Wells: a deeper answer
Thanks for an interesting article (see “When Your Customer’s Water Well Can’t Keep Up—A New Look at an Old Problem,” WC&P, December 2003, pp. 28).
It appears that Figures 1 and 2 got reversed? (Correction runs of page 14.) In the pressure tank example, the author states “the well pump will not come on to replace any of the water used from storage until nearly all the stored water is used.” That depends on how the well tanks are set up. A typical setup for a higher yield well will have the air charge in the tank set close to the cut in pressure of the pump and the author is correct. However, pressure tanks can, and have been, set up where this is not at all true. In one installation, a pressure tank was originally set up to run between 30/50 psig with the air pre-charge at 28 psig. At that pressure range, the (86 gallon pressure) tank held 29.2 gallons with only 2.8 gallons in reserve. The well was low yield, so extra stored water was desirable for that reason and extra water during power outages. Setting the pressure switch at 50/70 and the air precharge at 30 psig meant, when the pump turned on at 50 psig, there was still water in the tank, which was available to be delivered to meet demand. In this installation, the volume held between 50/70 psig is 14 gallons and the reserve volume held between 30/50 psig is 26.5 gallons (total: 40.5 gallons). Obviously, if three tanks were used, as shown in the article, you’d have a total of about 121.5 gallons of usable water [(14+26.5) x 3 = 121.5)]. With three tanks set up like that, the pump will turn on after 42 gallons (14×3) of water have been used still leaving 79.5 gallons (26.5 x 3) in reserve! With this higher pressure set up, the well pump runs more frequently and pumps less each time (same idea as the atmospheric tank) since there’s less water held in the tank between 50/70 than between 20/40 psig. At the higher pressure range, pump output is reduced so excessive start/stops on the pump are prevented.
It should be noted that the atmospheric storage option has a few drawbacks:
- Since the atmospheric tanks need to be vented they can get contaminated (airborne bacteria, viruses and mold) unless a sub micron air filter is installed and maintained. (It is hard to tell from the picture in the article whether one is installed.)
- Higher energy usage. (Once the water is pumped into the atmospheric tanks, another pump needs to be added to deliver the water to the house.)
- Added expense of extra pump and associated controls.
For extremely low-yield wells, atmospheric storage tanks may be the only option—but many wells with low yield can be successfully handled by pressure tanks. My advice would be to review each installation looking at both options to pick the optimum solution for the customer.
David Beretta, Senior Product Engineer
West Warwick, R.I.
The author responds: Of course, Mr. Beretta is correct both about the mislabeling of Figures 1 & 2 [Our apologies; see corrected version online—Eds.] and ability of pressure tanks to hold water in reserve when the pressure switch settings and air charge are as he suggests. With a set-up as detailed in his letter, there’s more of a safety margin when the well pump comes on and withdrawals from the well are smaller and more frequent, which is a good thing when well yield is low.
Still, there’s no way to schedule withdrawals from the well and no way to prevent over-pumping (pumping the well down). As many geologists will tell you, over-pumping can lead to the early demise of a well. In addition, plumbing performance will deteriorate the longer demand continues because pumps of the size most often installed in low-yield wells cannot provide the flow rates required to make a modern plumbing system function properly and their capacity to deliver will diminish as the water level in the well recedes.
Timed pumping systems don’t over-pump wells when they’re set to collect at a rate equal to or less than the well yield and collecting water this way can keep a well producing as long as necessary—up to and including 1,440 minutes per day.
Dave correctly points out atmospheric tanks need to be vented, but so do wells. The suggestion that water in atmospheric storage leads to contamination from airborne bacteria, viruses and mold is one that I hear often. Rather than start a debate as to whether or not this is true, let’s look at the dynamics of a low-yield well.
When a low-yield well is pumped, the water level drops fairly rapidly. The greater the pump capacity and the poorer the yield, the faster the water level drops. If you think about the receding water as a 6-inch diameter piston being withdrawn from a 6-inch diameter bore (the well), you quickly realize the well must be vented or the pressure in it will drop into the negative numbers, making it more and more difficult for the pump to withdraw water. That’s why well caps have vents in them.
A well with a 10-gpm yield and a 10-gpm pump doesn’t draw in much air because the water level stays relatively constant. A 10-gpm pump in a 1-gpm well is a different story. Water levels can vary greatly, drawing in air when the pump runs and expelling it when the pump shuts off and the static level rises.
The air a well breathes passes through screened openings in the underside of most well caps—too often only inches above the ground and frequently overgrown with grass and weeds. There’s no sub-micron air filter involved in this arrangement and there’s very little to prevent airborne bacteria, viruses, mold, lawn care chemicals, fertilizers, fumes from lawn mower and weed whip motors, and many other contaminants from being drawn in with the air.
If water in atmospheric storage is contaminated in this way, then so is the water in a low-yield well operated with a pressure tank. I don’t believe either is necessarily the case. Contamination in aboveground atmospheric storage most often originates in the well water itself and becomes a problem because too much storage is provided. When large amounts of water are held for long periods in an inactive tank—especially when exposed to the sun—water warms and growth begins. In time-based pumping, water is added to storage at regular intervals keeping the tank cool and active and, if storage is properly sized, the entire content is turned over at least once per day. The vents in the atmospheric tanks pictured are screened in the same manner as a well cap, elevated and in view at all times. We have atmospheric storage systems in operation for many years and the water in their tanks is as clear as it was on day one. There are others that showed signs of contamination with iron bacteria, or some other biological agent that needed to be addressed. Because the owners could see their water these problems were detected immediately and resolved quickly. Problems like this often go unnoticed in well systems operated by pressure tanks.
Mr. Beretta’s assertion that this kind of system requires more energy (electricity) is also correct. However, cost of operation is a secondary consideration for those tired of running out of water or just fed up with poor plumbing performance. And I would agree with his statement that atmospheric storage systems are more expensive than adding a pressure tank—but experience demonstrates it’s difficult, often impossible, to address the health of a well and make plumbing perform as it should using a low-yield well unless you separate the collection and delivery processes. To date, we’ve been unable to come up with a way to do that using one pump.
In conclusion, I would like to say we’re in agreement on two more points:
- Always look at all the options and select a system that will provide the performance your customer is looking for, and
Amtrol makes a great product (we use lots of their tanks).
J. Andrew Reid, CEO
Reid Plumbing Products, LLC
Sharing housing statistics
In the article on “Shared Mail” by David Martin in the December issue, it states that Advo distributes “mailers to over 60 million households weekly, reaching nine percent of the households in… the top 150 markets.” With a population of nearly 300 million in the US, either the 60 million or the 9% must be incorrect, or the households outside the 150 top markets are completely saturated.
St. Petersburg, Fla.
Editor’s note: Actually, the figure was correct. The issue was not population, but households. As pointed out in U.S. Census Bureau reports, the number of U.S. households totaled 106.3 million1 in 2001 and is projected to reach 115 million by 2010.2 The nine percent refers to the top 150 markets, not the entire country or the “60 million.” To be fair, though, I had to look at the question a couple of times before figuring out what you were trying to point out was a bit off. We’ll try to be more clear in how we present such data. Thanks for the tip.
- “American Housing Survey,” (see National Data, 2001, Introductory Characteristics, All Housing), U.S. Census Bureau, April 8, 2003: www.census.gov/hhes/www/ahs.html
- “Current Population Reports: Projections of the Number of Households and Families in the United States—1995-2010,” Document No. P25-1129, U.S. Department of Commerce, Economics and Statistics Administration, Bureau of the Census, April 1996: www.census. gov/prod/1/pop/p25-1129.pdf
Corrections: In WC&P’s December issue (“When Your Customer’s Water Well Can’t Keep Up: A New Look at an Old Problem”), captions for Figures 1 and 2 were reversed on pages 29 and 31. Also, in the January issue, a table was incorrectly labeled in Evan Koslow’s article, “Carbon Breakthrough: New Microbiological Reduction Capabilities Overcome Market Barriers.” It should have read, “Table 1. Microbiological reduction targets.” We apologize for any confusion. These are corrected in the online versions of the articles.