Why do floor drains dry out so fast in Arizona?

### Evaporation is the usual way a trap loses its seal

Sewer gas stays out of a building because of two things: the water seal in every trap, and the vents that release gas outdoors. Wikipedia's summary of sewer gas says the most common source of indoor sewer gas is water evaporating from traps in rarely used fixtures, and it names floor drains near furnaces, utility sinks and showers as typical culprits. A trap under a sink that gets used every day refills itself with each use. A seldom-used floor drain may not, unless its primer or another approved protection method is working.

The trap water in a floor drain is also more exposed than the trap under a sink. It sits in a wide opening in the floor, with room air moving across the surface all day. That makes it easy for a small pool of water to shrink.

### Phoenix and commercial buildings speed it up

Arizona stacks the deck. Phoenix air is dry, and commercial buildings run air handlers, exhaust fans and make-up air that move air across mechanical rooms, kitchens and back-of-house floors nearly all day. Dry, moving air takes water off a small pool faster than still, humid air does. We won't put a number of days on it, because it swings with the room, the season and the airflow. What holds steady is the pattern: a drain that gets no water for a few weeks, especially in summer, is a drain to check.

The code gives you very little cushion to start with. Each fixture trap must have a liquid seal of at least 2 inches and not more than 4 inches deep, according to the plumbing code summary from ASPE. That is not a lot of water to lose before the seal opens.

### The drains that dry out first

• Floor drains in mechanical, boiler, electrical and elevator machine rooms
• Mop sink and janitor closet drains
• Floor drains in unused storage, basements, loading areas and parking structures
• Floor drains in low-traffic restrooms
• Drains in vacant suites, seasonally closed spaces and buildings that sit empty in summer
• Emergency floor drains that only see water when something goes wrong

Any of these can go weeks with no water. If you manage a building with dozens of them, that adds up to a smell complaint waiting to happen.

### There is no code interval

The plumbing code does not say how often to pour water into a drain, because the right interval depends on the drain, the room and the season. A workable routine looks like this:

1. 1Walk the building and list every floor drain, floor sink, mop sink and rarely used fixture. Number them on a floor plan.
2. 2Pour water into each one until you see it stand in the trap. A gallon is plenty for most.
3. 3Recheck the ones that smell first. If a drain is dry again in a few weeks, it goes on a shorter schedule, or it gets a fix that does not depend on anyone remembering.
4. 4Log the date and the drain number. If you manage for an owner, that log is proof you were watching.

### A simple schedule that works for most buildings

Start monthly and adjust. In a Phoenix summer, shorten the interval for drains in mechanical rooms and anywhere air moves across the floor. In winter, most drains hold longer. A vacant suite gets a fill before the last person leaves and another at each walk-through.

Refilling is a stopgap. It works and it costs almost nothing, but it fails the first time someone forgets, which is why the drains that keep drying out need something better.

### How to tell the seal is really there

If the trap is safely visible, a flashlight may help show its water level. Not seeing water does not prove a dry seal: drain geometry, a grate or a barrier device can hide it. Water draining away normally also does not prove the trap is empty. Refill a conventional trap when appropriate and have a plumber check persistent odor, leaks, gurgling or the existing protection device.

### What the code allows

The plumbing code recognizes several ways to keep a trap seal in place where evaporation is a problem. ASPE's summary lists these options:

• Potable-water trap seal primer valves meeting ASSE 1018, with the discharge piped above the trap seal on the inlet side
• Wastewater-supplied trap primer devices meeting ASSE 1044
• Barrier-type trap seal protection devices meeting ASSE 1072, installed to the manufacturer's instructions
• A nearby fixture drain, such as a lavatory or hand sink drain, that feeds an emergency floor drain or floor sink in the same room, concealed in the construction

Wikipedia's trap primer article makes the same point in shorter form: a primer keeps the trap topped up so the seal survives the evaporation that empty drains suffer.

### Which one fits

A primer valve feeds water to the trap automatically, so it needs a water supply and piping. A suitable barrier device provides trap-seal protection when installed and maintained as specified, and it needs no water supply. Which one fits depends on the drain, the piping, the flow it needs to handle and the code edition your city has adopted, so it is a plumber's call. If the same drain keeps drying out, ask about protection for that drain instead of putting it on a refill list forever.

We are licensed and insured, and you get a free estimate and an upfront price before we start on any work like that.

### Sewer gas is not just an odor

Sewer gas is a mix that can include hydrogen sulfide, ammonia and methane. The CDC's toxicology summary says people can usually smell hydrogen sulfide at very low concentrations, and that at around 100 ppm they lose the ability to smell it after about 2 to 15 minutes. OSHA's hazard page describes the same loss of smell, called olfactory fatigue, at that level. So a smell that fades is not proof the problem is gone. It can mean your nose has quit reporting it. Deal with the drain, not the smell.

### What a tenant, guest or inspector sees

A dry floor drain does more than make a room smell. The same open trap that lets gas in also lets pests come up from the sewer, and in a restaurant a persistent sewer smell reads to a health inspector like a building nobody maintains. For a property manager, a documented refill schedule and trap protection on the problem drains is far cheaper than a stream of tenant complaints.

If every drain is full and the smell is still there, the cause is somewhere else. Work through how to find the source of a sewer smell, step by step, and see why a commercial building smells like sewer gas for the wider list of causes. If a drain gurgles or backs up as well as smelling, read why a floor drain backs up. For how floor drains, floor sinks and mop sinks differ, see floor sink versus floor drain versus mop sink.