“Scorched Air” Furnaces—They don’t have to be that way


You may have heard it before—“I heat with scorched air.”  Forced air heating has a bad name.  From the (purported) dryness it creates and the blasts of hot air and temperature swings to the noisy fan motor and banging in the ductwork.  The reality is that is what many people experience with forced air heat. 


But it doesn’t have to be that way.  The right variable speed furnace and good insulation, air-sealing, and duct-sealing can virtually get rid of these issues. 


Skip ahead it you want the solution.  I am going to walk through some of the problems and their causes, first.


First is sizing, and the are two parts to this.  Your furnace and system needs to be designed to keep you warm on the coldest day of the year.  (Geek speak:  we actually target a size that will hold your thermostat set point 98% of the time—but without getting too technical, you should be good 100% of the time.)  Older furnaces and even many new furnaces sold today are “single speed”.  This means they are pumping out enough heat to keep you warm on the coldest night every time they turn on. 


Think about that in terms of driving through town in your car.  What if every time you stepped on the gas, you had to floor it to get to 80 mph.  No feathering allowed.  No easing off for different situations like a short block, pulling into a parking space, etc.  You either had the pedal to the metal or you had the brake on.  The ride would no doubt be thrilling!  But you’d probably be overshooting stop signs quite a bit.  Wearing out break pads.  (And probably getting a lot of tickets.)  That’s what’s going on with a single stage furnace.  If you need a few BTUs (read: a very small unit of heat) or 100,000 BTUs, the furnace would churn out the same thing.  This means that on all but the coldest of days, the furnace is cranking out more heat than you need.  And like the car racing to a stop sign, it doesn’t stop exactly where you want it. It overshoots the thermostat setting, and you get too warm.  Then the temperature cools back down, and you do it again.


This can be fixed with multi-stage, variable speed furnaces.  These furnaces ratchet down the output when less heat is needed, just as you might not hit stomp on the gas if a stop sign was only 15 yards away.  When coupled with an efficient (ECM) fan motor, these furnaces can also have a big impact on the electricity you use in the winter.  As an added bonus, these motors, especially at low speeds, tend to be much quieter that the monsters going full blast all of the time.


Dryness is another factor.  In simple terms (that the physicists won’t like because I’m over simplifying a bit), how much moisture the air can hold is a function of the  temperature.  The warmer the air, the more moisture it can hold.  And for a give volume of air and water in the air, as the temperature goes up, the “relative humidity” or “RH” goes down.  So a temperature swinging up and down can give you an RH that swings up and down, increasing a sense of dryness.


That’s not the big cause of dryness in your home in the winter, though.  Air leaks are.  As air leaks out of your house (see my previous post on insulating)

the moisture in the air escapes with it.  And it is replaced by cold and very dry air from the outside.  This goes on 24/7 during the winter, the leakier the house, the worse the problem.  Then, add to this the fact that many ducts which move your heated air (and cooled air in the summer) are also very leaky and dump a lot of that air directly outside.  Just like leaks in the rest of your house, the more that goes out, the more cold dry air gets sucked in.  This is really what dries out your house.


You actually generate, by breathing, perspiring, showering, cooking, watering plants, etc, more moisture than you need in your home.  Most people don’t need humidifiers.  In fact humidifiers, especially if not rigorously maintained, can actually create problems.  If you seal up the major leaks, you’ll not only reduce drafts and be more comfortable and save on your energy bills, but you also won’t have to deal with super-dry air in your home.  You’ll want to provide adequate ventilation to provide some outdoor air and make sure you don’t have too much moisture, but it’s better to be able to control this rather than letting the wind and temperature decide for you.


The bottom line, whatever type of system you use, forced air, water, or even wood, isn’t as important as how your whole house is operating and how your heating system is integrated into the house.  For maximum comfort and efficiency with a forced air system, remember this:


  • Make sure your house is well-sealed and well-insulated.  A good contractor can verify this for you using a blower door and infrared camera.
  • Make sure your ducts are also well sealed.
  • Used an efficient (95% or better AFUE)–multi-stage furnace with a variable speed ECM motor.  Have your furnace professionally maintained at least once per year.  (See Patrick Herbert give an intro to furnaces.)
  • Make sure your house has adequate ventilation and use bath and kitchen exhaust fans to remove excess moisture at the source.

In a well-built or well-improved house, forced air can be a great heating system.  (Hot water systems can be great, too—but you still want to insulate and air-seal properly!)



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3 Responses to ““Scorched Air” Furnaces—They don’t have to be that way”

  1. Air vent Says:

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  2. Barney Pardieck Says:

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  3. Furnace Efficiency—more than just the label « GreenHomes America Says:

    […] lost up the chimney, through the cover of the equipment, and as the unit cycles on and off.  (See “Scorched Air” and […]

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