Technical bulletins
June 2014

Sump pumps failures: beware of floods

By Martin Morissette Eng., CFEI, CVFI,

Homes and commercial/industrial buildings may have one or more sumps built-in their lowest level floor, which is in the basement for most homes. The main function of these sumps is to collect water from the surrounding foundation drain, and sometimes other sources such as floor drains and garage dirt traps in residential installations. In these types of installation, the sump is typically limited in capacity, being of 200 liters in average, thus can become full quite rapidly during heavy rain or snow melting periods. An electrical pump is the best choice to drain the sump.

Main types of pumps

Figure 1: Submersible pump

The two most widely used types of pumps in residential installations, are the " submersible " and, the " column " type. Each one of these types has its advantages and weaknesses, which can strongly influence the choice when its time to acquire a pump. Typically, submersible models (fig.1) are generally more expensive than the column types (fig. 2), due to their more robust and compact design. This type of pump as also the advantage of being completely hidden in the sump, making its installation less bulky, but its motor is more vulnerable to water intrusion, due to its "submerged" design.

Column type pumps have the advantage of having their motor out of the water, which practically renders any contact with the latter impossible. However, this type of pump may be prone to mechanical failures of some components of its column, that are often made of plastic, and of dimensions that make them more fragile.

Figure 2: Column type pumpe

Most common causes for failure

Common failures for these two pump types that unfortunately often lead to water damage are presented in the following.

Presence of debris in the sump and their accumulation can obstruct partially or totally the pump inlet ports under the suction effect. This, situation can lead to the sump overflow. Fibrous debris (i.e. hairs, grass, fabric threads) can wind up around the impeller shaft and impair its rotation, sometimes until complete stop (fig.3). Stones can also enter the pump and wedge or even damage the impeller. It is to be noted that a sump pump is designed to pump clear water that is free of solid residues. If such residue is present, a "bilge" pump must be used.

Electrical motor failure is an often seen situation for the two types of pump.. This type of failure can have more than one origin, such as: main winding failure (open circuit or ground circuited), failure of the starting winding or condenser, or water intrusion following damage of one seal (submersible type).

Failure of a motor bearing, is not a rare situation and can generate an important casing overheat and lead to its destruction. Inevitably, those situations will harm relatively rapidly, the integrity of the motor.

Figure 3: Obstructed pump

The float switch can be a source of failure of the pump on which it is installed. This kind of switch is typically used on both types of pump, although its design can be different regarding the pump type. Water intrusion in the float, modification or damage of the fastening point of the float cord (submersible pumps only), presence of debris in the sump and, at last, a pump that is installed to close to a sump wall, are situations that can prevent proper movement of the float, thus, will prevent the pump to start or stop. Failure of the switch, that is the main component of the float switch, either by water intrusion or a mechanical problem, is also not a rare situation. On column pumps, the float is generally mounted on a metallic rod, which drives another switch that is located on the motor. Misalignment or the absence of the guide of that rod can also prevent the rod to move properly to activate the motor switch.

Degradation of the float switch electrical cable bracket or of its fastening screw (submersible pumps) is also a well-known problem. Galvanic corrosion is often responsible for that degradation, and involves metallic components. In simple terms, when some different metals are placed in an aqueous environment, an electrochemical phenomenon occurs, that causes rapid oxidation of one of the two metals. For example, steel will corrode quite rapidly in presence of copper.

On column type pump, damage to the rotation shaft, generally made of plastic, is also a common situation. This type of failure typically occurs at the coupling locations, at the shaft extremities (fig.4). Many factors can be responsible for that kind of failure, namely: excessive vibrations generated by a misalignment of the column with regard to the motor or a lack of support of the latter, sudden blocking of the impeller, for example resulting of the intrusion of a stone in the volute.

Figure 4: Broken shaft

Failure of the impeller is a less common occurrence that is met mostly on plastic impellers. Cases where impeller blades had been broken or worn-off have been observed. This situation can originate from a material defect or wear and tear from hard debris such as a small stone that intruded into the impeller housing. This situation would reduce flow capacity of the pump and possibly lead to sump overflow.

Sometimes, improper or abusive manipulations can lead to part failure. Multiple manipulations can damage the float switch, the electrical cord or even the impeller housing.

Other devices part of the sump installation can lead to overflow , such as :

  • clogged outlet pipe;
  • broken, clogged or missing check valve (creating backflow in the sump);
  • movement of the float switch impaired by debris or some obstacle;
  • 2-pump installation with insufficient room in the sump;
  • dried-out sump leading to a damaged or sticky mechanical joint at the pump axle;
  • improper adjustment of the float switch cord;
  • electrical supply failure;
  • blockage from (ferrous) red ochre.

Conclusion and recommendations

In conclusion, a few low cost precautions can be taken to avoid sump overflow, namely, installing a second, back-up pump in the sump, that would be connected to a different electrical circuit breaker or operate on a 12V marine grade battery. There is also a type of " emergency " pump operating on city water. These pumps are of low capacity and should be used as emergency back-up only. They are not permitted in some municipalities.