Forced Air from Ceiling
user-719615
| Posted in Mechanicals on
We are designing an energy efficient house, (1700SQFT), which wil be airtight and well insulated. The house is single story with no basement, (slab on grade), in order to make use of passive solar. The heating system is forced air so we will have to run all the ductwork above a drop ceiling, but within the house envelope. My question is will the house be just as comfortable if the supply air is blown down from the ceiling?
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Replies
Peter,
As long as you pay close attention to airtightness during construction — I recommend that you set an airtightness goal and conduct a blower-door test to verify your home's air leakage rate — and insulate your envelope well, including below the slab, then you shouldn't have any comfort problems due to ceiling registers.
Floor registers are an attempt to address the cold-floor problem. But the cold-floor problem is caused by air leaks. (When air leaves your house through ceiling cracks, outdoor air enters the house near the floor level.) If you can minimize air leakage, you shouldn't have a cold floor.
Remember, don't skimp on subslab insulation — and make sure it is continuous horizontal insulation, not just perimeter insulation.
Peter,
We’ve done the same system in the Southwest for many years with excellent results. Your envelope must be tightly seal, raise the top plate to 9’+ (it looks better) and use the hallways to furr-down 12” and place your supply runs there. Use ONLY hard ducts sealed w/ mastic. Usually a good distance to blow air in a room is in the 16’+ range; more than that, I use a coffer ceiling to get a supply run to “the other side” of the room.
Make sure your HVAC system is designed by an INDEPENDENT Mechanical Engineer and not the same HVAC Company you are buying stuff from (I know, they'll tell you they know how to do it right); you get better bids keeping everyone honest; and have your ducts tested and HVAC system and supply air balanced and tested.
Insulate the whole under-slab with 2”+ rigid insulation only (those radiant-bubble insulations do not work). Loosing heat through conduction is your goal to stop.
Peter,
Make sure the ceiling registers are oriented to move air down along exterior walls (and windows). If the warm air blows directly on your head, you will not be comfortable. Human comfort depends on warm feet and cool heads. Turbulent air has the perceived effect of doubling the actual velocity.
Indoor comfort factors include:
Vertical Air Temperature Differences from ankle to head should be less than 3°C (5.4°F).
Radiant Temperature Asymmetry from cold windows should be less than 10°C (18°F).
Radiant Temperature Asymmetry from warm ceilings should be less than 5°C (9°F).
Surface Temperature of floors should be between 19°C and 29°C (66°F and 84°F).
Relative Humidity should be between 30% and 70%.