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From: Larry N. Brown (no email)
Date: Sun Apr 27 2008 - 08:25:35 EDT
> All of the water heater installation manuals I've seen have recommended a
> check valve on the input, and an expansion tank in the system. However,
> I've
> yet to see a boat with an expansion tank, and my current boat does not
> have
> one.
>
> Does anyone know why the check valve is recommended? If it is left out,
> the
> expansion problem seems to taken care of by expansion of the plastic
> tubing
> in the hot/cold piping. I haven't put a water pressure meter in the line,
> so
> I don't know what the resulting pressure is.
>
> I'll have to make a decision on the check valve and expansion tank in the
> next few days, so I'd like to hear what the list's plumbers have to say.
>
> Kevin
Perfect question for which I have an answer from recent, intimate
experience. The check valve is to prevent thermal expansion pressure from
backing up into the cold water side. The expansion tank is there to allow
the hot water a place to go when it expands.
Now, here's the interesting part. Remember I started a thread a few weeks
back on Shurflo 5.7 pumps and premature diaphragm failure? Well, when I
plumbed in my water heater, I didn't install a check valve. I planned to do
so but since it was working fine without one, I just neglected to do so.
Well, the thermal expansion was backing up through the cold water plumbing
and into the pump, in effect making the pump diaphragm the accumulator. This
steady back pressure was causing the diaphragm failures.
Decided to remedy it with a check valve and a 1 gal expansion tank. Wrong
again. Remember the discussion of using a Shurflo with an expansion tank? I
can tell you why you shouldn't do it. The Shurflo doesn't have a pressure
switch but a flow rate sensor. It varies rotational rate to supply the
appropriate flow. If you employ an expansion tank, it tricks the flow rate
sensor into thinking it should run slower. This puts high torque-- and high
current demand-- on the motor at low speeds causing thermal cut out and
premature motor failure.
I discovered the above after numerous email exchanges with Peter Silva with
Shurflo's customer service department. I sent back two pumps and they had
their engineers disassemble them to discover the failure mode. Sent me two
brand new pumps. You might remember that I had had another pump fail and
figuring it had no warranty, I gave it to a friend who uses it to power his
sailboat autopilot. Shurflo replaced that pump just on my word. I cannot say
enough about their customer service.
Conclusion: Whatever you do, use a check valve. If you have a conventional
pump with a pressure switch, use a small expansion tank as well. If you have
a variable rate pump, such as the Shurflo, don't use the tank.
Regards,
Larry and Teri
M/V Cigano, 47' Prairie Sundeck Cruiser
Lying: Bear Point Marina
Orange Beach, AL
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