Sunday, December 29, 2019

Water Heater Safety

Do you have a water heater on your boat?

Here’s a fact that you probably know, but which has direct bearing on the question:

Virtually everything gets bigger when heated

And of course this includes the water in your water heater.  Why is this an issue?  Well, because on a boat, the water heater is in a closed system.  There is nowhere for the expanding water to go because the check valves in your water pressure pump do not allow backflow to the water tank.

The pressure that expanding water can build is tremendous - two to four atmospheres per degree centigrade.  With a 100 degree temperature rise, this is easily enough to rupture the water heater tank if it is not released in some way.  In your onboard plumbing system, there are only three ways that pressure is relieved:
  • The pressure relief valve on the water heater opens (never remove or defeat this!)
  • The entire plumbing system swells/expands to accommodate the additional volume
  • Something breaks
In a seemingly unrelated fact, most boats with pressurized water systems also have a "surge" tank (aka "pulse dampening" tank, etc) to smooth out the water pump running cycle.  The idea here is that this tank, with its enclosed air bladder, will fill with water when the pump runs, compressing the bladder.  When the pump reaches its shutoff pressure, the air bladder in the tank will force water out into the system when a valve is opened, eliminating the need for the pump to run, for awhile.

How this bears on water heater safety is this:  that surge tank also acts as an expansion tank.  As the water heater heats up its water, the expansion is accommodated by compressing the air bladder in the surge tank a little more.  Problem solved.

More and more municipalities are installing "backflow preventers" (aka check valves) on water lines entering houses, in an abundance of caution to prevent water that has been in household plumbing from re-entering the city’s supply.  This has made all water systems in these municipalities into closed systems, with nowhere for the expansion in the water heater to go.  Because of this, the market has ginned up, and there are now available a host of water heater expansion tanks.  Even the smallest of these (2 gallons) is more than adequate to serve as an expansion tank, flow smoother and rapid pump cycling preventer tank on any boat.  And they cost far, far less than the tiny pump cycle tanks sold at marine stores.  They should fit just about anywhere, with dimensions approximately 8" dia by 12" tall.

Installation is a snap:  Put a tee in your pump output water line, the closer to the pump the better.  And then hook up the side arm of the tee to the tank.  Some tank manufacturers give instruction about how the tank should be oriented for mounting, but I cannot see how this matters.  Perhaps someone out there will explain to me why this is important.

If you have a water heater but no expansion tank/pump cycle tank, you should install one.  It could save your fresh water plumbing.


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