natural gas furnace. Selecting and installing help.

This forum is for Wood Boilers and Furnaces. I would appreciate keeping it on topic.

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woodsmaster said:

I recently bought a investment home that I will be flipping. I'm going to install a new natural gas furnace in it. I want to put in something decent that wont break the bank. I will probably do most of the install myself, and have it inspected by a pro before using. I believe I need a unit in the 100.000 BTU range based on the old furnace. Any recommendations on brand or model and tips on installing ? It seems E-bay has the best deals from what I have seen so far.

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You probably don't care, but you should go online and do a heat load calc. Many units have been installed to large to begin with. Also if going with a 90% plus unit you will be getting 10 to 20% more heat out of the same size unit.

To me it seems to be a business decision regardless of which fuel it uses.
My past experience tells me that you never get paid back for those extra bucks you put on high end appliances.

woodsmaster said:

I recently bought a investment home that I will be flipping. I'm going to install a new natural gas furnace in it. I want to put in something decent that wont break the bank. I will probably do most of the install myself, and have it inspected by a pro before using. I believe I need a unit in the 100.000 BTU range based on the old furnace. Any recommendations on brand or model and tips on installing ? It seems E-bay has the best deals from what I have seen so far.

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A. Do a heat loss. We find nearly 8 out of every 10 furnaces we see are oversized.
B. Trane and American standard have highest brand approval and name recognition in the industry.
C. 95% vs 80%...... even in today's low fuel price market, efficiency sells. Also, check local codes on chimney liner requirements if an 80 is used. Many times an 80 will wind up being same installed cost if a liner is needed.
D. Mama makes the final decision. Remember that selecting the furnace brand as well as anything else you install in or do to the house.

I once installed an oversized furnace in a rental property and it affected the comfort and the efficiency. When the thermostat was satisfied and the burner shut down the residual heat being scavenged from the unit would overheat the space before the blower shut down.

In talking to my dad, if given the choice between oversized or slightly undersized, for AC and furnace he prefers to go undersized. First, for AC, one of the main things is if it's oversized it will tend not to run as much and therefore not remove as much humidity. When the AC is running, it's not only cooling but removing that humidity. You may find out you have a cool, damp house if it's too oversized. For heat, he prefers it to run more as well in order to limit the feeling of hot, then cold, back to hot, back to cold feeling you may get if the furnace is oversized and therefore running for shorter amounts of time. When it's slightly undersized, the furnace may run more which will give you more even heat. This makes complete sense to me.

woodsmaster said:

Does it hurt anything to oversize ? I'm actually finding better deals on over sized furnaces.

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Short answer.....Yes it most definitely does.
Being oversized causes the furnace to short cycle and most likely overheat if the duct system is not up to the task of carrying the required sq ft.
Starts/stops = more wear and tear on the equipment so shorter life. This is why the best furnaces on the market today are variable firing rate and variable air flow.
Efficiency also suffers to a far greater extent than most people realize.

In heating equipment I'd rather be 10% light than over sized.

woodsmaster said:

old furnace says 125,000 btu input and 100,000 bonnet capacity. What is the difference ?

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the difference is 25kbtu, or 20%. what youd expect with an 80% efficient furnace. you should do the furnace sizing calc. it takes 5 min. google it

another tip is to match the horsepower rating of the blower on old furnace (assuming it was sized correctly). you can also measure the duct size of your intake, and google a rule of thumb CFM flowrate which should match the blowers capability

http://www.homedepot.com/catalog/pdfImages/a7/a76f47be-7461-4edf-a6b5-352f2c23ef88.pdf

rule of thumb chart. i wouldnt go over the btu recommendation for your area, and agree with slight undersize per other comments.

i replaced my old furnace with a 99% efficient furnace and it uses pvc exhaust pipes, not the metal casing. if you want to reuse the existing metal exhaust, then youre stuck with an 80% model. read the manual of your selected furnace before you buy it!

I'd bet a Franklin you can easily heat that house with an 80,000btu high efficiency furnace if the old one is rated at 125,000 input. Maybe even a 60.

That bet is based on 20+ years of removing old beasts like that, doing a heat loss for the house and replacing with what is actually needed.


125 input vs 100 bonnet.........the 125 is what the furnace is burning, the 100 is actually going into the house (and that was when it was new, running under lab conditions as when tested). You're likely getting somewhere between 80-90,000 actual output and the rest is keeping the great outdoors warmer via the chimney.