furnace undersized?

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Old 11-05-08, 10:33 PM
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furnace undersized?

I've just done a manual J heat load calculation and the BTU for the heater came out to be 48k. I'm looking at Goodman heaters and the only options available are 45k, and 70k nothing in between.

Question is: will 45k unit be too small? what to do in this situation -- round up to 70, or round to the closest available size, 45 in this case?

the same load calculation calls for 2.5 ton AC. Goodman offers combos, and 45K is not available with 2.5 ton AC unit. I live in CA, so AC will be working harder than heater.
 
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Old 11-06-08, 05:53 AM
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Just to be clear what are your heat loss and heat gain (both sensible and latent) numbers? And do you live in a humid environment?

Manual J is supposed to be a little conservative, and it is okay to be slightly undersized your heat. If you are looking at a heat pump to do all your heating you could opt for larger heat strips in case the HP cannot keep up with demand.

On the cooling side it is hard to know what will work without the heat gain numbers and how humid it gets.
 
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Old 11-06-08, 07:20 AM
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Just out of curiosity, is your house in the range of 1000 square feet, single story, 8 foot ceilings, good windows and doors, insulated?

What kind of temp ranges throughout the year do you get? We can get like -30(yes, minus) to 95 here.

I'm curious as I service some condos that have furnaces in that range. And one with 40,000 is paltry cool heat that has to run and run and only puts out like 106 degrees out the registers, and when in A/c mode it takes forever for the thermostat to change 1 degree.
 
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Old 11-07-08, 01:50 PM
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thanks for your responses... here are some more details

house is about 1370 sq ft (internal sq ft -- i measured all rooms, bathrooms and closets) external sqft = 1570,
1 story 8ft ceilings, double pain windows, single pain skylights

summer temps are 95-110 its dry heat (I'm in northen CA)
winter temps are 20-40

Manual J numbers:

sensible gain = 28,461
Latent gain = 0
total heat gain = 28,461 (2.5 tons)
total head loss = 47,690


The heater i'm looking at is Goodman 95% efficiency 2 stage

here's a link to the specs of the system (both ac and furnace): Goodman Gas Furnace Air conditioner Systems, Buy Direct from ACWholesalers.com, Free Shipping.

I had a contractor come out, and he quoted me this exact system, but he didn't really measure anything, just went off the house being 1570 sq ft

lol and btw, he did quote me $11k for this system, that's why I'm not going with him

I'm leaning towards a a bigger furnace, I have a baby on the way, and would hate for the furnace to be too weak.

Any suggestions will be most welcome
 
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Old 11-07-08, 04:02 PM
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Id go with the 2.5 ton AC with what ever size furnace you can get! The 45k unit will work for you! My guess is it does not have a lot of air flow to handle the 2.5 ton coil. If the next size unit is the two stage 70k furnace you will be fine because it being a two stage! Why not a heat pump???
 
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Old 11-10-08, 11:44 AM
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Originally Posted by mihan View Post
The heater i'm looking at is Goodman 95% efficiency 2 stage

here's a link to the specs of the system (both ac and furnace): Goodman Gas Furnace Air conditioner Systems, Buy Direct from ACWholesalers.com, Free Shipping.
I think you're on the right track as that is a two stage furnace. Its first stage should do nicely and the second stage (if ever used) should give you that extra capacity.
 
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Old 11-10-08, 12:05 PM
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I am looking at the Goodman GMV950453BX, and that can run up to 3 ton A/C.

I'd rather have a furnace that you'll see 2nd stage be used, and not use it will be an expensive single stage furnace.

be sure you get a two stage t-stat.
 
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Old 11-10-08, 04:33 PM
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thanks for your replies guys. You're awesome!!

I've decided to go for the 45k btu 2 stage, with 2.5 ton AC.

I've looked at the heat pump option. I would still have to get the furnace for the cold nights, and the extra cost doesn't make sense to me. I'm gonna be selling or renting this house in the next 5-7 years anyway, so I don't think I'll be able to recoup the benefits.
 
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Old 12-06-08, 10:50 PM
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Like you I need a new furnace & want to do a Manual J to validate what I am being told. Can u share where you got the info to do your own Manual J - Thanks
 
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Old 01-13-09, 05:07 PM
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here's the link to the program I used -- sorry it took a while to reply

99 HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations 99
 
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Old 01-13-09, 05:59 PM
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It's easy mihan. Do you consider yourself to be an environmetalist? If so, use the smaller furnace. A little suffering is good for the soul.

And if so, do without the AC and think how many tons of carbon you can save.
 
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