How to Size a Pressure Tank – Factors to Consider

What Size of Pressure Tank Should I Get?

It is not unusual to experience low water pressure in the house for people on a well system just because the pressure tank capacity cannot cope with the water demand in the house. This is why it important to buy a pressure tank that can supply your household with sufficient water pressure even during peak hours.

To properly size a size a pressure tank, consider is drawdown capacity relative to your household water demand. A pressure tank’s drawdown capacity (in gallons) is the product of the well pump’s flow rate and run time.

pressure-tank

There is a difference between pressure tanks volume and its drawdown capacity. For instance, a 20–gallon pressure tank has drawdown capacity of 5 to 7 gallons depending on its cut-in and cut-out pressure.

A pressure tank can be too small but hardly too big. A big pressure tank will mean long but few pump cycles. Fewer cycles are important in avoiding the premature failing of the pump due to wear and tear as a result of overheating.

If your well pump is short-cycling, you can put 2 pressure tanks in series on one well. That will increase the lifetime of the pump and will also ensure that you have constant well pressure in your house. Alternatively you can install a pressure tank with a bigger drawdown capacity.

How a Pressure Tank Works

The pressure tank is the link between your house and the well. Water from the well is pumped to the pressure tank where it is stored under high pressure. Pressure tanks are however not your ordinary tanks.

They have an air bladder inside which looks like a balloon. When water from the well flows to the pressure tank, it compresses the air and hence it is stored under pressure. It is this pressure which allows water to flow to all of your fixtures even when the pump is off.

The pressure of water in the pressure tank is controlled by a pressure switch which is usually located very close to the tank and adjacent to the pressure gauge. A pressure switch controls the water pressure in the tank by turning the pump on and off.

pressure-switch

Pressure tanks have 2 pressures, the cut-in and the cut-out pressure. The cut-in water pressure is the pressure which if and when reached, the pressure switch triggers the well pump to start filling the pressure tank.

Cut-out pressure on the other hand is the pressure which when reached, the pressure switch turns the well pump off. Most pressure tanks have a cut-in pressure of 40 psi and a cut-out pressure of 60 psi.

The air pressure in the bladder should be 2-5 psi lower than the cut-in pressure. If it is way more than that you will have low water pressure in your house.

A large pressure tank does not necessarily increase or have more water pressure than a small one. What a large pressure tank does is give a longer draw-down time. Unlike with a small pressure tank, water will flow out of your fixtures for longer before the well pump kicks in.

Why You Should Size Your Pressure Tank

To understand the importance of a properly sized pressure tank, we will try and imagine a well system without a pressure tank. What would happen in such a scenario?

The first thing is short-cycling. Short-cycling happens when the pump is running frequently. Every time someone turns on a faucet to wash hands or flushes the toilet the well pump kicks in to compensate for the drawn water.

Not only is this energy-inefficient but it strains your pump causing wear and tear. A well pump should last for about 15 years. Without a pressure tank the pump will not last that long.

A running well pump generates a lot of heat which causes it to wear out faster especially if it is short-cycling. Replacing a well pump is very costly. If you have a defective (waterlogged) pressure tank that is causing your pump to short-cycle you need to have it replaced as soon as possible.

The second advantage of a pressure tank is that it allows constant water pressure across all of your fixtures. Without it, you would notice fluctuating water pressures across all of your faucets especially when several are being run at the same time.

This is why it is important to size your pressure tank in relation to the size of your household and your peak-hour water demands. If the pressure tank is too small, it is going to overwhelm the well pump and you will also experience low water pressure in the house from time to time.

How to Size a Pressure Tank

In order to properly determine what size of pressure tank you need for your house, there are a few terms that you first need to understand. After that you need to do a few simple calculations.

1. Drawdown

A pressure tank’s drawdown is amount of water (in gallons) that enters the tank from when the pump kicks in (at cut-in pressure) to when the pump stops (cut-out pressure). It is the volume of water drawn from the time the tank is full until the pump starts again.

Bigger tanks have a higher drawdown capacity than smaller tanks. Pressure tanks with equal capacity may have different drawdown capacities depending on the cut-in and cut out pressure.

For instance, a pressure tank with cut-in and cut-out pressure of 40/60 psi have a higher drawdown capacities than a pressure tank with a 30/50 psi. That is the same way a 20-gallon pressure tank will have a drawdown capacity of 5 gallons or 7 gallons.

Going by the example above, a person with a 20-gallon pressure tank would need to draw 5 or 7 gallons from their faucet in order for the well pump to start again. This is how drawdown is different from a pressure tank’s capacity.

2. Flow Rate

Flow rate refers to the volume of water in gallons per minute that the well pump produces. The highly rated (in terms of horsepower) the pump is the more its flow rate.

It is always a good idea to size the pressure tank with respect to the size of the well pump. With pressure tanks bigger is always better than smaller.

3. Run Time

The run time of a well pump is the length of time a pump takes to fill that pressure tank with water from the cut-in pressure to the cut-out pressure. As a standard, pressure tank manufacturers recommend a run time of less than a minute if the pump’s horsepower is less than 1HP.

For pumps with a horsepower of more than 1HP then a run time of more than 1 minute will be ideal. Always check out this with the pressure tank’s manufacturer.

From the above 3 factors, we derive the following formula which is important in sizing a pressure tank:

Pressure tank drawdown capacity (gallons) = Flow rate (GPM) x run-time (Minutes)

In sizing a pressure tank, use the following specifications

  • Pumps with a flow rate of up to 10 gallons per minute should have a pressure tank with a minimum drawdown capacity of 1 gallon for each GPM delivered by the pump
  • Pumps with a flow rate of 10-20 gallons per minute should have a pressure tank with a minimum drawdown capacity of 1.5 gallons for each GPM delivered by the pump.
  • Pumps with a flow rate of more than 20 gallons per minute should have a pressure tank with a minimum drawdown capacity of 2 gallons for each GPM delivered by the pump.

Example 1:

If you have a well pump with a flow rate of 7 gallons per minute and a motor rating of 0.75 HP, what will be the required pressure tank drawdown capacity?

Solution:

Since the pump’s motor has less than 1HP, we will assume that the required run-time is 1 minute. Using the formula above,

7 gallons per minute x 1 minute = 7 gallons (drawdown capacity)

Example 2:

Assume you have a pump with a flow rate of 16 gallons per minute and motor with 1.5HP. If the manufacturer recommends 2-minute run-time, let us calculate the pressure tank’s drawdown capacity.

Solution

16 gallons per minute x 2 minutes = 32-gallon drawdown capacity

Conclusion

Sizing a pressure tank is a simple yet very important step in solving your water issues at home. A bigger pressure tank is more expensive than a small one but if you buy one that is too small you might end up with low water pressure in the house and a short-cycling pump.

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