InspectAPedia ^TM

Water Tank Size, Volume, & Requirements PlumbingAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Water Tank Size - how much water is in the water pressure tank? How much water volume do we need to avoid short cycling the water pump?
• Water Pump Short Cycling & Water Tank Air - How & Why to Add Air
• Water Tank Repair or Replacement

Here we discuss Water Tank Size - how much water is in the water pressure tank? and we address the question of how much water volume we need to avoid short cycling the water pump? This website answers just about any question you may have about pumps, wells, and drinking water. Even before performing water quantity, quality, equipment function tests, there is an enormous amount we can determine about a building's water supply just by looking at the equipment. Articles here provide details on water pumps, tanks, controls, and wells and water supply inspection, diagnosis, and repair. The photograph shows a one-line jet pump, the water pressure tank, and a water softener. We know from the fact that this is a single line jet well pump that the well is a shallow one, probably less than 27' deep. Well depth may have implications for water quantity and quality and vulnerability to surface water contamination. The page top photo shows our client holding the open top of his well casing along with some unusual well casing exit plumbing at a drilled well with a modern steel casing. Finding the location of your well and inspecting the condition of the well piping and equipment are an important first step to assure a functional and potable drinking water supply - that is, having enough water supply and having water that is safe to drink. The articles listed below provide detailed advice on diagnosing and repairing problems with water pumps, water tanks, wells, and other water supply equipment.

How to Find Out How Much Water is in the Water Tank, Considering the Space Taken up by Air in a Water Pressure Tank

It's not how much water the tank holds that is useful to know. It's how much water we can get out of the water tank before the pump has to turn on. This is the "draw down" volume, which we can measure or calculate, or we can focus instead on how long (in time) we can run the water before the pump has to turn on. (See our discussion of "short cycling" water pumps at Water Tank Repairs: Diagnose "Water Pump Short Cycling" & Restore Air in a Building Water Tank. If your pump is short cycling you want to fix it, as we explain in that article.

How to calculate the volume of water in a water tank

IF we just wanted to calculate the volume of water inside of a round water tank, for simplicity ignoring it's domed top and maybe bottom, we can use the formula to calculate the volume of a cylinder. Just measure the height and circumference of the water tank.) You can use the online calculator at http://www.online-calculators.co.uk/volumetric/cylindervolume.php or use the formula:
2 * pi * radius * height (pi is 3.1416)

If we needed to be precise and if the bottom and top of a water tank are domed at the top and covex at the bottom (usually) we can measure these areas and calculate their volume using the formula for the volume of a sphere (or part of one). But I suggest skipping this detail. Probably the spherical volume lost from the convex tank bottom is about equal to the spherical volume of the tank top, so it's a wash and we can just use the tank's overall height and diameter.

A 30 gallon water tank does not give you 30 gallons before the pump turns on

Remember that a "30 gallon water tank" used to control water pressure and pump cycling in a building does not hold 30 gallons of water, but something less than that (say 20 gallons of water max and 10 gallons of air at the point of pump cut-off). Then as you draw water out (and the in-tank pressure falls down to the pump cut-on point) the pump is going to come on before all of the water leaves the tank.

So the maximum actual water you get out of the tank is less than the tank size, maybe 15 to 20 gallons max. The bladder-type pressure tank mfgs cite an "equivalent" draw-down water volume as that provided by the older bladderless tanks.

How to measure the draw-down water volume provided by a water tank

• After the water pump has just shut off at full cut-out pressure (the water tank is now as full as it's going to get), put a 5 gallon bucket under a bath tub spout or other convenient faucet or hose.
• Then turn on the water and let it run into your test bucket.
• Listen for when the pump cuts on, or if your pump is submersible (and you can't hear the pump) station an assistant next to the pump control and have them listen for the click that indicates that the pump relay has turned "ON" - they'll also see the pump pressure gauge start to move back up.
• When the assistant sees or hears that condition, turn OFF the water at your test fixture.
• Measure the volume of water in your bucket.
• That's the draw down volume.

What are some typical water tank draw down volumes?

A 10 gallon water pressure tank that starts fully empty and is pumped up to about 50 psi will contain about 3 gallons of air and 7 gallons of water. The water tank in normal operation does not draw down to 0 gauge pressure before the pump comes on.

The water tank provides outflowing water down to 20 psi (on a 20-40 psi system or down to 30 at a 30-50 psi system) when the pump comes on.

A water pressure tank with a total volume of 10 gallons and operating between 20 psi and 50 psi of pressure will have a draw down water volume of just 4.35 gallons of water.

A typical kitchen water faucet runs between 3 gpm and 5 gpm (varying as the water pressure in the system varies as the water pump cycles on and off), so we can expect to run the water at the tap for about a minute before the pump will come on with this theoretical water tank. Because the pressure drops as the water tank empties and then increases as the water pump comes back on, the water pressure at a faucet or other plumbing fixture will vary between the pump cut-in pressure (typically 20 psi or 30 psi) and the pump cut out cycle (typically 40 psi to 50 psi). quoting from Water Tank Pressure, Temperature, and Air Volume Calculations.

The above-cited article, which I admit is a bit unnecessarily complex (I was answering someone else's query) has the math you need to calculate the actual draw-down volume of water you get with a given sized tank, with a given in-tank water volume when the pump has reached its shutoff point.

More expert information on this plumbing supply and drain piping, pumps, water tanks, oil tanks, water supply, water testing, wells, septic systems

More Information about Plumbing and General Building or Home Inspections

...

InspectAPedia ^TM Home & Site Map - Building Inspection, Diagnosis, & Repair, Environmental Inspection & Testing - Research Website

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Home Inspection Construction Consulting Services & advice for home buyers

Contact Daniel Friedman for website content suggestions or for fee-paid consulting

	Enter your search termsSubmit search form
	Search This Website - InspectAPedia.com