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Re: Looking for Hot Water Heater Efficiency comparison

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Posted by nicksanspam on April 9, 2007, 11:56 am


The EEMAX EX55T tiny tankless electric has a 0.99 EF and costs $50 and
uses 5500 W and can accept hot or cold water and controls the output temp
to +/- 1 F regardless of flow. It can warm a 1.25 gpm low-flow shower 30 F,
so we might preheat water from 60 to 80 F in an electric tank heater set
lower and then heat the water to 110 with the tankless to make lots of
hot water more efficiently with no change in house wiring.

If the tank can heat 64.3 gallons of water per day from 58 to 135 F with
a useful output Eu = 8.33x64.33(135-58) = 41,242 Btu and a 0.95 EF and
an energy input Ei = Eu/0.95 = 43,413 Btu and the tank walls lose Ei-Eu
= 2171 Btu with a 68 F air temp, the wall conductance G = 2171/24h/(135-68)
= 1.35 Btu/h-F. At 80 F, the walls would lose 24h(80-68)G = 389 Btu (1 cent
per day at 10 cents/kWh), for an energy factor of 1-389/41242 = 0.99.

For greater savings, we might disconnect the lower tank heating element
and preheat the lower part of the tank with a greywater heat exchanger
(should we run fresh water pipes up through drainpipes in new apartment
buildings?) or sell the tank and build a solar pond water heater.


Posted by Epictitus on April 9, 2007, 1:47 pm
0.99 sounds good - the highest I've seen is 0.95

Where did you see the specs for the EEMAX EX55T?
Unless I'm missing something the Eemax product spec sheet doesn't give any
info on EF, FHR operating costs etc.  This system is a low volume, small
booster system not designed to run a household.

From what I've seen on electric tankless units is that their efficiencies
are lower than the better tank units.

See what I mean about lack of consistency on spec sheets?  I'd of thought
EnergyStar.org would have compiled a nice comparison sheet by now...

Posted by Solar Flaire on April 9, 2007, 2:05 pm
 It is electric with an immersed heater and no chimney. Where could
more than 0.1% go? How much noise could the WH make?

Posted by nicksanspam on April 9, 2007, 3:17 pm

... 0.99 makes sense for a small tankless electric. With no central chimney,
the only limit on an electric tank's EF is the thermal mass and insulation.
Reducing the mass so it can cool between uses and reducing the amount of
surface that needs insulation can help a lot.

The EEMAX EX55T pdf spec sheet says something like "efficiency: 0.99,"
which I'm certain means EF = 0.99.

That's the reason for starting with 80 F water and 5500 watts, in a house
that already has a 5500 W tank heater. A new house might have an EX95T
9500 W heater with no tank, or a tempering tank or a well pressure tank
to preheat cold water to 70 F. EF is the ONLY number we need to compare
electric energy efficiencies.

I suppose somebody has (Consumer Reports, or some state agency?). I couldn't
find one either. Perhaps this has something to do with politics. There may
be a list of certified Energy Star water heaters which includes their EFs.
Keeping it up to date as models change would be significant work.


Posted by Paul M. Eldridge on April 9, 2007, 2:06 pm
 On 9 Apr 2007 07:56:15 -0400, nicksanspam@ece.villanova.edu wrote:

Hi Nick,

One word of caution.  From what I've read, the optimal temperature for
Legionella growth in hot water tanks is between 32C and 35C (90F and
95F), although I understand it can still develop above and below these
temperatures.  A study of 211 Quebec homes found Legionella
contamination in 40% of the electric water heaters tested.

The following paragraph illustrates the risks you could encounter if
you were to disconnect that lower heating element:

"The optimal temperature for Legionella proliferation in water varies
between 32C and 35C, but it can easily proliferate at temperatures
of up to 45C. Usually, there is no growth above 55C, and a
temperature of over 60C has a bactericidal effect. Thus, the WHO
recommends that water be heated and stored at 60C (3). However,
studies in Quebec have shown, even when the thermostat is set at 60C,
a high percentage (approximately 40%) of electric water heaters remain
contaminated because of the lower temperature, about 30C to 40C at
the bottom of the tank. The probability of contamination will increase
considerably if the temperature setting is lowered to 49C. The risk
of contamination is much lower for water heaters operating with fossil
fuels, and is practically nonexistent for these heaters set at 60C."

Source: http://www.pulsus.com/infdis/15_01/leve_ed.htm


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