Trap Primer: Friend or Foe?

A 1960's style floor drain with a copper trap primer tube supplying water to the trap seal.

A 1960’s style floor drain with a copper trap primer tube

supplying water to the trap seal.

Trap Primer: Friend or Foe?

by Keith Tripp. Revised May, 2023

In Canada we have some of the best and cheapest water supplies anywhere in the world. This means wastage of water does not get much attention. However, water costs are increasing dramatically and frequently. In this article, I explain a plumbing system that can be the source of huge water wastage.

Officially called a trap seal primer, the purpose of this system is to maintain water level in the trap seal at the floor drain in the basement or any other drain that is not connected to a fixture. From here on in I‘ll just call it a primer.  The most common primers are serving floor drains in the basement. The purpose of the trap seal is to create a barrier between the house air that we breath, and the nasty air of the sewer pipes on the other side of the trap. Sewer gasses are hazardous to human health and since the early days of plumbing, trap seals have been employed in the drain systems to prevent exposure to sewer gasses. Failure of primer systems can result in sewer gasses entering the house, or in extreme waste of water.

All fixtures such as sinks and baths have traps installed in the drain just below the fixture. These traps receive water each time the fixture drain is used, so they are not at risk of drying out as long as the fixture is used at least once in a while. Toilets have built in traps so there is no trap installed in the drain pipe from a toilet. The water you see in the toilet bowl is acting as the trap seal. Floor drains do not receive water on a regular basis, so they are at risk of drying out. hence the requirement for the trap primer.

A traditional style laundry faucet with primer tube attached to the faucet body.

A traditional style laundry faucet with primer tube attached to the faucet body.

The most common water source for the primer system is the laundry tub faucet. At the underside of the traditional laundry faucet body is a fitting to which a tube can be attached.  See photo. Whenever the valves, hot or cold, are opened, water flowing in the body of the faucet runs through the primer tube. On older houses (60s and 70s) the tube was copper and is routed  through the cement floor slab and enters the floor drain pipe through the side of the vertical drain, just above the trap seal water level.  On newer houses, the tube is plastic and is routed through a larger plastic conduit that runs through the concrete floor slab to the drain. If there are multiple floor drains the tubes may be split to flow to all the floor drains. This type of direct flow primer system has been installed in original construction since about the early 1960s.


What can go wrong? Too much water! The only thing worse than a primer that doesn’t work is one that works all the time!  Failure of primer systems can result in a continuous flow of water to the drain, often going undetected. This can be hot water, which means energy as well as water are wasted. Worn washers or washer seats, or a partly open valve at the laundry faucet can result in water flowing continuously through the primer tube, often without detection. If water is leaking past the washers in the faucet, it will take the path of least resistance. Based on location, that path is the trap primer tube connected to the underside of the faucet body. The leakage can occur through either the hot or the cold side or both. The faucet won’t drip, so this condition will often go undetected. Sometimes a slight gurgling noise can be heard, as air is drawn in to the faucet to replace water that escapes through the faucet body to the floor drain. If hot water is leaking through, over time the concrete floor slab may get warmed up by this. Substantial amounts of water can be lost this way resulting in HUGE water bills and water heating bills.

What can go wrong? Not enough water! If the primer fails to provide water to the trap, the trap seal water in the drain may evaporate, resulting in an open trap and sewer gasses will enter the house. This is most common at drains supplied from a split tube where the flow really is not adequate to service more than one floor drain. Tubing or conduit could be damaged or poorly routed and preventing flow of the water to the trap. Strangely enough, the dried-out trap does not always emit an offensive odour to warn of what has happened. The extent of odour depends on where the drain is located relative to the rest of the sewer drains, and air flow through the house. I have however, walked in to cold rooms with dried out drain traps where the odour could knock you off your feet. This is relatively common in new construction where builders have failed to commission the primer system.  (note: not all cold rooms have floor drains.)

To test the primer, simply operate the fixture it is attached to, such as running the laundry tub water, and water should be visible or audible flowing in to the floor drain just below floor level and above the trap seal water.

The white tube is a primer tube, split to serve two floor drains.

The white tube is a primer tube, split to serve two floor drains.

If there is no laundry tub, or if the laundry faucet is not a traditional laundry faucet then it is important to confirm presence of a trap primer system. Sometimes its hard to find them. In a new house, ask the builder to identify the primer location if you can’t find it. Primer tubes of the direct flow type described above can also be connected to toilet flush systems, and to shower-head supply pipes.  The shower connections are becoming more popular as fewer standard type laundry faucets are being installed in new construction. There is potential for excessive water flow from the shower connections and that will include waste of hot water.

There are primer devices that are actuated by a pressure drop. These are used when a traditional laundry faucet is not available at a reasonable distance from the drain. They can be installed on any water line and will direct a flow of water to the drain when pressure drops because of water usage on that line. Valves connected to timers can also be sued as a primer system.


There are primer substitute devices that don’t require water flow. These are sometimes used in laundry room floor drains in new construction. Look down the drain. If the opening is blocked with a rubber diaphragm this is probably a trap sealing device. Don’t confuse this with a device that may be installed in a basement floor drain to prevent backflow.

If the primer doesn’t work, repair accordingly. In the short term, pour a few buckets of water down the drain every month or so to maintain water level in the trap.

Related Tips

Baby clothes washers? Recently I came across clients who had installed “baby clothes” washers in their laundry rooms. These are small washing machines that require the water hose to be connected to a faucet. The washing machine has a solenoid valve that controls flow of water in to the machine, so it is not necessary to shut off the water at the faucet.  The washer supply hoses were attached to the threaded laundry faucet spout, and the laundry faucet taps were in the open position. With this arrangement water was flowing full time through the primer tube to the floor drain even though no water was flowing to the washing machine.

Installing a new laundry faucet? Only the basic laundry tub faucets have the primer connection. If installing a kitchen style faucet at a laundry sink, a different primer system will be required.

Leaving a house vacant for more than a few weeks? A primer is only effective if water is run on the pipes or faucet that will trigger the primer action.  If a house is vacant, no primer water will be flowing. If a house is vacant, or a laundry faucet it not regularly used, then it’s a good idea to run water every few weeks for all fixtures including those that do not have primers.

Questions about trap primers or home inspections? Email to Keith Tripp ,