It was a brisk February morning and Johnny Tundra, the engineer from Big Timber, was on the road to Angela, Mont. The wind was kicking up a bit across the prairie, sweeping the snow across the highway in blankets of undulating white. The sun was bright enough to blind, and inside Johnny's International Harvester Scout station wagon it was easy to forget the outside temperature, failing to make it above zero.

Johnny was on his way to see his old friend Thatcher McGowan, the facilities manager at the Stony End Home for the aged. Thatcher was not a happy camper. Night time lows at Stony End were dropping to the seasonal norms of about -5 F to 15 F below zero, and there were four rooms that refused to heat at this low temperature.

"Why me?" Thatch's voice boomed out of Johnny's phone, just as the lanky troubleshooter was ready to install his new dryer vent as part of a perpetual remodel of his Big Timber bungalow. Johnny laid the dryer vent on the kitchen table.

"That you, Thatch?" Johnny asked, as the dryer vent rolled off the kitchen table and dropped onto the sleeping carcass of Gas Train, Johnny's aged basset hound. The dog responded with a howl of indignation and rolled over onto the copper tubing at his side.

"Johnny, do you remember when you were here last month and you fixed that balky steam trap--the one with the over-extended bellows that wouldn't drain?" Thatch asked. "Well, that same wing had a new air handling unit installed last summer, and it just won't heat. Everything was okay at first, but as the temperature has gotten colder, it's been harder and harder to get heating out of the AHU."

"I've checked everything," Thatch continued. "The heating water line, the strainer, the control valve, the coil, even the ductwork, and everything seems to be working fine. But the air coming out of the registers in the rooms is not warm enough to make the grade. We've had to move the residents out until the problem is fixed and the super is hoppin' mad. All I can figure is that there's a leak in the ductwork.

"Since the duct is cast into the concrete foundation, it's going be a disruptive job to dig it up, find the break, and fix it. But I don't think there's any other solution."

"Digging up that ductwork is going to cost you a fortune in downtime and labor," Johnny said. "Give me a few minutes to put on my Carhartts and warm up the Scout, and Gas Train and I will come over and sniff around a bit. We'll figure out how to get some heat into those rooms."

A little over two hours later, Johnny cruised into the back lot of the Stony End home. Thatch saw him coming and ran to the Scout so fast that Johnny had to hit the brakes hard enough to dump Gas Train onto the front floor, jowls first.

"This is bad, Johnny," Thatch replied. "The super called up his contractor pal in Wounded Bear, and he's on his way with jackhammers. If I don't figure out something, they're going to rip up the floor and pay for it with my maintenance budget. If that happens, I'm sunk."

"OK, then," Johnny said. "We'd better beat feet to those ice boxes of yours right away."

Thatch led Johnny down the main hall of the new wing and into the first of the four problem dorm rooms. The wall thermostat read 65 F, about 10 F below the normal temperature. The supply air felt cool, and Johnny's thermometer showed the supply-air temperature to be about 60 F--much too cool to heat the space to anywhere near the required 75 F, even with the fin-tube perimeter convectors heating at their full capacity. The other three rooms in the group showed similar conditions.

"The crazy thing is, these are only four of the eight rooms on the same AHU," Thatch said. "The supply duct comes out of the AHU and splits into two branches. One branch heats these four rooms to the north; the other branch heats the four to the south. The four to the south are heating fine. In fact, their perimeter heat isn't on at all. These four to the north are freezing!"

"Maybe we better go take a look at those warm rooms before we eyeball that AHU," Johnny replied. "Lead the way."

The two walked down the corridor and into the first of the four south rooms. The room was as cozy as could be. Johnny reached down and felt the fin-tube convector and then the air coming out of the supply register. The supply air was hot and the fin-tube was off.

"Now, let's go take a look at that AHU," Johnny said.

Once in the machine room, Johnny gave the unit a quick once-over. The AHU was laid out in a north-south orientation, close to the west wall of the machine room. The outside air came in through the west wall and was ducted directly to the far side of the mixing box. The return air came down from overhead and was connected to the near side of the box. The mixed air passed through the full-width heating coil and into the double-width double inlet (DWDI) 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," Thatch said. "Just look," as he pointed to the gauges on the local control panel. "Coil discharge temperature is right on at 70 F. 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 saddle-bags that'll be!"

Meanwhile, Johnny had 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 then taped his dial thermometer to the end of the yardstick and held the stick inside the AHU.

"Hey look, we don't have time for any games," Thatch said. "You better stand aside because we have to tear up this floor."

"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. 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. 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 only heated to 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. Surprising as it sounds, fans are not air mixers. If the air goes in stratified, it may well come 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. Replace those opposed blade dampers with parallel blade types and mount them to direct the airflow upstream from the coil. By directing the airflow 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. In fact, I bet I can even drum up an elk bone for that beautiful canine of yours, too!"