HVAC, fan not ON working in AUTO mode

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fobinidu

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Rothsay
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Electrical Contractor
I replaced an old (mercury type) thermostat with a new Honeywell thermostat model RTH2300B.
The wiring is quite straight forward based on the manual: red to R, white to W, yellow to Y and green to G.
There is no C so the 24VAC common is not connected. I have to install 2 AAA batteries per instruction.

Before installing this new thermostat, I perform a bench test as follows:
1) take a piece of cable with 5 wires in it (red, yellow, white, green and cyan)
2) connect 4 wires (as above) to the connector gang. The other end of this cable has 5 wires sticking out of the thermostat
3) connect the 24VAC hot to the red wire, common is NOT connected.

I use a voltmeter, set to 50 Volts AC range. The black wire connects to the 24V common of the transformer.
Test sequence is:
1) set fan to ON, voltage at the G wire shows around 24 volts (i.e. fan is turned ON)
2) set fan to OFF, and set temperature a few degree higher than current temperature: after a few seconds, the W wire shows 24 volts (i.e. heat is turned ON at the furnace). Problem is the G wire shows 0 volts even after 30 seconds the heat goes on.
3) lower the temperature setting to be lower than current temperature, after a few second the W wire shows 0 volts (i.e heater off)
All these tests are done without connecting any wire to the furnace/AC system. So the problem is somewhere in the thermostat. Bottom line: the fan only turns ON if set to ON. It stays off in AUTO mode no matter what.

I move forward to install this thermostat to my HVAC, the results are identical to the bench test.
I get an identical one from local store and repeat these tests. SAME RESULTS, FAN not working in AUTO mode.

I appreciate all advises I could get on this. ;)
 
It would help to know if you tested the auto setting heat mode, cool mode, or both?
As stated in post 2, heat mode auto fan control will be handled by the furnace’s plenum temp switch.
 
The furnace does control the fan cycle in heat on typical units and in cooling mode the T-STAT controls the fan with regards to old school design.
Modern T-Stats are constantly being re-eingneered and OVER engineered. Battery powered, also often times known as " Power Stealing " thermostats are not a good choice ever for any reason, because when the batterys go low they will not switch. You are 100x better running a common wire and getting rid of the nonsensical batteries which are wasteful and useless. Some of this Solid state TEK will behave strangely when attempting to run what would seem like logical DIAG. tests because it too loaded up with electronics and does not behave like relay logic. People that are not trained in electronics do not think in this way and for good reason. Modern furnaces no longer run on Temp based fan limits, but on time based ones which is a function of the control logic for that model. In the past is was on TEMP and the controls were mechanical bimetal in nature. This is with respect to gas heat. The old electric heat sequencers are time based devices that close after a delay which depends on the model. First stage electric SEQ. generally also triggers the blower fan. A major lot of product is bad out of the box these days. It happens. Also the programmable T-stats are the most over Engineered of all. I used to carry a library on my truck just for those, AND they make for a huge amount of service calls that would not otherwise ever happen. Time wasters in the large because most mortals cannot operate them correctly. They only work in certain settings with intelligent operation.
 
Is this for an electric furnace? I believe this is user error.

In gas mode, you power up W and the AHU does the fan control. The G is not powered unless the thermostat is trying to manually command the unit to turn on the fan without heat.

In elec mode, thermostat controls the fan, so in heat mode, thermostat powers both W & G.
That's the whole reason why there's gas/electric setting.
 
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