How to Determine Condensate Pump Size for a Furnace

Posted by: Mas Broto
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High-efficiency furnaces rely on condensate pumps to function correctly. Because these units are so efficient, they create a lot of condensation while working.

That condensation, along with assorted minerals, must leave the furnace quickly. This is where the condensate pump comes in. These small devices collect and remove water directly from a high-efficiency furnace or air conditioner. They come in a wide range of sizes and horsepower, but selecting a new condensate pump can be painless.

What is a Condensate Pump

A condensate pump moves the condensate (water) made by HVAC systems. They can also pump the condensate that forms when latent water vapor in any of the following gas mixtures comes in contact with a cooler gas.

Things to Consider When Choosing a Furnace Condensate Pump

When choosing a new condensate pump for a furnace, you need to know the exact head, GPM (gallons per minute), HP (horses per minute), and voltage rating. Getting the right size for a pump’s head volume and lift is important for how long the equipment will last and how much power it will use over its lifetime.

Pump Head Measurement

This is the linear vertical measurement of the highest height a certain pump can send liquid to the pump outlet. A pump can only move liquid up to a certain height because the weight of the liquid above the pump mechanism is greater than the forces that are trying to turn the pump or move the piston or diaphragm. The head measurement of a pump can be found by measuring vertically from the pump’s outlet to the point where gravity takes over the flow of water in the drain line.


The GPM, or gallons of condensate water the pump can get rid of in one minute, is usually two to three times the rate at which the system can get rid of moisture. This is normal for small receiver tanks. For larger condensate collection tanks, it’s worth looking at a system heat balance. It may be enough to have a capacity of 1-1/2 times the rate of condensation. Because the pump is bigger than it needs to be, it can run at full load for 30 to 45 minutes. Look in the manual with your furnace or condensing system to determine how much condensation it makes. Put a bucket under the furnace’s drain and track how much condensate comes out in an hour. This will give you a good estimate of how much condensate drains per minute (GPM).


Condensate pumps are usually small and rated at a fraction of horsepower because they are used in homes and individual heat exchangers. Horsepower is usually measured by how high the pump head is and how many GPM are needed. The average residential use is between 1/30hp and 1/50hp. Light commercial HVAC systems and systems that are bigger than average may need up to a 1/3hp pump and bigger collection bins. As the horsepower of a condensate pump goes up, so do the GPM and head pressure.


Most residential uses of electricity need either 115 or 120 volts. Larger systems or special uses need 230v pumps, which can have the same or more horsepower than 115v or 120v pumps.

To conclude, here are the 3 steps to determine the right condensate pump for a high-efficiency furnace.

  1. Refer to the owner’s manual that came with your furnace to determine the amount of condensation the furnace creates. Place a bucket under the furnace’s drain and record the condensate produced in an hour if you wish to be absolutely certain.
  2. Measure the height difference between the condensate pump’s location and the outlet’s level using a tape measure. Measure the horizontal length that the water must travel to be removed. Consider relocating the pump if this length exceeds 50 feet.
  3. Take the information you’ve gathered to your local home improvement store or HVAC specialty store. Select a pump designed for high-efficiency furnaces that slightly exceeds your furnace’s needs. Base your selection on the size of the pump’s reservoir, vertical pump capability, and materials. A well-made plastic condensate pump will not rust in high-humidity locations like basements. Research models online if the local stock is not satisfactory.

If you are simply replacing an old pump with an identical model, refer to the label on the worn-out model for the specifications.

Be aware that many furnace condensate pumps come in 120-volt and 240-volt models. The vast majority of residential settings should be 120 volts.


Find out how much condensation your furnace makes by looking at the manual that came with it. If you want to be sure, put a bucket under the furnace’s drain and write down how much condensate comes out in an hour. If you follow all the processes, you will find the right condensate pump for your furnace.

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Mas Broto
Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.