How to size a condensate pump and prevent double trapping

Avoid dissatisfied customers, severe water damage, bacterial growth, and overheating units by following our step-by-step guide on how to size a condensate pump and prevent double traps. We will review best practices for measuring total pump head, when to use a condensate neutralizer, the importance of safety switches, common installation mistakes, and more.

With high-efficiency air conditioners, furnaces, and other HVAC units producing additional condensate than less-efficient alternatives, it’s more important than ever to ensure you select the best-suited condensate pump for each heating and cooling project.

When is a condensate pump necessary?

Sometimes, it is impractical to use a gravity-fed drain to remove condensate from a furnace, AC unit, or other HVAC equipment. For example, when the drain is higher than the drip pan/uphill, the HVAC equipment is located in a basement with no natural slope, or the drain is far from the equipment. In these cases, a condensate pump is necessary to pump the extra water away, either into an existing drain or outside of the building.

How to size a condensate pump?

When selecting a condensate pump for standard residential and commercial applications, it’s important to size it properly for the HVAC system. You’ll need to assess the condensate drainage rate or flow rate, the discharge line’s pressure/head, and voltage.

Measure/calculate:

• Flow rate

• Total dynamic head/lift capacity

• Voltage

Below we will explain how to calculate each to properly size a condensate pump.

How much condensate does an air conditioner or furnace produce?

Relative humidity, temperature differences, run-time, efficiency ratings, and equipment status impact how much condensate the HVAC system creates. Generally, you can approximate 1 gallon/hour of condensate per 100K BTUs for standard residential and commercial applications. If the maintenance and operations manual is available, it will list the expected condensate amount. To ensure efficient pumping, shop for condensate pumps equipped to handle 2 to 3 times the calculated peak rate. Then, the pump can run less often, even under full load.

*Please note: On larger systems, take into account heat balance. This may impact the flow rate calculation for condensate.

How to calculate the amount of condensate an HVAC system generates?

Beyond a general estimation based on the system’s BTUs or manual, you can calculate condensate production by placing a bucket under the furnace's drain (for example). After an hour, measure the accumulated water. This will provide a close approximation; however, keep in mind that humidity, temperature, run time, and other factors may impact condensate production. It is still wise to select a condensate pump that’s rated to handle 2 to 3 times the recorded amount (or 1 ½ to 2 times for larger collection tanks).

How to calculate the total dynamic head or total lift capacity?

In order to calculate the total dynamic head or lift capacity required for your condensate pump, you need to determine the static head, friction head, and destination backpressure. This will tell us how much pressure the condensate pump needs to discharge the water. To convert your measurements (feet) to pressure, divide the total by 2.31. Then, round up. For example, if the total dynamic head is 22 PSIG, round up to 30 PSIG. This will account for any unknowns (such as a hidden elbow within the wall) or calculation errors.

How to calculate static head

The static head is the linear vertical distance that the condensate pump needs to pump the water. In other words, it’s the distance from the condensate outlet to the highest point of the discharge line, before it turns downward, allowing gravity to take over. This is important because as water moves upward, it becomes heavier and harder to pump. When buying a condensate pump, you’ll need to match the HVAC system’s flow rate with the static head measurement to ensure the pump has the correct total lift capacity.

How to calculate friction head, or friction loss?

Friction between the water and the pipe may slow down the flow and influence the pump head measurement. Small pipes with high flow have higher friction loss. Large pipes with low flow have lower friction loss. Refer to a friction head chart to approximate your measurement. Don’t forget to account for each 90º elbow, which has an equivalent straight pipe measurement. For example, a 1” 90º elbow is equal to 8 feet of straight pipe.

On average, aim to slope the discharge pipe ⅛ to ¼” per foot or at about 1% to keep the condensate moving. Avoid strictly horizontal runs and dips.

Do you need to account for destination backpressure in your total pump head measurement?

If the condensate discharges into a pressurized vessel, then account for additional pressure. For example, many deaerators operate at 5 PSIG, so we need to be sure the flow is able to overcome 5 PSIG. To calculate feet of head from PSIG, multiply it by 2.31. For example, 5 x 2.31 = 11.55 feet of head.

What voltage does a condensate pump need?

For standard residential and commercial applications, a 115/120-volt power source is often adequate for proper condensate pump wiring. Certain applications (for example, hardwired connections) and commercial systems may require 230/240 volts, and up to 460 volts for considerably-sized commercial and industrial applications. Always check available voltage and pump specifications before shopping for condensate pumps.

How to determine horsepower requirements for a condensate pump?

On average, condensate pumps have 1/30 to 1/50 HP for residential applications. Some commercial and larger systems may require up to ⅓ HP. The higher the horsepower, the higher the flow rate.

When do condensate pumps need a neutralizer?

Highly acidic condensate can erode pipes, deteriorate equipment, pollute groundwater, and contaminate the environment. To determine if a neutralizer is necessary, first check the local building codes to see if it is required on all systems. If it is not required, you may still need it. Add a neutralizer for all high-efficiency and 90%+ AFUE units. These systems generate acidic water ranging from 2.9 to 4 pH as a by-product of the combustion process.

Pro tip: Remind your customers to replace the marble chips and limestone neutralizers annually. Setting a calendar event will help ensure an on-time application and avoid equipment deterioration.

Avoid common mistakes when installing a condensate pump and prevent customer callbacks.

There are several common reasons for customer callbacks after installing a condensate pump. Here’s a quick installation checklist to ensure you get a 5-star rating.

• Connect the overflow safety switch

• Clean out the drain line

• Avoid double-trap issues

• Recommend regular cleaning/maintenance

Don’t forget to wire the condensate pump overflow safety switch.

Wiring the condensate safety switch to the HVAC equipment is a quick, effective way to prevent water damage. If the pump stops working, then the safety switch will turn off the furnace (for example) before the reservoir overflows. Review the manufacturer’s manual for condensate pump wiring diagrams. After connecting the overflow safety switch, pour water into the reservoir. Once the water level reaches the maximum level, the furnace should turn off.

Clean out and maintain the discharge line

New, high-efficiency HVAC equipment produces more condensate. It’s critical to clean out the drain line before installation to remove existing bacteria. This will prevent additional growth and reduce unexpected friction head. Use a condenser cleaner and flush the line several times.

How to avoid a double trap during condensate pump installation

Air blocks in double traps can cause intermittent drainage issues, making them difficult to troubleshoot after installation. Sometimes, double-trap problems lead to multiple callbacks and dissatisfied customers. Here are common locations for double traps and best practices to avoid subsequent issues.

Double Traps in Return Box Installations

WHAT’S HAPPENING

This is the most common double-trap scenario. After connecting the discharge line to the unit and to the corresponding T, you may have pushed the T line down to accommodate a better pitch. In doing so, the bottom of the T line could be sitting on the cement, which is typically lower than the subsequent chase joint. As a result, when the chase fills up with water, the elbow that’s resting on the cement does too (since it’s lower than the chase joint). This creates an air pocket and eventually a drain that won’t drain.

WHAT YOU MIGHT SEE/HOW TO TROUBLESHOOT

• The system may run without problems for a while before issues are noticed

• Intermittent dripping is reported

• Normal flow that suddenly stops and eventually begins again

• Gurgling

• Play or movement in the T line

• Dump a lot of water into the pan. If you see the water resting, rather than flowing, then pull the T up; if this releases the water, it’s likely you’re dealing with a double trap.

SOLUTIONS

If you’re opening the area beneath the HVAC unit, then set up a brace and strap the drain line to it. This will ensure you don’t accidentally push the line down when connecting the discharge pipe to the T. Bracing can also prevent future movement that will likely occur during system maintenance.

If you do not have access to the drain pipe beneath the HVAC unit, lift up on the T line before connecting it to the discharge line attached to the unit. Be cautious not to accidentally push the T pipe downward when making your connection. After installation, make sure the T line does not have a lot of movement, especially when pulling it upwards. If it does move upwards, you’ve likely shifted the bottom corner too low and can expect draining issues.

Double Trap Issues in Attics

WHAT’S HAPPENING

In attics, there are often long, straight drain lines that rest along the trusses. Over time, these lines can get shifted or start to sag. Furthermore, if a small change is made somewhere along the drain pipe, it can push the other end in such a way that it creates a dip.

SOLUTIONS

During every attic install, use braces and blocks to stabilize the entire drain line from the point it drops down into the wall back to the equipment. This will ensure it does not sag over time. If any change is made to the system, recheck the pitch and adjust bracing as necessary.

Attaching to a System that Already has a Trap

WHAT’S HAPPENING

In some cases, you may want to reroute where the water exits a system. For example, if the discharge pipe is coming straight out of the wall and dripping condensate onto the ground, you may consider moving the line so it drips further from the house. To accomplish this, you need to take the line underground, which creates a new trap. However, another trap is likely hidden within the wall. Without meaning to, you just created a double trap.

SOLUTIONS

• Don’t add a trap if you don’t have to.

• When adding a new trap, add a vent. This provides an escape for trapped air.

Review regular maintenance with your customer

After installation, instruct customers to check the system monthly. Damp areas and pooling water may signify a loose connection or worn out seal. Also check for bacteria, algae, or other growth. If they notice any buildup, they can run water through the line or use a vinegar mix and a bottle brush to clean the components. Regular maintenance prevents clogs and keeps the pump working efficiently.

Low-profile condensate pumps for mini-splits

Small, compact condensate pumps are available for mini-split systems. For example, the Rectorseal Aspen® Mini White Pump Kit offers a sleek profile that mounts below the evaporator. It is discrete, accepts a standard ⅝” drain hose, and has a filter cartridge that accepts neutralizer tablets for high efficiency units.

Where to buy condensate pumps for high-efficiency HVAC equipment?

Ferguson stocks condensate pumps from leading HVAC manufacturers. Browse our inventory, order online, and expect fast turnarounds.