Sun-Mar Excel Composting Toilet Review

What Is the Sun-Mar Excel and Who Is It For?

Composting toilets occupy a completely different product category from the standard gravity-flush and pressure-assist models you find in most homes -- toilets like the best flushing toilets from TOTO, Kohler, American Standard, and Woodbridge. Those units connect to a municipal sewer or septic system and rely on water to carry waste. The Sun-Mar Excel uses no water at all. It converts waste into a dry, stable compost material that most jurisdictions allow disposal of in the trash or garden (check local regulations before use).

Sun-Mar has manufactured composting toilets in Canada since 1971 and is among the longest-standing names in the category. The Excel model sits at the top of their residential self-contained line, positioned above the more compact Spacesaver and below the remote or central composting systems that separate the toilet from the composting drum.

What Are the Sun-Mar Excel's Published Specifications?

How Does the Sun-Mar Excel's Drum System Work?

The drum composting approach is the Sun-Mar Excel's primary engineering advantage over simpler batch-pile systems. Most low-cost composting toilets use a static pile: waste accumulates in a chamber with peat, aeration is passive, and the homeowner manages the pile manually. In static-pile designs, uneven decomposition is common -- wet pockets and dry zones form, odor control becomes inconsistent, and the operator must monitor pile conditions carefully.

The rotating drum solves the aeration problem mechanically. Each rotation introduces oxygen throughout the mass and prevents anaerobic pockets -- the condition that causes most composting toilet odor complaints. Sun-Mar patents this system under the Bio-drum name. The drum's fiberglass construction resists corrosion and is inert to the acidic environment inside a composting chamber.

The liquid management system is an equally important but less discussed component. Human waste is roughly 90% liquid by volume. The Excel's evaporation chamber below the drum receives excess liquid drawn by gravity. The optional 250W heating element in the evaporation chamber accelerates liquid removal. In climates with low ambient humidity and adequate ventilation, owners report that the drain connection at the rear of the evaporation chamber rarely sees active flow. In humid coastal or tropical climates, that drain port becomes essential -- route it to a small leach pit or gray water system.

How Does the Sun-Mar Excel Compare to Other Composting Toilets?

The key distinction between the Sun-Mar Excel and the Nature's Head is context of use. Nature's Head separates urine from solids at the seat using a diverter funnel, meaning the solids container stays drier and can be emptied less frequently -- an advantage in mobile situations where liquid disposal is difficult. The Excel's integrated drum handles both waste streams together, which simplifies the user experience but requires more active liquid management in humid environments.

Compared to a conventional flushing toilet -- a TOTO Drake II or Kohler Highline -- the Sun-Mar Excel requires far more owner involvement. Those water-flush toilets operate passively; they need no bulking material, no weekly drum rotation, and no vent pipe installation. The composting toilet's value proposition is for locations where sewer or septic service is unavailable or prohibitively expensive to install.

What Do Owners Say About Sun-Mar Excel Performance and Odor?

Across thousands of aggregated owner reviews, the Sun-Mar Excel earns consistent praise for two things: odor performance and build quality. Owners report that with the continuous fan running, there is virtually no bathroom odor. The negative-pressure ventilation created by the fan draws air down through the seat opening and up through the vent pipe, which means odors travel away from the living space rather than into it.

The most frequent owner complaints cluster around three scenarios:

• Liquid overflow in high-use periods -- when more people than the rated capacity use the unit over a weekend, the evaporation chamber can saturate before the heating element evaporates excess liquid. Most experienced owners preemptively connect the liquid drain port to a small gravel pit or leach line rather than relying entirely on evaporation.
• Confusion about composting completion -- new owners sometimes empty the finishing drawer before composting is complete, resulting in material that hasn't fully broken down. Sun-Mar's recommendation is to leave material in the finishing drawer for a minimum of 4 weeks after the drum stops adding new material to it.
• Cold-weather performance -- the optional 250W heating element is effectively mandatory for year-round off-grid use in climates that regularly see temperatures below 60 degrees Fahrenheit. Without the heater, evaporation rates slow significantly in winter months.

Long-term durability reviews are broadly positive. Owners with 5 to 10 years of continuous use report that the fiberglass drum and body remain structurally sound. The fan motor is the most commonly replaced part; Sun-Mar sells replacement fans, and the installation is straightforward with basic tools.

What Are the Installation Requirements for the Sun-Mar Excel?

The vent pipe is the most critical installation variable. Sun-Mar recommends a 4-inch diameter run that exits through the roof for maximum draft. A wall penetration can work, but a roof exit creates a more consistent upward draft because warm air from the unit naturally rises. The vent pipe should rise at a minimum angle of 45 degrees from horizontal wherever vertical routing is not possible, and the total run should not exceed 10 feet. A longer run reduces fan effectiveness.

For the electrical connection, the unit draws 10 watts continuously for the fan. If the optional 250W heating element is energized, total draw rises to 260 watts -- still well within a standard 15-amp circuit. Solar-powered off-grid installations need to account for the fan's 10W continuous draw in battery sizing calculations; 240 watt-hours per day is a reasonable baseline.

Floor support is straightforward -- the unit sits on any level floor. Some owners build a small platform to raise the seat height to ADA-level comfort, since the Excel's standard seat height can feel low relative to modern comfort-height toilets like the American Standard Champion 4 or the TOTO Drake II. A 4 to 6 inch platform brings the Excel to a comparable sitting height while also providing convenient storage space for bulking material.

Permitting requirements vary significantly by state, county, and municipality. Some jurisdictions in Vermont, Colorado, Oregon, and Washington have explicitly approved NSF/ANSI 41 certified composting toilets as primary sanitation systems. Others still require a conventional septic system to be in place even if the composting toilet handles day-to-day waste. Check with your local health department before purchase.

What Is the Ongoing Maintenance Schedule for the Sun-Mar Excel?

The weekly maintenance routine that successful Sun-Mar Excel owners describe follows a predictable pattern. After each use, one cup of coconut coir or peat moss goes into the drum through the seat opening, followed by 5 to 7 rotations of the drum handle. This takes approximately 30 seconds. The rotation aerates the composting mass and moves material toward the finishing drawer. Skipping this step consistently is the single most common cause of performance problems.

Monthly tasks include checking the evaporation chamber liquid level and cleaning the drum handle mechanism. Every 4 to 8 weeks, the finishing drawer is pulled out and the composted material -- which at this stage resembles dark, crumbly soil with no offensive odor -- is removed and composted further in an outdoor pile, buried, or disposed of per local regulations. Sun-Mar recommends a second 4-week outdoor curing period before the material contacts edible plants.

Annual maintenance covers the fan motor inspection, vent pipe cleaning (a long brush run through the pipe removes any condensation buildup or debris), and a thorough cleaning of the evaporation chamber with a 50-50 white vinegar and water solution. The drum seals should be inspected for any cracking that could allow liquid to escape into the evaporation chamber faster than intended.

Consumable cost is straightforward: a 50-liter bag of coconut coir or compressed peat runs roughly 3 to 4 months for a 2-adult household at the recommended dosage. This is the system's primary ongoing supply cost.

How Does the Sun-Mar Excel Handle Odor Control?

The physics of composting toilet odor control are straightforward. As long as the fan creates a slightly negative pressure inside the unit relative to the bathroom, air flows inward through the seat opening rather than outward. Any volatile compounds produced during decomposition travel up the vent pipe to the exterior. The Sun-Mar Excel fan operates continuously at 10 watts -- this is intentional and important. Turning the fan off to save power is a common beginner mistake that allows the pressure differential to equalize, which then allows odors to escape.

The rotating drum contributes to odor control by maintaining aerobic decomposition. Anaerobic decomposition -- the kind that happens when oxygen is absent -- produces significantly more volatile and offensive compounds, including hydrogen sulfide (the rotten-egg smell) and ammonia. The drum rotation ensures oxygen is mixed throughout the composting mass, keeping the process aerobic and odor-minimal.

Users who have installed vent pipes that are too short, angled incorrectly, or partially obstructed by nearby vegetation or structures report reduced odor control. The vent pipe must terminate above the roofline in a location where building exhaust does not recirculate back toward windows or air intakes. Sun-Mar provides a detailed vent installation guide with the unit.

Sun-Mar Excel vs. Standard Flushing Toilets: Is a Composting Toilet Right for You?

This comparison matters because composting toilet marketing sometimes suggests these units are a broadly desirable "eco" upgrade for any home. That framing is misleading. For any property with an existing septic system or sewer connection, switching to a composting toilet introduces significant complexity and ongoing maintenance without meaningful ecological benefit relative to modern EPA WaterSense certified toilets.

A TOTO Drake -- one of the best-performing toilets in MaP flush testing with a MaP score of 1,000 grams at 1.28 GPF -- already uses just 1.28 gallons per flush, and some TOTO Aquia IV dual-flush models drop to 0.8 GPF on the light flush cycle. The Kohler Highline Arc and American Standard Cadet 3 are WaterSense certified at 1.28 GPF with strong MaP scores. These are passive, maintenance-free systems with robust industry support.

The Sun-Mar Excel makes genuine sense in the following situations:

• Off-grid properties where digging a septic system would cost more than $10,000
• Seasonal cottages or cabins where a septic system would be unused for months at a time
• Locations where soil conditions (bedrock, high water table, tight setbacks) preclude conventional septic
• Tiny houses on wheels where a conventional toilet and black water tank system is impractical
• Eco-resorts or glamping facilities seeking to reduce water and sewer infrastructure costs

For a property that falls into one of those categories, the Sun-Mar Excel is the most capable self-contained composting toilet available. For any other property, a WaterSense certified gravity-flush toilet will serve better in every measurable dimension.

What Are Common Problems With the Sun-Mar Excel and How Are They Solved?

Liquid management is the most frequent source of frustration for new Sun-Mar Excel owners. When liquid in the evaporation chamber exceeds the evaporation rate -- either because use is heavy or ambient temperatures are low -- liquid can pool visibly at the bottom of the unit or, in severe cases, leak from the bottom drain port. The immediate fix is to connect the 1.5-inch drain port to a proper disposal point rather than leaving it capped. The long-term fix is to enable the optional 250W heating element, which dramatically increases evaporation capacity.

Fly infestations, particularly of phorid flies (also called drain flies), are an occasional reported issue. These small flies breed in moist organic material and can find their way into the drum through the seat opening. Prevention involves keeping the seat closed when the unit is not in use and ensuring the vent fan is always running. If an infestation occurs, a small amount of food-grade diatomaceous earth added to the drum discourages the insects without disrupting the composting process.

Drum rotation resistance increases over time as the composting mass grows and the drum mechanism accumulates residue. A small amount of food-grade vegetable oil applied to the drum's rotating mechanism every 6 months reduces friction. If the drum becomes difficult to turn, reducing the bulking material and temporarily adding extra turning frequency helps break down heavy accumulations.

For comparison, a conventional gravity-flush toilet's most common problems -- a running toilet, a ghost flush, a slow fill, or a clogged trapway -- are all solved with inexpensive parts available at any hardware store. The Sun-Mar Excel's failure modes are different in character: they require operational knowledge rather than plumbing skills.

See our guide to best composting toilets for side-by-side comparisons with competing brands, and our best eco-friendly toilets guide if you want a waterless or ultra-low-flow option that doesn't require a major lifestyle change.