FAQs about how to set the hidden switches or internal switches in room thermostats.

This article series describes the purpose and setting of tiny switches found inside of some room thermostats. These little slide or toggle switches may control the heating system on-off cycle rate or burner-on time in a manner similar to the heat anticipator feature found in other room thermostats.

How to Set the Cycle Rate or Fan Operation Switches on a Room Thermostat

These questions & answers about how to find and set various types of internal or "hidden" switches found in some wall thermostats were posted originally

at THERMOSTAT SWITCHES, INTERNAL - be sure to see the hidden thermostat switch examples and settings found there.

On 2019-03-06 by (mod) - Is it a problem to leave the thermostat switched to "GAS"?

Joe it won't be critical which way the GAS OIL switch is set as you'll get heat in either case.

The difference in those settings is an analog to the heat anticipator on some other thermostat designs, and simply tries to adjust the thermostat to account for the probable speed with which the heater will respond on a call for heat, thus avoiding "overshoot" in which the heat continues to warm the building past the point at which the thermostat is satisfied.

On 2019-03-06 by Joe - little switch under “fan on or auto” switch was switched in the gas

My AC/Heat unit operates on electric, but I just noticed the little switch under “fan on or auto” switch was switched in the gas position.

Even though my unit is electric. Is that a problem? What’s the difference if my house runs off electric.

On 2019-01-20 by Mark

With a CT87K I would like to have more of a temp spread between cycles, currently thermostat is set on off off with 2 wire. I have H/Air with oil thanks for your help Mark

On 2017-07-26 by (mod) -

Sorry Kathy I just can't guess from your message. Tell me the A/C brand and model and perhaps we can find the answer easily by finding the installation and operation or owners' manual for your unit.

On 2017-07-25 by KATHY

There are two cooling on my air conditioner box one blinks the other one does not which one should I put it on ?

On 2017-01-07 by (mod) - distinction among types of heat is important. Boiler, Furnace, Hot Water, Forced Air, Gravity, Steam, etc.

Thanks John. That note will help other readers.

Your distinction among types of heat is important. There's plenty of confusion to go around.

Many people use the term "furnace" as generic. With rare exceptions, a "furnace" is a forced-air heating system. A "boiler" is a hot water heating system.

"Boilers" are further subdivided into

hot water heating system boilers - hydronic heat (hot water circulates through heating pipes and radiators or baseboards or similar heat-emitting devices)
and

Steam heating systems - steam heat (steam circulates through pipes and radiators and condensate or condensed water from cooled steam returns to the boiler)

Hot water heating systems are further sub-divided into:

forced-hot water heat: a circulator pump pulls water around through the piping and radiators/baseboards
and

gravity-hot-water heat: no circulator pump is used, and hot water moves by natural convection (hot water tends to rise in pipes as it's less dense or weighs less than cold water, and cold water tends to fall in pipes - to return to the boiler).

People use the term "gravity" hot water or "natural convection" hot water heat for these older-type systems. Usually a gravity hot water system will have larger diameter pipes so that enough heat can circulate through the occupied space.

On 2017-01-07 by John

Forgot to say - I have a old boiler 2 wire setup. Neither Steam or Gravity sound correct to me. It`s hot water and it`s pumped around. The point is your average DIY guy maybe confused by terms given by techs who work on these everyday.

The Honeywell CT87K that Fred asked about is for heat only -on the front of the manual you will see which does what.

The manual provided is dual for N/K but they leave out words "hot water" in the ON ON first choice box -which maybe the confusion. Factory setting caused my system to short cycle in 5 minute cycles - try ON ON.

On 2016-12-15 by Jeremy

It's a HumCo 3443-IS wood burning furnace. All I needed was a heat only thermostat and bought the one that was being dicussed. There is no setting for wood. There is for all others, I thought you might have an idea

On 2016-12-14 by (mod) - cycle rate switches for a wood burning furnace

Jeremy I don't know the answer as here we're discussing thermostat switches used on oil or gas or electric heating equipment. What is the brand and model of your furnace and what does the furnace manual say about fine-tuning or internal thermostat switch settings for your heater?

On 2016-12-14 by Jeremy

What should I set the cycle rate switches to for a wood burning furnace

On 2016-12-05 by (mod) -

The arrow or indicator mark on a thermostat is the set point: you place the temperature you want at that indicator.

On 2016-12-05 by Anonymous

Pointer arrow should be at what mumber. Thermostat inside

On 2016-10-02 by Lori

My thermostat won't let me do anything. Almost like it's frozen.

On 2016-03-06 by (mod) - which settings give the longest cycle on the CT87K

Fred:

If you have had trouble finding the manual for your Honeywell CT87K thermostat let me know by Email (see the page bottom CONTACT link) and I'll help you get a copy at no charge.

There you'll see that there are two internal switches that are to be set in this thermostat according to the type of heat it is controlling:

Steam heat: Switch 1 ON, Switch 2 ON

High Efficiency warm air (furnace) heat (90% efficiency or greater): Switch 1 OFF, Switch 2 ON

Gas or Oil warm air (furnace): both switches OFF - this is the factory setting

Electric warm air (furnace): Switch 1 ON, Switch 2 OFF

There is also an internal FAN CONTROL switch in this thermostat

Set the INTERNAL Fan Control Switch inside the thermostat to F for Gas or Oil fired heating systems (this is the factory setting)

Set the INTERNAL Fan Control Switch inside the thermostat to E if your heat is electric or by a heat pump. As Honeywell explains,

"(This setting is for systems that allow the thermostat to control the fan in a call for heat, if a fan wire is connected to the G terminal.)"

There is no traditional heat-anticipator adjustment as was found on some older thermostats we describe in this article series.

So you cannot adjust the "on cycle length or duration" in the thermostat.

On 2016-03-05 by Fred Gruber

Please just tell me which settings give the longest cycle on the CT87K

...

Continue reading at THERMOSTAT SWITCHES, INTERNAL - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• HEAT ANTICIPATOR ADJUSTMENT
• HEAT ANTICIPATOR OPERATION
• MANUALS for HEATING & A/C SYSTEM CONTROLS
• THERMOSTAT DIAGNOSTIC GUIDE
• THERMOSTAT SWITCHES, EXTERNAL
• THERMOSTAT INSTALLATION & WIRING MANUALS

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