Facebook X-twitter Youtube
Get a Free Estimate

Thermostat or Furnace? How to Tell Which One Is Causing the Problem

When the house is cold and the furnace is silent, the thermostat on the wall and the furnace in the basement look like one machine. They are two, and they talk over a single low-voltage signal. Working out which end has the problem is the first useful question, because the answer decides whether it is a setting you can fix in a minute or a fault that needs a technician.

This guide explains how the thermostat tells the furnace to heat, using the furnace manufacturer’s own description. It covers the settings that make a working system look broken, what the thermostat’s position on the wall has to do with it, and one check a homeowner can make safely that separates a thermostat-side problem from a furnace-side one. It ends where the homeowner’s part ends.

How the thermostat starts the furnace

A gas furnace does not start on its own. It waits for a signal. Goodman’s installation manual for one of its gas furnace lines describes the start of every heating cycle in one line: “On a call for heat, the thermostat contacts close & the control board receives 24 VAC on the W terminal.”

That is the whole of the thermostat’s job. It closes a switch, and a small 24-volt signal reaches the furnace’s control board on the terminal marked W. Everything after that happens inside the furnace. The manual’s sequence continues with the control board running “its self-check routine”, checking its safety switches, and then energizing “the Induced draft blower”, the small fan that clears the burners before ignition. Ignition, burner flame, and finally the main blower follow.

The 24 volts do not come from the house wiring directly. The manual notes that the furnace “is equipped with a 40 VA transformer” and shows the thermostat wires connecting at a low-voltage terminal strip on the furnace, marked with letters: R, W, G, C and Y on the models in its wiring diagrams. W is the terminal that receives the heat call, C is the common connection the manual names for the 24-volt circuit, and the others serve the fan and cooling. The manual’s own start-up record has a blank for this supply, “Secondary Voltage (Measure Transformer Output Voltage) R – C”. Measuring it is a technician’s job, and it answers the question of whether the thermostat has power to send a signal at all.

The checks between the call and the flame are worth knowing, because each one can stop the furnace after the thermostat has done its job. The manual lists them in order: the control “verifies the limit switch is closed”, “verifies that pressure switch circuit is open”, and “performs a gas valve circuitry check to verify gas valve relay state and presence of voltage at the valve”. Only then does it start the inducer, and the pre-purge begins “once the pressure switch is detected closed”. A furnace that stops at any of those points has received the call and refused to go further, which is the control doing its job, not the thermostat failing.

This matters for the diagnosis because a thermostat can fail to call, the call can fail to arrive, or the furnace can receive the call and fail somewhere later in its own sequence. Only the first of those is really a thermostat problem.

Settings that make a working furnace look broken

Before anything else, check the thermostat itself. ENERGY STAR’s maintenance checklist starts with it: “Check thermostat settings to ensure the cooling and heating system keeps you comfortable when you are home and saves energy while you are away.” These are the settings that produce a cold house with nothing broken:

  • The mode. The thermostat must be set to heat. A thermostat left on cool or off will not call for heat, whatever the temperature.
  • The setpoint. The thermostat calls for heat only when the room is below the temperature it is set to. The manual’s own start-up steps say to “adjust the thermostat to a setting above room temperature” to create a call.
  • A schedule or hold. A programmable thermostat may be following a schedule that has the house cooler at that time, or holding a temperature someone set earlier. The thermostat’s own instructions explain how to view and change its schedule and hold.
  • The fan setting. With the fan set to on, the blower runs whether or not the furnace is heating, and air from the vents can feel cool between heating cycles. What that looks like, and why, is covered in furnace blowing cold air.

If any of those was the cause, the furnace should respond to the corrected setting on its next call.

When the furnace is meant to be off: dual-fuel homes

In a home where a heat pump and a gas furnace share the work, a silent furnace can be correct. The manual describes this arrangement, which it calls a fossil fuel application: “a combined gas furnace and heat pump installation which uses an outdoor temperature sensor to determine the most cost efficient means of heating (heat pump or gas furnace).” It also notes that “a heat pump thermostat is required” for it. In that kind of system, the thermostat or its controls can send the call to the heat pump instead of the furnace, depending on the outdoor temperature. A furnace that does not start while the heat pump is running and the house is warming is not a fault. What is worth checking is whether the house is actually reaching its setpoint. If it is not, the question is the heat pump’s, covered in heat pump repair.

A blank or unresponsive thermostat

A blank screen means the thermostat has no power, and there are two places power comes from. Some thermostats run on batteries, and some take their power from the furnace’s 24-volt circuit, through the R and C wires. Some use both, with batteries as backup.

If the thermostat uses batteries, replacing them as its instructions describe is a homeowner task and a reasonable first step. If the screen stays blank with fresh batteries, or the thermostat has no batteries, the question moves to the furnace side of the circuit. The manual’s troubleshooting chart lists, for a furnace that fails to operate, “Open fuse” with the corrective action “Replace fuse”, and “Short in low voltage wiring” with “Locate and correct short in low voltage wiring”. Both are inside the furnace cabinet and both are technician work.

The C wire and newer thermostats

Some thermostats need continuous power from the furnace’s 24-volt circuit, through the C terminal the manual names as the common connection. Whether a particular thermostat needs a C wire is stated in its own installation instructions, which the furnace manual points to: “Consult the instructions packaged with the thermostat for mounting instructions and further precautions.” Adding or reconnecting a wire at the furnace’s terminal strip is low-voltage electrical work inside the cabinet. It is not a homeowner step, and nothing here suggests attempting it.

Where the thermostat sits

A thermostat can work perfectly and still control the house badly if it is reading the wrong temperature. The furnace manual is specific about placement: “In an area having good air circulation, locate the thermostat about five feet high on a vibration-free inside wall.” It then lists what to avoid:

  • “Drafts, or dead spots behind doors, in corners, or under cabinets.”
  • “Hot or cold air from registers.”
  • “Radiant heat from the sun.”
  • “Light fixtures or other appliances.”
  • “Radiant heat from a fireplace.”
  • “Concealed hot or cold water pipes, or chimneys.”
  • “Unconditioned areas behind the thermostat, such as an outside wall.”

A thermostat in afternoon sun, beside a supply register or near a lamp can see a warmer room than the rest of the house and stop calling early. One on an outside wall or in a draft can do the opposite. The homeowner’s part is clearance: keep furniture, lamps and heat-producing appliances away from it. Moving a thermostat is a wiring job.

The safe check that separates the two

Here is the check a homeowner can make without opening anything. It uses the furnace’s own start-up sequence, quoted above, in which the first thing the furnace does after receiving a call is start its induced draft blower.

  1. Set the thermostat to heat, with the setpoint a few degrees above the room temperature, so it is calling for heat.
  2. Go to the furnace and listen, without opening any panel. After the control board’s self-check, the manual’s sequence begins with the inducer fan starting, a steady whirr that comes before the burners light.
  3. Note what you hear, or do not hear.

The result points one way or the other:

  • Nothing starts at the furnace at all. The call may not be reaching it. That points to the thermostat, its power, or the low-voltage circuit between them: batteries, settings, the transformer, the fuse or the wiring. After batteries and settings, this is technician work.
  • The furnace starts its sequence, then stops. The call arrived. The problem is inside the furnace’s own sequence: a pressure switch, ignition, flame sensing or a safety switch. The furnace side is covered in furnace won’t turn on, why a furnace won’t ignite and how a furnace’s safety switches work.
  • The furnace runs, then shuts off after a short time, repeatedly. That is its own pattern, covered in furnace short cycling.

If at any point you smell gas, leave the house and call the gas utility from outside. That is not a thermostat question.

What the furnace records

Some furnaces record their own faults. The Goodman control described in the manual stores fault codes, and the manual explains how a technician views them: “Press the center switch to view stored faults.” The same section explains how to erase them, and warns that pressing and holding erases them all. The display is on the control board inside the blower compartment, behind a panel. That makes it a technician’s check, and stored codes are worth leaving in place for whoever diagnoses the fault, because they are a record of what the furnace saw.

What a technician checks

With the homeowner’s check done, a technician’s diagnosis follows the same chain in more detail:

  • Supply at the thermostat. The R to C voltage, the figure the manual’s record sheet asks for.
  • The call at the furnace. Whether 24 volts reach the W terminal when the thermostat calls for heat.
  • The low-voltage fuse and wiring. The “Open fuse” and “Short in low voltage wiring” entries from the troubleshooting chart.
  • Stored fault codes. What the control board recorded.
  • The rest of the sequence. If the call arrives, the diagnosis moves into the furnace itself.

When it is the thermostat, replacement is straightforward in principle, but the new thermostat’s instructions decide what wiring it needs, including whether it needs a C wire. When it is the furnace, the detail is in furnace repair. For how this fits into heating a home here more broadly, see the guide to winter heating in Boulder.

Frequently asked questions

How do you know if your furnace thermostat is broken? Check the settings first: mode on heat, setpoint above the room temperature, no schedule or hold overriding it. Replace the batteries if it uses them. Then set it to call for heat and listen at the furnace. If the furnace starts its sequence, the thermostat is doing its job and the fault is in the furnace. If nothing at all starts, the call may not be reaching the furnace, and a technician can measure where it stops.

Why is my thermostat on but I’m not getting any heat? A thermostat can show that it is calling while the furnace fails later in its own sequence. The manual describes a sequence of self-checks, safety checks and ignition after the call arrives, and any of them can stop the furnace. Listening at the furnace tells you whether it received the call; if it starts and then stops, the problem is on the furnace side.

Will my heat still work if my thermostat is bad? No. The furnace waits for the thermostat’s call before it starts a heating cycle. The manual’s sequence of operation begins with it: “On a call for heat, the thermostat contacts close & the control board receives 24 VAC on the W terminal.”

How do I reset my furnace thermostat? The reset procedure depends on the thermostat, and its own instructions describe it. Resetting the thermostat’s settings is a homeowner task. Resetting or switching power at the furnace is not covered here, and clearing the furnace’s stored fault codes erases the record a technician would use.

Why isn’t my furnace getting power to my thermostat? A thermostat that takes power from the furnace draws it from the furnace’s 24-volt transformer through the R and C wires. The manual’s troubleshooting chart lists an open fuse and a short in the low-voltage wiring among the causes of a furnace that fails to operate. Both are inside the furnace cabinet and are for a technician.

What is the most common cause of a thermostat failure? There is no sourced answer to that, so this guide does not give one. What gets checked is the order above: settings, batteries, the thermostat’s position, whether the call reaches the furnace, and then the low-voltage supply, fuse and wiring.

Sources

Recent Posts

Contact Form

Our Boulder HVAC Projects