Replacing a Furnace in a Tight Basement With a Draft-Hood Water Heater
A basement mechanical room measures 14 ft by 12 ft with a 7.5 ft ceiling and is enclosed by a solid door with no louvers. It contains a 20-year-old natural-draft, draft-hood water heater with 40,000 Btu/h input, vented into a dedicated 8 in. by 8 in. clay-tile masonry chimney on an exterior wall. The owner is replacing a 60,000 Btu/h furnace with an 80,000 Btu/h unit, and the installer proposes to common-vent both appliances into that same chimney. Upstairs, a 110 cfm exhaust-only bath fan runs continuously, and a clothes dryer sits in the adjacent room. A carbon monoxide alarm on the first floor has gone off twice in the past year - always in winter, always reset by the tenant.
1. Is this mechanical room a confined space for combustion air purposes, and what would compliant combustion air look like? Show the calculation.
Read the worked answer
Yes - it is confined, and the current construction provides no combustion air at all.
- Room volume: 14 × 12 × 7.5 = 1,260 ft³.
- Unconfined threshold: the standard rule of thumb is 50 ft³ of volume per 1,000 Btu/h of total input.
- Total input after the change: 40,000 + 80,000 = 120,000 Btu/h → 120 × 50 = 6,000 ft³ required.
- 1,260 ft³ is roughly one-fifth of that. The room is confined, and a solid, non-louvered door means the only air path is leakage around the door - which is not a designed opening.
Compliant approach using two permanent openings to outdoors or to a freely communicating space:
- Each opening sized at 1 in² of net free area per 4,000 Btu/h of total input: 120,000 ÷ 4,000 = 30 in² each.
- If the openings are connected by horizontal ductwork, the sizing becomes stricter (1 in² per 2,000 Btu/h).
- Louver free area matters: a 30 in² net free area requirement with louvers at 75% free area means roughly 40 in² of gross opening. Sizing to gross opening is one of the most common field errors.
- One opening within 12 in. of the top of the enclosure, one within 12 in. of the bottom.
Given the exhaust fan and dryer, drawing combustion air from inside the house is the wrong choice here regardless of what the table allows - see the next question.
2. The installer wants to common-vent the new furnace and the existing water heater in the 8 in. by 8 in. clay tile chimney. What are the problems, and what would you do instead?
Read the worked answer
The chimney is being asked to do three incompatible things at once.
- The flue is oversized for the water heater alone and marginal for the pair. A masonry chimney on an exterior wall is a cold flue; a draft-hood appliance needs a warm, correctly sized flue to establish draft. When the furnace is not running, the water heater vents into a large cold cavity, cools its own flue gases, and loses draft. That is the classic winter spillage pattern described in the setup.
- Mixing vent categories is not a field decision. Common-venting a fan-assisted appliance with a draft-hood appliance requires that the combination be permitted and sized per the venting tables or the appliance listings, and frequently requires a listed liner. A 60-year-old clay tile flue with no liner, on an exterior wall, is not a safe assumption.
- The ignition risk is real. Carbon monoxide alarms tripping every winter is evidence of spillage, not a nuisance alarm to be reset.
Recommended solution:
- Direct-vent the new furnace - two-pipe, sealed combustion, drawing air from outdoors. This removes the furnace from the chimney, removes 80,000 Btu/h from the room's air demand, and makes the room's combustion air problem much smaller.
- Reline the chimney or install a properly sized listed vent for the water heater alone, sized to that appliance's input and the chimney's height and location. An aluminum or stainless liner sized for a 40,000 Btu/h draft-hood appliance is the standard fix.
- If a power-vented or direct-vent water heater is within budget, replacing both appliances with sealed-combustion equipment eliminates the depressurization problem entirely.
Do not 'fix' this by adding a chimney-top draft inducer and calling it done - the appliance listing and the vent sizing still have to work.
3. How would you verify the venting actually works, and what does the bath fan and dryer have to do with it?
Read the worked answer
Test the worst case, not the best case.
The appliances sit in a zone that two exhaust devices - a continuously running 110 cfm bath fan and a clothes dryer - pull air out of. A tight modern house with no makeup air can be driven negative enough that a naturally aspirating appliance reverses and spills flue gases into the basement.
Verification steps:
- Establish worst-case depressurization: turn on the bath fan, the dryer, and any kitchen exhaust, and close the door as the tenant would.
- Measure the pressure difference between the mechanical room and outdoors. If the room is significantly negative relative to outdoors, natural-draft venting is not reliable regardless of the flue.
- Perform a spill test at the draft hood - a smoke or flame test at the dilution air openings - and confirm draft with a gauge after the appliance has run long enough to warm the flue. Do it once cold and once at steady state.
- Combustion test: measure CO in the flue gas and in the room. A draft-hood appliance spilling will show elevated ambient CO.
- Provide makeup air for the exhaust devices or convert the appliances to sealed combustion - the exhaust appliances are not going away, and 'the tenant should open a window' is not a correction.
The point of the exercise: in a confined basement with exhaust devices, combustion air and venting are one problem, and the fix that survives the winter is sealed combustion plus a correctly sized flue - not a bigger opening in the door.