Part 2 of: The "1-2 Punch" to HVAC "Not Understoods"

Proper sizing of HVAC equipment is smaller, and smaller means more comfort.

Probably the usual thought in the HVAC industry by technicians is that the units need to be made bigger for comfort. Perhaps this comes from the idea of coming home from work, and needing a set back thermostat to do its magic, and get the home comfortable fast, before dinner.

What happens when we add too many Btuh to a space too fast? The air gets heated or cooled fast, and then the thermostat detects the rapid change in the air temperature and shuts off the system. After all it made the grade and the air is a good temperature. So all is well. Correct?

What is missed is this: The "average radiant temperature" of the house and furniture has not been changed correspondingly with the rapid change in air temperature. The radiant temperature of the house and furniture has a lot more to do with the perception of comfort than does the air temperature. 

If you remember "radiant heat - or cool" can travel instantly through space and air with little to no resistance. So even if the air is warm, the furniture will beam you with uncomfortable temperatures.

So with the big HVAC system, the air will rapidly warm to the thermostat set point, yes it is true. The house and furniture will then start to change temperature with the new air temperature changes, and then the HVAC system will start to cycle on and off - contributing to swings in temperature. Also the turning on and off of the system will be noticed, and distracting.

  • Like a big truck in traffic - the constant stop and go will use a lot of energy. It will be distracting.

Smaller HVAC is always better. Less Btuh per hour, and just enough, and maybe a little more, to overcome the losses in the shell, will provide maximum comfort. The house radiant temperature would be maintained at the set point of the thermostat. Because the system runs more constantly, the house radiant temperature is held at a steady state. Hot and cold rooms tend to vanish. Smaller systems are quiet and almost not perceptible when they are running. The consistency in internal thermal mass will help save energy, as will the cycling losses of the equipment. 

  • Like a small car on the highway - the constant speeds will use a lot less energy. It still gets you where you need to go. And if you don't notice your HVAC system, it is even more comfortable. Obviously it will get better fuel mileage, as well.

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Next up: How the 1, 2 punch work together to waste 50-80% of energy spent on HVAC costs. For more information please sign up at www.UpSmart.Tv and connect with the Super Efficient HVAC Series there.

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Comment by Christopher Cadwell on December 31, 2011 at 9:04am
If an AC contractor told me that it would not blow enough air, then my response would be to ask for the calculations to show me that there is more cfm needed. Of course they usually cant. So then ask them how they can make an assertion about something without calculations - then just direct them to my blog. Eventually I will have free courses for them to learn about it. Usually you can get about half of them interested, and the other half will just continue to defend themselves against things they do not understand.
Comment by Bob Mariani on December 28, 2011 at 11:55am
I work with inspectors in an attempt to educate. Works sometimes. Changing fan helps. Mostly I try to sell them on heat pumps
Comment by Bruce Navin on December 28, 2011 at 10:57am

So, Bob- How do you handle those objections?

Comment by Bob Mariani on December 28, 2011 at 7:59am

I have tried getting HVAC contractors to spec smaller units after deep energy retro fits.   But they claim the code still requires a certain heat load to be used.  Another response is the house can use a smaller Ton unit but the smaller unit will not blow enough air for the volume of the home.  Another issues is the sizes are no longer available in 1/2 ton.. so you still need to get a 4 ton unit when a 2 1/2 ton should work.

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