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Installation of hot air furnace return air recommendation?

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To offset high electric heat costs, 20 years ago we opted to install a forced air furnace oil heating system.  The furnace is in an enclosed crawl space (concrete walls and floor) with a ventilated entry door.  The furnace has been and contiues to satisfy our heating needs but a question has come up about the system NOT having return air ducting.  We are under the impression that as long a cool return air is provided to the space through our louvered entry door, there's no problem.  With this, are we / have we been in any danger or violation(s)?
asked in Heating System by thomas-h-brown-jr (140 points)
    

1 Answer

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Hi Thomas,
 
Based on your description I can’t give you any definite answers because there are a couple more variables this oil furnace installation safety and code compliance depends on. 
 
First item is the type of your oil furnace’s venting / combustion air supply; in case you have a direct vent oil furnace which uses combustion air (to support burning process) from exterior / beyond the crawlspace and not through the louver doors, your installation may be acceptable.
 
Direct vent oil furnace would have sealed combustion chamber and two separate pipes; exhaust and combustion air supply pipe (or one concentric pipe) protruding from the furnace’s enclosure and penetrating side wall (foundation wall) of the house or sometimes going through the roof.
 
Furnace's cold air intake (louver doors) may not be the ideal way to return air to the appliance, but if the furnace has a properly installed air filter, which should be mounted on its blower compartment, this configuration is correct. 
 
Second, if your oil furnace has a single vent pipe connected to a chimney, an open combustion chamber, and the combustion air is taken from within the crawlspace and your home (through the louver doors), you should definitely correct this installation.
 
Combustion air and cold air supply / return should not be the same, there’s a requirement of 10 feet minimum distance between them and in your installation they’d be both supplied through the louver doors.
 
The solution:
  1. Install cold air return duct system and position it at least 10’ from the louver doors. 
     
  2. Make sure that the louver doors provide adequate supply of combustion air for the furnace.

To calculate that you’d need:

  • oil furnace BTU’s  (they should be listed on its manufacturer’s plate)
  • access doors louvered section size
  • dimensions of an area the crawlspace access door is located in or dimensions of an entire area that supplies air to the crawlspace
  • presence of crawlspace ventilation ports open to exterior, / their dimensions, screens, louvers, etc.
  • crawlspace dimensions
For example: if the crawlspace access is from a 10’x15’x8’ room, equipped with a solid door / no other openings or vents connecting it with the rest of your house, very small crawlspace (just to accommodate your appliance), no crawlspace foundation vents, this room would be considered as the only combustion air providing area. 
 
Since the 10’x15’x8’ room area itself wouldn’t provide adequate amount of combustion air (click “combustion air” for more information), additional sources would have to provided.
 
Please let me know if you need more assistance and provide information required to complete steps described in “The solution” part of my answer (in case you don’t have a direct type furnace).
Just click "comment" below.
 
answered by admin (21,330 points)

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