Regular Guest
Hey all, I havent worked on many 90% and this is an older one in a dirty environment ran w/o filter for i dont know how long.. Heil model: NUGK100DH0Z.
primary heat exchanger looks ok but im wondering if the 2ndary exchanger could be plugged with dirt or something. Furnace goes thru ignition sequence and lights off, temp where fan limit/high limit sticks into heat exchanger reaches 200° then burner drops out. i know I have a bad fan limit, blower runs constantly even without a 24 volt signal from the stat. i can shut blower off by manipulating the switch next to the high limit in the assembly. Im worried if i just change out the fan limit i will still have the same problem. i have jumped each safety individually and tried to eliminate them as the cause. without high limit/fan control in place the furnace will run. pretty sure thats the issue, but i think im not moving enough air over the high limit. flue is clear, combustion air is from the space. any help would be appreciated. im a 4th year apprentice, not a home owner but like i said i dont do much work on 90%ers. i work commercial, and mostly as an installer. thanks all.
Professional Member*
Shouldn't be that difficult to determine if the secondary is plugged. Get down and dirty with a flashlight and safety glasses. While you're at it make sure the blower isn't caked with filth from no filter.
Why do you think the high limit is bad? What is it rated at?
"If common sense was so common, everyone would have it."
Banned
sounds as if you are on the right track .... not sure how that model furnace / heat exchanger is designed because when I did a search for that furnace the only search result that showed up was your thread. If the secondary heat exchanger is designed like most are, with coil fins, I would definitely be concerned the fins were covered with dirt / dust, that could definitely make the furnace overheat and cycle on limit. So yes, you should confirm the secondary heat exchanger is clean or you will most likely have the same problem, also, running a furnace hot can damage the heat exchanger .... as mentioned inspect the secondary heat exchanger and also make sure the blower assembly / squirrel cage / motor is clean ...... ( just cleaned a blower assembly / squirrel cage / motor and secondary heat exchanger the other day due to the air filter not being replaced as needed )Originally Posted by SeampYankee
are you referring to a combination fan / limit switch ?
measuring your return and supply air temperatures and comparing them to the ' temperature rise ' on the furnace data sticker is a good way to determine if the furnace is operating within its acceptable ' temperature rise '
Global Moderator
Pull the blower and look at the secondary HX. Don’t jump things - measure! A 4th year apprentice should know that.
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Regular Guest
yep, its a combination fan limit switch. Blower runs constantly so im pretty sure its bad. Return air was 62, supply air was 115 before it trips out. Max supply air temp on nameplate is 170°. it trips at 200° according to my probe. Unit runs with fan limit outside heat exchanger section.
Regular Guest
youre right.
Professional Member*
Does this have ac also?
"If common sense was so common, everyone would have it."
Regular Guest
no, no A/C coil
Regular Guest
I jump things all the time! Is that wrong? Why? Help me understand, I want to learn
Banned
you need to apply for professional status to go into further detail
in this instance the limit switch protects the heat exchanger from seeing excessive heat, you should never jumper or bypass any safety switch
Professional Member
if trying to figure out if its bad we do it from time to time but never more then for testing reasons and never to by pass one
Professional Member
A 90% furnace with " yep, its a combination fan limit switch" ?? Hhmmm
Professional Member
Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your God’s law, I will also forget your children.
"You've got to Stand for Something or You'll fall for anything" (A. Tippin)
Mat_15:24 But he answered, “I wasn’t sent to anyone but the lost sheep of the house of Israel.”
Professional Member
Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your God’s law, I will also forget your children.
"You've got to Stand for Something or You'll fall for anything" (A. Tippin)
Mat_15:24 But he answered, “I wasn’t sent to anyone but the lost sheep of the house of Israel.”
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