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Rainwater Harvesting - Pumps or Pressure Tanks
by Doug Pushard

If you are building or planning to install a rainwater collection system, water pressure (i.e. water line pressure) is one of the main issues you need to think about. There one several ways to achieve water pressure when you turn on the faucet. One is gravity. Another is installing a pump to ensure water pressure in your water lines. If gravity does not work for you, you will need to install a pump and there are several options to consider, some new and some old.

System components change over time, but system designers or installers might not change at the same pace. Sometimes it is the old adage “if it ain’t broke don’t fix it”, but it also could be just resistance to new ways or lack of information on why and when to change. Is this the case with new pressure sensitive pumps versus pressure tanks

In designing a system, sometimes it is one aspect of the overall rainwater system that makes one option better than another. Is this the case with the brand new inline pump controllers versus pressure sensitive pumps?

Pressure tanks have been around for a long, long time and have typically been installed on wells and whole-house rainwater catchments systems (i.e. households with no other water system). The tanks hold water for immediate use; consequently, there is no lag in turning on the tap and water coming out the spout as there would be with a standard pump.

Pressure-sensitive pumps are newer and sense the pressure in the water line and turn on when there is a pressure drop in the line (i.e. when the faucet is turned on). Both pressure sensitive pumps and pressure tanks work to accomplish the same end - water at the tap when required.

Newest of all, are inline pump controllers. These are a separate device (i.e. not integrated into the pump) that sense line pressure and turn on the pump. Inline controllers can be easily added to existing installations.

So with three different options, which is best? The short answer is - it depends. A summary of the pros and cons of each solution follows.

Pressure tanks are a proven and widely available and understood technology. They mimic what a municipal water utility does (i.e. pump water to a tank for later use), but on a much smaller scale. Pressure tanks as typically closed steel tank that usually hold about 35 gallons of filtered water. They have a plaster or rubber bladder (i.e. elastomeric) inside the tank which acts just like a balloon (i.e. when the neck of the balloon is opened, air is released, in the case of a pressure tank bladder it is water, not air) to provide the water pressure. As the pressure in the bladder declines the pump turns on to refill the bladder. With pressure tanks, it is necessary to buy a pressure sensitive pump that shuts off, when a specific pressure is reached (i.e. 30-50 psi).

Pressure tanks

Pros:

Cons:

Proven Technology

Requires occasional bladder replacement

Can be use with lower powered pumps (i.e. Solar systems) or low flow rate pumps

Entire tank needs to be protected from freezing

Easy to Service

Needs a separate enclosure

Widely available (most large hardware stores)

 

Occasionally a small pressure tank (i.e. 1-2 gallons) is used in residential installations to minimize pump cycling (i.e. pump coming on and off frequently) which will reduce the life of the pump. Pump cycling can occur due to a leak in the system or the use of low flow drip irrigation.

Prerainwater pumpssure sensitive pumps are one unit combining both a pump and a pressure switch. When a water pressure decline is detected, the pump automatically turns on, and as water pressure returns to normal in the water line the pump shuts off. Some of the more expensive pressure sensitive pumps also contain a internal bladder.

 

Pressure sensitive pumps

Pros:

Cons:

Little to no maintenance

Newer technology

Easy to install

More expensive

 

Most are not submersible

  If internal parts fail, whole unit will likely need to be replaced

 

Not as widely available

Inline Pump Controller is optional part that can be screwed directly onto the existing pump and water line. These devices detect pressure changes, as does a pressure sensitive pump, and turn on the pump. Some of these units will detect if the tank is out of water condition and turn off the pump so it does not burn out

Inline Pump Controller

Pros:

Cons:

Easy to add to existing system

Separate unit, another unit to maintain

Little to no maintenance

May not be submersible

Possible to use almost any pump

Can not be installed in tank, requires a separate enclosure

Less expensive than pressure sensitive pump

Least commonly available

Can be located in separate location from pump

 

As can be seen from the comparison above, all of the options have applications. The installation will sometimes drive the decision. For example, in an off-the-grid installation a pressure tank is probably the best solution, due to its ability to use a low-power 12V or a 24V pump. In these installations the pressure tank can be slowly recharged over time, requiring a much smaller and lower voltage pump. For existing installations that want the convenience of constant pressure, than an Inline Controller is a perfect solution.

Ultimately costs may play a factor in the decision of which option to choose. Below are typical costs of the different options.

Approximate Costs

 

Pressure Tank

 

Pressure Sensitive Pump

Inline Pump Controller

Unit Cost

$130* 

$575* 

~$100

Submersible ½ HP Pump

-

$150 

Standard 3/4HP Pump **

$220 

-

Controller

-

-

Misc parts***

$20 

$20

$20

Total Cost

$270 

$595 

$270

Notes: Standard submersible pump priced from local Home Depot Hardware store. Pressure sensitive pump and Inline Pump Controller priced from suppliers listed in Vendor directory. * A 42 gallon pressure tank. ** Smallest available pump with builtin pressure sensor. *** PVC pipes and pressure guage to connect to system. Will vary depending on type of connection required.

With the growth in the number of both wells and rainwater catchment systems, all three options will likely decrease in price over time as demand increases, but Inline Pump Controller and Pressure Sensitive Pump will likely see bigger price declines as more manufacturers begin to produce these units.

In a lot of installations any of the three options could be used. In these cases it may come down to what your designer or installer is comfortable with. So in you are working with a designer or a rainwater collection system installer, make sure you discuss which option is best for you.

A special thanks to Lee Jaslow of Pond Technology for assisting with this article. Pond Technology is a source for pressure sensing pumps and pump controllers

Links

Related Articles: Comparing Storage Alternatives
Related Articles: Pump Sizing for Rainwater Catchment Systems
Related Articles: Free Rain, Free Watering and Exercise All in One (Bicycle Pumps)

Related Website: University of Hawaii - Rainwater Catchment Guidelines
Related Book: International Plumbing Code
Related Book:
Uniform Plumbing Code

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