HVAC Controls & Thermostats

A thermostat seems simple until a customer calls about a system that runs too often, or a room that overshoots by four degrees. Those problems come from settings and control logic, not broken parts. If you know how a thermostat decides when to act, you can fix comfort complaints that part-swapping never will. This guide covers the decision-making side of controls: the control loop, thermostat types, key settings and how to trace a fault.

What You Will Be Able to Do

• Explain how a closed control loop works.
• Choose the right thermostat for the equipment.
• Set differential, deadband and setback correctly.
• Explain why location affects comfort.
• Trace a no-heat or no-cool control fault in order.

The Control Loop

Every HVAC control system follows the same closed loop.

1. Sense. A sensor measures something, such as temperature, humidity or pressure.
2. Compare. The controller compares the reading with a setpoint.
3. Act. The controller sends a signal to a device, such as a compressor or a valve.
4. Feedback. The sensor reads the result, and the loop repeats.

When a system does not hold its setpoint, the fault is in the sensor, the controller, the device or the logic that links them.

Thermostat Types

Low Voltage and Line Voltage

A low-voltage thermostat handles 24-volt control signals. A line-voltage thermostat switches the full 120 or 240 volts of a heater, using heavier wire such as 12 or 14 AWG. Line-voltage models come in single-pole, with no true off position, and double-pole, with a true off. Mixing the two types destroys the thermostat and creates a shock and fire hazard.

Before You Install a Smart Thermostat

Check the manufacturer's compatibility tool. Common problems include:

• Millivolt systems, such as some gas fireplaces.
• Line-voltage electric heat.
• Multi-speed air handlers that need special wiring.
• Heat pumps that need specific O/B and auxiliary heat settings.

Settings That Control Comfort

Setpoint. The target temperature.

Differential, also called swing. The range around the setpoint in which the system cycles on and off. Worked example: a heat setpoint of 70°F with a 1°F differential may start heat at 69.5°F and stop at 70.5°F. A narrow differential holds temperature tightly but cycles more often. A wider one reduces cycles but lets the room drift.

Deadband. The range between the heating and cooling setpoints where neither runs. Worked example: with heat at 68°F and cooling at 72°F, the deadband is 4°F. It stops an auto-changeover thermostat from fighting itself.

Anticipator. On older mechanical thermostats, a small heater warms the bimetal during a heating call, which shuts the system off slightly early. The heat already in the ducts and heat exchanger then finishes the job, so the room does not overshoot. Digital thermostats do this with a cycle rate setting.

Setback. An automatic temperature change during unoccupied or sleeping hours, set on a schedule. The U.S. Department of Energy says a setback of 7 to 10°F for 8 hours a day can save up to about 10% a year on heating and cooling. Large setbacks on a heat pump can bring on expensive backup heat, so use a thermostat with smart recovery.

Compressor delay. A timer keeps the compressor from restarting right after it stops, often for 3 to 5 minutes. This lets pressures equalize, so the compressor does not try to start against high head pressure.

Multi-Stage Systems

A staged system uses first stage for most of the day and second stage only when the first cannot keep up. Set stage delays or temperature differences in the thermostat. A setting that is too tight brings on second stage too often.

Location Matters

A thermostat reads the air around it. If that air is not typical of the room, the system acts on false information.

Avoid these locations:

• Direct sunlight, because the thermostat reads high.
• Lamps, TVs and other heat sources, for the same reason.
• Supply registers.
• Exterior walls.
• Doors to the outdoors.
• Kitchens and bathrooms.

Seal the wall hole behind the thermostat. Drafts from a wall cavity can fool the sensor too.

Short Cycling

Short cycling means the system turns on and off rapidly. It wastes energy and wears out equipment. Common causes include:

Tracing a Control Fault

Most no-heat and no-cool calls that are not airflow problems come down to a break in the control circuit. Work in order.

Warning: Testing live circuits is work for qualified people, or for a trainee under direct supervision. Line voltage may be in the same cabinet.

Step 1: Check power.

1. Is the thermostat display on?
2. Measure from R to C at the transformer's output. You should see about 24 to 28 volts AC.
3. If you have no voltage, check the fuse, the transformer and the line voltage feeding it.

Step 2: Check the call.

1. Set the thermostat to call.
2. At the thermostat, measure from C to the called terminal, such as Y for cooling or W for heat. You should see about 24 volts.
3. If voltage is missing, check the thermostat and its settings.

Step 3: Check the signal at the equipment.

1. Measure from C to the same terminal at the equipment.
2. If the voltage shows at the thermostat but not at the equipment, the wire between them is open.

Step 4: Check the equipment's response.

1. If you have 24 volts at the equipment, the problem is further on.
2. Check the board's fault code.
3. Check safeties, the contactor and relays.

A call for fan only tests the G circuit. Trained techs sometimes use a jumper at the board to confirm that the blower responds, then use that result to tell if a fault is on the thermostat side or the equipment side.

A blown fuse is a symptom. Disconnect thermostat wires one at a time, and test for a short before you replace it. Pinched wire at a nail plate, wire rubbing on sheet metal and wires touching in the wall box are common causes.

Commissioning a New Thermostat

1. Confirm equipment type, stages and fuel.
2. Set O or B correctly for a heat pump.
3. Set the fan to the correct mode for heat. Electric heat often needs the thermostat to run the fan.
4. Set differential, deadband, compressor delay and any stage settings.
5. Test heat, cooling, fan and any auxiliary or emergency heat modes.
6. Show the customer how to use the schedule.

Common Mistakes

Check Your Understanding

A thermostat sits above a TV, and the room is always cool in winter. What is happening? The TV warms the thermostat, so it reads high and shuts off the heat early.

Heat is set to 70°F and cooling to 74°F. What is the deadband? 4°F.

After a brief power outage, an outdoor unit waits a few minutes before it starts. Is that a fault? No. The compressor delay is letting pressures equalize.

You measure 24 volts at the thermostat from C to Y, and 0 volts at the equipment from C to Y. What should you check? The wire between the thermostat and the equipment.

Key Takeaways

• Every control system follows sense, compare, act and feedback.
• Differential sets how far temperature swings, and deadband separates heating from cooling.
• Anticipators and cycle rates prevent overshoot, and setback saves energy.
• Compressor delays protect the compressor by letting pressures equalize.
• Keep thermostats away from heat sources, sun and drafts.
• Trace a fault in order: power, call, signal at the equipment, then response.