:confused: Day & Night by Carrier central heating and a/c unit

a/c unit model # 518CXX057000MACA date of mgf=1791

heating unit model# 394JAZ060080AAFA date of mgf=4592

Efficiency= natural draft?

Type= forced air

Fuel Type= NG

Problem: After changing original Honeywell T-stat with newer Honeywell digital
model # CT3355A 1003-1, the fan blower motor is stuck on and cannot be controlled by new t-stat (auto or fan lever). I can only shut off by disconnecting the ac cord from the outlet or taking the heater grate off & safely disconnect. I tried putting the old t-stat back and blower is still stuck on. Secondly, the auto electric igniter for the heater is not working, so that the heater does not come on. Prior to changing t-stat all systems worked OK.

When I was disconnecting the colored wires from the old t-stat, I may have touched the Red with the White wire but other than that I was able to connect the W, Y, G and Rh wires which has brass jumper to Rc on the new unit. The digital display works with 2AA batteries.

Is there a reset button or a relay "stuck on" inside the fan blower unit? What and where is the circuit board for this unit? I was also thinking the new t-stat may have been defective? Could someone please advise me?

Thanking you in advance!

Sounds like an open limit switch in the furnace.

You have blown the low voltage fuse on the control board inside the furnace. It will be a 3 Amp or 5 Amp automotive fuse. If the fuse is blown the blower will run until replaced. It's a small yellow fuse center of the board get it at any hardware or auto store. Turn power off first before replacing.

Sounds like you know a little about the furnace so I will just tell you what I think and if that does not help then post more info.
Like NorthwestHeat said above: Sounds like an open limit switch in the furnace. that is one of the first places that I would check. One of the wires going to the limit switch and just before it connects to the limit is a small fuse that looks like a small firecracker, use a test wire and bypass this little fuse, if it is the problem the fan should stop as soon as you bypass it. If this is the problem the fuse will have to be replaced. ****Do Not Leave The Bypass Wire Connected********

Next I would look on the circuit board for a blade fuse just like an automobile fuse. It will be a 3 or maybe a 5 amp fuse. It should be located on the left side of the Circuit board.

Next make sure that the red and green wire at the thermostat have not become shorted together.

If it has not been fixed by now use your voltage meter, one that will Read Out 24 volts and check your transformer at the secondary terminals on the circuit board. On some of the newer furnaces if the transformer is burned out the fan motor will run as long at the unit has power.

Sounds like you know a little about the furnace so I will just tell you what I think and if that does not help then post more info.
Like NorthwestHeat said above: Sounds like an open limit switch in the furnace., that is one of the first places that I would check. One of the wires going to the limit switch and just before it connects to the limit is a small fuse that looks like a small firecracker, use a test wire and bypass this little fuse, if it is the problem the fan should stop as soon as you bypass it. If this is the problem the fuse will have to be replaced. ****Do Not Leave The Bypass Wire Connected********

Next I would look on the circuit board for a blade fuse just like an automobile fuse. It will be a 3 or maybe a 5 amp fuse. It should be located on the left side of the Circuit board.

Next make sure that the red and green wire at the thermostat have not become shorted together.

If it has not been fixed by now use your voltage meter, one that will Read Out 24 volts and check your transformer at the secondary terminals on the circuit board. On some of the newer furnaces if the transformer is burned out the fan motor will run as long at the unit has power.

Thanks T-Top and letmetellu for your comments:)

My thought is that something very simple is preventing the new t-stat from operating the heating & a/c unit?? I carefully opened up the blower access compartment by removing the grate and located the control circuit board behind a metal plate with yellow warning sign. It contained a transformer and the control circuit board with various solid copper wires connected to board terminals & also thinner colored t-stat control wires. There are two cube-like relays (one for cooling and one for heating). I looked several times at the whole board and did not find either standard fuses or any newer automotive fuses plugged in on this board. There is signage that states "This furnace is equipped with a fusible element or manual reset limit switch in burner compartment and/or draft diverter..." There is also an igniter/lockout control unit in the control access area. I cannot get a service appointment until next week :mad: , so I'm open to any and all suggestions until then!:confused:

Thanking you in advance!

It still sounds like a low voltage problem to me. If it worked before you changed out the thermostat and you did not turn the power off while changing I think you shorted the 24 volt circuit. At that time some of the HK boards did not have a fuse. If you have a cheap volt meter to read 24 volt AC, check from R to C at the furnace control board. To do this you will have to put some tape on the door switch so power will be on.(removing bottom door will open the door switch and kills power going to unit) Do not forget to remove tape from door switch when finished.

It still sounds like a low voltage problem to me. If it worked before you changed out the thermostat and you did not turn the power off while changing I think you shorted the 24 volt circuit. at that time some of the HK boards did not have a fuse. If you have a cheap volt meter to read 24 volt AC, check from R to C at the furnace control board. To do this you will have to put some tape on the door switch so power will be on.(removing bottom door will open the door switch and kills power going to unit) Do not forget to remove tape from door switch when finished.


Thanks T-Top

I did the above steps and only got 0.00 volts between R and C on the control board. I tested a few other positions and got same 0.00 reading. This was while the fan blower was running and the t-stat was on but had no control over heating nor cooling. What did you mean by "HK boards" in above answer? Thanks for your responses and does anyone else have any ideas on how to reset my Day & Night unit and turn off the blower unit?

:confused: Day & Night by Carrier central heating and a/c unit

a/c unit model # 518CXX057000MACA date of mgf=1791

heating unit model# 394JAZ060080AAFA date of mgf=4592

Efficiency= natural draft?

Type= forced air

Fuel Type= NG

Problem: After changing original Honeywell T-stat with newer Honeywell digital
model # CT3355A 1003-1, the fan blower motor is stuck on and cannot be controlled by new t-stat (auto or fan lever). I can only shut off by disconnecting the ac cord from the outlet or taking the heater grate off & safely disconnect. I tried putting the old t-stat back and blower is still stuck on. Secondly, the auto electric igniter for the heater is not working, so that the heater does not come on. Prior to changing t-stat all systems worked ok.

When I was disconnecting the colored wires from the old t-stat, I may have touched the Red with the White wire but other than that I was able to connect the W, Y, G and Rh wires which has brass jumper to Rc on the new unit. The digital display works with 2AA batteries.

Is there a reset button or a relay "stuck on" inside the fan blower unit? What and where is the circuit board for this unit? I was also thinking the new t-stat may have been defective? Could someone please advise me?

Thanking you in advance!
Check to see if your unit has relays, sometimes when they go bad, the operation of sequence does not work properly.

Check low voltage on control board at sec 1 and sec 2. That is the low voltage from the transformer if you don't have 24 volts you have blown the transformer (cheap part to buy) if you do have 24 volts you have a open limit switch.

check to see if your unit has relays, sometimes when they go bad, the operation of sequence does not work properly.

Thank you for responding! There are two cube shaped relays (labeled CFR & HFR) hard soddered? On the blower control board. I tapped on both with end of screwdriver but no response!

A stuck blower relay will not keep the heat from working.

Check low voltage on control board at sec 1 and sec 2. That is the low voltage from the transformer if you dont have 24 volts you have blown the transformer (cheap part to buy) if you do have 24 volts you have a open limit switch.

Thanks T-Top:)

There was no voltage 0.00 between sec 1 and sec 2. There was 120.0 volts between line 1(black) and line 2 (white) on the board which looks like ac power to disconnect switch. There was 120.0 volts between PR 1(black) and PR 2(white) on the board which looks like ac power to transformer. I'm off to the Sat open electric store to look for a replacement
24v transformer. Is there anything you would advise me before I try to replace, so that I don't short out again?

Thanks again T-Top for your help:)

You have it fixed 120 volt 30 va sec 24 volt ac transformer will fix it. Get it at a supply house or hardware store. If you replace it yourself besure to turn power off first.

You have it fixed 120 volt 30 va sec 24 volt ac transformer will fix it. get it at a supply house or hardware store. If you replace it your self besure to turn power off first.

I checked a couple of similar size 120v to 24v transformers 40va and the output voltage read 27.5v and the other was 26.2 v. Is this +10% voltage differential going to be a problem for the control board or the t-stat? I was afraid to purchase or replace until I was positive?
Secondly, will the panel breaker for the heater/blower be on a single 15 or 20 amp circuit?
The reason I ask is that the blower motor has a grounded outlet cord which plugs into the double receptacle in the wall next to the unit. I believe this supplies the only 120v to the blower motor & disconnect switch, the 24v transformer and the control board?

Thanks again T-Top and everyone else who contributed:)

That's the one you want it will work fine. You can also buy a inline low voltage fuse holder to keep from blowing the transformer. 5amp low voltage fuse will work. Good luck and let us know how it turns out.

Thats the one you want it will work fine. You can also buy a inline low voltage fuse holder to keep from blowing the transformer. 5amp low voltage fuse will work. good luck and let us know how it turns out.

I presume the inline fuse holder is on one side of the 24vac lines out? I'm guessing that just unplugging the blower motor 120v ac from the wall will turn off power to control board & heater? I'm also planning to shut off the natural gas until transformer is replaced & checked. When it is less hectic, I'll probably have a licensed HVAC company double check system and perform routine maintenance.

Thanks again T-Top:D

I have taken out the fan on my Carrier and the "fan itself is not connected to anything on the motor, is just free spinning and make laud noise,

How should the fan be connected to the motor, can anyone help?

In Advance Thank you
Noise,

I have taken out the fan on my Carrier and the "fan itself is not connected to anything on the motor, is just free spinning and make laud noise,

How should the fan be connected to the motor, can anyone help?

In Advance Thank you
noise,


It sounds like you need a new blower wheel

I have the same problem and same experience with changing the t-stat to a digital Lux and experienced the same results of the blower not shutting off unless I unplug or shut the circuit. Before I decide to invest in a new furnace, I would like to attempt to reset the limit switch. Can someone tell me (show me with a link) where the open limit switch might be or looks like? Thanks for the help!

https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313-2.html#post269193

Thats the one you want it will work fine. You can also buy a inline low voltage fuse holder to keep from blowing the transformer. 5amp low voltage fuse will work. good luck and let us know how it turns out.

TIP: 2 fully insulated female terminals make a "quick and dirty" blade type fuse holder.:D

Simply cut the hot secondary lead, strip and crimp one fully insulated 1/4" female spade terminal onto each cut side, and plug one leg of fuse into each terminal. Tape if desired.

Show to bypass board to make fan run on the air handler on central air and heat unit.

Are you asking a question, or responding to this "oldie"? Start a new thread & provide brand/model# of indoor unit.