06-06-2012, 09:18 AM
I am very new to the world of furnaces and would truly appreciate some input. We just purchased a house and looking to replace the existing furnace which is very old. The house is about 3000 SqFt and not sure if we should look at a high-efficiency or the 80% one. I am looking for a reliable brand that doesn’t cost me an arm and a leg. Also need info. on the approx. product and installation cost.
06-06-2012, 02:50 PM
Need to understand what fuel you'd use: NG, propane, oil, other?
Then, do NOT just replace with one the same size...it is highly likely that the one you have there now is 2-3x bigger than needed, and thus, is not giving the efficiency it was designed or capable of when new, let alone now. Then, check with the local utility company as there may be rebates that would only apply to certain brands and/or efficiency levels.
So you can compare apples and apples, you need to either pay for or approximate a reliable heat loss calculation. If you have or can get last year's energy bills, there are ways to use those for an estimate, otherwise, you may need to pay someone to run the numbers.
All good heating designs start with a heat-load calculation based on realistic outside (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) and inside design temperatures, not some fudge-factor using the 50 year record low temp "just to be sure".
Most hot-air furnaces out there are at least 2-3x oversized for the actual design condition load, and while it doesn't take nearly the performance hit of a hydronic boiler from oversizing, being oversized results in a noisier, draftier system and reduced comfort. If you have (or can get) fuel use numbers of the prior owner over the past year or two it's pretty easy to throw some reliable stakes in the ground for what the approximate design condition load is at the 0F ( the 99% outside design temperature (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) for most of Chicago.) It's shocking the number of well-intentioned installers who "adjust" the numbers to come up with something WAY beyond the real load conditions, even when using ACCA Manual-J type calculation tools. But fuel use numbers don't lie- the heat load can't be higher than what the fuel use numbers imply by more than 25% unless the house was literally unheated much of the time.
It's worth retrofit-sealing any ducted HVAC system to a reasonable level of tightness, and insulating at least the supply ducts (especially where they pass through unconditioned or semi-conditioned space), which can make as much difference as moving from an 80% AFUE furnace to a 92% AFUE furnace on fuel use. Sealing ducts takes a lot of whistle & hiss out of some systems too. Sealing with duct mastic on all seams & joints are preferred, but on shiny-clean galvanized or aluminum ducts FSK tape can work.
Even pretty-good 95% AFUE 2-stage (or even varible-speed modulating) condensing furnaces in the small to mid sizes are pretty cheap- under $2000, and the difference in installed cost between a 1-speed righ-sized 80% and a 2-stage 90%+ won't usually be huge either- maybe grand, but probably not even that much. The exhaust of a condensing furnace can't be vented into the old chimney flue, but it can use cheap PVC vent pipe and are easily side-vented in most applications. But even if you went with an 80% version that was right-sized for the actual load you'd still probably have to use a flue-liner to narrow it down sufficiently for the lower BTU input if a 3x oversized beast was it's predecessor. Comfort factors on 2-stage & right-sized is high, and well worth it even if the difference in fuel cost doesn't have an easy financial rationale. (If you go with a condensing unit and there's a gas-fired tank sharing a flue with the old furnace you'd be well advised to move it to a narrower flue or get a power-vented version. The flue will be too fat for just the water heater, and you risk backdrafting & flue condensation issues. google "orphaned water heater".)
But start with real heat loss calculation (or work backward from fuel use) to size the burner output for max-comfort/minimum-noise and work from there. If you have pro do it, don't settle for a "Lessee, 35BTU a foot time 3000 feet comes ta hundred thou BTU and hour, let's call it 125K just to be sure.", which will reliably oversize it for even the draftiest least-insulated 3000' house in IL. (The way they end up being 3x oversized in the first place, is badly-vetted rules of thumb like that.)
A true heat load calc for a reasonably tight reasonably insulated 3000' house with double paned (or storm) windows will likely come in between 35-55KBTU/hr @ 0F depending on it's actual construction, geometry/shape and air-tightness. If the ducts are sealed & insulated there's no reason to add in a duct-loss factor. A 2-stage where the low-fire output is below the calculated heat load and the high stage slightly above, you're golden- it'll only hit high-fire during recovery from overnight setbacks, and almost all operation will be long efficient low-fire burns with the air handler running on "low" for low noise & low draft.
Goodman has a lot of pretty-good smaller 2-stage units, but it's a competitive market, with many good vendors. The quality of the installer/contractor is ultimately going to be a bigger factor in satisfaction & reliability than the name-plate on the furnace. Call around tell them what you have and what you want- those who offer to do a real heat load calculation as part of the proposal process without prompting move to the front of the line, even if they charge for it (that charge often comes off the installation price if you go with their proposed system). But look at their calculation and question it if they use a sub-zero outside design temp or 75F inside temp, since that indicates a thumb on the scale. Old habits die hard, and nobody want's the 5AM call from the shivering irate customer should they err to the low side, but that feared scenario almost never happens in real life.