Kitchen Gas Appliances - Roof Forced Air Unit

I have a Carrier unit on the roof top. A/C & Heat. The heater and a/c have been working fine. About 3 days ago the heater was on, but after a while we noticed no heat was coming out of the vents, just cool air.

Tried the thermostat (electronic) in all positions Heat & Cool with negitive rusults. Went up and opened the cover on the heating side of the unit to check. Nothing was working not even the inducer motor.

The only control I have is from the main electrical panel. I can turn the main blower on and off from the electrical panel only.

Could this be a transformer problem. I have fixed this unit before. Two years ago I replaced the ingnition control box.
At that time the inducer fan was working, and the A/C worked. This time
everything is dead?
Frustrated.....Help if you can
Lee

Hello: Lee. Welcome to my Gas Appliances topic & our Do-It-Yourself Web Site.

First item to check is the door panel safety switch. Also known as an interlock safety device. If the switch is defective, not getting current or not fully close, nothing works. Shuts off all power.

Another item is the power vent fan. Check for current at the terminals and at the power source. Transformer should be okay but worth checking also. Line voltage in and 24 - 28 volts DC out.

If the T-Stat has no power, which means it will not turn on the A/C nor just the blower fan, check it. Remove it from the wall plate, check for DC current at the wires. Red & White for heat.

No current there, check that door switch interlock, transformer, all terminal connections on the terminal block, control valve etc. We are looking for a power source break.

Hand spin the induction fan blower to be sure it is not frozen or stick, blades not in contact with the blade cage, etc. Check the vacuum lines to and from the inducer fan.

Often times moisture collects in the vacuum lines. Check for vent restrictions beyond the forced draft induction fan blower, the vent cap, etc for restrictions or blockages.

The ignition control module could be defective and or burned out again. Have it tested at the appliance parts store or local heating agent dealer in your area. Check for current and inspect the terminals to the ignition module, etc.

Unit may be in permanent lock out. Cycling the power may or may not reset the main control board or circuit. Usually happens when a major safety device shuts down the unit and or after the unit cycles several times in an attempt to fireup.

If you need further assistance, use the reply button to add any and all additional information, etc. By doing so, the additional information you add will remain within this posting. Using this method also moves the topic back up to the top of the list automatically.

Regards & Good Luck. Sharp Advice. TCB4U2B2B Business Management Serivces. Web Site Host, Forums Monitor, Gas Appliances Topic Moderator & Multiple Forums Moderator. Energy Conservation Consultant & Natural Gas Appliance Diagnostics Technician.

Personal Reminder:
Buckle Up & Drive Safely.
"The Life You Save, May Be Your Own."

This is a roof top hvac it doesn't have a door panel safety switch.

I have already done this. "Hand spin the induction fan blower to be sure it is not frozen or stick, blades not in contact with the blade cage, etc. Check the vacuum lines to and from the inducer fan."

I checked the T-Stat. It has no power.

What is the power vent fan. Is that like an inducer fan?

Why wouldn't you suspect the transformer since there is no control of any of the systems?

Listen to me. Ha Like I know anything.

Although I have worked on this unit before, I am dreadfully ignorant of the names of some of the parts you are mentioning and what they may look like, but I feel if I have enough info I can sift out the problem.

Thanks for the initial response and I hope you have more after this reply.

Thank You,
Lee

Hello: Lee

Either term can apply to the power vent fan also refered to as a power inducer (induction) fan. All the same. Terms often get interchanged by all of us. I used power vent because that one is commonly known to a wider audience of readers in the forum.

Thinking Out Loud:
Door panel has no safety interlock switch. (Older model)
No power to the T-Stat. (Possible transformer)
120 volt supplied. (Main blower fan operates)

Power vent fan spins freely. No cage contact. No vacuum lines blockage.
List Of Additional Possibilities:

Possible transformer burned out. (Unknown at this time)
No transfomer electrical tested done yet.
Test needs to be done on transformer. (Results posted back)

Test for 120 AC volts in and 24 to 28 volts DC out.
Determine transformer test results. (Need to know)

Vent restriction? (unknown) Check
Possible flu restriction? (unknown) Check

Electronic circuit board? Unknown. (Need to know)

Does unit has an IID pilot lighting ignitor? (spark ignition)

Pilot flame senser? (Element in pilot assembly) If So:
Check pilot flame senser connection to module. (Green ground wire must be clean terminal with connection to ground on frame.)

Fuseable link? (Looks like a transistor) May be burned. (Often but not always) located on front above gas control valve.

Flame roll out safety? (Looks like a transistor) Usual location on front of furnace, just above flash back plate on the firebox.

Lee. Hope all or some of the above helps you to continue the diagnostic testing and thought process of possible causes.

Word of caution. Since the unit has A/C there is 240 volts going to and inside the unit. Use extreme caution when live testing the unit for current. Contacting 240+ volts is highly dangerous.

Good Luck
Work Safely
Sharp Advice

Power vent fan does spin freely. No cage contact. There are no vacuum lines blocked.

The test for 120 AC volts in and 24 to 28 volts DC out is done. The transformer is not burned out, and is working properly.


There are no vent restrictions, and no
flu restrictions.

Electronic circuit board? This unit does have one.

Does unit has an IID pilot lighting ignitor? YES

Pilot flame senser? YES

If So: Check pilot flame senser connection to module. (Green ground wire must be clean terminal with connection to ground on frame.)

I checked this and everything is OK.

Fuseable link? (Looks like a transistor) May be burned. (Often but not always) located on front above gas control valve. If there is one, I haven't been able to spot it.

Flame roll out safety? (Looks like a transistor) Usual location on front of furnace, just above flash back plate on the firebox.
This unit doesn't have one.

One thing I did determine is that the power vent fan motor was not recieving voltage from one of the circuit board leads. I jumped from a known source of
current and the motor started running.

Come on back.
Lee Latting

Lee

Possible cause and problem may have been found. Power vent fan must be operational or the ignition system goes into lock out mode. Which means the fan is not defective if it runs.

More likely the lack of current from the main circuit board causing the problem. Which means if the is no current from the board, it will have to be removed and possibly tested.

Not all heating supply parts stores will do the testing, since they do not make repairs to any parts on the board. However, if that is the cause it will have to be replaced.

Some savy electronics techs can determine which part is defective or circuit is defective on motherboards. Finding such a tech is a problem in and of itself.

Unless you can see a burned up part, find a replacement part (usually at electronic part stores) and make the repair yourself. The price of main circuit boards are rather pricey. Good Luck.

Sharp Advice

If I did want to buy a board, are there any online sites I could buy from?

The other problem I have is finding a serial #/Model # on this unit. The only information I can find is unreadable. Over the years the sun and weather have oxidized the surface.

Any Ideas?

Lee Latting

Lee

I personally do not recommend and rarely suggest buying parts of this type and cost online, unless it is directly from a mfg web site. No brand name parts do not ensure quality, only price.

In my opinion, you should visit the local heating agents retail stores with as much info as possible and the part in hand. Doing so will likely be productive, since the parts counterpersons know what parts where used in which brands and models.

In my opinion, anything other is risky should the mail order part be an incorrect part, fail prematurely, etc. Too risky to buy without knowing exactly what is needed and the quality.

Your choice. I prefer an in person purchase from a local business should you have other needs, questions, installation problems or after sale warranty needs.

About all the info on this last suggestion as I can provide. Find the part, install it and post back the results. Hopefully you'll have heat and the furnace will be a repair project completed.

Good Luck
Sharp Advice