The Attic Furnace and the Winter Lockout
A new 2,600 sq ft house has a condensing furnace and an HRV in an unconditioned attic. The furnace sits on the bottom chord of roof trusses, the attic hatch is 600 mm by 900 mm with no service platform, and the condensate drain runs through the exterior wall and discharges 150 mm above grade. The furnace is direct-vented through the roof and draws combustion air from the attic. Ducts in the attic are wrapped, but the joints are taped only. At -18°C the furnace locks out, ice forms at the vent termination, and the homeowner reports cold rooms and a noisy HRV.
1. As plan reviewer and site reviewer, list the design and code issues you would flag before occupancy, distinguishing safety, performance and documentation issues.
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Safety issues include the lack of a service platform and safe access to the furnace, clearances to combustibles, structural loading on the truss bottom chord, vent termination clearance and ice formation, and combustion air drawn from an attic that may be obstructed or contaminated. The condensate drain is a freeze risk: a line that runs through an exterior wall and discharges above grade can freeze and back up into the furnace. The attic hatch is too small for practical service and may not provide a safe access route.
Performance issues include duct leakage at taped-only joints, attic duct insulation and location, HRV balance and noise, room-by-room air distribution, and equipment sizing. Documentation issues include the load calculation, equipment selection, venting design, structural review for the platform, commissioning records and manufacturer’s instructions. Part 6 general requirements still apply where Part 9 does not address a detail. None of these should be deferred to the homeowner.
2. What commissioning tests and records should be required for the attic installation and HRV?
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Require a commissioning record that includes: heating and cooling load calculation; equipment model and capacity; gas supply pressure; manifold pressure; temperature rise compared with the nameplate; external static pressure; combustion analysis for CO and CO2; vent system inspection and termination clearances; condensate trap and drain test; and confirmation that the furnace operates at design conditions without nuisance lockout.
For the HRV, require balanced supply and exhaust airflows, measured at the specified fan speed, with confirmation that the house pressure remains acceptable when exhaust fans operate. Record the HRV controls and filter maintenance. Require a cold-weather verification plan: the condensate drain must be proven freeze-protected, not just observed in mild weather. Provide the homeowner with operating instructions, filter information and the commissioning report.
3. If the builder proposes a heat tape on the condensate line as the only fix, how do you respond?
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Heat tape alone is not an acceptable sole fix. It does not address the trap, the exterior termination, power failure, or the underlying decision to locate a condensing appliance in an unconditioned attic without freeze protection. The furnace can still lock out if the trap freezes, if the heat tape is not energized, or if the drain discharges where ice blocks it.
Require a freeze-protected condensate system: route the drain to a conditioned space where possible, slope it continuously, provide a trap and freeze protection designed for the attic temperature, and add a secondary protection or alarm. If the appliance remains in the attic, require a service platform, safe access, clearances and a cold-weather commissioning test. The heat tape may be part of the solution, but the builder must demonstrate that the complete system works at design temperature and that failure modes do not cause water damage or loss of heat.