Hooking Up Two Well Pressure Tanks: What Michigan's Manual Says Changes

When a second tank is on the table

In Michigan's sizing method, if the pump meets its Table 1 minimum, "supplemental storage is not required." If it is smaller, storage "is necessary to meet peak demands" Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1. Michigan's one-tank example shows extra volume can also come from pre-charge and switch settings Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1; the tank guide's settings section has those numbers.

Arizona says "a tank cannot compensate for demand greater than your pump or well capacity" Source 3: University of Arizona Superfund Research Program, Well Owner's Guide3. Michigan's passage on a pre-charge just below pump-on pressure says when demand exceeds the pump capacity, "the pressure will drop, and the supplemental supply from the tank will be used to meet the demand" Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1. The two are worded differently; neither source reconciles them.

Michigan's low-capacity-well section says changing when water is used can reduce peak demand. Simple changes in water-use habits "may be enough" where shortages "occur infrequently" Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1.

Two tanks in Michigan's example

In Michigan's example, a household complained of running out of water. Its 13 usable gallons from one tank became 39 from two, after three changes made together. For an added storage tank, the manual's general sentence says its pre-charge "should be lower than that of the primary tank," and the switch range "should also be set closer" Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1.

The household's seven-minute peak use is 70 gallons. Its well makes 5 gallons per minute, so the pump supplies 35 gallons in that time and 35 must come from storage. Michigan's Table 1 lists a 10 gallon per minute minimum pump for a 70-gallon peak without supplemental storage Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1.

• First tank: a 42-gallon diaphragm-type tank, running at 30 to 50 psi and pre-charged to 30 psi. It gives about 13 usable gallons.
• Second tank: another 42 gallons, pre-charged to 20 psi, added with the switch moved to 40 to 60 psi Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1.

Afterward the first tank gives 17 usable gallons and the second about 22, together delivered from 20 to 60 psi.

If demand keeps pulling the pressure down, the example's tanks empty in stages Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1.

• Pump switches on. At 40 psi, about 5.7 gallons remain in the first tank and 10.7 in the second.
• First tank empty. At 30 psi, the first tank is essentially out and 5 gallons remain in the second.
• Both tanks empty. At 20 psi, the only supply is the pump.

What the example leaves out:

• Three changes came at once. Michigan gives no result for a second tank with the settings left alone.
• Closer is not defined. Michigan does not say what "set closer" means. In the example the range goes from 30 to 50 psi to 40 to 60 psi, the same 20 psi wide, so this page does not read closer as narrower.
• One household. These are one household's figures in a manual's example, not settings for your tanks.

What two manuals show

Washington's design manual is written for public water systems, with guidelines for design engineers. It says they "may use Equation 9-1 to determine the number of bladder tanks of a certain gross volume, based on the pump-on and pump-off pressure settings for single-speed pumps controlled by a pressure switch" Source 2: Washington State Department of Health, Office of Drinking Water, Water System Design Manual, DOH 331-1232. Its Example 9-1 picks "six 86-gallon bladder tanks for pump protection, pre-charged to 55 psi" Source 2: Washington State Department of Health, Office of Drinking Water, Water System Design Manual, DOH 331-1232. All six share one pre-charge. That is engineering guidance for those systems, not a layout to copy.

Michigan ties a lower second pre-charge to peak demand Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1, so the two manuals answer different questions.

Michigan's text gives no procedure for connecting household tanks. Its figure for the example shows two tanks along one line with a single pressure switch labeled 40 to 60 psi. That is a picture of a finished layout, not a method, so this page gives no steps.

Who does the work, and what to ask

This lands at professional-only. That is this site's verdict, not a ban found in these sources. The example moves the switch range, which on this site the professional sets. An owner's part ends at flipping a breaker off.

When hiring, NGWA says to check a contractor's state license or registration, and that not all states require licensing Source 4: National Ground Water Association, Well Owner's Guide, 2021 edition4.

These questions are this site's advice, not a sourced list.

• How much added storage comes from the second tank, and how much from changed settings?
• Can the settings be written down? See the tank guide's hiring section.
• Does your work call for disinfection and testing, and who does it? Michigan names drilling a well, and repairing a well or household piping, as ways bacteria can get in Source 1: Michigan Department of Environment, Great Lakes, and Energy, Water Well Construction Manual1. It does not name adding a tank, so this is a question. See well water testing.

The sources used here do not say whether adding a tank needs a permit. That is a county question. Requirements vary by county. Your county health department has the final word on permits, inspections, and what is allowed on your property.

Where this fits

Rapid on-off cycling is a different question; see well pump short cycling. For the switch, see the pressure switch post.