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Posted by hallerb@aol.com on May 18, 2008, 10:11 pm
> If you put the electric DHW downstream of the boiler's tankless coil and
> have it's thermostat set slightly below the aquastat on the boiler, when
> the boiler is running during the heating season the electric HW will not
> operate much at all and will act as an insulated accumulator tank. When
> the boiler is shut off in the summer the electric heater will operate
> since the incoming water will be cold. No fancy controls or valves
> needed.-
EXCELLENT SOLUTION!! And no fancy wiring or controls:)
KISS at its best
try the electric tank and see how much it saves, if you have
houseguests in the summer turn on the boiler too, for extra capacity.
the electric tank wouldnt be as good as the boiler, but might be fine
for your needs
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Posted by Paul M. Eldridge on May 18, 2008, 11:32 pm
wrote:
>If you put the electric DHW downstream of the boiler's tankless coil and
>have it's thermostat set slightly below the aquastat on the boiler, when
>the boiler is running during the heating season the electric HW will not
>operate much at all and will act as an insulated accumulator tank. When
>the boiler is shut off in the summer the electric heater will operate
>since the incoming water will be cold. No fancy controls or valves
>needed.
Hi Pete,
Excellent suggestion. As odd as this sounds, I'm working from the
opposite direction having just installed a small, 120-volt/1.5 kW
electric water heater to pre-heat the water that is feed to my
indirect DHW tank. This indirect tank is powered by our oil-fired
boiler and accounts for roughly two-thirds of our remaining fuel oil
consumption. Last year, we consumed a little over 700 litres/185
gallons of fuel oil for space heating and DHW purposes and now with
this electric water heater in place, that number should fall to 250
litres or less. At current rates, we expect to save about $150.00 a
year, but that figure should increase substantially with each passing
year.
A picture of the new tank can be found here:
http://www.datafilehost.com/download-458c4344.html. Although this is
hard to see, the tank sits on 80 mm of styrofoam insulation to help
minimize heat losses through the base.
The tank is just 67-litres/18 gallons in size so its storage capacity
is limited, but it should still serve our needs well. I've set the
thermostat at 70C/160F and the main tank that it feeds at 50C/120F so
that the water entering the main tank upon initial draw will be
substantially hotter than what is being taken out. We expect the
hotter water being pulled through the main tank will not only satisfy
our fairly modest requirements, but also compensate for most if not
all of the standby losses (the SuperStor Ultra is rated at 0.5F per
hour). The standby losses of the electric tank itself are quite
reasonable due to its small size, but the higher set point will bump
that up somewhat; in addition, tank life when cranked up to this level
will take a bit of a hit. With that in mind, I plan to monitor its
performance closely and slowly reduce the setting to a point just high
enough to prevent the boiler from kicking on; we should be able to
drop it during the summer months when supply temperatures are higher
then, if need be, bump it back up come winter when inlet temperatures
are lower and when draw downs are typically greater.
We could have by-passed the main tank altogether and just called it a
day, but this arrangement works better from our point of view because
we can rely on oil to help out during times of unusually high demand
(e.g., when we have overnight guests) and, furthermore, I didn't want
to shut down the boiler for an extended period of time for fear that
this may result in operating problems down the road. Our boiler has a
Tekmar control system, so unless the DHW tank or one of the zone
thermostats calls for heat, the boiler is allowed to fall to room
temperature -- thus no oil is wasted. In addition, every two days it
turns on the circulator pumps, opens up all the zone values and
circulates water through the entire system for about five minutes as a
preventative measure and, obviously, if we were to turn off power then
that would no longer happen.
So, again, with this pre-heat arrangement, we can leave the boiler
turned on, but unless the electric water heater can't keep up with
demand it will never actually fire-up *and* because it *will* fire-up
when truly needed we'll never have to worry about running out of hot
water during times of unusually high draw. And, as noted, because we
don't have to kill power to the boiler, the Tekmar can continue to run
its exercise programme every other day to prevent things from seizing
up.
Given that our ductless heat pump supplies nearly all of our space
heating needs and now with this new electric hot water tank in place,
I don't expect our boiler to fire-up again until sometime mid next
February.
Cheers,
Paul
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Posted by tman on May 19, 2008, 8:14 pm
Pete C. wrote:
>
> If you put the electric DHW downstream of the boiler's tankless coil and
> have it's thermostat set slightly below the aquastat on the boiler, when
> the boiler is running during the heating season the electric HW will not
Gosh, why didn't I think of this. This is such a simple, but good idea.
My only concern is will the coil in the tank be too much of a
restriction to flow -- I know that the rate of hot water flow is pretty
limited today going through the coil, and that is with at least some
water going through the mixing valve. I think I can make do with a
bypass to shunt some proportion of the cold water around the coil, and
pose less of a restriction to flow....
T
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Posted by KJPRO on May 19, 2008, 11:31 pm
> Pete C. wrote:
>
>>
>> If you put the electric DHW downstream of the boiler's tankless coil and
>> have it's thermostat set slightly below the aquastat on the boiler, when
>> the boiler is running during the heating season the electric HW will not
>
> Gosh, why didn't I think of this. This is such a simple
Cause Engineer and Simple don't mix... DUH!
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Posted by KJPRO on May 19, 2008, 12:03 am
> However, being a bit of an engineer, I want to complicate this setup --
Go figure!
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