Riding along the snow-covered road on the way to Rock Springs, Mont., was just as peaceful as could be. The road was in poor repair, but the snow and ice had filled in most of the potholes to where the ride was smooth enough. Winter sub-zero temperatures had made way for more moderate spring weather, at least slightly above freezing. Johnny Tundra's trusted basset hound, Gas Train, was riding next to him in the cab, asleep as usual. A bump in the road raised the hound to semi-consciousness and caused a general relocation of his capacious jowls.

"No need to get excited," Johnny said.

Their destination was the Rosebud County Vo-Tech, a sprawling facility of a dozen buildings or so, totaling a little over 80,000 sq ft, set in the middle of the eastern Montana prairie. Being dry as a bone and clear most of the time, temperatures dropped rapidly at night, sometimes 30 degrees or more. So, the traveler always wanted to be prepared.

The Vo-Tech was completing a new addition to the original Library-Classroom (LC) building to house more computer training rooms. The LC building was built in the early 1900s, and a 1988 remodel had converted the steam heating system to hot water. Last year, Johnny had stopped in to eyeball the planned expansion and solved a rooftop air-handling unit (AHU) freeze 'stat problem for his old pal, Jonas "Wooly" Wolcott, so-nicknamed in honor of his national-forest-sized beard.

Johnny's solution to the freeze 'stat problem was a key factor in Seltzer Architects' selection as the lead design firm and, of course, Seltzer's selection of Johnny as the mechanical engineer. The owner, Jerry Seltzer, had known Johnny since college and had a long-standing practice of cooperation through constructive disagreement.

Earlier in the day when Johnny was about to connect his dryer vent as part of his perpetual home remodel at his Big Timber bungalow, his phone rang. On the other end was Jerry Selzer and he was frothing at the mouth.

"This mechanical system is supposedly finished, and the contractor says he can't get the chiller to run, no matter how hard he tries," Selzer said. "The thing keeps tripping off-line with some readout like 'high head something or other.' But there's no way the chiller can have any load on it when the temperature outside is 15°. We have to run all the boilers just to heat the building to 60 degrees to dry out the sheet rock. I'm about ready to disconnect this piece-of-junk chiller and get one in here that works!"

Johnny told Jerry to settle down before he ruptured his relief and he'd be right over and eyeball the situation. Johnny took the Rock Springs exit and slid the trusty Scout into the facilities services parking lot in the back of the maintenance shop. He and Gas Train dismounted and went inside.

"Anybody home?" he called out upon entering the cramped shop area, piled to the ceiling with racks and shelves of boxes and equipment.

"Yeah, back here," Jonas replied, from the BAS operator workstation at the rear of the plant. "That chiller tripped again just a few minutes ago, and I'm checking out the diagnostics on the workstation. The readout says 'high head pressure,' but that's an error. There's no thermal load on the chiller and without a load there can't be high head pressure."

Wooly went on, "I locked all the AHU cooling coil valves closed through a global command at the BAS. Even if they were open, there's no load in the building at this temperature.

"Your pal Selzer is about ready to fire the mechanical contractor because he can't get the chiller going, and we need it to cool those computer rooms. No chiller, no computer rooms, and no summer session equals a giant lawsuit!"

"Well, I reckon that's why we have engineers on these projects," Johnny said. "Let's go take a look at that reefer of yours first-hand and maybe we can figure something out."

The pair left the maintenance area, headed for the new addition, and quickly came to the boiler/mechanical room that led to the chiller vault. The existing mechanical room had been expanded to accommodate the additional boiler and equipment for the new addition. The new boilers were running full-out to heat the semi-completed addition, and the uninsulated piping and equipment made the mechanical room too hot for comfort. The wall thermometer was maxed out at 100°F.

Johnny noted the big unit heater, wires dangling down, and thought, "Don't want to forget to hook that up."

The chiller room was cool and comfortable. The mechanical contractor was hunched over the chiller control panel and was talking with Jerry the architect, mostly in words that were not in Webster's.

"Hi Jerry, how's it going?" Johnny asked. "Terrible! This controller is busted," Jerry said. "The chiller is locked out every time we try to start it. Says 'high head pressure,' but there's no load. They sent us a defective controller, and I am going to sue the pants off 'em for it."

Johnny gave the chiller a fast once-over. The unit was piped and wired, and the installation was complete right down to the insulation. He saw the chilled water supply and return lines disappear into the pipe chase and two branch pipes take off into the machine room.

"This installation looks fine to me," Johnny said to the foreman, "except for the thermometers on the chilled water lines. Where are they?"

"They were value-engineered out of the project and the test fittings haven't been installed yet," Jerry said. "No big deal. We can't do final balancing until we get this chiller started anyway. And, it looks like that isn't going to happen for awhile. I guess I may as well get started unhooking this controller and get a replacement on the way as soon as possible."

Johnny bent down over the chilled water circulation pumps and put his hand on the pump casings. Then he glanced again at the chilled water branch piping to the machine room. A smile formed in the creases of his weathered mug.

"Put on those hobbles, Jerry," Johnny said. "Your problem isn't in the controller; it's in the machine room."

"Johnny, you have really lost it this time," Wooly said. "What in the world are you talking about?"

"Just take a minute and put one of those test ports on the chilled water return line," Johnny replied. "We should have had thermometers all the time, but those test ports will work to demonstrate why the chiller won't start."

Ten minutes later a portable temperature indicator was in the chilled water return line and the three watched the indicator climb from 60°F to 70°F and finally over 90°F.

"There's no way that thermometer can be right," Jerry exclaimed, "There isn't a cooling load in this building that's 90°."

"Just the opposite," Johnny replied. "The only load in this building is quite a bit warmer than that. The machine room!

"You were right, Jerry, when you said that there was no load in the AHUs because it was too cold and the valves were all locked closed. But you forgot the one load in the building that wasn't an AHU. As you may recall, we were concerned about overheating the machine room because the heat could damage the air-cooled compressor. So, we put in a unit heater and piped it into the chilled water circuit to cool the room, if need be.

"That unit heater was shipped complete with its own solenoid valve to control water flow. Because the unit was intended for heating, the valve on the unit fails open on a loss of power. The heater isn't wired yet, but it's piped. So, the coil has full flow in that hot machine room. It made no difference that you locked out the cooling because that unit isn't connected to the BAS.

"So, with the circ pumps on and all the AHU valves closed, the maximum possible flow was being run through the unit heater in the hot machine room. That made the return chilled water too hot for the chiller to handle and it tripped out on high head pressure before it could cool the loop.

"If you close the isolation valves to the unit heater and stop that branch's flow, your chilled water temperature will cool down to the point where you can start the chiller. Once the loop is cool and the chiller is running, you can connect whatever loads you want."

"Johnny, I hate to say it," Jerry said, "but it looks like another round for the engineers on this chiller mystery. And value engineering or no, I am going to get a change order in right away to put those permanent instruments back into the chilled water lines."