Garage Unit Heater Clearances and Vent Penetration
A homeowner wants a 45,000 Btu/h natural-gas unit heater installed in a 24 ft x 24 ft attached garage with a 10 ft ceiling. The heater has electronic ignition and a 4 in. single-wall vent connector. The proposed location is 6 in. from the combustible ceiling and 4 in. from a wood-framed wall. The vent connector will pass through the combustible wall to a listed Type B vent terminal 2 ft above the roofline and 4 ft from a second-storey window that can open. The gas line is 1/2 in. black iron, 25 ft from the meter, with a shutoff and union. The heater will be mounted 7 ft above the floor, but the burner and ignition control are 4 ft above the floor. The garage floor is 4 in. below the house floor, and the customer stores gasoline in approved containers near the heater. The heater rating plate requires 1 in. clearance to combustibles at the top and 6 in. at the sides.
1. Identify which parts of the proposed installation are acceptable and which must be changed.
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The ignition height is acceptable: the burner and ignition control are 4 ft above the floor, which exceeds the usual 18 in. minimum for garage appliances. The top clearance of 6 in. also exceeds the 1 in. required by the rating plate. The side clearance is not acceptable: 4 in. is less than the 6 in. required, so the heater must be relocated or the wall protected in a way the listing permits. The single-wall vent connector passing through a combustible wall is not acceptable without a listed thimble or vent shield that maintains the required clearance. The vent terminal height and distance to the operable window must be checked against B149.1 and the manufacturer; 4 ft may be adequate for some appliances, but the wall penetration and termination details still need correction. The 1/2 in. gas line must be verified by sizing calculation, not assumed adequate.
2. The installer argues that because the burner is 4 ft above the floor, the garage ignition-height rule is satisfied and no other protection is needed. Is that correct?
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No. The ignition-height rule is only one garage requirement. The appliance must also be protected from physical damage by vehicles, which normally means bollards, a curb, or mounting the heater out of the vehicle path. Clearances to combustible walls and ceiling must match the rating plate. The vent connector and termination must comply with B149.1 and the listing. The heater must have adequate combustion air and must not be installed where flammable vapors are likely to be drawn into it. Storing gasoline containers near an ignition source is a hazard and should be stopped. The gas line must be sized for the connected load and pressure drop. Satisfying the 18 in. ignition height does not make the installation compliant.
3. What corrective actions should be required before the heater is placed in service?
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Relocate the heater or provide listed protection so the 6 in. side clearance is maintained. Replace the through-wall section with a listed thimble or vent shield and maintain the vent connector clearance. Verify the vent termination against B149.1 and the manufacturer for height, distance to the operable window, and distance to any air intake. Install vehicle-impact protection such as bollards. Verify combustion air for the garage. Remove gasoline containers from the heater area and instruct the homeowner on safe storage. Perform a gas-line sizing calculation for the 45,000 Btu/h load and upsize if needed. Pressure-test and purge the line, check manifold pressure and combustion, and confirm the heater operates without spillage or CO before leaving it in service.