Jim Wheeler: Indoor air movement
The sad thing about having worked in our industry for about 50 years is that I’ve had to watch many of the great HVAC engineering advancements fall by the wayside and give way to less-comfortable designs, especially in the residential heating/cooling field because of either declining design skills or to make way for new technology.
And this problem has especially manifested itself in the area of designing for comfortable indoor air movement.
The problem with air motion is you can’t see it happening so most people don’t think it’s anything to worry about. Have you ever had to stay in a motel that was heated and cooled by a through-the-wall unit that only brings on the indoor fan when there is a call for heating or cooling? I have many times and I can tell you that first the air gets very uncomfortable (because the unit thermostat is over there next to the window) and then everything turns on with a loud bump, whoosh and a hum (which wakes me up).
Of course, most motel owners aren’t worried as much about my comfort as they are about saving electricity so they ask for the units to be set to not allow continuous airflow during heating and cooling operation (all through-the-walls can be set to provide constant air flow in heating and cooling cycles).
The fact is on a properly-designed system, it always is more comfortable to keep the indoor fan running. Yet, in most residential applications the fan is set to cycle in order to save energy. But then the consumers buy ceiling fans that continuously run and use the same energy to get comfortable again.
However, now we have new variable-fan-speed air-handler technology, which may keep some air moving all the time. This is a great idea because all the air in the building is being mixed and drawn past the thermostat and results in more even temperatures in all rooms.
However, when a variable-speed fan is used in connection with a variable-speed compressor in the cooling mode, there often is not enough pressure inside the ducts and at the diffusers to provide sufficient air velocity to throw and diffuse the chilled air throughout the room and the air just drops instead of circulating. This too is uncomfortable.
Understand that with a proper diffuser selection and location, the discharge air is set to be evenly and fully distributed throughout each room so there are no “drafty” or “dead-air” locations. Unfortunately, even in many commercial applications, this isn’t how things usually work out because, as everyone knows, you can’t see air so proper air flow isn’t that important. And variable-speed cooling operations are making this problem even more difficult to solve.
Here in the south we use a lot of heat pumps and as common knowledge has it, heat pumps are drafty and uncomfortable… right? Yes, it is true heat pumps blow harder in the heating mode than do gas furnaces, so there is more air movement, thus the air is cooler and it doesn’t feel as comfortable.
Yet, though much of the problem of drafty heat-pump operation could be solved by better diffuser selection and location, most of the down-and-dirty low-ball companies in this area can’t “see” they are creating a problem. And, of course, now more total variable-speed systems are being sold that can exacerbate the challenge of creating comfortable air-conditioning.
This article was originally titled “Indoor air movement” in the May 2018 print edition of Supply House Times.