Furnace and AC equipment, explained.

Almost every estimate compresses the equipment into one line that looks something like “Carrier 96% 80k BTU furnace, 3-ton 15.2 SEER2 condenser, matched coil, two-stage.”

That line is doing a lot of work. Unpacking it is the difference between comparing two estimates and guessing between two numbers.

The two machines

Most River Forest homes with central air have a split system, which is two machines sharing one duct network.

In the basement sits the furnace, which burns gas to make heat, and contains the blower that moves air through your ducts all year. Sitting on top of or inside the furnace cabinet is the evaporator coil, which gets cold in summer. Outside, usually beside the house, is the condenser, the unit with the fan that runs in July. A pair of copper tubes called the line set connects the coil to the condenser.

When you replace “the furnace and the AC,” you are replacing four things: furnace, coil, condenser, and usually the line set. The ducts, in most cases, stay.

This is why the blower lives in the heating equipment even though it matters most for cooling, and why a bad furnace choice can hurt your air conditioning.

Capacity: BTU and tons

Heating capacity is measured in BTU per hour. An 80,000 BTU furnace is common for a house of moderate size around here. Note that the number on the estimate is usually input; the useful output is input multiplied by the efficiency, so an 80,000 BTU input furnace at 96% delivers about 76,800 BTU of heat.

Cooling capacity is measured in tons, a leftover from the days of ice. One ton equals 12,000 BTU per hour of cooling. Residential systems here are typically two to five tons. Sizes come in half-ton steps, so there is no 3.2-ton unit; there is a three and a three and a half.

The important idea: capacity is not a quality rating. It is a fit. Too small and the system cannot keep up on the worst day of the year. Too large and it creates its own problems.

Why sizing gets done badly, and why it matters

Oversized equipment short-cycles. It blasts to temperature, satisfies the thermostat, and shuts off. On the cooling side that is worse than inefficient: an air conditioner removes humidity only while it runs, so a unit that reaches temperature in eight minutes leaves the house cold and clammy rather than cool and dry.

Capacity should come from a load calculation, commonly called a Manual J, which accounts for square footage, insulation, window area and orientation, and air leakage. In practice, many replacements are sized by reading the label on the equipment being removed.

That shortcut is understandable and often lands close. It also inherits any mistake made in 1988, and it ignores work you have done since. If you replaced the windows, insulated the attic, air-sealed the rim joist, or finished the third floor, your house’s load has changed. Houses near Concordia that have been through two renovations rarely need exactly what they needed forty years ago.

It is entirely reasonable to ask: did this size come from a calculation, or from the old unit?

Furnace efficiency: AFUE

AFUE stands for annual fuel utilization efficiency. It is the percentage of the gas you pay for that ends up as heat in your house. The rest goes out the vent.

Two families exist in practice. An 80% AFUE furnace vents hot exhaust up an existing chimney or flue. A 96% AFUE furnace is a condensing furnace: it extracts so much heat that the exhaust is cool enough to vent through PVC out a sidewall, and the water vapor in the exhaust condenses into acidic liquid that needs a drain.

So the efficiency choice is also an installation choice. Moving to 96% can mean cutting an exterior wall, new vent pipe, a condensate line, and sometimes a pump if there is no floor drain nearby. It can also mean addressing a water heater that was sharing the old chimney and is now alone in it.

Neither is the right answer for every house. But the estimate should tell you which one you are getting, and the installation scope should reflect it.

AC efficiency: SEER2

SEER2 is the current seasonal energy efficiency ratio, measured under federal test procedures that replaced the older SEER method. Because the test conditions changed, a SEER2 number reads slightly lower than the SEER number for comparable equipment.

That creates a real trap when comparing estimates. If one page says “16 SEER” and another says “15.2 SEER2,” the second is not necessarily less efficient. Ask both contractors to state SEER2, and compare on the same scale.

Higher efficiency costs more up front. Whether it pays back depends on how much cooling you actually use. A house that runs the AC hard from June through September is a different calculation than one where the windows are open most of the summer and an Italian ice from Johnnie’s handles the rest.

Staging and blowers

This is where comfort actually lives.

• Single-stage: full output or off.
• Two-stage: a low gear used most of the time, with full output held in reserve for the coldest or hottest days.
• Variable or modulating: continuously adjusts output.

Alongside that, the blower is either a standard PSC motor or a variable-speed ECM motor. The ECM ramps up and down, runs quieter, and uses considerably less electricity when running long cycles at low speed.

Longer, gentler run cycles are what fix an uneven house. If your second floor is five degrees off the first in August, staging and blower type will do more for you than a higher efficiency rating will.

This is also the largest cost spread in the equipment half of an estimate. Two pages that both read “furnace and AC” can be three thousand dollars apart entirely on staging.

The matched pair and the line set

The outdoor condenser and the indoor evaporator coil are tested and rated together as a system. The efficiency number on your estimate only holds if the pair being installed is the pair that was rated. Putting a new condenser on an old coil usually forfeits both the rating and, often, the warranty.

So “matched coil” on an estimate is not filler. Its absence is a question.

The line set matters for a similar reason. Modern refrigerants and oils are not always compatible with residue in old tubing. Depending on what is being installed and what is there now, the correct answer is a new line set, or a thorough flush, or reuse after inspection. What you do not want is silence, because silence means a decision was made without telling you.

Heat pumps, briefly

A heat pump is an air conditioner that can run backwards, producing heat in winter by moving it rather than burning fuel. Cold-climate models now work at temperatures that would have been unrealistic a decade ago, and they are increasingly quoted in this area, sometimes paired with a gas furnace as a dual-fuel system that switches over when it gets cold enough.

If a heat pump appears on your estimate, the relevant questions are what happens on the coldest days, what the backup heat source is, and how the changeover is controlled. It is a legitimate option worth understanding, not a substitution to discover after installation.

Thermostats and controls

Two-stage and variable equipment need a thermostat that can actually speak to them. Some manufacturers use communicating controls, where the thermostat and equipment talk over a proprietary protocol. Those systems work well and lock you into that brand’s thermostat.

There is nothing wrong with that, but it is worth knowing before the wall plate goes on. Ask which thermostat is included, whether it is communicating, and whether you could replace it later with something generic.

What to write down

For each estimate, capture these nine items. If you can fill them in, you can compare two pages in five minutes.

• Furnace model number, BTU input, AFUE
• Condenser model number, tons, SEER2
• Indoor coil model number, and whether it is a matched pair
• Staging, for heating and for cooling
• Blower type, PSC or variable-speed ECM
• Line set: new, flushed, or reused
• Venting method, and any effect on the water heater
• Thermostat, and whether it is communicating
• Whether the size came from a load calculation

That list is the whole vocabulary. Nine lines, and the equipment half of any estimate stops being a foreign language.