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.
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. |
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, 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.
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) |
NEXT
Please
read the companion to this page that helps you better identify which gas
system you have.
NEXT
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.
ICE
RINK EQUIPMENT FOR SALE
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
To
solve a problem use our furnace service fill out form
To
solve a gas ignition problem use this service fill out form
To
solve a heat pump or air conditioning problem use this form
Written By: Scott Meenen
N3SJH of:
G&S MECHANICAL SERVICES.
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.
Contact
us
If I can improve this page please let me know.
Email us at: jsmeenen@toad.net
This text written by: Scott Meenen
All files
can be seen at the G&S Mechanical Framed home page
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