Rainman watermakers are simple, low maintenance systems, but there are a few important consumables to be replaced periodically. A frequently overlooked part is the impeller. Since it is out of sight, it is often ignored. This article is designed to provide technical understanding and stress the importance of proper impeller maintenance.
The impeller is a vaned rubber component inside the lift pump. On an electric Rainman, the lift pump and black high pressure pump are mounted on opposite ends of the electric motor and spin concurrently. On a petrol (gasoline) Rainman, it is driven through a pulley system connected to the Honda motor.
Its purpose is to draw water up through the intake hose, push it through the prefilter, then keep positive water pressure into the high pressure pump.
How Does The Impeller Wear?
There are two primary types of impeller wear.
Through normal system use or simply old age, the impeller vanes may crack, tear or pit. This degrades performance and eventually small pieces or entire vanes will break off.
Pickling the system in the normal sodium metabisulfate solution will cause impeller swelling over time. Using a stronger pickling solution mix will cause swelling more quickly.
What Can Happen If You Don’t Replace The Impeller?
A worn or swollen impeller can manifest itself in a number of non-obvious ways.
|✓||✓||Poor lift pump performance causes HP pump cavitation|
The impeller will not draw sufficient water to feed the high pressure pump, causing it to cavitate. This cavitation may cause one or more downstream symptoms, such as failing to maintain pressure, excessive noise, excessive vibration of the black high pressure hose, or reduced flow rates. Longer term this cavitation may wear on the high pressure pump packings and lead to salt water drips under the high pressure pump. A packing change in the high pressure pump will eventually be required.
|✓||Rubber piece breaks and jams in prefilter hose|
A small piece of rubber may detach from a vane and jam in the white prefilter tube leading towards the prefilter. This may restrict flow and lead to the same cavitation issues as described above. If you change an impeller after this has already occurred, you will need to clear the stray impeller piece out of the tubing.
|✓||AC system draws excess power|
An AC system should draw approximately 1250 watts (~11 amps @ 115VAC or ~5.5 amps @ 230VAC). A swollen impeller increases friction in the lift pump and force the motor to work harder than usual.
|✓||AC system trips breaker on generator or inverter|
The increased current draw described above may cause the breaker to trip if the threshold is only slightly above the nominal current draw.
|✓||Prolonged use strains AC motor/capacitors|
The extra load from a swollen impeller adds load to the motor. Over an extended period of time, this may wear the AC motor and/or its capacitors, leading to premature failure.
|✓||Petrol (gasoline) pressure supply unit not run as smoothly|
The extra load from the swollen impeller will make the Honda engine not start as easily as normal. It may also require more choke or not run as smoothly as usual.
|✓||Squealing sound from lift pump|
A swollen impeller is squeezed inside the lift pump. This can lead to a high pitched squeal as it spins.
Rainman offers Jabsco Profile D 22405-0001-P as replacement impeller, but Johnson 09-808B-1 works just as effectively. Other equivalent impellers exist that likely work well (eg. Volvo, Oberdorfer, etc.), but Rainman has not specifically tested them.
The various Rainman operations manuals describe how to change the impellers.
Impellers are inexpensive and relatively small consumable components of a Rainman watermaker. Three main recommendations for proper impeller maintenance:
- Always keep a spare impeller on hand.
- Consider using propylene glycol for pickling the Rainman. If using sodium metabisulfate, do not mix solution stronger than described in the operations manual.
- Change impeller annually or after extended storage.
We have also prepared a video tutorial on impellers for those that prefer to watch.