HVAC terms, explained

A room-by-room calculation of how much heating and cooling your home actually needs, based on its size, insulation, windows and which way it faces. It’s how a new system should be sized, instead of copying the old unit.

Manual J is published by the Air Conditioning Contractors of America (ACCA). The residential building code used in Virginia calls for heating and cooling equipment to be sized from a load calculation like this. Many replacements skip it and simply match the old unit, which may have been oversized from the start or may no longer fit a home that has new windows, insulation or a finished basement.

How cooling capacity is measured. One ton equals 12,000 BTU per hour of cooling. Most homes need between 1.5 and 5 tons.

Bigger isn’t better. An oversized air conditioner cools the house quickly, shuts off before it has pulled the humidity out, and turns on and off more often, which can leave rooms feeling clammy and wear the equipment faster.

British thermal unit, a measure of heat. Furnace and heat pump output is listed in BTU per hour (BTU/hr).

Cooling efficiency for an air conditioner or heat pump, like miles per gallon for a car. A higher number means lower summer electric bills.

SEER2 replaced the older SEER rating in 2023. It uses a tougher test, so the SEER2 number for a unit reads a little lower than its old SEER number. When you compare quotes, make sure they use the same rating.

Furnace efficiency: the share of the gas a furnace turns into heat for your home. An 80% furnace turns 80% of its fuel into heat.

Standard 80% furnaces vent through a metal flue. High-efficiency furnaces (90% and up) vent through PVC pipes and need a drain for the water they produce, so switching between the two can add venting and drain work.

How a system runs. Single-stage is either fully on or off. Two-stage has a gentler low setting it uses most of the time. Variable-speed adjusts in small steps. More stages run quieter and more evenly and remove more humidity, at a higher price.

An outdoor unit that cools in summer and, by running in reverse, heats your home with electricity in winter. It moves heat instead of burning fuel.

A heat pump paired with a gas furnace. The heat pump heats most of the winter, and the furnace takes over on the coldest days.

The indoor parts of the system. The coil cools the air as it passes over it. In homes with a gas furnace, the coil sits on top of the furnace; in all-electric homes, an air handler holds the fan and coil.

The two copper pipes that connect the indoor coil to the outdoor unit. Sometimes they can be cleaned and reused; sometimes they need to be replaced, depending on their size, condition and the refrigerant the new system uses.

Replacing a line set that runs through finished walls or ceilings is real work, and it’s one of the most common differences between a low quote and a high one.

The ductwork that pulls air from your rooms back to the system, through the grilles that air doesn’t blow out of. If it’s too small, any system runs louder and less efficiently.

Cubic feet per minute: how much air moves through the system. A common rule of thumb is about 400 CFM for each ton of cooling.

Cooling pulls water out of the air, and a drain line carries it away. An overflow switch shuts the system off if that drain clogs, before water damages a ceiling or floor.

CSST is flexible gas piping, usually with a yellow coating. It needs to be bonded (connected) to your home’s electrical grounding to reduce the risk of damage from a lightning strike.

The pipe that carries a gas furnace’s exhaust outside. Standard furnaces use a metal flue; high-efficiency furnaces use PVC pipes. Changing furnace type can mean changing the venting.

The switch box next to the outdoor unit that lets a technician safely cut its power. It’s often replaced along with the outdoor unit.

The written list of exactly what a job includes: the equipment, its size, and the work around it, like lines, ducts, electrical and the permit. When every contractor prices the same scope, their bids actually compare.

Virginia’s Department of Professional and Occupational Regulation licenses contractors. HVAC work requires a contractor license with the HVAC specialty, and you can look up any company online.

County approval for the work, followed by an inspector checking the installation. Most HVAC replacements in Northern Virginia need one, and the rules vary by county.