Attic Exhaust Fan

[Cleveland Apprentice]

Hello,

I replaced a line thermostat on an attic exhaust fan a few weeks ago for HO. At the time of the install, it was cool so I tested the thermostat by turning it down to the minimum setting (60 degrees if recall correctly) to see if fan kicks on. The fan did kick on, so I changed the setting at 100 degrees and left. Recently, the HO called me and said that the fan does not run on hot days, but during the night when the temperature starts to drop, the fan kicks on until it gets hot outside again. The thermostat is operating the exact opposite the way it should and I don't understand this. I don't believe the thermostat is defective because it's brand new. There are only two black wires to the thermostat and doesn't matter which is the line or load per the instructions. Has anyone else encounter anything like this? Thanks.

 

[ActionDave]

It sounds like you have a heating stat instead of a cooling stat.

 

[GoldDigger]

Hello,

I replaced a line thermostat on an attic exhaust fan a few weeks ago for HO. At the time of the install, it was cool so I tested the thermostat by turning it down to the minimum setting (60 degrees if recall correctly) to see if fan kicks on. The fan did kick on, so I changed the setting at 100 degrees and left. Recently, the HO called me and said that the fan does not run on hot days, but during the night when the temperature starts to drop, the fan kicks on until it gets hot outside again. The thermostat is operating the exact opposite the way it should and I don't understand this. I don't believe the thermostat is defective because it's brand new. There are only two black wires to the thermostat and doesn't matter which is the line or load per the instructions. Has anyone else encounter anything like this? Thanks.

It may not be your thermostat that is controlling this, but rather an overload temperature switch at the fan motor. Unlikely, yes, but possibly if it is defective. It could be turning the fan off when it is hot and turning it back on when the temperature drops to where the air can cool the fan for continued running. But in that case when the temperature got below 100 the fan should have been shut off by the thermostat instead.

Or the thermostat may just in fact be broken, even though it is brand new. Is this a screw adjusted attic fan thermostat designed to be mounted with the fan in the attic? If not, it may possibly have a switch to allow the two wires to be connected either to the normally (cold) open or normally closed contacts.

Your report that it turned on when set to 60 and off when set to 100 when first installed does not support the idea that it was originally a heating thermostat.

 

[roger]

It sounds like you have a heating stat instead of a cooling stat.

Yes it does

Roger

 

[GoldDigger]

It may not be your thermostat that is controlling this, but rather an overload temperature switch at the fan motor. Unlikely, yes, but possibly if it is defective. It could be turning the fan off when it is hot and turning it back on when the temperature drops to where the air can cool the fan for continued running. But in that case when the temperature got below 100 the fan should have been shut off by the thermostat instead.

Or the thermostat may just in fact be broken, even though it is brand new. Is this a screw adjusted attic fan thermostat designed to be mounted with the fan in the attic? If not, it may possibly have a switch to allow the two wires to be connected either to the normally (cold) open or normally closed contacts.

Your report that it turned on when set to 60 and off when set to 100 when first installed does not support the idea that it was originally a heating thermostat.

Pardon the typo above; "does not" should be "does". Or "does" should be "does not" or both. Or not, depending on whether I was right the first time or not.

If you turn the stat down and the fan comes on it is NOT a heating stat. But if the temperature rises and it turns off, it would be a heating stat. The solution is obvious. Somebody replaced the stat when you were not looking.

 

[kwired]

It sounds like you have a heating stat instead of a cooling stat.

except for the part where he turned the setting down at installation time and that made it run.

 

[augie47]

except for the part where he turned the setting down at installation time and that made it run.

I agree,,,since it tuned on when you lowered the setting, I would think it was the correct thermostat. Golddigger's suggestion that the overload (overheat) sensor might be causing it to turn off has merit.
Did you measure the current compared to the nameplate ?
(Motor overload may have been the original problem making them think the thermostat was bad)

 

[roger]

except for the part where he turned the setting down at installation time and that made it run.

A heating T stat would close on drop in temperature, a cooling T stat would close on rise.

Roger

 

[ELA]

The proper thermostat for an attic fan will close on a temperature rise, or when you lower the setpoint ( simulating a temperature rise), as the OP said he did to test.

Is it possible that the unit also has a built in humidistat?

 

[kwired]

A heating T stat would close on drop in temperature, a cooling T stat would close on rise.

Roger

Correct, OP said he turned the setpoint down at installation time and it closed, indicates to me it is a cooling thermostat, as once it is turned below ambient it will be calling for cooling.

 

[roger]

Correct, OP said he turned the setpoint down at installation time and it closed, indicates to me it is a cooling thermostat, as once it is turned below ambient it will be calling for cooling.

I was agreeing with you, I should have started my post out with that.

Roger

 

[ActionDave]

I was up late when I posted. I am agreeing too. I am also agreeing to check the motor.

 

[gadfly56]

Hello,

I replaced a line thermostat on an attic exhaust fan a few weeks ago for HO. At the time of the install, it was cool so I tested the thermostat by turning it down to the minimum setting (60 degrees if recall correctly) to see if fan kicks on. The fan did kick on, so I changed the setting at 100 degrees and left. Recently, the HO called me and said that the fan does not run on hot days, but during the night when the temperature starts to drop, the fan kicks on until it gets hot outside again. The thermostat is operating the exact opposite the way it should and I don't understand this. I don't believe the thermostat is defective because it's brand new. There are only two black wires to the thermostat and doesn't matter which is the line or load per the instructions. Has anyone else encounter anything like this? Thanks.

Can you supply the make/model for the thermostat and fan motor please?

 

[Cleveland Apprentice]

it's a ventamatic xxfirestat:

 

[Cleveland Apprentice]

sorry I don't recall what brand/model the actual fan is. I need to check. Thanks all for replies

 

[GoldDigger]

it's a ventamatic xxfirestat:

Oddly enough of the 30+ reviews of more or less this same model on Amazon, three of them complained that it worked backwards.
There may be a manufacturing or design problem here. For example, if the bimetal spring was damaged in some way it might actually force the contacts apart as the temperature increased rather than pushing them together.

The label on the case "Fan Off Temperature" may mislead some. It is designed with a 10-15 degree hysteresis, so that fan should turn on when the temperature rises to 10-15 degrees above the set point and turn off when it drops back to exactly (more or less) the set point number. Based on this, setting it to 100 may have been too high for best cooling.

I would just try getting a replacement from the manufacturer.

BTW, the Firestat part is required by code in some areas and is essentially an over temp fuse which insures that the fan turns off and stays off if there is a fire in the attic. I do not think that it resets but rather has to be replaced if it ever actuates. But without the instructions (if any) I cannot be sure of that.

One quick thing to check: With power disconnected, measure the resistance from one end of the diode-like Firestat element to the other. There should be continuity there whether the switch is open or closed.

 

[gadfly56]

I checked out the Amazon reviews and I'd say GoldDigger is correct. Somehow your unit is working backwards. Don't try to fix it, just return for replacement.