need new furnace for 1200 sq. f.t. house

geargirlMarch 25, 2010

Greetings all,

I've been checking out the site and decided this is a great place to get some expert advice on our looming "new furnace" decision!

We're finally at the point where we're ready to exchange our good ole 80+ year old monstrosity of a gravity-feed furnace! You know, the kind that looks like an octopus and takes up over half the basement?! It's definitely served its time, has been running pretty flawlessly for me for nearly 12 years, but the old soldier is running at lss than 50% efficiencyƂand I'd kind of like to have some basement space!

We're going with a local co. here in WA state that carries Lennox and is highly regarded with BBB, our power company, etc. My question is this: with relatively mild temps year-round, a fairly small 1.5 story 1921 home (1200 sq. ft.), insulated attic/basement ceiling/blown-in wall ins. and new vinyl, double-paned windows throughout the house, which would be the better furnace: the Lennox G71MPP-36B-070 furnace or the G71MPP-36C-090?

The rep has recommended the 70 over the 90, but with no way to heat the top story (no place to run ducts) we're concerned that the 70 wouldn't do as good a job as the 90. Right now we seal off the stairwell and use space heaters upstairs to warm up the bedrooms in cold weather.

We like the idea of a smaller, less-expensive furnace, but want to make sure it's going to meet our needs. We'll also have a vent for our unfinished basement, too, so we can eventually heat it when we convert it to a workshop space.

Any thoughts? Oh, and the furnace is gas-powered. Also, thermostat thoughts are most welcome!!

Thanks so much for any thoughts and recommendations!


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It's hard to do anything but throw a rule of thumb heat loss calc without a whole lot more info. Old houses are tricky and without a visual inspection of your actual R values, window area, construction type, exposed wall areas, etc. it isn't possible to come up with much in the way of accuracy. But in a well or fairly well insulated home such as described and with the improvements you've described, probably the 070 (that is the part of the model number representing the btu input) will put you comfortably above the load required. Not being at hand familiar with the model nomenclature of Lennox equipment, (I never joined their "club") I'll assume that this model represents an at least 95% AFUE, condensing gas furnace of some configuration. Increasing the size or oversizing your furnace to compensate for a non-ducted upper floor area will do you more harm than good. Oversized furnaces, in spite of their duct system - or lack of it - will short cycle (start and stop frequently) which is wasteful in terms of energy, dissatisfactory in terms of comfort, and hard on components, especially ignition systems.
Ask your installer for ideas to get some duct upstairs. Oftentimes some thought and imagination can find a way. But I'd go with his recommendation as far as size.
Think of your furnace as you would an automobile. Short trips around town are bound to do it more harm and use more fuel than long treks on the open road.

    Bookmark   March 25, 2010 at 7:53PM
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also if you seal the air leakage in the house it will be easier and more affordable to heat and cool.
have you had any hvac companies out to look at your home?

    Bookmark   March 26, 2010 at 1:52PM
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Hello all,

Thanks so much for your input. Yes, we've been making improvements to the house over the years, which includes new vinyl windows, insulation, a new door and we're working on sealing up various air leaks as well. :) Now we're pretty snug during the winter, and I'm sure we'll be shocked how much more comfortable the house is (not to mention cheaper gas bills!) once we get our coal/oil/gas old timer furnace replaced by a new one!

I've had a couple of HVAC companies out, and we've pretty much decided on the company that carries Lennox. Their bid was better, plus we can get some rebates through going with them via Costco, so we're feeling good about the install. I was just wondering about the smaller furnace, but snoringcow confirmed what I'd been suspecting, so I'm sure we'll be happy with a slightly smaller model.

My last question: is the Honeywell Vision Pro IAQ thermostat a good choice, or should we request another model? This forum has taught me that thermostats are also something to be considered, so let me know if there are more suitable models out there. However, we're not doing AC, so maybe the thermostat issue is not an issue at all! But, just thought I'd toss that out there for y'all, just in case.

Thanks again!


    Bookmark   March 26, 2010 at 3:42PM
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70 and 90,000 BTU 95% furnaces for that small of a house in a mild climate both seemed very oversized.

These furnaces need to move a LOT of air to be efficient and variable speed blowers have the get up and go to do it. Could be very loud.

If you go that big, better off skipping insulating and new windows, you need all the heat escaping you can.

    Bookmark   March 28, 2010 at 5:42PM
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The furnace's BTU is 63,000, so I think we're OK. Plus, the furnace is in the basement, which we use primarily for storage and working on crafts, etc.


    Bookmark   April 1, 2010 at 2:30PM
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Do you have the paper white asbestos on your ducts that are going to be removed, if so what is that costing. later Paulbm

    Bookmark   April 3, 2010 at 6:01PM
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