Learn · HVAC

HVAC, in plain English.

Furnaces, heat pumps, AC, ductwork and keeping the house comfortable for less.

The lingo (18)Guides and reading (9)Articles (14)From the Pros (0)Questions (0)Ask the Pros

The lingo.

AFUE
Annual Fuel Utilization Efficiency, the percent of fuel a furnace turns into heat. An 80 percent furnace vents through a metal flue; a 95 percent condensing furnace vents in PVC and drains condensate.
Air handler
The indoor unit that moves air in a system without a furnace, typically paired with a heat pump. Contains the blower and evaporator coil.
BTU
British thermal unit, the energy to raise a pound of water one degree. Furnace output and AC capacity are both rated in BTUs per hour.
Condenser
The outdoor unit of a central AC or heat pump: compressor, coil and fan. It rejects the heat pulled from inside the house.
Evaporator coil
The indoor coil, usually sitting on top of the furnace, where refrigerant absorbs heat from the air. A dirty or iced coil is a common cause of weak cooling.
Heat pump
An air conditioner that can run in reverse to heat the house. Modern cold-climate heat pumps hold full capacity well below freezing; a backup heat source covers the coldest days.
HSPF2
Heating Seasonal Performance Factor, the heating efficiency rating of a heat pump. Higher is better.
HVAC
Heating, ventilation and air conditioning: the furnace or heat pump, the air conditioner, the ducts, the thermostat and everything that moves and conditions the air in the house.
Manual J
The industry load calculation that sizes heating and cooling equipment from the house's actual insulation, windows, orientation and climate. A Pro who sizes by square footage alone is guessing.
MERV
The filter efficiency rating, 1 to 16 for residential. MERV 8 to 11 balances filtration and airflow; a very high MERV filter in a system not designed for it starves the blower.
Mini-split
A ductless heat pump with a small outdoor unit feeding one or more wall or ceiling units indoors. Efficient, quiet and ideal for additions, garages and houses without ducts.
Refrigerant
The fluid that carries heat between the indoor and outdoor coils. R-410A is being phased out for lower-warming refrigerants such as R-454B and R-32; a system low on refrigerant has a leak, not a thirst.
SEER2
Seasonal Energy Efficiency Ratio, the efficiency rating on a central air conditioner or heat pump under the current federal test. Higher is more efficient; 14 to 15 is minimum, 18 and up is high efficiency.
Short cycling
Equipment that turns on and off every few minutes instead of running full cycles. Causes include oversizing, a bad thermostat, a clogged filter or a failing component.
Static pressure
The resistance the blower works against in the ductwork. High static pressure means undersized or restricted ducts, and it shortens equipment life.
Ton
A measure of cooling capacity, not weight. One ton removes 12,000 BTUs of heat per hour. A typical house needs one ton per 400 to 700 square feet depending on climate and insulation.
Variable speed
Equipment whose compressor or blower runs at many speeds instead of just on or off, holding steadier temperatures and humidity with less energy.
Zoning
Dampers and multiple thermostats that let one system heat or cool parts of the house independently.

Guides.

Reading list.

Articles from around the internet that our Pros trust. They open on the source's site.

  • Central air conditioning · U.S. Department of Energy

    How central AC works, sizing, efficiency ratings and when to replace.

  • Furnaces and boilers · U.S. Department of Energy

    Efficiency ratings, retrofits and replacement of gas and oil heating.

  • Heat pump systems · U.S. Department of Energy

    Air-source, ductless and geothermal heat pumps, and what to expect in cold climates.

  • Indoor air quality · U.S. EPA

    Ventilation, filtration, humidity and the sources of indoor air problems.

Articles from HomeMaxx.

All articles
Seasons

The fall checklist: twelve things to do before the first freeze

An afternoon of small jobs in October prevents most of the emergency calls Pros get in January. Here is the list, in the order to do it.

2 min readSafety

Dryer vents, extension cords and space heaters: the three fire starters in most homes

Most house fires are not electrical faults inside the walls. They start with three ordinary things that get used wrong. Here is what wrong looks like.

3 min readMaintenance

Humidity, mold and the musty basement

Mold does not need a leak. It needs air over 60 percent humidity and a cool surface, and every basement in summer has both. What the number should be, how to get it there and when a musty smell is a bigger problem.

3 min readSafety

Wildfire smoke and the house: keeping the air inside clean

Smoke from a fire hundreds of miles away now reaches most of the country some summer weeks. How to seal up, filter and clear the air inside, and what the numbers on a filter mean.

3 min readSeasons

The summer checklist: keeping the house cool, dry and standing

Summer is hard on a house in quieter ways than winter: heat in the attic, water in the basement air, sun on the paint, and the storms. What to do in June so August is uneventful.

2 min readMaintenance

Getting the air conditioner through a heat wave

An air conditioner is designed to hold the house about twenty degrees below the outside temperature, and on a 105-degree day it is running flat out. What to do to help it, what not to do, and the signs it is about to quit.

3 min readSafety

Summer power outages: the generator, the food and the sump

Summer storms take the power out more often than winter ones now, and the two things that go wrong are a generator run in the wrong place and a basement that floods while the pump sits idle. The rules for both.

3 min readSeasons

Heat in the attic: why the upstairs is ten degrees hotter

An attic in July can hit 140 degrees, and everything under it feels it. Ventilation, insulation, the ducts that run through it, and the fixes in the order they pay.

3 min readSeasons

The spring checklist: finding out what winter did to the house

Freeze and thaw, snow load, ice dams and four months of heating leave marks. A spring walk-around finds them while they are small. Here is the list, outside then in.

3 min readMaintenance

What a real air conditioning tune-up includes

A spring tune-up is worth it if it is a tune-up. Here is what a Pro should do in the hour, what you can do yourself, and how to tell a maintenance visit from a sales call.

2 min readImprovement

Heat pump or new furnace? How to decide

The old answer was that heat pumps are for the South. The equipment changed, the incentives changed, and the answer now depends on your fuel, your climate and your ductwork.

3 min readSafety

Carbon monoxide: the alarm every home needs, and where it goes

You cannot see it or smell it, and it comes from the furnace, the water heater, the car and the generator. What an alarm costs, where to mount it and what to do when it sounds.

2 min readSeasons

The winter checklist: getting a house through the cold months

Fall was for preparing. Winter is for watching. The habits that get a house through to March without a burst pipe, an ice dam or a furnace call at midnight.

2 min readSeasons

Ice dams: why the ceiling leaks in February, and what actually fixes it

The icicles are the symptom. The cause is warm air in the attic, and the fix is insulation and ventilation, not a new roof. How ice dams form, how to stop one, and how to stop the next.

3 min read

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