Can I safely keep the fan on my oil furnace on longer?

Reply

  #1  
Old 01-06-11, 10:55 AM
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 4
Received 0 Votes on 0 Posts
Can I safely keep the fan on my oil furnace on longer?

I have a Rheem oil furnace (Model # ROBB - 084C) That is approximately 30 years old but has been well maintained and works fine. I recently insulated the air ducts in the basement and have discovered that I am leaving a lot of heat in the ducts when the furnace fan goes off. Most of that heat is then lost before the furnace turns on again. Currently the fan runs for about 2 1/2 minutes after the furnace goes off. I would like the fan to run for about 12 minutes. This would result in a substantial reduction in the amount of time per hour that the furnace is on, based on the measurements that I have made. After 12 minutes we start to get air that is colder than the room air.

The unit has a small Honeywell controller on the front, but I could not find a model number. The controller has a small metal wheel at the top that says “CAUTION, do not rotate – hold dial when setting pointers.” Along the bottom of the wheel are too narrow metal cutouts. Along the top of the left cut out it says “Off On” and on the top of the right cutout it says “Off.” The left middle cutout has markings from 50° to 140° and the right hand cutout has markings from 150° to 250°. The pins in the left hand cutout are set at 80° and 140° and the single pin in the right hand cutout is set at about 170°.

The furnace recently had its annual servicing and I asked the technician (who works for the company that supplies my heating oil) to change the settings so that the fan would stay on longer. He said he could not move the low end pin below 80° because it would “burn a hole in my heat exchanger.” When I asked him to explain why, I did not get any kind of clear reason.

Instead, he moved the upper end pin in the left cutout to 140° and said that would fix my problem. Of course, since that’s the high-end cutoff, nothing changed. He also said that I could leave the fan on all the time, but as I mentioned above, that doesn’t do any good because eventually it starts blowing cold air into the house.

My questions are should the right hand pin in the left cutout be set all the way to 140° or should it be set at a lower level? And can I set the lower pin down to 70° or 65° in order to keep the fan on longer?

Thanks.
 
Sponsored Links
  #2  
Old 01-06-11, 09:05 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 13,980
Received 9 Votes on 8 Posts
What the tech told you is just plain dead wrong. The lowest setting is the temperature at which the fan will shut off. Hold the dial with one hand & lower the "fan off" setting about
10º to start. If that doesn't keep the fan on long enough go another 10º. Normally the "fan on" (middle pin) is set somewhere around 130º. Settting lower will bring the fan on sooner, & higher will hold the fan off longer before coming on.
 
  #3  
Old 01-07-11, 01:32 PM
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 4
Received 0 Votes on 0 Posts
Thanks

Thanks Grady.

I set the "on" pin to 130 and after some trial and error found that the fan will turn off after 11 minutes when the "off" pin is at 60. That's perfect.

Unfortunately, your answer has made me a little suspicious (paranoid?). The tech supposedly did an efficiency test - there is a form that he pastes onto the front of the unit which says that the efficiency is 81.25%.

Is that possible for an older unit like this? Or are they just saying that to keep me from buying a newer unit that would use much less heating oil?
 
  #4  
Old 01-07-11, 07:35 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 13,980
Received 9 Votes on 8 Posts
The test he performed is combustion efficiency. This is only part of AFUE. Combustion efficiency will always be higher than AFUE but one of the biggest things to efficiency is proper sizing. It is not at all uncommon to see an oil burner 2-3 times the capacity it needs to be.
 
  #5  
Old 01-08-11, 07:09 AM
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 4
Received 0 Votes on 0 Posts
The house is 2,900 sq. ft. The furnace is rated at an input of 105,000 BTU/hr. and a bonnet capacity of 84,000 BTU/hr.

I did try to look up the components of AFUE online but could not figure it out. What are its components?

Are there other things I can change to make it more efficient?

Thanks.
 
  #6  
Old 01-08-11, 12:09 PM
Grady's Avatar
Forum Topic Moderator
Join Date: Jan 2005
Location: Delaware, The First State
Posts: 13,980
Received 9 Votes on 8 Posts
I don't know all of the components of AFUE testing but electrial consumption does come into play. If your exhaust temperature is high
(550º+) you can add a device in the vent pipe with a fan to extract some of that heat & put it into the basement.
 
  #7  
Old 01-08-11, 01:16 PM
Jay11J's Avatar
Member
Join Date: Sep 2001
Location: Rochester, Minnesota
Posts: 18,427
Received 0 Votes on 0 Posts
The 60˚ off setting may be too "low" when mild weather comes, and the fan will never shut off.
 
  #8  
Old 01-10-11, 07:32 PM
Member
Thread Starter
Join Date: Jan 2011
Location: USA
Posts: 4
Received 0 Votes on 0 Posts
Thanks so much for all of your help.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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