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gotoman for ac
gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 4766
Experience:  51 years of experience install design and repair of all makes and models of AC
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the blower continues to run even after furnace has completed

Customer Question

the blower continues to run even after furnace has completed it's cycle. then blows cold air. I've checked thermostat settings and they are set to have the fan on auto. could this mean a new thermidistat is required? except for this problem furnance and ac units run fine.
Submitted: 3 years ago.
Category: HVAC
Expert:  Douglas replied 3 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

Remove the blower door and disconnect the Green thermostat wire from G and run the unit again, the blower will stay on for about 2 minutes after the burner shuts down. If it stays on, let me know.
Customer: replied 3 years ago.

OK. give me a few minutes.

 

1. disconnected G wire on blower.

2. turned unit back on.

3. furnace came on, heated air, & blower continued to operate.

4. after furnace stopped heating air blower stayed on.

 

Q: Can tthere be a setting on blower unit wherein the blower stays on all the time

no matter setting on thermostat?

Expert:  Douglas replied 3 years ago.
This is a bryant 90i? That would make it something like 15-20 years old? Just so I am on the right unit...
Customer: replied 3 years ago.
About right. first 4 digits of serial # 2802 indicating week & year of mfg. I recall this unit being installed in mid 90s.
Expert:  Douglas replied 3 years ago.
Ok... let me look at a few things... just a moment.
Expert:  Douglas replied 3 years ago.
Can you give me the model number?
Expert:  Douglas replied 3 years ago.
If this is the one I am thinking of, I want you to power it down and look at the control board. In the lower left corner, is there a small dial (Very small) that has the word blower below it, 60 on the left with an arrow clockwise around the switch to 240 on the right? IF so, lets try turning that dial counterclockwise slightly so its straight up and down.

Customer: replied 3 years ago.

OK. give me a few minutes.

no I do not see the dial on the control panel. I am looking at the control panel hidden by the blower panel at the bottom of the furnace cabinet. It matches Fig 24 in Bryant's installation manual for this furnace. But I do see dip switch assignments for the CF fan operation all in the default position. According to Bryant's notes in their schematic this may allow for continuous fan operation no matter the signal from the thermostat?

 

Expert:  Douglas replied 3 years ago.
I would need the actual model number to look at that exact manual to what we are looking at. Im looking at the Carrier version which should be the same. Depending on the model all should be down but 6 should be up for natural gas and down for LP.

I meant if you look to the left of the swtiches there are two dials, do you see them? The one we want to turn is the furthest one
Customer: replied 3 years ago.

No we must be looking at different things. The control panel I am looking at has no dials at all. I have looked al around the blower unit and traced the elec lines - both line voltage & 24V. no dials I can see.

 

Here is what I will do - I'll call in a local company to see if they can trace down the issue. It does appear as if the thermostat has been bypassed in some way inside the furnace unit.

 

Thanks.

Expert:  Douglas replied 3 years ago.
Thats why I need the specific model number.. If you can provide that I can look into it. 90i is just a series.
Customer: replied 3 years ago.
355MAV060120F
Expert:  Douglas replied 3 years ago.
Ahh ok thats different.

Ok when the burners shut off, are you seeing any of the codes appear on the LED lights?or is just the blower staying on?

Customer: replied 3 years ago.
blower just staying on. no codes
Expert:  Douglas replied 3 years ago.
Hmmmm... Regarding the continuous fan note, thats talking about which speed you would want IF the fan switch were on.

I want to do a test.

Turn the power off...

Remove the red thermostat wire

FInd the dip switches marked SW1 and turn switch 6 on, repower the furnace and let me know what it does.... does the blowe motor shut off at the end of the test?
Customer: replied 3 years ago.

yes blower motor just turned off after component test.

Expert:  Douglas replied 3 years ago.
Ok. rehook the red, make sure the thermostat is not calling for heat, power it up and do a heat cycle again by turning thermostat up, leave it calling for heat until at least the burners have been on for 1 minute before turning the thermostat back down.
Customer: replied 3 years ago.
leave SW-6 in same position?
Expert:  Douglas replied 3 years ago.
no, sorry, you can put it back to off
Customer: replied 3 years ago.

after rehooking red, resetting SW-6 and powering up furnace I turned up thermostat.

furnace cycled thru burners on, blower cycled up to high sped then reset at lower speed. I turned down thermostat, burners shut off. blower remains running at lower speed.

Expert:  Douglas replied 3 years ago.
That blower is an ECM motor, its powered all the time by line voltage, it switches signals to turn on, off or change speed by manipulating low voltage on the 16 pin connector (even though only about 5 wires are there)

In the test, the motor shut off so I know the module is not corrupt on the motor.

The delay is preset in the software, but we may need to revisit SW1 switches 7 and 8. Note those two switch positions, turn the furnace power off switch each of those two opposite of what they are now. Repower the furnace and try a call for heat again. These two switches deal with the blower off delay in heat.
Customer: replied 3 years ago.
same result. blower remains on at lower speed blowing cool air well after burners have shut off.
Expert:  Douglas replied 3 years ago.
Is this something that recently started?
Customer: replied 3 years ago.

I am not sure. I encountered this in a home I recently acquired. My information comes from the former owners. One of the family members has breathing issues and needed a constant supply of air circulating in the home to help her breathe. My guess is that the furnace/ a/c unit was installed to constantly circulate air.

 

The former owners just built a brand new home with medical quality air handling & filtration in addition to being chemical-free. I am trying to get the furnace back to what I would anticipate would be 'normal' mode.

 

Expert:  Douglas replied 3 years ago.
If there is no wire connected to G terminal, the fan should switch off at the end of a call for heat.

Now we need a wire on G for cooling to work but it would go directly to the G on the thermostat and not be hooked to anything else.

So if someone shorted 24v to the Green wire or G terminal the blower would stay on low.

Was anyting but the green wire from the thermostat bundle connected to G
Customer: replied 3 years ago.
no just one green wire at the terminal. I wonder if there are hidden connections/shorts that are not apparent?
Expert:  Douglas replied 3 years ago.
well, didnt we try it without the green wire attached?
Customer: replied 3 years ago.

yes and without improvement. I bet at this point I'll get a local firm to sort thru the controls. I am betting they will find a very specific wiring set-up done to accomodate the requests of this family when the unit was first installed.

 

Thank you for your time and help.

Expert:  Douglas replied 3 years ago.
Lets try one more thing...I may be missing something.

Do you know how many tons the Air conditioning is?

See SW2 (the upper bank of 3 dip switches?)
Expert:  Douglas replied 3 years ago.
For the sake of this test, power it off again, Find SW3 switch number 2 and move it to the on position, power up and try one more time.
Expert:  Douglas replied 3 years ago.
If that works, we need to know how many tons the outdoor unit is. Power the furnace off and set the SW3 switches like this...

1 1/2 ton - 1 on, 2 and 3 off
2 ton - 1 off, 2 on, 3 off
2 1/2 ton - 1 and 2 on, 3 off
3 ton - 1 and 2 off, 3 on.

If you dont have cooling, it doesnt matter use the 2 ton setting.

Customer: replied 3 years ago.
tried switch #2. same result as before. blower stays on at low speed. I have just set SW3 switches to 2-ton settings as I don't know AC tons. also same result with blower on low speed.
Expert:  Douglas replied 3 years ago.
ok...well I wish I knew what to suggest next. Not being there, I cant really do much else.

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