What is a Demand Pump and How Does It Work?

Posted by
John Woodard on September 13, 2019

Demand pumps are small pumps used for transporting water from its storage location to the point of use. Demand pumps are simple, versatile, and affordable ways to control the movement of water in your home, camper, boat, or garden. Join John Woodard, our Master Water Specialist, as we explore what a demand pump is and how it can work for your home. 

What is a demand pump? 

A demand pump is a pump that kicks on when water is demanded and moves that water from a storage tank to the intended point of use. Demand pumps are also known as “on demand pumps” and “demand and delivery” pumps. Demand pumps allow you to control the movement of your water and have it delivered to you when and where you desire it. When a drop in pressure is detected, like a faucet turning on or a toilet flushing, the demand pump is activated. As long as the faucet is running and using water, the demand pump will continue to supply water through the open line. When the faucet turns off and the water demand concludes, the demand pump will automatically shut off. Unlike water booster pumps, demand pumps to not add additional pressure to the water supply. Demand pumps are designed to fulfill a wide variety of fluid transfer applications  

Demand mode vs. delivery mode 

Demand pumps can operate in two modes: demand mode and delivery mode. A demand pump will usually operate in demand mode. In this mode, the pump senses pressure changes within the system and automatically delivers water to the source demanding the water. A pump in demand mode is operated by a built-in pressure switch. Pumps in delivery mode are activated by an external switch rather than the pressure switch built in to operate the pump. When water is desired, the switch is manually turned on and the delivery pump will send the water. Turning off the switch will turn off the water supply from the delivery pump. 

How does a demand pump work?   

Demand pumps work by sensing alterations in pressure and responding by transporting water. When an appliance is used, a solenoid valve opens and alerts the demand pump that water is required. On demand pumps are diaphragm pumps. Unlike rotary vane pumps, which move water with a spinning propeller in a chamber, demand pumps move water by oscillation. When the diaphragm inside the head of the pump expands, water flows into the pump. The rubber diaphragms have cups within them, and as they rotate, the cups squeeze together. This creates suction, and when the cups are pulled apart and the suction is broken, the water is ejected from the pump. This process of suction and ejection are what creates the draw and push inside of the pump.   

Demand pumps utilize their built-in pressure switches to assist them in operation. When demand is initiated, the pump begins to move water. When the demand is met, the pump will continue to run until the pressure setting in the pressure switch is met. For example, if a pump has a 65 psi pressure switch, the motor will run until the pressure in the line meets that shutoff setting of 65 psi. When a shower is turned on or a faucet opened, the pressure will drop and the pump will restart and water will begin to move. These pressure switches typically have about a 10 to 15 psi drop before they turn the motor back on. So a 65 psi shutoff switch won’t reactivate until the pressure reaches around 50 or 45 psi. 

Where are demand pumps used?

Demand pumps are effective pumps in any application where water needs to be moved from storage to point of use. RVs and campers are one of the most common applications for an on demand pump. They are small, lightweight, and easily portable, making them ideal for being taken on the road. Demand pumps can circulate water from the RV’s water storage tank to the toilet, shower, and kitchen sink. Marine applications, like yachts or fishing vessels, are also very common for demand pumps. Demand pumps can pull water out of the storage tank aboard the ship and to the galley or the ship’s plumbing. 

Demand pumps are often paired with a reverse osmosis system’s storage tank to deliver the filtered water to faucets throughout the house. If distilled water is being stored and distributed Demand pumps can also be hooked up between a water filtration unit and a natural water source like a river or lake for those camping or living off-the-grid. Hydroponic systems use these pumps to circulate oxygenated and nutrient-rich water from storage to their plants. Food and beverage applications utilize demand pumps to deliver fluids to their ice makers and water dispensers. Demand pumps can be used with fish ponds, irrigation, and car washes. A demand pump can be used anywhere a relatively small volume of water is being transported.

Explore more about how hydroponic systems work. | Learn how to fix RV water pressure issues.      

Pressure and flow: the pump curve 

When you examine the label of a demand and delivery pump, you’ll notice it has a gallon per minute rating. These can be deceptive. The gallon per minute rating listed on the pumps is describing the gallon output at open discharge. That is the absolute maximum potential output of the pump, based on water blowing out of the pump without any sort of connection to tubing or appliances. However, once the pump is hooked up to its intended application, it is going to run at a different pressure. To see what the actual output will be for your pump, you’ll need to consult the pump curve. For example, a pump rated at 3.5 GPM will actually output 2.31 gallons per minute at 30 psi.

demand pump pressure curve

As the pressure increases, the flow is restricted. This is the reason why the gallon per minute output reduces as greater pressure is applied to it. Think of a garden hose. If you turn a garden hose on and allow water to pour freely, you will see a high flow rate out of the hose but at a weak pressure. If you put your thumb over the end of the hose, it’s going to start spraying at a higher pressure. The amount of water exiting the hose will diminish as you tighten your thumb over the mouth, but the spray will reach further and at a higher pressure. The pump curve demonstrates the relationship between gallons per minute and the pressure you’ll get at that specified flow rate. Knowing what your desired pressure and flow rate is will help you determine which pump is appropriate for your intended application.

How do I maintain my demand pump?

Demand pumps require minimal maintenance. Periodically, you will want to run a sanitizing solution through the pump to ensure the water it is delivering is clean and bacteria does not grow within the pump. 

To sanitize an on demand pump: 

    1. Fill a clean bucket with four gallons of warm water.
    2. Add half a cup of non-sudsing detergent to the water and stir it until the detergent is dissolved evenly throughout. (Dishwasher detergent works best here.) 
    3. Add one-fourth of a cup of bleach to the mixture. Stir it gently.
    4. Run a line from the solution through your demand pump and to a drainage site. 
    5. Pump half of the sanitizing solution from the bucket through the pump. 
    6. Wait for ten minutes, allowing the sanitizing solution to sit inside the pump. 
    7. Pump the remainder of the solution through the demand pump.
    8. Purge the pump. After the bucket has been drained, reconnect the demand pump to the water source. Run clean water through the pump until it has been primed and completely purged of sanitizing solution. Taste the water to make sure there is no funny aftertaste. 

demand pump sanitization chart

What fluids are demand pumps intended for?

Demand pumps are rated to carry water. They perform optimally when delivering relatively clean water sources. Avoid pumping water with large amounts of solid waste and organic matter. If you are pumping water from a well or circulating a nutrient-rich hydroponic solution, you will want to sanitize the system more frequently. These pumps are not intended to carry chemical solutions. If you plan on pumping anything chlorine heavy or laden with chemicals through the pump, make sure it is compatible with Santoprene. Santoprene is a thermoplastic polymer and the material the pump is made from. Certain chemical solutions will degrade the rubber and render the pump unusable.

Explore our pump collection.

| Learn more on how water booster pumps work. | Learn about how peristaltic pumps work. | Learn about how sump pumps work. |



Comments 1-4 of 4
Leave a comment