Furnace in attic or crawl space (horizontal)

What is going on

Placement rules for a horizontal furnace in an attic or crawl space. Mostly about clearances, access and keeping the unit dry and supported.

Likely causes

• Horizontal furnace going into an attic, where service access and framing contact matter
• Horizontal furnace in a crawl space where moisture is a concern
• Suspended installs hung from joists or rafters
• Replacement jobs where the old platform is too small or in the wrong place

Check in this order

1. In an attic, place the furnace on the left side (as viewed from front in vertical position)
2. Build a service platform large enough for clearances on all sides and front service access, because a tech has to reach the blower and burners
3. Make sure only line contact occurs between the furnace casing edges and joists, studs or framing
4. In a crawl space, set the furnace on a pad or noncombustible material to keep it dry
5. If suspended, use 3/8 in all-thread rods with angle iron at both ends and the midpoint
6. Make the angle iron at least 32 in long so panels can be reached
7. Check the finished install for clearance to combustibles per the rating plate

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)