Ecobee Blowing Cold Air Instead of Hot

I’m Nathan, and I’ve been an HVAC technician for over 10 years. One of the most common winter service calls I get starts like this:

“My ecobee is set to heat, but the vents are blowing cold air.”

Sometimes it’s a simple thermostat setting. Other times it’s a heat pump reversing valve setting, loose wiring on the O/B terminal, or even a furnace safety lockout that shuts the burners off while the blower keeps running.

In this guide, I’ll walk you through the same troubleshooting steps I use in the field so you can pinpoint the real cause and fix it safely.

Before You Start: A Quick Reality Check (Cold Air vs Not-So-Hot Air)

A lot of homeowners expect “toasty” air from the vents in heating mode. But the type of system you have matters.

What vent temperatures should feel like

Here’s a practical baseline I use:

Gas furnace (normal operation)	100 to 140 F	Clearly warm
Electric furnace / heat strips	90 to 120 F	Warm, not as hot as gas
Heat pump (normal heating)	85 to 100 F	Can feel lukewarm, especially at low airflow
Heat pump in defrost	Can drop temporarily	Often feels cool for a few minutes

Safety Notes (Important)

You can do most of these checks safely. But do not open electrical panels or handle high voltage wiring if you’re not comfortable.

If you’re going to pull the ecobee off the wall to check wiring:

• Turn off power to your HVAC system at the breaker first
• Confirm the screen turns off
• Take a clear photo of the wiring before touching anything

The Most Common Reasons Ecobee Blows Cold Air on Heat

In my experience, these are the usual culprits, in order of how often I see them:

Heating and cooling seem reversed	O/B reversing valve setting is wrong	Heat pumps
Blower runs but air is cold	Furnace safety limit tripped (often dirty filter)	Furnaces
Air blows cool at the end of a heat cycle	Fan dissipation or fan set to run	Any system
Outdoor unit not running in heat	Breaker, disconnect, defrost issue, failed outdoor components	Heat pumps
It worked before, now it doesn’t after thermostat install	Wiring loose or landed on wrong terminal	Any system
It heats sometimes, then cool air	Defrost cycle or aux heat not coming on	Heat pumps

Now let’s troubleshoot properly.

Step 1: Confirm You’re Actually in Heat Mode (Not Cool or Auto)

On the ecobee:

1. Tap the thermostat
2. Select the system mode
3. Choose Heat
4. Set the temperature at least 3 to 5 degrees above room temperature for testing

Also check:

• If you’re in Auto, the thermostat may be switching between heating and cooling based on your thresholds.
• If you use a schedule, it may be changing the setpoint without you noticing.

What I tell homeowners: for testing, force it to Heat and raise the setpoint enough that it should clearly be calling.

Quick sign it’s calling for heat

On most ecobee models, you’ll see a heat icon or a message indicating it’s heating. If it says it’s heating but the system blows cold, keep moving.

Step 2: Fix the Most Overlooked Setting: Fan Control in Heat Mode

This one causes a surprising amount of “cold air” complaints, especially after installs.

If the thermostat is controlling the fan incorrectly, the blower may run when it shouldn’t, pushing cool air.

Set Fan Control in Heat Mode to HVAC

On ecobee, go to:

• Main Menu
• Settings
• Installation Settings
• Equipment
• Furnace
• Fan Control in Heat Mode
• Set it to HVAC (not thermostat)

What this does:

• HVAC mode lets your furnace or air handler decide when the blower should run during a heat call
• That prevents the fan from starting too early or running when there’s no heat output

Also check your regular fan setting

Make sure your fan is not set to On all the time. For most homes, it should be Auto.

If the fan runs constantly, it will blow room temperature air between heating cycles, which many people describe as “cold air.”

Step 3: If You Have a Heat Pump, Check the O/B Reversing Valve Setting (Big One)

If you have a heat pump and the O/B setting is wrong, your system can literally run in cooling mode while the thermostat thinks it’s heating.

How to tell if you have a heat pump

Most of the time:

• You have an outdoor unit that runs in both summer and winter
• You may have “Aux Heat” or “Emergency Heat” options
• You likely have an O/B wire connected at the thermostat

What O and B mean in real life

Heat pumps use a reversing valve to switch between heating and cooling. The thermostat tells that valve what to do using O or B logic:

• O is energized in cooling on many brands
• B is energized in heating on some brands

If ecobee is set opposite of what your system expects, you get reversed operation.

Toggle the O/B setting on ecobee

Go to:

• Main Menu
• Settings
• Installation Settings
• Equipment
• Heat Pump
• O/B Reversing Valve
• Switch it to the opposite option (O to B, or B to O)

Then test again:

• Set thermostat to Heat
• Raise temp 3 to 5 degrees
• Give it a few minutes

What should happen if this was the issue

If the reversing valve setting was wrong, this change usually fixes it immediately. You’ll notice the air warms up and the system stops acting “backwards.”

Step 4: Check for “End of Cycle Cold Air” (Fan Dissipation and Similar Settings)

Some systems keep the blower running after the heat call ends to “use up” leftover heat or to circulate air. That can also push a short burst of cooler air.

If your complaint is:

• “It heats fine, but near the end it blows cold for a minute”

Then check ecobee settings for any features that keep the fan running after a cycle.

I generally recommend turning these off if they’re causing comfort complaints, especially on heat pumps where supply air is already not super hot.

Step 5: Inspect the Air Filter and Airflow (This Causes Furnace Lockouts)

If you have a gas furnace and the blower is running but the air is cold, one of the first physical things I check is airflow.

Why a dirty filter can create cold air

When a filter is clogged:

• The furnace overheats
• A safety limit switch trips
• The burners shut off
• The blower often keeps running to cool the furnace down

To you, it looks like: heat was on, then suddenly cold air, and the fan keeps blowing.

What to do

• Replace the filter with the correct size
• Make sure supply vents are open and not blocked
• Make sure the return grille is not blocked

Step 6: Check Wiring at the Ecobee (Especially O/B, W1, Y1)

Incorrect wiring or a wire not fully seated can absolutely cause “cold air on heat.”

Safest way to do this

1. Turn HVAC power off at the breaker
2. Pull the ecobee display straight off the wall plate
3. Check that each wire is fully inserted and not loose

The most important terminals for this problem:

• O/B for heat pump reversing valve
• W1/AUX for furnace heat or auxiliary heat strips
• Y1 for compressor call (heat pump heating uses the compressor)
• G fan
• R and C power

Typical wiring map (general guidance)

This varies by system, but this gives you a reference for what each terminal usually does:

R	24V power (hot)
C	24V common
Y1	Compressor stage 1
O/B	Reversing valve
W1 / AUX	Aux heat or furnace heat call
G	Indoor blower fan

If any wire looks damaged, too short, or poorly clamped, that’s a real problem.

Step 7: For Heat Pumps, Confirm the Outdoor Unit Is Running in Heat Mode

A heat pump needs the outdoor unit to move heat. If the outdoor unit isn’t running, you’ll often get indoor air blowing that feels cold or room temperature.

What to check (homeowner-friendly)

• Is the outdoor unit running when the ecobee calls for heat?
• Is the fan spinning?
• Is the breaker tripped?
• Is the outdoor disconnect pulled out or switched off?

One more heat pump detail: defrost mode

In cold weather, heat pumps periodically defrost the outdoor coil. During defrost:

• The system may temporarily blow cooler air indoors
• Aux heat may come on to compensate (if it’s working)

If you’re getting cold air for 3 to 10 minutes every so often, that can be normal defrost behavior. If it’s blowing cold constantly, it’s not.

Step 8: Stuck Reversing Valve or HVAC Mechanical Failure (When Settings and Wiring Check Out)

If you’ve confirmed:

• Ecobee settings are correct
• O/B is set correctly
• Wiring is solid
• Outdoor unit runs

Yet you still get cold air in heat mode, the issue may be mechanical.

Stuck reversing valve

A reversing valve can stick, especially on older systems. Sometimes you’ll hear a distinct changeover sound when switching modes. If it never changes, it may be stuck or the solenoid could be failed.

This is not a DIY repair. It can involve refrigerant work and specialized tools.

Low refrigerant or restricted system

Low charge, a restriction, or a failing compressor can all cause poor heating output. The system may run but not actually deliver heat.

Again, that’s a technician diagnosis with pressure and temperature readings.

Step 9: Re-Run Ecobee Equipment Configuration (After Any Wiring Changes)

Any time you correct wiring or suspect the thermostat is misconfigured, re-run the equipment detection so ecobee knows what you actually have.

On ecobee:

• Main Menu
• Settings
• Installation Settings
• Equipment
• Reconfigure Equipment

This step matters more than people think. I’ve seen ecobee set up a heat pump as a conventional system or miss auxiliary heat, which leads to comfort problems.

Fast Troubleshooting Checklist (What I’d Do in 5 to 10 Minutes)

If you want the quickest path, do this in order:

1. Set system to Heat and raise setpoint 3 to 5 degrees
2. Set fan to Auto
3. Set Fan Control in Heat Mode to HVAC
4. If heat pump, toggle O/B setting
5. Check filter and airflow
6. Verify outdoor unit runs (heat pump)
7. Power off, check wiring is tight on O/B, W1/AUX, Y1
8. Reconfigure equipment

If you get stuck, it usually means it’s time for a mechanical diagnosis.

When You Should Call a Pro (No Guessing)

Call an HVAC technician if:

• Breakers keep tripping
• You suspect a gas furnace issue and burners won’t stay on
• The outdoor heat pump runs but never heats
• You suspect a reversing valve issue
• You see ice buildup that doesn’t go away
• You changed wiring and now nothing works

In a service visit, I’m typically checking:

• Temperature rise across the system
• Heat pump pressures and superheat/subcooling
• Limit switch status and fault codes
• Proper staging of aux heat
• Control board outputs to the reversing valve and compressor contactor

FAQ

Closing Thoughts (From the Field)

Most ecobee “blowing cold air on heat” cases I see come down to two things:

• The O/B reversing valve setting is wrong for the heat pump
• The fan is being controlled in a way that pushes unheated air