square footage geat requirement
Can anyone tell me the rule of thumb how to know if a furnace is the proper size to properly heat a home? Is there so many btu’s per square foot or how would I know I am not buying a unit that may be too small and have to work too hard to heat and cool my home?
Is there a certain life expectancy for a furnace? Are there certain brands that are more economical than others? I am looking at a good heating and air conditioning unit so any opinions please let me know.
Edited 3/9/2007 8:39 pm ET by bigbossman
Replies
If the home doesn't get too cold on the coldest night of the year.
Otherwise you need to know the coldest temp, the quality of insulation in the walls and ceiling (and floor, if crawl), the square feet of all walls/ceiling/floor, the square feet of windows, the amount of air infiltration, and several other things.
Re "working too hard", a properly sized furnace will run maybe 80% of the time on the coldest night of the year. It doesn't work "harder" when sized smaller, just longer. And the "right-sized" furnace will generally run a hair more efficiently than an oversized one.
But for an example, for our house (1000 sq ft footprint/roof area, approx 2000 sq ft wall area, "average" windows with no big window walls) in southern Minnesota (coldest temp about -35F), 1976 conventional frame construction but has been tightened up quite a bit and gotten extra insulation in the attic, a 65K BTU furnace appears to be completely adequate. (The "Manual J" calculation on our house came out to something like 55K BTU, but the 65 was the closest in most manufacturer's lines.)
Life expectancy is all over the map. Many furnaces still in operation are 30/40/50 years old. The only reason they need replacing is that they're much less efficient than newer units. OTOH, some of the early high-efficiency units sold in the 70s died a pretty quick death. (But newer high-efficiency models appear to be quite robust.)
When we were checking out furnaces I came to the conclusion that, at least in our area, it's a pretty competitive business and you get what you pay for. Cheaper units are more cheaply constructed, outfits that low-ball installation figures are going to cut corners.
Different heating contractors sometimes have little "quick and dirty" formulae for estimating required furnace size developed from years of experience, and generally the resulting furnace/boiler will be significantly oversized so the the customer doesn't complain about being cold. Oversized is inefficient! Take your floor square footage, wall and window area, and insulation data to a heating contractor who will, or have his supplier, perform a heat loss calculation for your house, and recommend a poperly sized unit. A furnace is most efficient when is runs continuously, so as DanH mentioned, on the coldest, windiest days, it should be running nearly all the time. The big, commercial burners use several changeable nozzles(orfices) to maximize burner time. See a heating pro.
Is there any brand names to look for since you sound like you know a lot more about this than me? Is it a difference in squirrel cages in the unit that helps it to be more efficient?
I just dont want a unit too small for my place 2900 sq ft with 8ft ceilings not a terrible amount of windows with an new insulated roof and I'm not sure of the insulation in the walls.
Find an installer you feel comfortable with. He'll recommend the right equipment. Most of your major brands put out decent units.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I second DanH's post. It's the design of the heating unit, and the BTU input(nozzle/orfice size)/BTU output. With a wood fired unit, you stoke it once or twice a day and rely on the flywheel effect of the mass of the unit or the stored hot water. With a "regular" furnace, your "flywheel" is the mass of the house, the drywall, plaster, tile, etc.
"Rules of thumb" vary greatly by location. What works for a Texas winter won't cut it in New York.
The best way to do it is through a load calculation. This place: http://www.hvac-calc.com sells a time limited license for short money, and the program is easy to use. Compared to a load calculation, "rules of thumb" will significantly oversize the equipment- which costs more money upfront and over the lifespan in operation.
A properly sized furnace or air conditioner will run 100% of the time on a design day (coldest or hottest day of the year). If the outdoor temperature exceeds the design temp, then the temp in the home will rise or fall to match. Air conditioner ratings are based on an 80* indoor temperature.
Two stage, variable speed equipment will offer much greater comfort and reduced energy costs if sized and installed properly.
The quality of installation is far more important than the name on the cabinet.
There is a lot of advertising money behind the big names.
Did I mention that the quality of the installation is more important than the name on the cabinet?
Unfortunately, homeowners and builders view HVAC as a commodity to be obtained at the lowest price. There is a difference between HVAC that is "done so it works" and "done right". It is impossible to tell what you are getting in a line item bid, most HVAC users shop using "free estimates" and HVAC contractors do not want to waste time with tire kickers that won't spend $.02 more than they have to, so it is a Catch 22 problem.
Lots of luck.
We only had wood till we got a new gas furnace, my friend is a heating man, i told him get whatever i need, He said at his supply house they had brand new ones half price as the finish was damaged, I bought a new one for my rental too, These were only the 80 percent ones not the 90, Very surprized how much we saved, Make sure you ask about that