Thursday, December 8, 2011

Heating in Residential Applications

Now that winter is upon us, we are getting the “How do I heat with a high ceiling?” questions. High ceilings in residences, especially here in Texas, are often the rule, not the exception. Heating is always a problem, especially from a 24 ft ceiling often found in Texas houses. Hot air rises and cold air falls, and with fixed air outlets and high ceilings this causes conflicts. In commercial spaces, I often recommend strategies not typically available in residences. These include using a VAV box to shut off interior located diffusers thus increasing the air flow from perimeter outlets, in heating.

Generally, residential diffusers are fixed deflection grilles. The use of an adjustable curved blade diffuser, such as the Krueger 180 (http://www.krueger-hvac.com/ecatalog/model.aspx?refid=852), is a much better solution, and should always be adjusted to direct the air horizontally, along the ceiling. Unfortunately, occupants are seldom aware of this, and often set the blades to blow supply air down into the space. Assuming the outlet is 24’ from the floor, this isn’t likely a problem if it is set up properly. Horizontally supplied air will have a much higher mixing ratio, result in more uniform room air temperatures, and will avoid drafts in summer. With high discharge temperatures in heating, however, there will likely still be stratification.

The most practical solution in residences seems to be the use of ceiling fans. While it depends on the room configuration, setting the fan to blow up in heating, while it would seem to be counter-productive, usually causes air to flow down the walls, and takes the warm air at the ceiling with it, and minimizes air motion in the room, which is usually objectionable when in heating mode. Chairs, of course, are best placed a short distance away from the walls. If a variable speed fan is employed in the central system, discharge temperatures in heating should be kept as low as possible, by running the fan at a high speed when heating from the ceiling, to minimize stratification.

In cooling, the ceiling fan is set to blow down, increasing room air motion (at least away from the walls) and this is often desirable, and may allow a slightly higher thermostat setting in the summer.

Authored by: Dan Int-Hout, Chief Engineer Krueger