Replace 4 speed Direct Blower with 3 speed?


  #1  
Old 02-08-08, 10:27 AM
Z
Member
Thread Starter
Join Date: Feb 2008
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Replace 4 speed Direct Blower with 3 speed?

I have an old (out of business) furnace with a direct drive blower. I have a replacement blower but it's only a three speed and the currently installed one is a four speed; is there a problem with just installing a three speed? Will the furnace explode?

-z
 
  #2  
Old 02-08-08, 10:36 AM
Ed Imeduc's Avatar
Member
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 17,505
Upvotes: 0
Received 1 Upvote on 1 Post
Wink

You dont say what the new motor speeds are. But a lot of the older furnace had only a 3 speed motor in them. With low on heat and high for AC. You should be ok. Set the speed so it does blow warm air.
 
  #3  
Old 02-08-08, 10:43 AM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
Never mind the number of speeds a blower/motor would have, only two are needed...a higher speed for the summer, and a lower for the winter.

The important part is that the motor you're planning on installing as a replacement performs like the old one. How do you find this out? I could go on and on with a disertation on motor horsepower, CFM's, static pressure, etc., etc., and that the new motor specs should match the old motor specs (this is actually the right way to go). But a practical approach would be as follows:

Once the new motor is in place and the unit running on heating mode (blower's lower speed hooked-up), the air temperature rise across the furnace must be within what the manufacturer tells it must be. Which is???? well...the mfr's brand and unit M/N are needed. Some smaller units accept a 30-to-45F, the larger size ones 30-to-60F or so.

And in the cooling mode (blower's higher speed hooked-up), if the evaporator coil does not ice-up and off the registers the air temperature is somewhere between 57-and-60F, you'll be O.K.
 
  #4  
Old 02-08-08, 11:21 AM
Z
Member
Thread Starter
Join Date: Feb 2008
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks! The motor's are identical in size and power. Though the new one apparently needs a larger run capacitor. The original motor had four wires. High, 2 Mediums and a Low. The new one only has the three (High, Medium and Low). Testing the wiring I'm only hooking one of the medium lines up and leaving the other medium wire unconnected. It appears to work. Should I need to do anything with the extra medium wire, or is it safe to leave it 'as is'?

-z

Originally Posted by pflor View Post
Never mind the number of speeds a blower/motor would have, only two are needed...a higher speed for the summer, and a lower for the winter.

The important part is that the motor you're planning on installing as a replacement performs like the old one. How do you find this out? I could go on and on with a disertation on motor horsepower, CFM's, static pressure, etc., etc., and that the new motor specs should match the old motor specs (this is actually the right way to go). But a practical approach would be as follows:

Once the new motor is in place and the unit running on heating mode (blower's lower speed hooked-up), the air temperature rise across the furnace must be within what the manufacturer tells it must be. Which is???? well...the mfr's brand and unit M/N are needed. Some smaller units accept a 30-to-45F, the larger size ones 30-to-60F or so.

And in the cooling mode (blower's higher speed hooked-up), if the evaporator coil does not ice-up and off the registers the air temperature is somewhere between 57-and-60F, you'll be O.K.
 
  #5  
Old 02-08-08, 11:53 AM
pflor's Avatar
Member
Join Date: Dec 2007
Location: U.S.A.
Posts: 976
Upvotes: 0
Received 0 Upvotes on 0 Posts
just cap it off so it does not touch anything metal.

There may be "park" terminals somewhere there too...if so, hook up the unused speed to one of the "park" terminals
 
  #6  
Old 02-08-08, 01:54 PM
Z
Member
Thread Starter
Join Date: Feb 2008
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks. I really appreciate the help. I don't pretend to be a HVAC person (or even play one on TV).

Is it possible for the blower wheel to rub during the initial start-up then move freely? I'm getting a "grrrr" sound in the first couple seconds when the blower starts, then it just moves air like crazy.

Originally Posted by pflor View Post
just cap it off so it does not touch anything metal.

There may be "park" terminals somewhere there too...if so, hook up the unused speed to one of the "park" terminals
 
  #7  
Old 02-08-08, 02:00 PM
Ed Imeduc's Avatar
Member
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 17,505
Upvotes: 0
Received 1 Upvote on 1 Post
Wink

Check and reset the blower wheel. So it dont hit the sides there.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: