I recently had a contractor build a wall which split one room into two. The new room is an office with two computers, and four monitors. It gets very, very hot, until the air conditioning turns on, then it gets very, very cold.

My contractor does not seem to know that much about sizing ducts.

There are two ways we can run a duct to that room.

Option A - The most direct way would be to run a four inch duct (the only size that will fit in that route) approx 20 feet with four right angles.

Option B - The other option is to run a duct (up to 12 inches) 60 feet around the basement up through the house and through the attic, about 60 feet with three right turns.

I think that Option B is the way to go, he seemed to think that a four inch duct would be fine for a small room (about 90 square feet).

Is there any way that a four inch duct would actually do anything over that distance with that many angles? Neither one of us has any ideas on how to size ducts (but at least I am aware of my ignorance)

  • Why doesn't your contractor contact an HVAC subcontractor? why are you telling your contractor how to do his job? sounds like you need a new contractor.
    – Tester101
    Jun 5, 2012 at 10:51
  • His prices are great, and so is the quality of his work, and he's done great work for all of my extended family, so looking elsewhere isn't much of an option. Jun 5, 2012 at 14:55
  • a 4-inch duct is quite small... I hesitate to use 4 inch ducts even for bathroom exhaust fans. Consider that a 12-inch round duct is NINE times the capacity of a 4-inch round duct. Often you'll have problems with people improperly combining duct sizes, too. For example, I have a room which calls for an 18" round and the HVAC "professional" told me that two 12" rounds were way more than a single 18" round....
    – Matthew
    Aug 7, 2012 at 21:45
  • Are the return and supply both in the same room?
    – Matthew
    Aug 7, 2012 at 21:52
  • They would be in the same room, correct. The supply would be in the floor, and the return would be in the ceiling, ideally. Aug 8, 2012 at 22:34

1 Answer 1


It is difficult to properly cool two spaces with vastly different heat loads and only one thermostat.

Assuming typical computers and monitors and two users, you probably have about a kilowatt of heating or 3400 BTUs per hour.

Rather than run ducts, have you considered a dedicated cooling unit for your office? You can go with a small window unit. Even the smallest one is 5000 BTU and only uses about 500 watts when running. That's just a few pennies per hour and even that is offset by the reduction in load on the main unit. With the thermostat in the same room, its load response will be much better and the temperature extremes you have been suffering with will be eliminated. Plus you can turn it off when no one is there and save energy that way.

Lately my thinking is that ducts are basically parasites and efficiency goes up when the heater/cooler is in the same room as the load. In the past, the equipment cost was lower with a shared unit but operating costs (energy) have gone up a lot (and will continue to go up) and that could change the equation on selecting shared or dedicated equipment. Comfort will always be superior with dedicated equipment.

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