Could a heat limiter be interfering with 24 volts getting to the furnace valve?
Yes. There could be a couple of high limit safety devises in the circuit that can open the circuit to the gas valve. Pending your furnace type If your thermostat is calling for heat and the furnace tries to start your transformer is working. High limit safety controls are normally closed. If you have a open limit switch you will not have any continuity through it using an ohm meter or if you test it with a volt meter you will have 24 volts across the terminals of the safety devise. Other problems can be pressure switches, hot surface igniters and control boards. most newer furnaces have a control board that will flash a light code to help trouble shoot the system. Count the flash pattern and look on one of the furnace doors for the flash code directory.
SOURCE: a carrier furnace
check the relay
the primary should have a low resistance
there should be a distinct click when 24 volts is applied to the primary
the secondary should be conductive when the primary is energized (contactors can corrode)
not a solution something to check
SOURCE: I have a Trane gas
almost a classic sign of a bad gas valve. pilot is lit so gas is available. FAN COMES ON AND 24 VOLTS TO gas valve so there is a call for heat . gas valve doesn't open so it looks like its bad.
SOURCE: AC Control Transformer keeps failing.
well it has blown the fuse on the transformer, then you have coil switch bad.It could be the coil on the contath in for cooling.But can check by taking the yellow wire a loose and see if the don't blow. The red wire is one side of the transformer and the com could be any color.But look at the red wire because it is the main wire that travel the voltage throuhg the unit.
SOURCE: My ac unit does not
Please exercise due caution in checking voltages in live circuits!
24 volts sourced by the transformer, through the thermostat, then through the contactor (turns on power to compressor/fan) and returns to the transformer, If the thermostat is closed (calling for cool) you will see no voltage across it, but instead the contactor should be energized. Please note, 200 volts seems odd, you should have between 215 and 235. It sounds more like the contactor relay coil may be open. Is the contactor/transformer one piece?
SOURCE: Blown Transformer heat pump
blown transformers are usually from a low voltage short the first thing i would do is wire a automotive or resetable 3amp fuse in line with the transformer so you dont blow transformer after transformer the wire coming off the transformer that say 24v take that wire a couple of inches down and cut it and install the fuse in series there
24v----------------
24v----------x----------
24v--------{3ampfuse}----------
now you can go through your low voltage wires one by one and test them unhook all the wires to your outdoor condenser at your indoor unit(furnace or airhandler) and try turning it to cool now if you blow the fuse the short is between the tstat and the indoor unit if the fuse is still good leave it on cool and hook up one wire at a time to your outdoor unit starting with R a lot of times your contactor or your defrost board will short out on you also look for pinches in wires wires with cracks in the insulation and wires touching metal this situation can be a real pain sometimes i would recommend calling a hvac company if you start to get stumped
hope this will help you
PROBLEM #1 - HOT SURFACE IGNITOR DOES NOT GLOW RED
(Remember to wait for purge time, 17/34 seconds, on models so equipped).
Possible causes
A. No main power
B. Faulty transformer
C. Faulty thermostat
D. Faulty limit switch
E. Faulty blower interlock switch
F. Faulty hot surface ignitor
G. Faulty ignition control
Solution
Step #1
Perform normal system checks of main power, transformer, thermostat, limit
switch, blower interlock switch, and replace faulty component.
Step #2
With power on and thermostat set at its highest position, check voltage
between "TH" and "TR" on HS780 ignition control. If 24 volts is not present
check output at transformer. If no, replace transformer. If transformer
checks out OK look at other controls in the 24 volt circuit I.E.: thermostat,
limit switch, blower interlock switch. Check for 120 volts between "neut"
(L2) and "120" (L1). Check for 120 volts at the ignitor across "IGN" and
"IGN" terminals on the HS780 ignition module. If 120 volts is present,
replace ignitor. If no, replace the 780 series module
PROBLEM #2 - IGNITOR GLOWS RED BUT MAIN BURNER
WOULDN'T LIGHT
Possible causes
A. Improper ignitor alignment
B. Faulty ignition control
C. Faulty gas valve
D. High inlet gas pressure
E. Polarity reversed
F. No earth ground
Solution
Step #1
Check for availability of gas at gas valve. Make sure the manual valve
upstream of the gas valve and the gas valve are in the full on position. Also
make sure gas is being supplied to the system at the proper pressure, too
high will lock-up the valve.
Step #2
Check proper polarity of "NEUT" and "120" on the ignition control by reversing
the wires, if reversed gas valve will not open. If this does not solve the
problem, replace wires as they were.
Step #3
Check for proper ignitor position.
Step #4
Check for a good earth ground by using a meter connected between L1
and furnace chassis. Should read 120 volts, if no, check and/or repair ignition
ground wire or ignition control mounting screws.
CAUTION POLARITY MUST BE OBSERVED
Step #5
Check for 24 volts at the gas valve terminals; "1" and "2" on a 7200 series,
"M" and "C" on a 7100 series, "TH" and "TR" on a 7000 series. If yes,
check wire integrity or change the gas valve. If no, change the 780 series
ignition module.
That is pretty much all of the troubleshooting required.
Thanks for choosing FixYa,
Kelly
999 views
Usually answered in minutes!
×