How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

Why does AUX HEAT (auxiliary heat) light keep coming on at my thermostat, even when it's not that cold outside?

Wednesday, June 13, 2018

Compressor To Aux Heat Delta

Most heat pump thermostats have a setting called “Compressor to Aux Heat Delta,” which is defined as the number of degrees in difference between the set point (desired temperature setting) at the thermostat and the actual indoor temperature. It is a minimum at which the system will turn on the auxiliary heat to run simultaneously with the heat pump as a supplemental “surge” for additional heating capacity. The default number is usually 3º F, but can be adjusted at digital programmable thermostats.

     When you turn the heat setting up to 70º while the indoor temperature is 65º, for example, then the system will add the electric resistance heat coil to hasten the temperature increase. If, however, the indoor temp is 65º and you set the thermostat to 67º, the AUX HEAT will not come on as a supplement. In day-to-day ordinary usage, the system would restart before the temperature differential reaches 3º, so the “Compressor to Aux Heat Delta” setting would only kick in when you do a big temperature setting increase, such as after arriving back from vacation on a cold day.

When AUX Heat Activates

The only two other times that the AUX HEAT should come on are when the outdoor temperature is below approximately 35º—which is when the heat pump can’t produce enough heat by itself—or if the condenser (outdoor unit) is in defrost mode. Otherwise, the heat pump system is malfunctioning and it’s time to call for professional repair.

    Also see Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • 

  To learn more about heating and air conditioning systems, see these other blog posts: 

• How long does a home thermostat last?

How can I find out the SEER of my air conditioner? 

My air conditioner won't turn on. What's wrong? 

How can I find out the size of my air conditioner? 

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

What is wrong with an air conditioner when the air flow out of the vents is low?

Why has the thermostat screen gone blank? 

Why does it take so long to cool a house when an air conditioner has been off for a while? 

Why is my air conditioner not cooling enough? 

What are the most common problems with wall/window air conditioners?  

Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

Water Heaters

Water Heater Age

"What Are The

Signs Of..."

Septic Tank Systems

Structure and Rooms

Plumbing Pipes

Termites, Wood Rot

& Pests



When It First

Became Code

"Should I Buy A..."

Park Model Homes


Shingle Roofs




Wind Mitigation

Roof and Attic

"Does A Home


Pool and Spa

"What Is The Difference Between..."




Concrete and

Concrete Block

Metal Roofs


Modular Homes

Rain Gutters

Mold, Lead & Other Contaminants


Older and

Historic Houses

Crawl Spaces

Mobile-Manufactured Homes

Building Permits

Life Expectancy

Clay Soil





Exterior Walls

& Structures


Common Problems

HUD-Code for

Mobile Homes

Garages and Carports

Flat (Low Slope) Roofs

Electrical Panels

Sprinkler Systems

Electrical Receptacle Outlets

4-Point Inspections

Hurricane Resistance

Frequently Asked Questions (FAQ)

Home Inspection

Heating and Air Conditioning

Building Codes

Fireplaces and Chimneys

Inspector Licensing

& Standards

Energy Efficiency

Washers and Dryers



Doors and Windows



Electrical Wiring

Click Below  

for Links

to Collections

of Blog Posts

by Subject

Plumbing Drains

and Traps


Smoke & CO Alarms

Aging in Place

Top 5 results given instantly.

Click on magnifying glass

for all search results.






Air Conditioner & Furnace Age/Size


Electrical Switches





Water Intrusion

Electrical - Old

and Obsolete


Foundation Certifications

Tiny Houses

About McGarry and Madsen



Buying a home in North/Central Florida? Check our price for a  team inspection by two FL-licensed contractors and inspectors. Over 8,500 inspections completed in 20+ years. In a hurry? We will get it done for you.

Moisture Problems