Furnace service, repairs and efficiency improvement

by G&S Mechanical Services:

With emphasis on service and repair not replacement 

    I get many inquires about problems with the pilots on gas furnaces. Unless you have a newer furnace with an electronic or electric ignition system you probably have a standing pilot. That is a system where the pilot is always burning and is used to light the main burner. There are two basic systems used with standing pilots. The first is the thermocouple system that uses two dissimilar metals to provide electricity to hold open the pilot valve on the gas valve, once you light the pilot. The main valve is operated by 24 volts from a transformer.

    The thermocouple is relatively generic. There are some versions that put out slightly more voltage than others and have a longer life rating. However the ones that you buy at the home stores should work fine.

Standard standing pilot 24 volt gas valve. Notice the connection for pilot tubing on the left and thermocouple in the center closest to you. This is the most common
Image of standard standing pilot gas valve (4 inches wide). Notice pilot tube connection on left side of valve and thermocouple connection in center of valve. Only standing pilot systems have a pilot position on the valve knob (you must push to light the pilot).
If you have an electronic (intermittent pilot ) system The valve will have the designations MV MV/PV PV (main valve pilot valve and common) and no connection for a thermocouple. Hot surface ignition/direct spark valves will not have a pilot tube connection.
Standard thermocouple for standing pilot system. Note the Milli-Volt Thermopile image below
Image of standard standing pilot thermocouple (available at Lowes or The Home Depot). Notice the connection on the end is the same as the connection on the gas valve to the left. This device is used on all standing pilot systems including water heaters and commercial gas equipment except Millivolt or some General Electric furnaces they have a special gas valve that has a pilot gas output but no thermocouple connection just a main valve (proven pilot). if you need to replace one of these turds, replace it with a standing pilot valve or an intermittent kit.
    The second system is called a Milli Volt system. The pilot heats a larger device that generates a bit more electricity, enough to operate the main valve too. The theory is that if the pilot is not lit then the main burner won't work and you don't need a 24 volt transformer. This system is used a lot on mobile home, RV furnaces and swimming pool heaters because they will work with out utility power.
    If you have problems where the pilot won't stay lit after you release the button or knob then replace the thermocouple. If you still can't get the flame to stay try another thermocouple then replace the gas valve (sorry there is nothing else that can be done). The problem is not with your thermostat or the furnace transformer. Note: the thermocouple has nothing to do with the main valve opening so don't replace your thermocouple if the pilot is already lit.
The exception is on some old furnaces they have a box with the thermocouple and a rest button for the main burner (usually made by Penn Baso). I will show one when I get a picture.
Image of Millivolt thermo-pile. Notice the Cabling and the lugs on the end of the wires make this guy unmistakable.
Image of Millivolt Thermo-pile, the wire ends give it away for sure. If you need to replace one of these my personal preference is to convert it to a standard standing pilot 24 volt system.
Newer furnaces will have intermittent pilot, hot surface ignition (HSI) or direct spark. The intermittent system uses a gray control box made by Honeywell, Robertshaw, Johnson controls, Fenwall, or White-Rodgers to spark the pilot and detect that it is lit, these boxes are very reliable but when they fail you are looking at at least $100 for the box.
    Another system is called cycle pilot. The pilot is lit by a pilot re-light module and a mercury flame sensor allows the main valve to open. This same module can be added to a furnace that has a nuisance pilot that blows out frequently.
    Proven pilot has a mercury sensor in the flame to allow the main valve to open (used on general electric).
    Hot surface ignitors are being used more and more today. The major advantage is the controls are cheaper. They just need a flame sensor to detect the main burner and open the main valve. The problem with them is the ignitor burns out and you are cold until you can get another one. when they fail you can either replace the ignitor or convert the sensor board to direct spark. If the sensor board fails I would just convert it to single wire direct spark, that way you eliminate future problems with the ignitor (you need a different one for every brand of furnace). To test an ignitor connect a 120 volt bulb (about 25 watts) in its place or use a multi-meter. If you get 120 volts but the ignitor doesn't glow then it is bad. When replacing it be careful not to touch it with your fingers. Oil on you skin can shorten it's life.
    Standing pilot is the most reliable and fool proof but intermittent, HSI or direct spark is good for situations where lighting the pilot is a nuisance and property damage could result.
Picture of Hot Surface Ignitor (HSI). From Carrier furnace. This one is Burned out.
Image of hot surface ignition  (HSI). This one is broken. 
It is a good idea to get a few of these and keep them near the furnace so when they burn out on the coldest day of the year when it is snowing you will have a spare. Some new systems like Lochinvar boilers use this device as the flame sensor (isn't that sneaky)

Please read the companion to this page that helps you better identify which gas system you have.
Please view this page which has other images of an intermittent pilot system.
This page is a better explanation of how the mercury flame sensor system works.
If you need to set your heat anticipator you will need a clamp on ammeter.
If your pilot light blows out or is a nuisance to relight add a relight module
Read about repairing an Amana furnace with a combustion blower./
If you also have a heat pump this page will help you repair it.
What is a cracked heat exchanger.


 Read more that I have written on this subject at the freegas page
 and the question page .
To find out more about fan and blower motors click here
To find out what controls the fan in the heat mode or proper settings
To get parts look to Grainger or Johnstone.
To solve a problem use the link to the form below.
 Scott Meenen

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Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation.
Anything having to do with Heat and Energy.
Serving Maryland, DC, and Northern VA.

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                 This text written by: Scott Meenen
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