[VAC] Re: tankless water heaters - more

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

Water heater tanks do vary in their insulation performance. My dad has a
Rheem with external elements that seems to be decently insulated. He has
it wired through a huge toggle switch on the kitchen wall in his house.
He gives it a few hours advance notice of expecting a large hot water
consumption like laundry, otherwise he leaves it off for days at a time.

No matter the quality of tank insulation, heat loss from a tank type
water is easily documented. Just wire the electric heater through a watt
hour meter, or the gas with its own gas meter and don't use any water
for a week and read the meter to see the energy consumed holding
temperature. The gas water heater undoubtedly looses its most
significant heat through the draft up the chimney. The inside pipe is a
heat exchanger and puts heat to the water from the flame or takes it out
to the stack draft equally well. So adding a blanket to the outside of
the gas water heater may have limited energy savings. There'd be benefit
to adding a stack damper, but that has a bit of hazard if the burner
goes on and the damper doesn't open. I've had a dose or two of CO from a
stack too fat for the water heater to keep warm under certain
atmospheric conditions. I don't use a gas hot water heater in the house
any more.

It is practical to add insulation to a tank type water heater and it is
possible to detect the energy saving. A hot water faucet drip is
probably a far greater energy looser.

A tankless water trades heat leaked through its insulation for large
peaks in energy consumption. And there's still a tank. Its not big but
there's a quart to a gallon of water to have enough volume to hold the
heating element or heat exchanger and enough water so the heat input
after the faucet is off and the thermostat shuts off won't boil that
water. 

Running a tankless water heater on LP in the RV could have a problem
getting enough propane vaporized in the tank. Vaporization depends on
external ambient around the tank. Rapid consumption could easily burn
all the available vapor in a 20 or 30 pound tank and then sputter way
back on output on a cool day. I have a handbook on LP and could figure
that. I know that I can't run my 5 KW Onan standby generator on small
tanks. It takes two 200 pound bottles to have enough vaporization for
all weather conditions to supply that engine with LP. Large peak loads
are not welcomed on electric power systems either. Far less welcomed on
systems of limited size as are typical of older RV parks. The tank type
water heater trades peak energy consumption for spreading out the peak
of energy consumption over a longer time period. RECs sometimes don't
like water heaters with 4KW heating elements, but prefer them to be
smaller just for that peak flattening. Makes it take longer to recover
with the smaller element but the voltage regulation is better. And the
REC may avoid paying peak demand charges to their power generation
source.

In my grand house design, I plan on a 5 gallon tank type water heater in
the counter under the kitchen sink, while the main water heater will be
just under the bathroom. That way (with good insulation blankets) I'll
have the convenience of nearly instant hot water in either location
without the peak loads of the tankless water heater and without the
price of the tankless water heater. I'll feed hot water to the "cold"
water inlet on that kitchen tank. And I'll insulate the 1/2" copper hot
water lines to hold in the heat. 1/2" to minimize the volume of water
exposed to cooling.

When adding insulation to the outside of an electric water heater its
important that the supply wiring be oversize with insulation rated for
higher temperature than the water temperature because that insulation
blanket will cause the wire to run warmer, sometimes warmer than Romex
is rated.

It is possible that the "energy saving" from minuscule storage in the
tankless water heater is offset by energy lost in wiring and the power
system from the large peak consumption. Or in the gas case by poor
combustion and heat exchanger efficiency caused by the small size
compared to a high efficiency central furnace that can afford fancier
controls, forced draft and enough heat exchanger to keep the exhaust
really cool. Most likely these values vary with vendor and size of
tankless water heater.

The emperor's clothes may well be in need of adjustment depending on his
tailor of the day. In energy situations, many "solutions" to energy
waste have come from persons who didn't take any engineering or math
courses and have less than perfect validity. Temperature set back
thermostats in some situations are a heat looser, not a saver.

Gerald J.