't it be easier to just discount the sale price for $2-3k and the new owner can replace it?

Only if the heat exchanger is bad or it needs a very expensive repair would I replace in this situation.
 
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Old 10-06-19, 04:09 PM
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The past few seasons I've had to make a repair to keep it going. Luckily I was able to do it myself. The gas valve, the 3 wire ignitor assembly, a safety sensor, etc. The other morning it wouldn't light. Was making a weird clunking sound, kept lighting the pilot only to shut off immediately. Kept rapidly repeating that. Not sure if the gas valve was malfunctioning or if the ignition control was goofing up. I made sure all the wire connections were good and tapped on the gas valve solenoid. It now appears to be working, but the spark ignitor still constantly sparks throughout the whole heat cycle. It should stop sparking once the burners light.

Anyways, I was thinking it would be nice if we had a reliable furnace, even if it's only for a year.
 
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Old 10-06-19, 05:23 PM
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3 wire pilot ignition system with problems? that changes everything - time for a new furnace, don't put a cent into it. i've looked and the parts are rare and expensive - it's an obsolete ignition system only carrier used.

not a good idea to sell a house with an unreliable furnace.

A cheap single stage goodman 96% will do.

Assuming you know about how the house was constructed (brick vs framed, insulated or not, r-values), do your own load calc.
 
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Old 10-06-19, 06:10 PM
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Thanks for your help! I have an installer willing to do a side job for me, so probably will have him stop by.
 
 

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