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The problem with ELECTRIC tankless plumbed in series as backup in coolwater
country is flow- you don't get even 1 shower's worth of flow out of it if
your solar tank ever drops much below 50F (which it might.) And the upfront
cost of upgrading your electric service for the dedicated 100-150amp 240V
service for big 'un is also quite daunting.  To take 50F water up to 110F
tub-filling temp at 5 gallons per minute takes ~44kilowatts (44,000 watts)or
180+ amps at 240V.  A 20KW tankless still requires a dedicated 100 amp
service, and it would support just one 2.5gpm shower.  A 12KW tankless could
hang on a 50 amp/240V service, but wouldn't really keep up if the solar tank
drops below ~80F. (Even people in FL with 70F water coming in from the
street need more tankless than that.)

The problem with a PROPANE or GAS-fired tankless as backup is that the
minimum modulated fire is too high- it can't tolerate higher temp incoming
water without short-cycling itself into low efficiency and early demise,
some will spit out error codes and lock out until you reset them when used
in this mode. They REALLY suck in this applicaion- don't go there- you pay a
heluva lot up front for a big PITA later. (And at current propane prices at
the as-used efficiency you'd actually get out of it you'd be better off
using electricity.)

The problem with using either of them plumbed as mini-boilers to maintain a
min-temp on the solar tank is lower collector efficiency due to higher
collector temps, resulting in a lower solar fraction.

The cheaper/better solution in most cases is to plumb a dumb electric tank
heater in series with the output of the solar store, and set T-stat on the
electric tank to 120-125F, but put a thermostatic mixing valve on the output
set to 115F or whatever works for you. The standby on an electric tank is
quite modest, and can be made even lower with a tanky-blanky and using
5/8"-wall or thicker closed cell pipe insulation on all of the plumbing
between the solar tank and the electric tank, as well as all of the hot
water distribution plumbing, and even the temperature/pressure valve &
overflow.  As-shipped a decent electric tank has about a 50-60watt standby
loss @ 125F storage temp, or about 35-45 kwh/month.  With near-tank pipe
insulation you can cut that by about a third, and with a retro-wrap on the
tank itself it's possible to bring that into the 20s or lower, and that's
only when you're feeding it with water colder than setpoint. When the temp
out of the solar tank is above the tank setting MOST of the time you don't
even pay the standby losses in electricity, taking only a low single-digit
percentage loss in solar fraction, a far lower hit than the hit in
collection efficiency you take if you maintain the solar tank at 120-125F.

The standby losses of gas/propane tank heaters is quite high and much harder
to reduce in comparison to electric tanks (due to convection in the
center-flue heat exchangers) and would be a lousy choice.

If your heating system is hydronic (pumped hot water,
baseboards/radiators/radiant-floor etc.) there's some argument for using an
indirect-fired tank running off the boiler plumbed in series with the
solar-store.  The mass of the indirect keeps the boiler from short-cycling,
and efficiency will still be pretty decent. But even condening propane
boilers might be more expensive to operate than resistance electricty,
depending on your electric rates and cost per gallon.

dana


-----Original Message-----
From: Vermont Skiing Discussion and Snow Reports
[mailto:[log in to unmask]]On Behalf Of Goodrich, Chris
Sent: Monday, October 10, 2011 10:00 AM
To: [log in to unmask]
Subject: [SKIVT-L] OT Solar Hot water


Anyone have experience with tankless water heaters? I am having a solar hot
water system installed and my 80 gallon off-peak electric heater is probably
a little overkill for backup. The 25 cent per day meter charge will eat up
any savings that I gain from off-peak since most of my energy will now come
from the sun. I also have a plan to re-use the old tank as heat storage for
the wood boiler. So I was thinking that an electric tankless heater would be
the simplest and most efficient backup system.
Chris

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