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Chicago, Illinois
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14 Posts
Discussion Starter #1
This may be a long thread. I'm trying to fix the sheet metal/new furnace install that just doesn't seem right.

When told to shop for TWO new furnaces for TWO of my 1000 sq ft apartments I was steered toward two Goodman GKS90905DX AFUE: 92.1%; BtuH Input: 92,000; BtuH Output: 86,000; CFM @ 0.5" ESP: 1,978; Depth: 28-3/4"; Height: 40"; Min. Circuit Amps: 13.2; Nom. Tons: 5; Vent Size: 2"; Width: 24-1/2. with matching 3 ton A coils above it.

We'll tackle 2nd floor later but for the first floor, Sheet metal guy installed 16 x 8 sheet metal throughout both apartments with no reducers after each vent. GRRRRR. This just didn't seem right. I've been reading about improper sizing of units/sheet metal so now this concerns me before I put the drywall back up. Is there a good website that may help me get started in picking the correct unit for these apartments? I have dimensions for all rooms, windows, house is a north south facing house nestled in between two huge apartment buildings and it's in Chicago. No insulation but is Lathe/Plaster then drywall over it. 1st Floor apartment has an insulated subfloor with r25 fiberglass with an un-heated basement. I was told that my unit is too powerful for the ductwork so I was thinking of downsizing the heating unit since it is much easier than to rip the soffit down to get at the duct work. I'm thinking of a 75k but goodman. AFUE: 92.0%; BtuH Input: 80,000; BtuH Output: 73,680; CFM @ 0.5" ESP: 1,202; Depth: 28-3/4"; Height: 34-1/2"; Min. Circuit Amps: 9.6; Vent Size: 2"-3"; Width: 17-1/2"; Wt. Lbs.: 115; with a matching 2.5 ton A coil. Am I right to go about it this way. The sheet metal tears at me if I have to rip it all out just to accommodate an oversized furnace. It's just much easier for me to haul a 100 pound $700 dollar furnace in than redo all the other labor.

Would love to find a website that simplifies a general rule of thumb rather than having to dish out more for a proper load calc for the apartment.
 

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Hvac Pro
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22,781 Posts
You should do a proper load calc.

However I think that a 70 or 75,OOO (M) unit is a better size. I have a 70 in my house and my ducts are 18x8 with a 24x10 return drop/duct. The duct size you have now sounds way to small for a 90.

There are no rules of thumb and in Winnipeg which is colder than you we can heat a 1000 sq ft home with a 70 even with poor insulation. Never had to go bigger than that. Seal the doors and windows properly as drafts are the worst for heat loss. Cannot do much about the walls. Insulate the attic to R40.

http://www.hvaccomputer.com/gtarget1.asp?kwx=6&adx=3&gclid=CMjGvu7Kw7UCFe4-MgodPF4AoQ
 

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Probably find out you only need a 50,000 BTU input 90% plus furnace. 16x8 duct is okay for about 40,000 BTUs output.

If the duct work is less then 28 foot long, it doesn't need to reduce.
 
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Chicago, Illinois
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14 Posts
Discussion Starter #4
Thank you Gentlemen, I rushed the completion of my reno and left the 16x8's up in the ceiling. I'm dealing with the insane rush of heat that pours out and the quick dissipation of heat once the air settles. I set my thermostat to 72 and my room heats up to 78. Kills me every time. I was told that the lower btu units will heat up my space slowly and evenly and these monsters I have will rush the heat into the room not allowing the heat to penetrate into the walls. I'm about to install r70 in attic. I'll see how that helps my 2nd floor tenants.
 

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You may be using thermostats with too much of a swing.

A t-stat which cycles the equipment more rapidly may help mitigate the swing. Once you have a furnace you should just keep it until it dies, at least for the length of the warranty. Can also avoid the high limit trips that way.

At the end of the day this stuff is a science, there are no rules, if it's done wrong you're stuck. Done by code doesn't even mean done right, the codes have more to do with safety than everything.

With this stuff if you don't know or don't hire an engineer/designer who does know, you will get screwed because the guys doing it may know just enough to get by.
 

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Hvac Pro
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Thank you Gentlemen, I rushed the completion of my reno and left the 16x8's up in the ceiling. I'm dealing with the insane rush of heat that pours out and the quick dissipation of heat once the air settles. I set my thermostat to 72 and my room heats up to 78. Kills me every time. I was told that the lower btu units will heat up my space slowly and evenly and these monsters I have will rush the heat into the room not allowing the heat to penetrate into the walls. I'm about to install r70 in attic. I'll see how that helps my 2nd floor tenants.
If the thermostat is set at 72 it MUST shut off at 72 if it is a electronic good quality one. They are accurate to within 1 deg F. Has nothing to do with furnace size. If they are el cheapo Chineez or mercury bulb or spring type those may be out by 2-3 deg but not 6.

Try keep them away from return and air supply vents and drafts and sunshine or they will short cycle.
 

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Even with mercury bulb t-stat you can adjust the heat anticipator to prevent overheating.

The furnace fan runs for 2 minutes +/- after the burners shut off and with very oversized furnace overshooting can be an issue.

The anticipater heats the bimetal coil and causes it to satisfy before the temp is reached. If the place heats another 2f (which can happen with extreme oversizing) once satisfied as the furnace cools down, it will always overshoot.

The electronic honeywells have their own intelligent adaptive anticipation and you can cut the swing by increasing the CPH setting.
 
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