Thursday, January 8, 2015

A Magnetic Personality

This post originally appeared on Windborne in Puget Sound

Like the rest of the country, the Pacific Northwest has been experiencing unseasonably cold temperatures of late (well, ok - for us, 32° is unseasonable).  And of course you know what this means...  our heating system failed.  Just like roof leaks only appear when it rains, heating plants never fail in the summer.  Oh well.

The first clue was that the thermostat display was completely blank.  Well, and the boat was cold, too.  Some back and forth with Marinaire, the heat pump manufacturer (great customer service, by the way), disclosed that there was a fuse on the main circuit board - a fuse hidden beneath a blue vinyl cover.  Yep, it was blown.  When it was replaced, the replacement blew immediately as the fan and circulation pump tried to start.  Blowing a couple more fuses revealed that the problem was the circulating pump - the pump that provides sea water to the heat pump.  (It is by the chilling of this sea water that the heat pump produces heat.)

Here's the pump after I pulled it out:
Salt water short-out
Yup - the shaft seal failed and sea water was trickling back along the shaft and into the electric motor.  Bzzzt!

So I bit the bullet and ordered a new pump.  This one has a magnetically driven impeller - that is, there is no shaft seal.  The motor drives a cup-shaped magnet; the pump body extends into the cup but has no opening.  The impeller has an imbedded magnet, and is thus driven by the motor without any mechanical coupling and without a shaft seal.  As you might expect, this kind of pump is more expensive.  But the technology is worth it.

As a bonus, the pump body itself (the white plastic portion) is considerably larger than in the old pump, and is much more substantially made.  The inlet and outlet are larger as well.

Since the pump is physically larger, it wouldn't fit where the old one had been.  So there was some fooling around involved in finding a location that...
  • was below the water line as far as possible - centrifugal pumps are not self-priming,
  • was not actually on the floor of the bilge compartment, since that would promote rusting of the motor base,
  • did not interfere with access to the nearby battery, 
  • minimized the required plumbing changes,
  • and finally, did not block the access door you see in the background to the right.
And of course I had to change the plumbing to use the new location.  If you can believe it, I actually ended up with fewer fittings in the new installation!

Based on the appearance of the discharge water stream, I'd estimate that this pump is delivering twice as much water as the old one, even tho both are rated at 500 GPH.

The boat is warm!  And now I expect to be able to forget about this pump for a long while, just as I have been able to with the refrigeration circulating pump.

Magnetic personality?  I must have one.  Can I make a recommendation here?  Avoid sea water pumps that have shaft seals wherever possible.  Like the cosmetic ad says, they're more expensive, but you're worth it.


  1. Bob,
    I've been reading for a while but don't currently have a boat. Is this type of magnetic drive available for drive engines and propellers? Just thinking ahead.

  2. Boy, that would be nice, wouldn't it Tony! But magnetic drives are limited by the relatively low torque they can handle. And driving a boat propeller requires a *LOT* of torque (our transmission gears the engine down 2:1, doubling the torque, to drive our prop... and it's a diesel engine).


  3. Agreed. I believe these are called "March pumps" ( least mine is. Only issue is that it's AC-driven, and it plus the Marineair 12,000 BTU heat pump I have kicks at about 17's 6 amps to run...and this taxes both my 2000W inverter and my 2000W Honda genset. One solution is getting a pair of U.S. Hondas (one a "companion" model) and hooking them together. Another is the Dometic "Soft Starter" ( which I still have to research a bit, but of which I hear good things.

  4. Rhys -
    While March do indeed manufacture magnetic drive pumps, they are by no means the only one who do. In fact, the circulating pump for our refrigeration is a 12V magnetic drive pumpmanufactured by Johnson Pump.

    Our Marinaire (note the slight spelling difference) heat pump has the soft-start/inrush management built in. Dometic's add-on may be the right answer for you...



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