Let me start on a Tuesday morning in January. I was standing in the north office at our plant, looking at a thermostat that said HEAT ON, while the room sat at 58°F. The set point was 72°F. I had two options: call a technician and hope the fix was small, or start troubleshooting it myself. I chose the second because I manage the maintenance budget and I thought I could avoid a service invoice.
I am not a technician. I am the procurement manager at a 45-person light manufacturing company. For the past six years—seven in August, actually—I have managed the service and maintenance budget and tracked every major HVAC invoice. That experience made me confident enough to think I could handle a heat pump problem on my own. The story that follows is proof that confidence is not the same as competence.
Why I thought this was a thermostat problem
Our building is hydronic. The main plant includes McQuay heat pumps that heat and chill water, and the indoor spaces use McQuay fan coil units to push conditioned air through the offices. When the north office thermostat calls for heat, a valve opens and a fan moves air across a hot water coil.
I knew the concept, but I did not know the difference between a problem in the heat pump plant and a problem in the fan coil unit. The thermostat was powering up and showing a setpoint, but the room was not getting warmer. I assumed the thermostat was bad. That was mistake number one.
The 'mcquay heat pump manual' did not save me
The old thermostat looked guilty. It was a programmable unit with the clock wrong by four hours and a scratched display. I figured a thermostat replacement would be a clean, low-cost repair. I turned off power, took a photo of the wiring, and pulled the thermostat off the wall.
Before ordering the replacement, I searched for 'mcquay heat pump manual.' I found a PDF for the heat pump plant quickly. It covered water temperatures, safeties, and controller wiring. I skimmed it, told myself I had enough, and closed the file. I did not look up the fan coil unit manual because I assumed one McQuay manual would cover the whole system. Mistake number two.
The thermostat replacement that did not fix anything
At the supply house, I told the counter person, 'I need a thermostat for a McQuay heat pump.' He asked if I had a model number. I said it was a McQuay heat pump, and I thought that should be enough. I left with a universal heat pump thermostat. We were using the same words but meaning different things. I meant the whole hydronic system, including the fan coil units. He probably heard a conventional forced-air heat pump with an outdoor compressor.
The install went smoothly. I labeled the wires, mounted the thermostat, restored power, and set it to heat. The fan coil started. I felt like a genius for about 20 minutes, until the air from the vent stayed lukewarm and the room temperature got stuck somewhere around 62°F. Maybe it reached 64°F on a good afternoon. I want to say the temperature did not matter much because nobody wanted to work in that office anyway.
Two cold days and a buddy heater
By the next morning, I still did not have heat. The new thermostat said it was calling, but the room was not responding. I bought a small buddy heater at a hardware store, mostly to keep the two employees in that office comfortable while we figured out the real issue. I do not love portable propane heaters indoors, so we kept the exterior door cracked, stayed in the room only while the heater was on, and turned it off whenever the office was empty.
The buddy heater helped with morale, but it was not a building solution. It just gave me time to stop guessing.
Which way to put air filter in furnace, and in a fan coil unit
Late that afternoon, one of our maintenance people asked if I had checked the air filter. I had changed the filter in October. I remembered doing it. He opened the access panel on the fan coil unit and pointed at the filter. The arrow was facing the return grille. I had installed it backward.
I know the keyword answer people are looking for here: which way to put air filter in furnace? The arrow on the filter points in the direction air is traveling. In a standard furnace, the arrow points toward the furnace body or blower compartment, not back toward the return duct. In a horizontal fan coil unit, the same rule applies: point the arrow toward the blower wheel.
I had searched for that exact question when I changed the filter in October. I even told myself, 'the arrow goes toward the furnace.' Then I got under the unit, second-guessed myself, and put the arrow in pointing backward. The filter still looked okay, but it had been restricting airflow for three months. The fan coil could not move enough air across the hot water coil, so the room stayed cold no matter what the thermostat said.
What I learned after the thermostat replacement
I replaced the filter with the arrow facing the blower, closed the panel, and let the fan coil run. The room came up to temperature. I did not check a stopwatch, but it was a lot faster than the two days I had already lost.
I still kick myself for not spending ten minutes on airflow before buying parts. The thermostat cost around $140. The buddy heater cost about $90 at the hardware store. The filter was less than one-third of the thermostat, if I remember the receipt correctly. Direct costs do not even include the two cold days, the employee frustration, or the fact that I installed a thermostat that might not have needed replacing.
That said, I do not want to overcorrect. A thermostat replacement is the right fix in many cases: a blank display, a stat that will not hold a schedule, a dead control board, or obvious wire damage. If the heat pump is throwing error codes, short cycling, or making sounds I cannot identify, I will call a technician. The lesson is not 'never replace parts.' The lesson is to check the cheap and obvious things first and to get the full model number before buying anything.
I also added the model numbers of our McQuay fan coil units to the maintenance log. When I need information again, I will not assume a generic McQuay heat pump manual will cover the whole system. If you have a similar setup, do yourself a favor: before you order a thermostat, check the filter, check its direction, and confirm what kind of unit is actually behind the wall. It sounds like small stuff. In my experience, small stuff is where the budget goes to die.
