• Share:
How to Adjust a Mechanical Thermostat

HVAC units that cycle on and off quickly or erratically may just need minor thermostat adjustments. Check level first, then take a look at the heat anticipator.

By the DIY experts of The Family Handyman Magazine

Remove the thermostat cover and check for problems

If you have a mechanical thermostat and your gas- or oil-burning furnace cycles on and off a lot, or the room gets too hot or too cold before the furnace kicks on, you may be able to fix the problem with a couple of simple adjustments.

First, remove the thermostat cover and make sure the subbase is mounted level on the wall. If it’s not, the mercury switch inside won’t work properly, which can affect the accuracy of the temperature readings. Your subbase will either have leveling posts where you can lay the level, or a flat area at the top or bottom where you can set the level (Photo 1). Move the subbase around until it’s level and then tighten the mounting screws.

Next, adjust the heat anticipator, which fine-tunes the point at which the thermostat turns off the furnace burners. Locate the heat anticipator adjustment lever arm (Photo 2). If the furnace is cycling on and off too often, move the lever away from the “longer” setting by one calibration mark. If the room temperature is either too hot or too cold for long periods, move the lever closer to the “longer” setting by one calibration mark. Let the furnace run for two to three hours and, if necessary, repeat the above adjustment. If the problem persists, you may need to replace your thermostat.

Back to Top

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • 4-in-1 screwdriver
    • Level

Comments from DIY Community Members

Share what's on your mind and see what other DIYers are thinking about.

1 - 5 of 5 comments
Show per page: 20   All

January 11, 3:49 PM [GMT -5]

The Text under the second picture states:
"If the furnace is cycling on and off too often, move the lever AWAY FROM the "longer" .....

However, if you click on the picture to zoom it, the text states the opposite:
"If the furnace is cycling on and off too often, move the heat anticipator adjustment lever arm CLOSER TO the "longer" .....

The second sentence appears to be correct.

January 11, 9:42 AM [GMT -5]

The Text under the second picture states:
"If the furnace is cycling on and off too often, move the lever AWAY FROM the "longer" .....

However, if you click on the picture to zoom it, the text states the opposite:
"If the furnace is cycling on and off too often, move the heat anticipator adjustment lever arm CLOSER TO the "longer" .....

The second sentence appears to be correct.

January 11, 9:28 AM [GMT -5]

The Text under the second picture states:
"If the furnace is cycling on and off too often, move the lever AWAY FROM the "longer" .....

However, if you click on the picture to zoom it, the text states the opposite:
"If the furnace is cycling on and off too often, move the heat anticipator adjustment lever arm CLOSER TO the "longer" .....

The second sentence appears to be correct.

January 11, 8:58 AM [GMT -5]

The Text under the second picture states:
"If the furnace is cycling on and off too often, move the lever AWAY FROM the "longer" .....

However, if you click on the picture to zoom it, the text states the opposite:
"If the furnace is cycling on and off too often, move the heat anticipator adjustment lever arm CLOSER TO the "longer" .....

The second sentence appears to be correct.

October 03, 11:05 PM [GMT -5]

If you have a mechanical thermostat, go to the closest home improvement store and buy a 20.00 digital stat and install it. If you can do the steps in this tip, you can change the thermostat. If you have a working household, and there are times of the day you are regularly gone or the house is empty, you may want to look into a programable stat, as they have become much more user friendly, and the prices have gotten very reasonable. They can also save you a lot of money. A plain inexpensive digital thermostat is easy to install, doesn't need to be level, is preset to operate your equipment much more efficently than the old mechanical thermostat, so there are no adjustments to worry about and about the only thing you will need to do is change the battery. The stat will have a battery low indicator on the screen to let you know when to change it. The battery will last for years.This is well worth the 20 dollar investment. A touch screen, 7 day, digital, Honeywell 8000, can be bought new for 80.00, shipped free on fleabay...er..ebay. This is not a 5 and 2 day stat, but each day can be programmed, but I should leave it at that for now and let you look up the different kinds of stats available before this becomes another article.

+ Add Your Comment
closeX

Add Your Comment

How to Adjust a Mechanical Thermostat

Please add your comment
closeX

Log in to My Account

Log in to enjoy membership benefits from The Family Handyman.

  • Forgot your password?
Don’t have an account yet?

Sign up today for FREE and become part of The Family Handyman community of DIYers.

Member benefits:

  • Get a FREE Traditional Bookcase Project Plan
  • Sign up for FREE DIY newsletters
  • Save projects to your project binder
  • Ask and answer questions in our DIY Forums
  • Share comments on DIY Projects and more!
Join Us Today
closeX

Report Abuse

Subject
Reasons for reporting post

Free OnSite Newsletter

Get timely DIY projects for your home and yard, plus a dream project for your wish list!

Follow Us