The furnace bid that didn't mention the chimney

You get three quotes to replace a twenty-year-old furnace. One is under five thousand, one is near nine, and the expensive one keeps talking about your water heater, which you didn't ask about and which is working fine.

That contractor is not padding. They noticed something the cheap quote either didn't look at or decided not to mention.

Line one

What size, and how they decided

The fastest way to size a furnace is to read the label on the old one and order the same thing. It is also the most common way to end up with a furnace that's too big.

Oversizing doesn't show up as a failure. It shows up as short cycling: the furnace blasts, satisfies the thermostat quickly, shuts off, and repeats. You get temperature swings, cold rooms at the end of long duct runs, more wear on the ignition and blower, and worse humidity control. And if anyone has added insulation or replaced windows since the original install, the house needs less heat than it used to, so matching the old unit bakes in a mistake that may be decades old.

The alternative is a load calculation, generally done to the ACCA Manual J method, which works from your actual square footage, insulation, window area and orientation. It takes real time, which is exactly why the quick quote skips it.

Ask whether they performed one. "Same as what's in there" is an answer, just not a good one.

Line two

Where the exhaust goes, and what gets left behind

This is the one in the drawing, and it's where most of the money hides.

An older mid-efficiency furnace sends hot exhaust up a masonry chimney, usually sharing that flue with your gas water heater. Both appliances contribute heat, and together they keep the chimney warm enough to draft properly.

A high-efficiency condensing furnace extracts so much heat from the exhaust that what's left is too cool for a chimney. It vents through the wall in plastic pipe instead. Efficiency is the whole point, and it's real.

But the water heater stays behind. It's now the only appliance in a flue sized for two, producing a fraction of the exhaust that flue was built to carry. The exhaust can cool before it reaches the top, which weakens draft and lets moisture condense inside the masonry. Two things follow: acidic condensate slowly attacking the mortar, and in the worse case, combustion gases that don't leave the building the way they should.

The fix is ordinary and well understood: a correctly sized liner in the chimney, so the orphaned water heater has a flue it can actually keep warm. It typically runs into four figures, and it is the single most common line item separating a cheap furnace quote from a complete one.

Worth knowing: this isn't automatic. Whether a liner is required depends on the chimney's existing condition, its size, and your local code and inspector. What should be automatic is that somebody looked and told you the answer.

Line three

Where the condensate goes

Condensing furnaces produce water — several gallons a day in cold weather — and it's mildly acidic.

It needs somewhere to drain. If there's a floor drain below the furnace, gravity handles it and the line item is small. If there isn't, you need a condensate pump. Some jurisdictions and some plumbing situations also want a neutralizer cartridge before that water reaches your drain line.

There's a cold-climate wrinkle too: a condensate line run through an unheated space can freeze, which backs water up and shuts the furnace down on the coldest night of the year, which is when you notice.

None of this is expensive. All of it is annoying to add later as a change order.

Line four

The ductwork nobody quotes

A new furnace is connected to your existing ducts, and those ducts were sized for the old one.

A variable-speed blower can compensate for a lot, and it's a genuine comfort upgrade — quieter, steadier, better at moving air to the far end of the house. It also costs more, and a quote comparing a single-stage unit to a variable-speed one is comparing two different products, not two prices for the same thing.

The thing to ask about is return air. Undersized returns are extremely common in older homes, and they choke a new furnace no matter how good it is. A contractor who measured static pressure, or who at least looked at your return situation and said something about it, is telling you they plan to install a system rather than swap a box.

Worked example

The same house, priced two ways

The dollar figures below are illustrative. They exist to show the shape of the gap, not to price your job.

Quote A is a legitimate product. An 80% furnace vented into the existing chimney keeps the water heater company, needs no liner and no condensate handling, and will heat your house. For a rental, or a house you're selling next spring, it may well be the right call.

What it is not is the same job as Quote B for four thousand dollars less. It's a different job, and the quote doesn't say so.

Before you sign

Six questions that make quotes comparable

That last one catches people out. Manufacturer parts warranties are often much shorter unless the unit is registered online within a window after installation, and the labour warranty is a separate promise from the contractor. A quote advertising "10-year warranty" is usually describing parts, which is the part you weren't going to pay for anyway.

Get those six answers in writing from everyone bidding and you stop comparing three numbers. You start comparing three jobs.

Venting, liner and condensate requirements vary by chimney condition, appliance type and local code, and your building official's interpretation governs. Written for homeowners and offered as general guidance only. It is not a substitute for a written, itemized proposal from a licensed contractor or for an on-site inspection. If you suspect a combustion venting problem, treat it as urgent and contact a licensed HVAC contractor or your gas utility.