Outdoor thermostat-How and when to use them?


If you have a heat pump in your house you probably have heard of the name outdoor thermostat.Why? Well as you probably know that not all indoor thermostats are made for heat pumps and even if it is you may have problems in winter when you either want your supplemental heat to turn on when outside temperature falls below tripping point.Or you want to not allow supplemental heating to turn on unless it is below certain point.This is where you can install an outdoor thermostat.



Outdoor thermostats are mechanical thermostat switches that senses outside temperature and can be installed in conjunction with your present indoor thermostat.Its purpose is to cutoff compressor and bring on the auxiliary heat when temperature falls below a point at which your heat pump is no longer effective in meeting the load requirement of house.

Conversely it can also be used to keep auxiliary heat from starting unless temperature is below a  set point band thus maximize efficiency.

Modern programmable thermostat do come with special algorithmic time delay to do this by sensing outdoor temperature with an outdoor sensor like Honeywell vision pro thermostat but in case you have basic heat pump thermostat and want to add an extra level of control an outdoor thermostat can do that job for you.


A heat pump is designed like an air conditioner with most part like condenser evaporator etc common to Ac system except that it can transfer heat both ways. When functioning like AC it takes inside heat to outside. But when functioning in heating mode it does the reverse by squeezing heat from outside and bringing it inside.It does it with a 4 way reverse valve.



During winter the air outside is cold so there is limited capacity up to which the heat pump can work efficiently.

So there is a provision of auxiliary heat.This heat comes into play when there is more than 2 degree difference in ambient temperature and set point and also when the load capacity of heat pump is less than load

This heat can come from a electric heat strip or a furnace. System that uses furnace as auxiliary heat are called dual fuel system.

One mistake that people do with heat pumps is that they switch on auxiliary heat but turn off the compressor or heat pump when outside temperature falls below certain point. In this mode the heat pump is running in emergency heat mode and not supplemental heat mode.

Most heat pump have electric resistance wire as auxiliary heat source which is very costly in terms of producing heat. So running the heat pump on supplemental heat alone greatly increase your electricity bill.

Best practice is to keep the heat pump running but add supplemental heat to make for the load demand that the heat pump is not able to meet up.



From above graph you can see that around 30 there is a balance point between load and capacity of heat pump. Below that temp the capacity is lower than the load of the house but it is not zero. So turning off heat pump condenser completely is not effective way of running a heat pump.


For Compressor cutoff-Although it is not recommended to completely cutoff the  the mechanical heating sometimes it is desired because low temperature can cause many defrost cycle or to shutoff the noise when its relatively less efficient. But overall it should be avoided.

For Heat holdoff-In this mode the thermostat prevent the supply of supplemental heat which is costly and will only allow if outside temperature is below certain point. This is better way of using outdoor thermostat.




Many thermostat like Honeywell vision pro, Honewell T10, Goodman Comfortnet CTK04AE thermostat , Honeywell thermostats with truezone panel etc and countless others can sense the outdoor temperature and also comes with an internal algorithms to switch between the dual fuel or electric and mechanical heating as stated in the starting of the article. Keep in mind that some thermostat like Honeywell 9000 series can use weather forecast to sense the outdoor temperature by ntering your local zip code into the system so the use of a temperature sensor is not required.

For these type of thermostats there are certain sensors that can do the job. You just have do install it at the location where you want to sense the ambient outdoor temperature and the thermostat will decide based on the setpoint to run the Auxilaary heating lockout or Compressor lockout based on the mode of operation.

Two such popular temperature sensor are

1.Honeywell C7089R1013 Wireless Outdoor Sensor

This sensor is made for wireless Honeywell thermostats. It comes with upto 5 years lasting 2AA lithium batteries. It pairs easily with Honeywell thermostat like Vision pro 8000. You need to have a Honeywell thermostat with Red link capacity. Although the sensor uses wireless communication for transmission it doesn’t interfere with the other wireless instruments around it.

You can add this device in your thermostat using the installer code of the product. It can gather both temperature and humidity information of its surroundings. You can use it compressor lockout like at low temperature where auxiliary heat kicks in and auxiliary heat lock out at higher temperature when it shuts of the auxiliary heat. You can use the humidity and temperature reading to control the humidity inside by locking out the vent when dew point is above certain level.

Some user find the temperature to be wrongly calibrated like around 6 degree so you can check and make adjustment or it will not give efficient performance when used for thermostat functions. Also one sensor can be used to read temperature in more than one thermostat units if you require.Be sure to install it under a shade at the north side of the house as it is sensitive to the sun.

Here is a link to the manual of the product.


Click here for more reviews rating and price of Honeywell C7089R1013 sensor

2.Honeywell C7089U1006 Outdoor Temperature Sensor

This light weight wired temperature sensor comes with a 2.25 x 0.375 inch sensor head with 60 inch lead wires wihich can be stretched to maximum of 200 feet by adding more length to it using 18 gauge 24 V thermostat wire. There is a mounting clip that allows for easy fit on siding or soffit. The listed accuracy of the +- 1 degree F at 70 degrees F.

It works with Honeywell Honeywell Th8000 – TH 7000 series. However it can not be used with the RTh series as it uses wifi to retrieve outside temperature. Its works well with heat pumps and dual fuel systems. It is sensitive to the sun so try to install it under a shade.

Click here for more reviews rating and price of Honeywell C7089U1006 Sensor


Well outdoor thermostat are not that much required with modern new thermostats having the ability to sense outdoor temp and optimizing the efficiency. But they are still good additional switch that will ensure your system heat cutoff and cut in capabilities.

Switch types

But first I think you should know various electronic switch code that are common with these thermostats.



stands for Single pole single throw which is just a simple on-OFF  the switch has it has a single pole which when thrown(makes contact) conducts otherwise it breaks the circuit.



In this type of switch is single pole connected to common toggles between two position which are both conducting, ie completes a circuit.

TPI LB110 Series LB Bi-Metal in-Built Thermostat, SPST Heat Only


TPI is American company which manufactures heating, ventilation, lighting, thermostats and and replacement heating element .

This thermostat has a bi metal temperature sensor instead of thermocouple. Comes with 6 feet/4 Gauge of lead for installation.

TPI LRD100A Series LR Bi-Metal SPDT Outdoor Thermostat


It also has a bimetallic temperature sensor with temperature marking in steps of 20 degree. Comes with 6 Feet/14 gauge lead for installation. It is dual switch which can toggle in two position of On. If not in use one terminal can be connected to common.

Goodman Outdoor Thermostat Kit OT18-60A

Goodman company which manufactures furnace, AC, Heat pumps and also HVAC replacement components.

This set is also a complete kit for installation. Temperature sensor is thermocouple and temperature marking in multiple of 5.

Following models of outdoor thermostat are available for purchase on amazon.

OT18-60A – Janitrol OEM Replacement Heat Pump Outdoor Thermostat


This is a replacement kit for heat pumps thermostats from Janitrol which was acquired by Goodman. It also make furnace, Ac, heatpump and HVAC replacement parts. This thermostat has a thermocouple lead as temperature sensor .You would need power tools and maybe a handyman too.

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