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Old 02-20-2010, 05:43 PM   #1
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Lennox furnace blower runs slow


I have a Lennox G60UHV-36B-090-02 Furnace converted to LP I just installed in my shop. It fires fine but the blower runs slow. I am using a simple 2 wire thermostat connected to W1 & R on the board. The thermostat jumper is set to single. What do I need to change to get the blower to run at a faster speed?

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Old 02-20-2010, 06:42 PM   #2
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Lennox furnace blower runs slow


Do you have a full set of ductwork on the unit like that of a house? Where did you get the furnace from. The reason I ask is that is a fairly large furnace with a variable speed ECM motor fan and it needs proper ductwork to work against or it may not get up to speed properly. Why do you think it is too slow? Have you done furnace work before? Post some pics of the furnace and ductwork. I am a Lennox tech and know that furnace very well.

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Old 02-20-2010, 06:50 PM   #3
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Lennox furnace blower runs slow


switch number 11-12/ position/ heating speed
on+on/low
off+on/med low
on+off/med high
off+off/ high

switch 7 on and switch 8 off will increase speed 10%

Last edited by Marty S.; 02-20-2010 at 06:52 PM.
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Old 02-20-2010, 07:05 PM   #4
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Lennox furnace blower runs slow


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Originally Posted by yuri View Post
Do you have a full set of ductwork on the unit like that of a house? Where did you get the furnace from. The reason I ask is that is a fairly large furnace with a variable speed ECM motor fan and it needs proper ductwork to work against or it may not get up to speed properly. Why do you think it is too slow? Have you done furnace work before? Post some pics of the furnace and ductwork. I am a Lennox tech and know that furnace very well.
Good point,,, ECM motors read the air resistance felt through the blower wheel so with no duct work and open air, it's going to think that there is very low resistance and run at a slower speed naturally.
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Old 02-20-2010, 07:07 PM   #5
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Lennox furnace blower runs slow


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Originally Posted by Marty S. View Post
switch number 11-12/ position/ heating speed
on+on/low
off+on/med low
on+off/med high
off+off/ high

switch 7 on and switch 8 off will increase speed 10%
I assume you are talking about the dip switches? I have one on the upper right of the circuit board which is mounted vertical with 3 switches, and 2 switches near the bottom of the board which have 4 switches each mounted horizontally. How are they numbered? Right now there is no ductwork connected to the furnace, we are just getting it running. It will have one 16' long heat run off a 3' tall plenum. It will just have a filter holder right on the furnace, no return ductwork. The furnace was a changeout due to the house being remodeled and the size doubbled. There was nothing wrong with it when it was removed. Right now the blower barly moves any air when it goes on.

Last edited by ventport; 02-20-2010 at 07:11 PM. Reason: add to info
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Old 02-20-2010, 07:11 PM   #6
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Lennox furnace blower runs slow


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Originally Posted by ventport View Post
Right now there is no ductwork connected to the furnace, we are just getting it running. .
There you go, with no duct work you have very little resistance, so the motor is designed to run @ a lower speed when there is less resistance. To test this, try blocking off part of the return and supply with some cardboard and watch how the motor will automatically ramp up in speed because this will simulate some static resistance found in ductwork

Last edited by Giianee; 02-20-2010 at 07:13 PM.
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Old 02-20-2010, 07:12 PM   #7
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Lennox furnace blower runs slow


You need to have some return air and supply ductwork and dampers and resistance or that fan will NEVER work properly. It is designed to work against a typical set of house ductwork and maintain a static pressure. This is all programmed into the circuit board. Add some decent ductwork and a fairly tight filter and it will work better. If not the motor will hunt and not last too long.
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Old 02-21-2010, 04:09 PM   #8
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Lennox furnace blower runs slow


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Originally Posted by Giianee View Post
There you go, with no duct work you have very little resistance, so the motor is designed to run @ a lower speed when there is less resistance. To test this, try blocking off part of the return and supply with some cardboard and watch how the motor will automatically ramp up in speed because this will simulate some static resistance found in ductwork
OK I tried blocking off part of the return and that made no differance in the blower speed, so I tried blocking off part of the the area where the plemum attaches too also and it made no difference. On reading my first post is the thermostat connected to the correct place. How are the dip switches numbered so I can try changing them. Any other suggestions? Thanks for all the help so far.
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Old 02-21-2010, 05:10 PM   #9
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Lennox furnace blower runs slow


Are you sure it was running too slow?

In first stage it won't run very fast. Have to wait until timer goes to second stage for the blower to increase in speed.

Should only be moving 850 to 950 CFM in first stage.
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Old 02-21-2010, 05:42 PM   #10
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Lennox furnace blower runs slow


Dip switches are numbered. You guessing on the cfm being low or using the blinking cfm counter built into the control board?

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