How to Choose the Right Size Water Pressure Tank
As a pre-purchase acreage inspector, I often find tanks that are waterlogged or too small for the pumps’ flow rate. The question that usually follows is, "What size tank do I need?" In this blog, I will try to answer that question and provide a comprehensive guide on choosing a proper-sized water pressure tank.
A water pressure tank is a crucial component of your water system. It's a water storage reservoir that helps maintain a constant water pressure level. If you're considering replacing an old water pressure tank, you might wonder about the capacity you need. The capacity of a water pressure tank depends on several factors, including the flow rate, pressure switch setting, and minimum run time. Another factor is the pressure tank's drawdown capacity.
Below are some terms required for the final calculation of the pressure tank size:
Pressure tank drawdown: A pressure tank drawdown refers to the amount of water the tank stores and makes available when the pressure switch is off. In simpler terms, it's the amount of water the pressure tank can store and/or deliver between the time the pump shuts off and restarts. Considering the air and unused water storage inside the tank, the volume of a tank should ideally be four times its drawdown capacity. The larger the pressure tank, the more water you can store. If you have a larger drawdown, your tank can run longer and will go through fewer pumping cycles.
Flow rate: The required water pump flow rate is usually determined and tested when a new well is drilled. The pump's flow rate is measured in litres or imperial gallons per minute.
There are several ways to determine the flow rate, but the simplest method is called “the bucket test.” Take a 22.75 litre (5 imp gal) bucket, turn the water flow to full capacity near the pump or at the bathtub spout, and determine how long it takes to fill. For example, if you filled the bucket in 45 seconds, calculate 22.75 litres (5 imp gal)/ 45 seconds x 60 seconds = 30.33 litres (6.67 imp gal) per minute. 30.33 litres (6.67 imp gal) per minute is your flow rate.
Pressure switch settings: This is the pressure at which the pump turns on and shuts off. This setting can affect the tank’s drawdown.
There are three pressure switch settings for pressure tanks: 20/40psi, 30/50psi, and 40/60psi. The first number in these settings is the lowest pressure at which the pump turns on, and the last is the highest at which the pump turns off.
Most water pressure tanks come with a chart to tell you the drawdown capacity based on your pressure switch settings, and there’s one single point to heed in these calculations. As the turning-on point of a pressure switch increases, its drawdown capacity will decrease. This is because of the difference in water pressure ratio. If a tank can have a 40/60 pressure switch, it makes only ⅓ of its water capacity available, while that number would be ½ for a 20/40 psi pressure switch.
This only has a small effect on the calculations you need to make. All pressure tank manufacturers provide their own charts that explain the relationship between drawdown capacity, pressure switch settings, and, of course, tank size relevant to their products. So, it’s best to consult the manufacturer's charts before purchasing.
Minimum run time of the well pump: This is how long your pump will run. Smaller pumps, under 1 HP, are usually designed to run for a minute and are most efficient with a minimum one-minute run time. Again, the longer the minimum run time is, the fewer the pump cycles. However, you'll need to consult the pump's manufacturer to understand its specific run time better.
Now that we know the terms, we can determine the right pressure tank size.
Determining the Right Pressure Tank Size
Pressure tanks come in various sizes, including 20-, 30-, 50-, 85-, and 120-gallon capacities.
You can determine the pressure tank drawdown by knowing the flow rate, minimum run time, and pressure switch setting. Knowing the drawdown will help you determine your tank size. The drawdown capacity is calculated as follows:
Drawdown capacity = Pump flow rate \ Minimum run time
For example, if you have a 30.3 litres (6.67 imp gal) flow rate and a 1-minute minimum run time, your drawdown capacity would be 30.3 litres (6.67 imp gal).
To determine the tank size you need, multiply the drawdown number by four (or as per pressure switch requirements) because only about ¼ of a pressure tank’s volume consists of usable water. This means the tank size would be 121 litres (26.5 imp gal) and rounded up to the next tank size of 136 litres (30 imp gal). You may want to consider a larger tank, especially if you plan to irrigate with the system. The larger the tank, the less often the pump will cycle, increasing the pump's longevity.
In conclusion, choosing the right size for your water pressure tank is crucial for the efficiency and longevity of your water system. By understanding your system's specific needs and doing some simple calculations, you can ensure you select the right tank for your needs.
Disclaimer: I am not a professional plumber, so I recommend you follow the manufacturer's instructions or use a qualified plumbing contractor to help size and install your pressure tank. Happy plumbing!