# How to change "Fan On" speed on a variable speed blower



## rwh442 (Nov 18, 2008)

This is my first post on this site since I started viewing it in May.  Great site guys.  It has helped me tremendously.

I installed a Thermo Control 2000 boiler and first fired it up this Friday.  The boiler is feeding a DHW sidearm and furnace plenum HX.  No problem with the boiler.

I have a Trane XV90 propane furnace with a variable speed blower.  I connected a second thermostat to the circuit board to run the blower (R to G).  However, this only runs the blower at 50% of available speed.  The AC circuit (Y) and heat circuit (W) interact with the variable speed blower controler but I cannot find any settings for changing the "Fan On" speed (G).  I looked over the manual schematics and it is not clear (if any) how to get the blower to run at 80% or 100%.  I would like this higher flow to better feed the farthest rooms from the blower and to speed up the heating (my thoughts).

Any suggestions or ideas?

Thanks.


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## Medman (Nov 18, 2008)

I don't know about Trane, but on my air handler (Goodman)  the speed of the blower is initially set by DIP switches on the control board inside the unit.  This only sets the max possible speed, however.  When the fan comes on in response to a call for heat, it does not go to this max speed right away, - it takes some time to ramp up to max.  There is also a trim setting on the air handler (2 switches) that allows for the speed to be increased by 10% of max or decreased by 15% of max if required.  I also find that I have much more control using a multi stage thermostat to control the blower.


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## rwh442 (Nov 18, 2008)

Medman,

My blower in my propane furnace does exactly like you are saying - it ramps up/down depending upon time periods and if/when the heat demand is met.  My original thermostat is two stage and it all worked fine with propane.

I want to keep the original thermostat as a backup in case the boiler cannot supply the heat required.  I checked all the dip switches concerning the fan speeds and times and all those appear to work only with the call for heat (W1 and W2) and call for cooling (Y) terminals on the board.  Just energizing the fan (G) seems to me to be a fixed 50% fan speed only.  So I guess what would happen would be to possibly find the dip switches to run the fan at 100% for BOTH the call for heat (W1 and W2) and fan (G)?  So it appears I cannot keep my original propane "setup" with the variable speed and make the fan-on speed 100%?

I thought I read in another post that a user disconnected his call for cooling (Y) terminal and used it for the second thermostat for the heat exchanger call for heat so that he could use the variable speed controls in that circuit.  Essentially the AC would be "disconnected" for the heating season then?  Would this work?  Just thinking.


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## RJP Electric (Nov 18, 2008)

Rob H said:
			
		

> This is my first post on this site since I started viewing it in May.  Great site guys.  It has helped me tremendously.
> 
> I installed a Thermo Control 2000 boiler and first fired it up this Friday.  The boiler is feeding a DHW sidearm and furnace plenum HX.  No problem with the boiler.
> 
> ...





Pull off the wire that is on the Y terminal for the season your a/c,  then put in a jumper in from Y to G terminal, this will turn your "fan on" to 100%.


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## Medman (Nov 18, 2008)

The "fan only" speed in my unit is pre-set to 33% of the max cooling speed - averages about 400 cfm.  You could easily use the Y connection for the wood-only thermostat.  
On my unit there are separate DIP switches for the cooling speed, based on tonnage of the condenser used.  I am going to install a protection circuit with a snap-disk sensor and relay so that when using the Y connection for wood heat it will not be possible for the condenser to run when the plenum HX is hot and vice-versa.
I have a similar sensor and relay in place for the electric element backup - the electric element will not energize until the water temp in the supply to the HX is lower than 120 F.


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## rwh442 (Nov 18, 2008)

Thanks Medman, RJP.  I will jumper the Y to G tonight and try that.

However, I am confused about the snap disc sensor.  This is the second time during my research that I have seen this mentioned.  How can the AC kick on if the Y wire going out to the AC unit is disconnected?  My water to air heat exchanger is above the fan and AC condensor coils probably by about 1 foot.


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## rwh442 (Nov 19, 2008)

The G to Y jumper worked and the blower now runs 50% for a couple of minutes then kicks into 100% and then back down to 50% before finally shutting off.  I actually have some flow to the back rooms now!  Thanks.


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## Medman (Nov 19, 2008)

Snap disc sensor is used to energize/de-energize a relay which switches the Y contact.  When plenum HX supply is hot, disc closes, relay energizes and connects Y(tstat) to G(tstat), breaking the connection between Y(tstat) and Y(condenser).  When supply is cold, disc is open, relay is non-energized and Y(tstat) remains connected to Y on condenser.


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