A homeowner sent us three quotes to replace a twenty-two-year-old gas furnace. The prices were $4,900, $7,400, and $11,200. All three were from licensed contractors with good reputations. None of them were trying to cheat anybody.
The spread was not about markup. It was about scope — and the scope was mostly invisible, because two of the three quotes were a single page with a model number on it.
Here is how to read what you were handed.
Start with the nameplate, not the price
Every piece of mechanical equipment carries a metal plate stamped with what it actually is: model number, input rating, output rating, efficiency, electrical requirements. That plate is the only part of the transaction that cannot be rounded off in a sales conversation.
Two numbers on a furnace matter most:
- Input, in BTU/h. How much gas the furnace burns. This is the number people mean when they say “an 80,000 BTU furnace.”
- AFUE, as a percentage. How much of that fuel becomes heat in your house instead of heat going up the flue. An 80% furnace turns 80,000 BTU/h of gas into roughly 64,000 BTU/h of delivered heat. A 96% furnace turns the same gas into about 76,800.
That second point is the one that trips people up. If your old furnace was 80% and the new one is 96%, matching the input rating gives you a meaningfully larger heating output than you had before. Quotes frequently do this — they copy the old input rating onto the new equipment because it is fast and nobody complains. The result is a furnace that heats the house in short bursts, cycles more often, and holds temperature less evenly than a correctly sized one.
Ask whether anyone calculated the load
The honest way to size a furnace is a load calculation — an estimate of how much heat your particular house loses on a design-cold day, based on its square footage, insulation, windows, orientation, and air leakage. In the residential world the standard method is called Manual J.
You are looking for one sentence in the proposal, something like “Manual J load calculation performed” or “heating load: 48,000 BTU/h.” If no quote in your pile says anything like it, ask each contractor directly:
What is the design heating load for my house, and how did you get that number?
Three answers are possible. “We ran a Manual J, here it is” is the good one. “Based on the square footage and my experience in these houses” is common and is sometimes fine, particularly from someone who has worked your neighborhood for twenty years — but it is a rule of thumb, not a measurement, and you should know which one you are buying. “It’s the same size as what’s in there now” is not an answer at all; it inherits whatever mistake was made in 2004.
In that homeowner’s three quotes, exactly one contractor had done a load calculation. His number came out about 20% below the existing furnace’s output. He was the middle price.
Find the scope, or find that there isn’t one
A furnace does not exist by itself. It sits in a system, and replacing it touches the parts around it. A thorough proposal says what happens to those parts. A thin one leaves them for the install day, where they become change orders or, worse, get skipped.
Things worth seeing in writing:
| Item | Why it matters |
|---|---|
| Flue / venting | High-efficiency furnaces vent differently than old ones. This can mean new PVC piping and a new exterior penetration. |
| Gas piping | A different input rating can require a different pipe size or a re-check of the existing line. |
| Condensate handling | A condensing furnace produces water. It needs somewhere to drain, sometimes with a pump. |
| Electrical | New disconnect, new circuit, or a thermostat with a common wire the old one didn’t need. |
| Ductwork connection | New plenum and transitions where the furnace cabinet size changes. |
| Permit and inspection | Almost always required. If it is not on the quote, ask who is pulling it. |
| Startup and commissioning | Someone should measure gas pressure and temperature rise after install, and write the readings down. |
| Old equipment removal | Disposal is not free. |
| Warranty terms | Parts warranty comes from the manufacturer; labor warranty comes from the contractor. They are different lengths. |
The $4,900 quote covered the furnace, basic installation, and haul-away. The $11,200 quote covered all of the above plus new venting, a condensate pump, a gas line correction, and two duct transitions the installer had photographed in the basement.
Neither one was wrong. They were quoting different jobs. Nothing in either document made that obvious to a homeowner reading them side by side.
Get the quotes onto the same footing
You do not need to become an engineer to compare proposals. You need every contractor answering the same questions. Send all three the same short note:
- What is the design heating load for my house, and how was it determined?
- What are the input and output ratings and the AFUE of the unit you are proposing?
- Does your price include venting, gas piping, condensate, electrical, duct transitions, permit, and startup measurements? If any are excluded, say which.
- What is your labor warranty, and what is the manufacturer’s parts warranty?
- What in this job could turn into a change order once the old unit is out?
The last question is the useful one. A contractor who has actually looked at your basement will name one or two specific risks. A contractor who has not will say there shouldn’t be any.
What the spread usually means
When quotes are far apart, one of three things is happening: they are not the same scope, they are not the same equipment tier, or one contractor is very busy and priced the job accordingly. All three are legitimate. All three are also knowable — but only if the proposal is written well enough to tell you, or you ask until it is.
That homeowner went with the middle quote. Not because it was in the middle, but because it was the only one that told her what her house needed before it told her what it cost.