One air-conditioning installation or aftermarket add-on that is easily forgotten by HVACR installers and service techs is the condensate drain-pan overflow switch. A condensate drain-pan overflow can potentially cost the end-user hundreds if not thousands of dollars in water damage, mold remediation and interior furnishings replacement. Secondary drain pans are excellent redundancy methods. But without a UL-508-tested electronic or mechanical overflow switch that deactivates the equipment and/ or alerts the building owner, it too can overflow undetected after succumbing to whatever clogged the primary drain pan. Therefore, the overflow switch is another form of redundancy that protects the customer’s property and the contractor’s liability associated with drainpan overflow damages.
Ironically, data centers have redundant precision air-conditioning because cooling microprocessors must be failsafe. Even at home, the HVACR contractor most likely has a surge protector for redundancy against power spikes to electronic equipment. Therefore, why not install a redundant safeguard protecting an air-conditioner from overflowing gallons of water into a building?
The International Residential Code (IRC) Section 307.2.3 calls for one of the following for redundancy:
A secondary drain line higher than the primary drain;
A drain pan with a drain pipe under the equipment;
A drain pan with interlocked detector; and
A water-level detection device interlocked with equipment, which is also referred to as an overflow switch.
The International Mechanical Code (IMC) calls for similar requirements. Some contractors prefer multiple protections beyond the unit manufacturer’s primary drain pan, such as a secondary drain pan and an overflow switch. While it might seem like overkill, some jurisdictions, such as Houston, TX, mandate all three on particular applications where potential property damage is greatest.
Aside from codes, common sense dictates that an air-conditioning system positioned in an area where potential overflow damage is greatest should have an overflow switch in additional to a code-mandated secondary drain line.
Best applications for overflow switches
Not every air-conditioning system location or application has propertydamaging potential. The most ideal applications for overflow switches are:
Air handlers and coils in attic locations, above ceilings, closets and finished basements, especially those in secondary residences that are not occupied year-round where an overflow switch can continue undetected for long periods;
Wall mini-split evaporators, which in most cases never have a secondary drain pan or drain line. Even mini-splits with condensate pumps are required by code to install an overflow switch; and
Rooftop systems where the supply duct descends through the unit bottom in a down-shot style, which can act as channel for condensate overflow into the building.
These applications should represent approximately onehalf
of the installations and service work a contractor comes
in contact with, which further illustrates the need to become
familiar with the industry’s diverse models of overflow switches
and their applications.
Choosing the right switch
Sorting through literally dozens of overflow switches available from many HVACR industry brands can be frustrating, especially when not every model is suitable to every application.
Some contractors prefer to put the same switch on every application out of habit. However, there is no one-size-fits-all solution in overflow switches, especially with the myriad of switch styles developed over the last 10 years to accommodate nearly every imaginable application.
Most switches fall under the categories of either mechanical or electronic devices that sense rising condensate levels and deactivate the blower unit and/or compressor motor.
Mechanical models detect water level through a mechanical float that breaks the electrical circuit. Electronic models sense conductivity to detect water level and use a logic software-controlled microprocessor to break the electrical circuit.
Mechanical water-level detection switches can be further categorized into reed, micro- or magnetic switches. Reed switches are very small switches that activate through the use of a magnet. These are also commonly used on doors and windows on residential alarm systems.
Advantages: Their smallness allows for tight-fit installations that would not physically accommodate larger overflow switches. They are also extremely reliable.
Disadvantages: Their sensitivity to over-voltage conditions and electrical surges can damage them and compromise the protection if a clean, stable supply of power is not available.
Micro-switches are miniature snap-action devices that open or close decisively with very little movement. They are also capable of normally open and normally closed points.
Advantages: They can handle higher voltage/current ranges than reed switches.
Disadvantages: While the term “micro” suggests smallness, these switches are larger and need more space to accommodate their size. They also have more moving parts and utilize normally open and/or normally closed contacts to break the control circuit during an overflow.
Advantages: These relay types can also be powered by either dc or ac voltage.
Disadvantages: While capable of being used in many more applications than the power-robbing models, the control boards require more complex wiring that tend to require a larger unit.
Forwards power models are wired up to the equipment control power. If an overflow is not detected, it “forwards” the voltage on through the circuit it is designed to break.
Advantages: It is an affordable electronic switch that also allows for normally open and/or normally closed points.
Disadvantages: It can only be used for ac voltage and is incompatible with communicating systems.
Electronic water-level detection devices
Electronic models vary by the method used to break the circuit. The electronics also extend benefits beyond what mechanical devices can offer, such as time delays to prevent equipment short-cycling or diagnostic LED lights for troubleshooting.
Power-robbing circuitry uses powered microprocessors to monitor the sensors and break the circuit.
Advantages: This design allows for extremely simple and easy-to-wire devices.
Disadvantages: They are limited to electromechanical circuits because microprocessors used in thermostats and equipment control boards are sensitive to millisecond power interruptions, which is the heart of this technology. However, electromechanical devices, like relays and contactors, are not affected and work quite well.
Electronic vs. mechanical
In most cases, a mechanical switch can accomplish the same overflow-switching tasks as its electronic counterpart. All switches require similar installation times, however, electronic switches are 20%–50% more expensive than their mechanical
Electronic switches have a couple of distinct advantages over their mechanical counterparts, such as smaller probes/ sensors, which physically can be used on nearly every type of install or application. The higher price of electronic switches can be a worthwhile tradeoff for the benefits of diagnostics and warning LED lights, which could pay back the extra cost with faster troubleshooting results on just one service call.
Typically, electronic switches have red and green light LEDs. A building owner can easily tell the service contractor over the phone which light shines when an air-conditioner does not activate—a red light indicates a condensate drain problem; a green light indicates the unit is draining normally and the air-conditioner itself has a problem. Some electronic switches have an amber light in addition to red and green.
An amber light can indicate a historical account of the overflow switch intermittently deactivating the unit sometime in the last several days, but the drain problem is not currently deactivating the unit.
Furthermore, the contractor will know what tools to bring to the job. The diagnostic electronics can also be remotely mounted to an accessible area for observation, which can reduce the number of 130°F attic crawling trips a service tech makes during a service call. As time goes by and our equipment uses more complex electronics and equipment, the use of electronic overflow switches will also most likely increase.
There is no one-size-fits-all solution, so the service tech must fit the switch to the application. Here are a few examples of applications needing specific types of overflow switches:
A system with a secondary drain pan would be best served with a primary pan switch with a second probe that can be attached to the secondary pan, vs. the expense of putting an independent switch in both the primary and secondary drain pan;
A system might be best served with an easy-to-clean, trap-type of device in the primary condensate port and an external overflow switch in the secondary port (if there is one);
Rooftop units and mini-splits typically require a very small sensor that is designed especially for their application;
Units reporting to communicating equipment, such as thermostats, building- automation systems, and onboard microprocessor controllers will need to use overflow switches with dc voltage or dry contacts;
High-efficiency equipment using variable-speed motors can possibly damage some reed switches, therefore a magnetic switch may operate more reliably; and
The secondary drain pan material (metal or plastic) can determine the type of overflow switch that is needed.
Plastic pans sometimes have foldedover lips that do not provide a flat enough surface to attach the switch body or sensor. Contractors that take advantage of today’s low-cost overflow switch technology and use the right product for the application can increase their customer’s redundancy and protection against unforeseen condensate overflows and property damage, and bring additional profits to their service work.
James Bowman is the National Technical Manager—HVACR for RectorSeal Corp. (Houston, TX), which offers a variety of air-conditioning condensate overflow switches and secondary drain pans under several brand names. Bowman is a former service tech that is EPA- and NATE-certified, holds a Class A contractor license in Texas, and has installed and serviced every type of overflow switch.