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Furnace

In Minnesota, furnaces are typically found in your basement or utility closet. Some are found in attic areas. They act as the blower or fan for both the heating and the air conditioning systems. Furnaces are also the electrical control center for your HVAC system.

When your furnace turns on in the winter, it ignites the natural gas through burners that heat up your heat exchanger (this would be like putting a coffee can over your stove burner - the fire's contained, but the can gets very hot. If you blew a small fan over the can, the air would be quite warm). When the blower blows air across that heat exchanger, the warm air is then circulated through your duct system and into your living area. The fumes from the burned gas are expelled through a flue system and the whole operation is monitored by a number of electrical and mechanical checks to make sure you're safe.

In the summer, the furnace is used as the blower and control center that circulates air through your evaporator coil and cools your home.

Defining 80% & 92% Efficient

There are two basic classes of furnaces: 80% efficient and 92+% efficient. When a furnace is 80% efficient it means that 80% of the heat energy is captured by your duct system when the gas is ignited to create heat. The remaining 20% escapes up the flue or emanates out the face of the furnace. I bet you now know what 92+% efficient means.

Old furnaces were anywhere from 50 - 70% efficient. If your old furnace was sized correctly to your home's heat load, then old vs. new would be something like this:

Old furnace:

100,000 BTU @ 60% efficient = 60,000 BTU of heat

New Furnace:

80,000 BTU @ 80% efficient = 64,000 BTU of heat

A new 80% furnace should be smaller than your old one. So, what is heat load?

Your house determines what size and efficiency your furnace should be. The elements that make up the shell of your home (walls, siding, windows, doors, shingles, etc.) all have insulation ratings. Using these insulation ratings, the square footage of your home and the number of windows and doors you can figure out how many BTU's it would take to keep your home continuously heated to 75F inside if it was -20F outside. The resulting number is your home's heat load.

Let's say that your homes heat load is 61,500 BTU. An 80,000/80% efficiency furnace provides 64,000 BTU. That is a good fit.

A 100,000/80% efficiency furnace provides 80,000 BTU. That would be overkill. You home will never reach optimal comfort because the heat will rise faster than it takes to cycle all of the cold air through the furnace. Your furnace would short cycle. That is bad.

The optimal choice will always be the model with the closest output that equals or just exceeds your heat load need.

Duct sizes are also crucial to furnace performance. They must correspond to the amount of air that the blower will force through the system.

So, you've had your heat load determined and now you need to choose a corresponding size and efficiency. Here is a difference between 80 and 92+%. An 80% efficient furnace can make use of the existing metal flue. A 92+% must be PVC vented with limitation on flue distances. Over the long haul, a 92+% efficient furnace will save you money on your utilities, but it might take 5 to 10 years. In either case, one size does not fit all. Seek trusted advice.

So we have two classes (80% and 92+%). There are three types of furnaces: Single-Stage, Two-Stage, and Two-Stage Variable. Unscrupulous contractors sometime sell the mythical Multi-Speed furnace - more on that later.

Single-Stage Furnaces

Your furnace and fan is either on or off. Pretty simple, huh? A salesperson can say that a single-stage furnace has multiple speeds. Why? Every home has a unique heat load that is supposed to be calculated for you by your heating and a/c contractor. HVAC manufacturers wisely put options in the furnace fan speed. If the heat load requires significantly less than the amount of heat that the furnace would provide - the furnace fan speed should be hard wired to its medium high option. The single-stage furnace does have optional speeds, but it is hard wired to one.

Two-Stage Furnaces

Two-stage furnaces were developed with comfort in mind. When the thermostat activates the furnace, it comes on at two-thirds strength (burning gas at 65% of maximum). If, after 10 minutes of operation, the thermostat is still calling for heat, the furnace will switch to 100%. This will heat all of the cool air in your house. If a furnace kicks on at full power and dumps hot air into the home to satisfy the thermostat, it will shut off leaving cool air still circulating through the home. This means the furnace will have to kick in again and again.

Two-stage furnaces are more efficient and more effective at heating your home. Furnaces are like light bulbs, they operate better if they are turned on and left on. If you flick a light switch on and off, over and over, that bulb is toast. The same goes for furnaces. Running at 65% for 9 minutes is better than 100% for 3 minutes several times an hour. It uses less gas and is easier on the machine.

For those that like cooking, this analogy might help. Your home is like a large oven. You can heat it with broil or bake. Broiling will heat faster - but tends to leave things undone at the center. Baking takes longer but provides much more even heating. In northern temperate zones, a minimum of a 2-stage furnace is recommended.

Two-Stage Variable Furnaces

Two-stage variable furnaces were developed with the knowledge that subtle circulation of heated air is the most effective way to heat a home. Not all manufacturers offer a true two-stage variable furnace. The furnace part is the same as two-stage furnaces; the difference is in the blower motor.

Multi-Speed Furnaces

A sly contractor will sometimes offer the mythical multi-speed furnace as an alternative to a much more expensive variable speed furnace. The multi-speed furnace has a regular electrical fan motor that is capable of a range of speeds (hence, the multi-speed), but must be hard-wired to one setting. Don't fall for that one.

In single-stage furnaces, if you want your fan on for circulation - the fan is always on high. It is the only option. Not only is that hard on the utility bill, but a fan on high means the warm air is moving fast - warm air moving fast creates a cool breeze. In variable furnaces, the fan turns over slowly to maintain air circulation while being easy on the utility bill. By the way, most air cleaners and filtration devices are only active when the furnace blower is on. So the best way to utilize these accessories is to have a variable speed blower. Switching its blower fan from a/c current to d/c current only draws 1/10 the electricity.

Total Comfort offers furnace installation, repairs and service for Minneapolis/St. Paul, Minnesota area residents. If you have additional questions about furnace maintenance or repair, please contact us.