Once in the machine room, Johnny gave the AHU a quick once-over. The unit was laid out in a north-south orientation, close to the west wall of the machine room. Outside air came in through the west wall and was ducted directly to the far side of the mixing box. Return air came from overhead and went to the near side of the box. Mixed air passed through a full-width heating coil and into a double-width-double-inlet fan plenum. At the AHU discharge, the supply duct split west and went down into the slab to the adjacent north zone. The east branch curved down into the slab and went under the AHU to the adjacent south zone. All of the ductwork was heavily insulated.
“The heating coil and control valve are working fine. Just look,” Thatch said, pointing to the gauges on the local control panel. “Coil discharge temperature is right on at 70 degrees. There has to be something wrong with that ductwork between here and the north wing. And we’re going to have to jackhammer up this concrete to figure out what it is. What a pain in the saddlebags that will be!”
As Thatch spoke, Johnny opened the mixing-box damper-access hatch and scanned the dampers with his flashlight. He then motioned to his buddy.
“Thatch, my boy, hand me that yardstick for a second.”
Johnny taped his dial thermometer to the end of the yardstick and held the stick inside the AHU.
“Hey, look, pardner,” Thatch said, exasperated. “We’ve got no time for reindeer games here. You better stand aside because we’ve got some floor to tear up.”
“Whoa there, cowboy,” Johnny said. “Your heating problem isn’t in that cast-in-place sheet metal. It’s in these mixing-box damper blades.
“You’re right in saying your heating coil is working fine,” Johnny explained. “It is working fine, on the average, and the average temperature is all you can read on the control panel because the sensor is an averaging sensor. It senses the temperature all across the coil.
“The problem is, the outside air entering the coil on the far side is a lot colder than the return air entering the coil from the top,” Johnny continued. “The mixing-box control dampers are opposed-blade dampers, which allow for the best control, but the opposed-blade dampers don’t throw the two air steams into each other to promote mixing. The air streams are drawn directly through the coils without much mixing. As a result, the air on the far side is heated to only about 60°F, while the air on the near side is heated to about 80°F.
“These two air streams are stratified as they enter the fan, and they stay stratified through the fan,” Johnny said. “Surprising as it sounds, fans are not air mixers. If the air goes in stratified, it may well come out that way. As it happens, the colder portion of the air stream exits on the west side of the discharge, and the warmer portion exits on the east. The east branch goes to the south zone, and the west branch goes to the north. Hence, your chilly north rooms and cozy south rooms.
“All you have to do is put the brakes on those concrete cutters and get some sheet-metal workers in here instead,” Johnny concluded. “Replace those opposed-blade dampers with parallel-blade types, and mount them to direct the airflows upstream from the coil. By directing the airflows upstream and directly into each other, enough turbulence will be caused to ensure mixing and bring an end to this nasty stratification.”
“Johnny, you’ve done it again!” Thatch exclaimed. “C’mon down to the shop for a cup of jake.”