For a small bathroom that must retain a 3-inch exhaust duct, the Broan-NuTone 688ES is our first model to compare: it has a native 3-inch connection, a 50-CFM rating and a 2.0-sone sound rating. Broan recommends it for rooms up to 45 square feet. If you need an integrated light, consider the 688ESL; if the room needs more airflow, investigate the Air King AS70 and its substantially higher noise rating.
The duct diameter is only one part of the decision. A compatible outlet does not guarantee adequate installed airflow, a matching ceiling opening or compliance with your local requirements. For a larger bathroom, a long duct run or a very quiet installation, improving the ductwork may be the better project.
Reviewed October 4, 2026. This guide compares manufacturer specifications and installation documents; we did not conduct hands-on airflow or noise tests. As an Amazon Associate, we earn from qualifying purchases. Product links do not determine our recommendations.
Bathroom fans with native 3-inch duct connections
All four models below specify a 3-inch round connection. Airflow and sound figures are manufacturer-published ratings at 0.1 inches of water-gauge static pressure, not measurements in your bathroom. Housing dimensions are width/length/depth dimensions from the manufacturer, not the grille size or necessarily the required rough opening.
| Exact model | Rated airflow / sound | Housing, inches | Best reason to consider it |
|---|---|---|---|
| Broan-NuTone 688ES | 50 CFM / 2.0 sones | 7-1/2 × 7-1/4 × 3-5/8 | Fan-only replacement; ENERGY STAR certified |
| Broan-NuTone 688ESL | 50 CFM / 2.0 sones | 7-1/2 × 7-1/4 × 3-5/8 | Integrated LED light; ENERGY STAR certified |
| Air King AS50 | 50 CFM / 4.0 sones | 7-1/4 × 7-1/4 × 3-3/4 | Basic fan with a square housing |
| Air King AS70 | 70 CFM / 5.5 sones | 7-1/4 × 7-1/4 × 3-3/4 | Higher rated airflow through a native 3-inch collar |
Noise trade-off: the Home Ventilating Institute recommends 1.0 sone or less for quiet bathroom ventilation. None of these four meets that recommendation. The Broan models have the lowest published sound rating in this shortlist, but we would not describe them as silent. HVI bathroom-fan guidance.
1. Broan-NuTone 688ES: the fan-only starting point
The 688ES combines the smaller native duct connection with ENERGY STAR certification and a lower published sound rating than the two Air King options here. It is a sensible candidate for a small room where the housing fits and the existing exhaust route can deliver the required ventilation.
Limitations: it has no built-in humidity or motion sensor and no light. Broan’s 45-square-foot room recommendation is worth respecting rather than assuming every 50-CFM model is intended for the same room. Have the installer check the existing opening and duct access before treating it as a straightforward replacement.
The linked Amazon listing was verified as the 688ES fan-only model. Confirm the model and seller when ordering; do not confuse it with the lighted 688ESL or older 688.
Broan-NuTone Bathroom Exhaust Fan, 50 CFM Modern Grille, Energy Star Fan, 2.0 Sones
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2. Broan-NuTone 688ESL: when you want a light too
The lighted version adds a 750-lumen, 3000K LED. Its specification sheet states that the light and fan run at the same time and that the unit is incompatible with dimmer switches or separate switches.
Choose it for: a small bathroom where combined fan-and-light operation is acceptable. Skip it if: you want to leave the light on with the fan off, control them independently or dim the light. Those are meaningful buying constraints, not details to discover after opening the ceiling. Check the exact 688ESL model.
3. Air King AS50: a basic square-housing option
The AS50 specification sheet recommends it for bathrooms up to 50 square feet. It calls for at least 4 inches of mounting clearance and a 7-3/8-inch square rough opening. Check those requirements separately from the housing dimensions in the table.
Its 4.0-sone rating makes noise the main compromise. The published performance table also shows airflow falling to 39 CFM at 0.25 inches of static pressure, illustrating why a 50-CFM label cannot guarantee 50 CFM through every existing duct.
Choose it for: a compatible basic replacement where its sound level is acceptable. Order the complete fan; a replacement motor or trim kit is a different purchase. See Air King’s AS series.
4. Air King AS70: more airflow, with a noise penalty
The AS70 is the higher-airflow candidate in this group. Air King recommends it for bathrooms up to 70 square feet and reports 62 CFM at 0.25 inches of static pressure. It uses the same stated housing size, minimum clearance and rough opening as the AS50.
The trade-off is its 5.5-sone rating. That makes it a poor fit if low noise is a priority. Also, more rated airflow does not fix a blocked, poorly routed or inadequate exhaust system. Have an installer assess whether retaining the existing duct is appropriate before choosing it solely for the larger CFM number.
How much airflow does your bathroom need?
For intermittent bathroom ventilation, HVI’s sizing guidance recommends 1 CFM per square foot below 100 square feet, with a 50-CFM minimum. Above 100 square feet, it uses fixture-based sizing: 50 CFM each for a toilet, shower or bathtub, and 100 CFM for a jetted tub.
- A 6 × 7-foot bathroom: 42 square feet. A 50-CFM model is a starting point, subject to the model’s room guidance and actual duct performance.
- An 8 × 10-foot bathroom: 80 square feet. The simple sizing guideline points to 80 CFM, above the nominal capacity of every fan in this shortlist.
These are selection examples, not a local-code determination. High ceilings, enclosed toilet areas and the actual ventilation layout need additional consideration. Ask a qualified installer to confirm the required airflow, sound and efficiency requirements for your project before purchase.
Check the existing duct before choosing the fan
Air must travel through the whole exhaust route, not just the fan’s connector. HVI identifies duct length, material, bends and the exterior termination as contributors to airflow resistance. Its duct-design discussion supports short, appropriately designed runs and attention to their resistance. The performance differences in the Air King sheets show why installed airflow matters.
Have the installer check for a crushed or disconnected duct, excessive bends, a sticking damper and a restricted outside termination. The duct must exhaust outdoors. A fan replacement alone will not correct a route that ends in the attic or a ceiling cavity.
Do not buy a 4-inch-outlet fan and assume a reducer preserves its advertised performance. Only consider an adapted installation when the exact manufacturer instructions permit it and the required airflow can be established for that configuration. Otherwise, choose a native 3-inch model or have the ductwork redesigned.
Fit and installation checks before ordering
- Read the old model label. A familiar grille shape does not identify the housing or establish that a new motor assembly fits.
- Measure the opening and available depth. Compare the rough opening, mounting arrangement, joist or stud position and grille coverage with the new model’s manual.
- Confirm access. The 688ES installation guide distinguishes room-side installation with flexible duct from attic-side installation with rigid duct. Do not assume a “no attic access” product description covers every replacement.
- Confirm controls and location. A qualified installer should check compatible switching, the circuit and all wet-location requirements. The listed models’ documents condition over-tub/shower use on a GFCI-protected branch circuit.
- Confirm delivered ventilation. Ask the installer to check actual airflow once the complete system is installed.
Electrical and installation work must follow the exact product instructions and applicable local requirements. This is a buying guide, not a wiring procedure.
Frequently asked questions
Is the old Broan 688 still the best option?
Broan now marks the 688 as discontinued and points to the 688ES replacement. Its 671 page also says discontinued. Distinguish old stock and replacement parts from a current complete fan, and confirm physical compatibility.
Can I use any 50-CFM fan with a 3-inch duct?
No. Airflow and duct-connection size are separate specifications. For example, Air King’s AS54 uses a 4-inch collar even though it is also a 50-CFM model. Check the exact model’s duct requirement, not just its series name or CFM. Air King model comparison.
Which is the quietest model in this shortlist?
The 688ES and 688ESL tie on their published 2.0-sone rating. If a quiet bathroom is your main goal, discuss a lower-sone fan and suitable ductwork with an installer rather than restricting the purchase to a legacy duct size.
Can I replace only the motor?
Possibly, if the manufacturer specifies a replacement assembly for the exact existing housing. That approach does not change the existing duct or establish its airflow. Compare the correct repair assembly with a complete-fan replacement before deciding.
How we made this shortlist
We required an explicitly documented 3-inch connection, an exact model number, published performance figures and accessible manufacturer instructions. We checked current Broan-NuTone and Air King pages, specification sheets and HVI guidance on October 4, 2026. The Amazon link was checked separately for model identity. We excluded unrelated dryer accessories, portable fans and products whose duct requirements did not match this guide. No retail price, stock status, installation time or numerical review score is promised.
Image note: The featured image is an AI-generated editorial illustration, not a photograph of a reviewed product.
