Originally published at: Masonry Heater Outside Air Details - Firespeaking
Providing outside or “make-up” air directly to a masonry heater is a modern technique to ensure that the firebox receives the air it needs for efficient combustion and does not compete with other appliances such as kitchen exhaust hoods for adequate venting.
Please note that the ASTM standard does not make a mandatory requirement but defers to the local jurisdiction. We have taken care in our design work (eg. 5-Run with Oven) to make it easy (for both installation and use) to use either outside or room air since there is a crossover between the secondary air manifold where the outside air is usually plumbed and the area serviced by the ash door which provides air from inside the room.
Our current recommendation for our standard-sized firebox is to install a 6″ outside air duct and use the Dayton 6″ Control Damper available through Grainger which has a nice silicone seal. The 1/2″ shaft on this product is shorter than required so you will need to extend the shaft and can use our Handle & Trim Plate for 1/2″ Shaft product to operate it.
In ContraFlow or other base venting models where a flue channel is required behind the ash drop and a single U-shaped secondary air manifold is not possible, we recommend splitting the outside air into two 4″ ducts:
Here is an example of a foundation for a masonry heater being constructed with outside air being plumbed:
Recommended Additional Reading
Masonry Heater Foundation Details
An in-depth article including detailed drawings of a basic foundation strategy for masonry heaters including footing, stem wall, base slab, insulation, and rough opening specifications.
Masonry Heater Foundation Construction Sequence
This article complements the Masonry Heater Foundation Details article by providing real-world process examples. All framing is placed 2″ beyond the heater footprint to meet clearance requirement. Only the subfloor bridges across that clearance (since it is a membrane that can dissipate heat and is necessary practically in construction). Although pink board was used in […]
Codes & Standards for Masonry Heater Construction
A synopsis of codes and standards for masonry heater construction, including information regarding their efficiency for home owners and building officials.