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Old 04-02-2011, 01:38 PM   #1
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Bryant Plus 90t with New Thermostat


Hello,

My city is sponsoring energy efficiency audits in my neighborhood and my wife signed up to have an audit done while I was at work yesterday. As part of the audit they replaced our non-programmable thermostat with a cheap programmable one. The furnace is dual stage (Bryant Plus 90t), but my understanding is that it does not need a dual stage thermostat to operate in dual stage (our previous thermostat was not dual stage). I typically leave the fan ON to keep air circulating throughout the house, this furnace runs the fan at about 60% when the heat or AC is not running. Well this morning I realized the fan was set to AUTO, so I changed it to ON. Much to my surprise the fan turned on at 100%, I left it there to see what would happen and the fan continued to blow at 100% for 20 or 30 minutes. When temperature dropped enough for the heat to kick on it actually dropped to fan setting down to the 60% that it typically runs at when the heat is not on. Once the heat kicked off the fan cranked back up to 100%. It sounds to me that the thermostat is not communicating properly with the furnace. I spoke to the company that installed the furnace (not the company that swapped the thermostat) and he said the Bryant furnace needs to be programmed to properly communicate with the thermostat that is installed. Of course the company that installed the thermostat cannot be reached today.

Do the symptoms that I explained make sense to anyone as something that could actually happen? Is it an easy fix? The company that installed the furnace will charge expensive weekend rates to come diagnose the problem today.
Is this something that could damage my furnace, or even worse be dangerous for my family?

Thank you for your help.

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Old 04-02-2011, 03:05 PM   #2
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Bryant Plus 90t with New Thermostat


Should not be possible to have that happening with a basic 4 wire thermostat. In the ON position it energizes the G terminal and whether it is programmable or not nothing changes. There is the very real possibility that they mixed up the wiring and it is energizing the AC wire. If the AC has the high voltage shut off for the winter it won't run and get damaged. It will energize the contactor for it and tell the circuit board to switch the fan speed to 100%. You definitely need someone to properly check the wiring soon. The circuit board may now be damaged. In the mean time leave the fan in Auto and it should be safe. Shut off the breaker for the AC or its outdoor switch. It may be off due to the low ambiant control and as soon as it hits 50 deg F could start and get damaged. If damage has occurred then print this post as most qualified techs will understand it and you may have grounds for a complaint with the 2nd company. Cheap thermostats have cheap electronics in them and can damage circuit boards. I would not necessarily keep it.

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Last edited by yuri; 04-02-2011 at 03:13 PM.
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Old 04-02-2011, 03:38 PM   #3
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Bryant Plus 90t with New Thermostat


Depends what thermostat your original one was.

When do you get the energy audit report?
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Old 04-02-2011, 07:45 PM   #4
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Bryant Plus 90t with New Thermostat


The original thermostat was a round Honeywell version. The audit was done yesterday (Friday). I think I'm going to get in contact with the company that swapped the thermostat first thing on Monday and give them a chance to send someone out and make things right.
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Old 04-02-2011, 07:47 PM   #5
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FYI, it was a Honeywell T8775C.
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Old 04-03-2011, 10:32 AM   #6
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Quote:
Originally Posted by yuri
Should not be possible to have that happening with a basic 4 wire thermostat. In the ON position it energizes the G terminal and whether it is programmable or not nothing changes. There is the very real possibility that they mixed up the wiring and it is energizing the AC wire. If the AC has the high voltage shut off for the winter it won't run and get damaged. It will energize the contactor for it and tell the circuit board to switch the fan speed to 100%. You definitely need someone to properly check the wiring soon. The circuit board may now be damaged. In the mean time leave the fan in Auto and it should be safe. Shut off the breaker for the AC or its outdoor switch. It may be off due to the low ambiant control and as soon as it hits 50 deg F could start and get damaged. If damage has occurred then print this post as most qualified techs will understand it and you may have grounds for a complaint with the 2nd company. Cheap thermostats have cheap electronics in them and can damage circuit boards. I would not necessarily keep it.
How can a cheap thermostat damage the electronics??
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Old 04-03-2011, 11:22 AM   #7
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Bryant Plus 90t with New Thermostat


Low voltage due to weak or poor quality fluttering contacts. Had one send 18 volts to a board and damage it.
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Old 04-03-2011, 05:45 PM   #8
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Quote:
Originally Posted by yuri View Post
Low voltage due to weak or poor quality fluttering contacts. Had one send 18 volts to a board and damage it.
Just curious, how would low voltage damage a board wouldn't low voltage just not let the board initiate its call. I understand why low voltage to a motor isn't good, but this is just a control. Not disagreeing with u just trying to understand
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Old 04-03-2011, 06:19 PM   #9
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Low voltage to the W terminal on the board caused the board to not start the ignition sequence properly. Boards are voltage and polarity and grounding sensitive. That is why the cheap power stealing tstats cause problems also. They can cause the ventor fan relay to pull in for no reason but not the rest of the relays etc.

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