T&T: A/C Pump Problems

[Dave Cooper]

One thing that is often over looked in the raw water side of AC systems is
the size and length of hose runs and if there are any fittings in the run.
For example the outlet of the pump is 1/2" Id and you connect a piece of 5/8
hose to it that 10' long which goes directly to the heat exchanger. The AC
unit is 3' above the water line and the 5/8" discharge hose leaves the unit
and goes to a thru hull a foot above the water line which adds another 6' of
hose.
So we have 16 ft of hose plus the 3' of heat exchanger and the restriction
of the thru hull which the discharge hose connects to. This would be
slightly less than 1/2" ID in most cases. If we have no sharp turns to add
additional flow loses this is adds about 4' to the head perhaps a bit more
so we have a total of say 7' of head in the system.

Using a typical 5.5 GPM pump would in fact not make it for our 4-5 GPM flow
for a 12-16K unit. So we need to go up a size to have a bit of "headroom" as
they say.

So as most things we need to look at the total system not just the
components in order that it is reliable and can accommodate a bit of wear,
tear and not give out on us at the most inopportune moments. Most of the
kits we have seem from the AC manufactures are sized on the extreme low
size. IE zero head with the unit a couple of feet from the
pump/intake/discharge. This might work in small boats but not in our larger
trawlers. Better to have a good Marine AC guy from the tropics spec out your
systems if you expect to be in hot water other wise you'll be in hot air and
that's not why you spent the money ;-)

We had a March series 5 for our forward 12K AC unit and thought we could get
by with a series 3. We did for 8 months till we hit some warm water again
and had to go back to the series 5 pump to keep from tripping the HP limit
switch.

BTW the March series 5 pump has a date stamp on it of June 1988!! A bit
rusty but still working well! The oil holes are on the front and back top
side and just need a couple of drops of oil periodically when you have the
oil can in you hand.

As always YMMV....

Dave & Nancy
Swan Song
Roughwater 58
Caribbean Cruise '07