New well pressure tank. Residential Well System:

Replacing A Pressure Tank

(And Fixing A Small Problem With The Pressure Switch)

In This Article:

A worn-out well pressure tank is disconnected. Fittings are removed and attached to a new tank. New iron pipe is installed to connect the new tank to the system. A problem with the pressure switch is resolved.

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Skill Level: 3 (Intermediate) Time Taken: About 6 Hours

By , Editor

What Is A Pressure Tank?  Why Do You Need It?

When a house is built in an area that is not served by a municipal water works, a private well system is used. The well pumps used in most systems are quite powerful, often capable of delivering more gallons of water per minute than any single faucet could draw.

Everybody I have spoken to in the plumbing or water-well business says that well pumps can burn out prematurely if they are made to start and stop more than necessary. By using a holding tank, the water can be used in a leisurely manner over a long time, and the pump will run briefly to refill the tank.

A pressure tank can be as simple as a big metal chamber with one hole in the bottom for water to flow into. As the pump pushes water in, the air in the tank is compressed. At some point, called the cut-out pressure, a switch interrupts the power to the pump and no more water enters the tank. Since there is compressed air above the water, there is enough force acting on the liquid to push it out of the tank, through the pipes, and all the way upstairs (possibly several flights of stairs) to the faucet.

But... there is a flaw with this simple design. Air can be dissolved in water, believe it or not. Eventually a simple tank with compressed air above the water will experience a loss of this compressed air. So the air space above the water gets smaller day by day. What you notice is that the pump will run for very short periods, perhaps 5 or 10 seconds, and do this frequently whenever water is being drawn.

That is exactly what was happening in my own home. The remedy is simple: just add more air to the tank with an air compressor. For years I have been doing this about every 2 months. It only takes a couple of minutes, but it's still a maintenance chore. Besides, there's a better way.

A proper well pressure tank does not use a simple empty steel can, it uses a flexible bladder or bag inside the tank, with the bottom of the bag pointing to the bottom of the tank. The bladder occupies a portion of the total tank volume. The steel tank contains compressed air. The operation is the same as the plain tank example above, but the bladder keeps the water separated from the air. The air never meets the water, so the air does not become dissolved in the water.  The tank never needs to be charged with air, unless the fill valve (which is like a tire valve) leaks.

But... eventually the bladder can leak. After all, it flexes every time the water is used, so wear and tear is expected. Once the bladder leaks, the tank reverts to a plain style of pressure tank (with a ripped rubber bag inside), so the compressed air slowly bleeds out. Plus you can get chunks of rubber in your water lines, which has its own set of problems.

That is why we replaced our pressure tank. Simple wear-and-tear.

But on this job I had a skilled helper: my good friend Tommy Skusa, a licensed plumber with 17 years of experience in all sorts of residential, commercial and industrial projects. I let Tommy do most of the work, while I took some pictures and tried to stay out of the way.


Old well pressure tank that leaked. The old pressure tank.


This is the Schrader valve used to add air to the upper part of the tank.


These two photos show the tank and control apparatus, and the pipe that enters the house from the well. Note there is also a wire that leads to the well.

As you can see, all this plumbing is under the basement stairs, so it made for crowded working conditions and less-than-perfect pictures.


I turned off the breaker that supplies the well pump.

This is a 240-volt breaker, which is common for well pumps.


Tommy drained the system with a short piece of hose and a bucket. This took about 5 minutes.


He removed the plastic cap covering the pressure switch.


I removed the wires. The outer wires were the supply lines.

Being a 240 volt device, both lines are switched. So this is a 2-pole switch.


Removing wires to well pump pressure switch. The inner two wires go to the pump.

I removed these carefully because it was obvious that they were quite old wires. That cloth wrapping was used a long time ago, I would guess this was installed in the late 1950's or early 60's. The lack of a ground wire also dates the wire. I understand that by 1964 grounded wiring was required by the National Electrical Code.


I removed the cable clamps too. They weren't holding very well.


The pressure switch after being disconnected.


We loosened the the dielectric union that connected the newer copper main pipe to the older galvanized steel pipe.

All of these pipes here are one inch diameter.


Disconnecting union fitting between well point and pressure tank. Tommy used a pair of pipe wrenches to loosen the union fitting in the line between the well and the tank.


He disconnected the union while holding up the pipe, so it would not tip the tank over.


Tommy just twisted the tank around and walked it out of its hiding spot beneath the stairs...


... and then set it on a workbench so we could dismantle the old parts.

The tank didn't weigh much... maybe 25 pounds with all those extra pipe fittings.


After removing the pressure switch (it sat on a ¼" nipple, about 4 inches long) we removed the pressure gauge with a crescent wrench. This was also a ¼" fitting


We removed the boiler drain valve, and we unscrewed the pipe that fed into the multi-port well tank fitting.


We tipped the old tank over.


We carefully removed this section of pipe from the old tank, because I wanted to save the tank for possible use with compressed air.


Reconnecting the pressure switch:

Tommy wrapped the ¼" nipple with Teflon tape. Note the direction of wrap. When the pipe is screwed in, the tape won't be peeled back.

It's important to use Teflon tape at the pressure switch connection, because paste thread compound can break away and gum up the switch.

It's a good idea to replace the simple rubber diaphragm in the pressure switch at this time, but we forgot about that and it's a long way to the hardware store. So we re-used the old switch as-was.

He installed a new pressure gauge, being careful not to overtighten and strip the ¼" threads...


... and he replaced the pressure switch

Note that the multi-hole fitting has been cleaned up. I took it out to the shop and used my buffer (a bench-grinder-mounted wire wheel) to remove some of the dirt and rust. This can also be done with a wire brush, but that is slow and tedious.

Apparently this piece was not galvanized. I also cleaned up the drain valve.

With the tank gone from the corner, I took a minute to clean out the debris that had fallen from the stone rubble foundation.


I cut off the 1 inch supply pipe that leads from the tank to the rest of the house. Since the new tank is much wider than the old one, the pipes will need to be moved a bit.


Tommy assembled this section of piping and carefully screwed it into the new pressure tank. The black part on the bottom of the tank is a plastic fitting, which made Tommy kinda uneasy. But I think the plastic is glass-filled nylon, which is really tough.

Still, he didn't want to over-tighten the connection and strip the plastic threads, causing a big delay in getting the water restored.

New well pressure tank and plumbing.


We put a few pieces of pipe together to see what would be the best route to the tank from the old pipe


Take 2:

We tried a different approach, going away from the wall on a 45° angle and then turning.

Note how this new lateral pipe turns downward before entering the tank. We had to do this because the new tank was farther from the wall than the old tank, and we needed a vertical pipe to connect to the copper pipe above.


We placed the tank on a pair of 4" cement blocks.

I wanted the tank to be higher off the ground than the previous one, so I could get a pail under the drain valve

I have seen many instances of pressure tanks sitting on the basement floor, and having great difficulty draining the system for repairs because the drain valve was only a half-inch above the floor.

Using a dinner plate to drain a tank is not a quick process. But that's what you do when you forget to bring a piece of garden hose.


Tommy likes to make sure his supply pipes are nice and level. I've never seen any other plumber do this. But Tommy's work looks good... and functions well too.

This is a plumber who carefully lays out his supply pipes, snapping chalk lines and keeping hot and cold lines perfectly parallel at 8 inches apart. And level too, or a slight slope towards a drainage point.

Pride in workmanship. Now that's cool.


I reconnected the wires at the pressure switch.

The pump wires (red arrows) did not instill confidence, but I connected them anyway, since replacing the wire in the middle of winter, under four feet of frozen ground, is not gonna happen.

First I installed new cable clamps because the old ones were messed up.


I connected the supply wires to the outer terminals. There is not much room under the cover of this electrical junction box.

I was leery of the insulation cracking and crumbling on those old wires, so I kept the bending and flexing to a minimum.

Wiring of well pressure switch.


Tommy reconnected the copper line.


The one-inch main pipe runs just over head at this point...


... and comes from the dielectric union just downstream from the pressure tank.

When the copper pipe was finally reconnected, and the wiring to the pressure switch was done, we closed the main ball valve (visible in the photo above) and turned on the pump breaker.

Since there was no pressure at the switch, the contacts were closed, and the pump started right away. The pressure tank filled up and the pump shut off. The clicking of the pressure switch is quite audible, and the action of the pressure gauge also tells what is happening.

Then I slowly opened the main shut off valve. Water filled the pipes and was visible in the swirl-down clear-bowl filter nearby.

But then... the needle on the pressure gauge fell to zero and stayed there. What the...?

It Didn't Work!

I took the cover off the pressure switch. It was still closed, as it should be.

Using a non-contact voltage detector I probed for power at the wire that runs from the switch to the pump. The sensor beeped, indicating power.

I was stumped. Maybe the pump took a dump?  But Tommy noticed something.

The photo at right shows the pressure switch when it is OPEN, that is, the pressure in the tank is high, has reached the "cut-out" point, and the pump is not needed.

The red arrows point to the two brass conductive connectors. At each end of these is a round dot of metal (bronze, perhaps) that touch opposing round dots, when the switch closes.

Contacts in well pressure switch.

These round dots are the contacts, and they take a lot of abuse. Whenever the contacts open (as the pressure reaches the upper limit) there is the visible flash of arcing current. Like miniature lightning. This arcing creates heat and oxidizes the metal. Most metal oxides are not good conductors.

The pressure switch when closed, when the pressure in the tank has dropped to the "cut-in" point and the switch now energizes the pump.

Notice how the opposing contacts are now touching, creating a path for current to flow.

What Tommy noticed was a tiny bit of oxidation on the contacts, visible as a light-colored surface.

The solution was simple: I got a little strip of emery cloth and folded it lengthwise to form a double-sided abrasive strip.
  • I manually opened the switch
  • I slipped the emery cloth between the contacts
  • I let the switch snap shut
  • I simply pulled the strip out.

I did this 4 or 5 times for each of the 4 pairs of contacts. (Of course I turned the power off)

Cleaning contacts with emery cloth, well pressure switch.

I turned the power back on and the pump worked fine.

But why did the voltage sensor detect power and the pump not run?

One wire must have been energized, and the other dead. It only takes a break in one of the two wires to interrupt power to the device... but for reasons of safety 240 volt well pump pressure switches are double-pole switches, interrupting both wires.

Why: Because residential 240 volt systems employ two hot wires. One wire is 120 volts higher potential than ground, and the other wire is 120 volts lower potential than ground. The potential difference (also called voltage) between the two hot wires is 240 volts.

Remember - just because a 240 volt device is not running doesn't mean there is no power to it. You can still get shocked if one line is live.

The completed installation. Sort of.

The wiring to this system is a big 'ol code violation. These cables need to be inside conduit, either rigid or flexible.

But I will be rebuilding the stairs in the future, and at that time I will also be correcting the gaggle of poor wiring in this area.

This is a Sears Craftsman Captive Air® tank, model 390.291658, which is a 36 gallon tank. The label says it is equivalent to an 82 gallon standard tank.

This tank has a replaceable bladder, which should save money in the long run as complete tank replacement would not be necessary. But bladder replacement involves draining the system and removing the tank. The air valve can also be replaced... it is an ordinary car tire valve.

With cut-in/cut-out settings of 40 and 60 PSI, this tank has a draw-down of 9.3 gallons. That means that I can draw over 9 gallons of water before the pump will kick on. We were experiencing the pump turning on once every time a 3.5 gallon toilet was flushed, and when the air got low the pump would kick in 3 or 4 times per flush.

Now I hardly ever hear the pump turn on.


Tools Used:

  • Pipe Wrenches, 10", 14", 18", 36"
  • Adjustable (Crescent) Wrenches, 8", 10", 12", 15"
  • Soldering Tools For Copper Pipe
  • Screwdrivers
  • Pliers

Materials Used:

  • Pressure Tank, Sears Craftsman model 390.291658
  • Galvanized Pipe, Nipples, Fittings
  • Pressure Gauge
  • Teflon Tape
  • Pipe Thread Compound
  • 3/8" Cable Clamps
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Copyright © 2005

Written February 1, 2005