How do I wire a bathroom exhaust fan and do I need a permit?

Bathroom Exhaust Fan Wiring in New Brunswick

A properly wired and vented bathroom exhaust fan is essential for moisture control — critical in NB's climate where winter humidity condensation causes mould, peeling paint, and structural damage in bathrooms.

Replacement vs. New Installation

Replacing an existing fan (same location, same wiring):

• No TSANB permit required


• Disconnect old fan, connect new fan to existing wiring


• DIY-friendly if you're comfortable with basic electrical


• Cost: $30–$150 for the fan + 30–60 minutes

New installation (no fan existed before):

• TSANB electrical permit required (new wiring)


• Must be done by or supervised by a licensed electrician


• Requires running new cable, installing a switch, and venting to the exterior


• Cost: $400–$800 total (fan + wiring + venting + permit)

Choosing the Right Fan

Sizing by CFM (cubic feet per minute):
The minimum fan capacity should be 1 CFM per square foot of bathroom, with a floor of 50 CFM:

| Bathroom Size | Minimum CFM |
|---------------|------------|
| Small (40 sq ft, powder room) | 50 CFM |
| Standard (60–80 sq ft) | 70–80 CFM |
| Large (100+ sq ft, ensuite) | 100–150 CFM |
| With jetted tub or large shower | 150+ CFM |

Sound rating (sones):
Lower is quieter. 0.3–1.0 sones is whisper-quiet; 3.0–4.0 sones is noticeably loud. Spend the extra $20–$40 for a quiet model — you'll hear it multiple times daily.

Features to consider:

• Humidity sensor (auto-on when humidity rises) — $80–$150


• Motion sensor — $60–$100


• Timer switch — $15–$40 for the switch


• Built-in LED light — $60–$120


• Heater built-in — $150–$300 (requires dedicated circuit)

Wiring Options

Option 1: Fan on the existing light switch
Simplest wiring — the fan and light turn on together. Connect the fan's black wire to the same hot wire as the light fixture. No new switch or wiring needed.

Pros: Free, no rewiring
Cons: Fan runs only when light is on; may not run long enough after showering

Option 2: Separate switch for the fan
Run a 14/3 NMD90 cable from the switch box to the fan location. The extra conductor (red wire) provides independent control of the fan.

• Black wire → light
• Red wire → fan
• White wire → neutral (shared)
• Bare → ground

Two separate switches in the box control light and fan independently. Requires: 14/3 cable, double-gang switch box, TSANB permit if new wiring. Cost: $200–$500 (electrician)

Option 3: Timer switch
Install a countdown timer switch ($15–$40) that turns the fan on for a set duration (5, 10, 20, 30, or 60 minutes). The fan runs after showering and shuts off automatically.

This is the most practical option for moisture control — set it for 20–30 minutes after each shower. Can be wired on the existing fan circuit with no new wiring.

Option 4: Humidity-sensing fan
The fan has a built-in humidity sensor that turns the fan on automatically when moisture levels rise and off when they drop. No switch operation needed — the fan takes care of itself.

Cost: $80–$150 for the fan. Wiring is the same as a standard fan.

Venting (Critical — Often Done Wrong)

The exhaust must vent to the EXTERIOR of the home. Common violations found in NB homes:

Wrong:

• Vented into the attic (moisture causes mould, ice dams, and structural rot)


• Vented into the soffit (moisture gets sucked back into the attic through soffit vents)


• Duct disconnected or never connected (fan runs but exhausts into the ceiling cavity)

Right:

• Insulated duct from the fan to a wall cap or roof vent


• Duct should be as short and straight as possible


• Use rigid or semi-rigid duct, not flexible vinyl (which sags and traps moisture)


• Insulate the duct in cold spaces (attic) to prevent condensation inside the duct — critical in NB where attic temperatures drop well below freezing


• Vent cap should have a damper to prevent cold air backdraft

NB Climate Considerations

Condensation in the duct: In winter, warm moist air from the bathroom hitting a cold duct in an unheated attic condenses and can drip back into the fan or leak into the ceiling. Solutions:

• Use insulated rigid duct (pre-insulated 4" duct, $3–$6/foot)


• Keep duct runs through cold spaces as short as possible


• Slope the duct slightly toward the exterior so any condensation drains out

Backdraft damper: NB winds can force cold air backward through the vent cap, cooling the bathroom and wasting heating energy. Install a backdraft damper at the fan and at the vent cap for double protection. Cost: $5–$15 each.

Run time in winter: Run the fan for 20–30 minutes after each shower or bath. In NB's dry winter air, bathroom moisture dissipates faster than in summer, but the warm moist air condensing on cold windows and walls is more damaging in winter.

Installation Summary

| Scenario | Permit? | DIY? | Cost |
|----------|---------|------|------|
| Replace fan (same location, same wiring) | No | Yes | $30–$150 |
| Replace fan + add timer switch | No | Yes | $50–$190 |
| New fan installation (new wiring) | Yes | No — hire electrician | $400–$800 |
| New fan + separate switch + venting | Yes | No — hire electrician | $500–$1,000 |
| Fan/light/heater combo (new dedicated circuit) | Yes | No — hire electrician | $600–$1,200 |

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