Forced air heat register location
bradrh
| Posted in Mechanicals on
My heating contractor wants to put the heat registers along the outside wall, near windows, the traditional way of doing things. My thought was to minimize ducts and the associated losses and keep them centrally located, with at least one in each room. With all the talk here of using centrally located minisplits and that working out OK, I don’t see why I need heat regs along the outside wall. I can see it being a little better, but not necessary in a house with pretty good windows, air tightness, and insulation. You don’t need passive house levels to pull this off do you?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
You certainly need a proper design and an efficient envelope. I ended up with most of my ducts on exterior walls since that typically the best location to ensure they wouldn't be blocked.
Heating registers in the floor directly in front of windows counteracts the convection draft of cool air falling down the face of the window when it's cold outside. That made a lot of comfort-sense with U-1.0 single-panes, (despite raising the heat load by putting the warmest air next to the lossiest surface), but with better-than code U0.25 or lower windows it matters a lot less.
When putting the registers somewhere else it can be important for comfort and more uniform temperatures to either ceiling-mount the registers using a louvered grill that directs the air toward the exterior wall or window, or wall mounted blowing toward the exterior wall/window, and with a narrowing register boot to give the air higher exit velocity. The further away from the exterior wall/window the register gets placed, the more important exit velocity or "throw" becomes.
See John Semmehack's pictures in his response to this thread:
https://www.greenbuildingadvisor.com/community/forum/mechanicals/93846/mitsubishi-minisplits-horizontally-ducted
Brad,
Lots of energy-conscious builders have had success with the approach you describe: locating registers on interior walls. Usually, these registers are located high on the wall, and the registers are chosen to have good "throw" characteristics.
This will only work if the builder pays close attention to airtightness (verified with a blower door). It's also a good idea to have above-code levels of insulation and windows that have a lower U-factor than the maximum U-factor permitted by code. (In cold climates, this often means specifying triple-glazed windows).
For more information on this issue, see All About Furnaces and Duct Systems.
Brad,
It's also worth thinking about the acoustic implications of short, centralized duct runs which can allow sound transmission between adjacent rooms.