HVAC Questions? Ask an HVAC Expert for Answers ASAP
Hi,This is risky business in the blind, not responsible for damages on your end. Click 'accept' when satisfied.. or we can get another expert.
You can jumper the high limit as a test.. if that works, take the jumper OFF for sure its dangerous. replace the limit switch.. you can get a new one at Grainger industrial supply. take this one in for a match up/
I dont know what the other gismo is. we will look at that if its not a bad limit switch. I will be here another half hour this evening, then back in the AM SF calif time, Phil
is there another way to find if switch is bad- i dont have a wire to jumper around it- but have a multimeter to test the voltage etc... when reset button is up (tripped), the voltage between two ends of the button is at 26V approx..
Use a paper clip for a jumper. its only 24v
I powered off the unit, used a paper clip across the switch, same results as before => ignition came on, gas fires up, after few minutes, gas went off, although the fax is running and power is still on => so i am removing paper clip and connecting the leads back to the switch
try pulling one wire off of the limit switch, see what happens. the button popping up tells 90 percent of the story.. some of these fail closed, others fail open.. thats why these oposite tests.
See you in the AM SF calif time. Phil
Hi Phil, Let me repeat to see I understand. I had removed the paper clip, put the 2 wires back on the limit switch. Now you are saying, remove 1 wire off the red button limit switch and see if the unit fires up - I will try in the morning. The other device I had mentioned in the very 1st message is a capacitor
Yes try removing one wire from the limit switch.. some are wired at the factory to operate the furnace when OPEN.. most operate the furnace when ClOSED, we jumpered to check that.
Now, in case its wired to operate the furnace when open, and it closes when its tripped, we are testing it by pulling one wire off.
Get back to me we can go from there, Phil
Phil - OK, to summarize, these are the tests we did so far
(i) power off, power on, press the limit red switch down, ignition comes on, gas comes on, after few mins, switch trips up, and gas goes off.... (ii) power off, remove wires from end of switch, jumper them direct, power on, ignition comes on, gas comes on, after few mins gas goes off.... (iii) power off, remove one end of wire, press the limit red switch down, no activity on ignition or gas, repeat with the other end of the wire, no activity on ignition or gas... (iv) power off, power on, press the limit red switch down, ignition comes on, gas comes on, pull one wire out while gas is on, it goes off right away, repeat same with the other wire, same results... repeat item (i) again to see if anything changed, same results as in (i) before.. PERHAPS time to call some HVAC to check it out in person
Hello again, thanks.. but something doesnt add up... if the switch trips and turns off the gas, then when you jumper it, the switch tripping should have no effect because you have bypassed it.
If its a 2 pole switch and you jump the poles.. the gas should stay on.. take a look for any errors you might have made in jumpering the switch.. does it have more than two terminals on it or something?
did the jumper make good contact on each end you touch to the switch terminals... odds are in the 95% range the problem is with how you jumpered it.
take another look.. we can go from there, Phil
Phil, Yes, this was puzzling me too.. The switch has 1 terminal on each side. I will get some real wire and jumper it properly (rather than a paper clip). Will try it this eve after work. Maybe I should just take the switch out and get a new one to rule out that as a defective piece.
Either way should work..
Phil, your analysis is correct. I hardwired around the jumper and unit comes on.. after few mins flame goes off.. after few mins ignition and flame comes on again.. however the blower sucking the air into the unit does not come on at all and there is no draft in the rooms.. I then went to thermostat and changed from AUTO to ON and the blower works with draft of hot air.. I notice there is a capacitor with 2 leads taped on the blower (perhaps that is defective ? and is this the reason the red limit switch trips when not bypassed..) .. i will test it further later tonight after work
So far it sounds like blower is not comming on because of a bad fan control switch, then the furnace is going off on is high temperature limit safety.
BE VERY CAUTIOUS HERE... DO NOT LEAVE ANY JUMPERS IN PLACE. those are only to test
Find the 'fan control switch' on the furnace wiring diagram.. then locate it on the furnace.. its probably bad.
get back to me. we can go from there/
if the fan motor runs with the fan switch on, the capacitor and fan motor itself are most likely OK