Hi, my house currently has a DHW coil sitting in a relatively OK oil
boiler (Burnham RSA 110). I put a Hobbs meter on the boiler, and find
that the thing is blowing somewhere around almost 3/4 gal of oil up the
chimney in standby losses per day -- that is just the standby usage with
no DHW or heating load, just to sit there. That is with the Aquastat
set to 125 for the low setpoint, which is about as cool as I want to get
it since I like my water to the washer, dishwasher, etc at least that hot.
So I am thinking about putting in a 50 gal high-efficiency electric hot
water heater to augment it, so that I can turn the boiler off when the
domestic heat is not needed, e.g. over the summer. I understand others
have done this, and have done quite alright saving money with a manual
switch-over, so that their DHW is electric during the warm months and
the oil boiler is "off" during the summer months.
However, being a bit of an engineer, I want to complicate this setup --
can anyone give me advice or gotchas on my plans?
- Electric HW heater is on full-time.
- DHW comes from either electric, or boiler, and is switched by a zone
valve, e.g. like a Taco 3-way.
- A second Aquastat / temp switch installed on the boiler flips the zone
valve to draw hot water from the boiler whenever the boiler temp is over
a certain setpoint, say 120F.
- A timer is rigged into the boiler Aquastat, so that the boiler is
normally off (full cold), but goes on to the lower setpoint (~130F) when
the timer is on -- this would be for anticipated high hot-water demand
days / times, I am thinking 2x / week when a lot of laundry is done.
The boiler is also on to the higher setpoint (~180F) whenever there is a
call for domestic heating, (of course).
My aim is to utilize the lower fuel costs of the oil boiler when the
thing is hot anyways due to a domestic heat call, or at certain limited
times of the week when it switches on in order to supply a heavy
hot-water demand (and economize the fuel cost). The aim is to keep the
boiler cool, and supply hot water from the electric tank, at all other
Kind of different, does anyone see any gotchas?
And two more questions along this thread, if anyone has any advice.
- Where can I get an idea of anticpated future electric costs -- I am in
CT, and pay $0.16 / KWH which is kind of high -- but the price has not
escalated to the rate that oil, gasoline, etc. have in the last 4-6 months.
- Where can I get some info on the standby heat loss from an electic hot
water heater -- I don't see much on the manuf websites.
Thanks much for any advice!!!
is a water heater comparision site, chock full of info.
the cost of oil hasnt been updated but electric still appears cheaper.
might well be cheaper than using the boiler oil at any time.
one needs to coinsider complexity and reliability,,,,,,,,,
while your idea may save a few bucks one malfunction where you home
doesnt heat might cost you far more:(
have you checked the efficency of new boilers? and upgrading
My opinion is to install a manual bypass system that allows you to use
either system independently, don't create a control and wiring nightmare,
use the oil burner during the winter and the electric during the summer and
drain it when not in use
KISS. Keep it simple sexy. As others suggested you don't want a maintenance
nightmare. I just spent 3 full days wiring up a heat and hot water system
with 14 pumps and 4 controllers and a few relays for a three floor
mcmansion. I pity the poor tech person who will be servicing the system in
a few years.
I am in NJ and my electric rate is at its peak during the day in the summer
so I would not want more electricity consumption during those peak months.
Maybe you have a fixed rate year round.
Is it possible to convert to an on demand high efficiency boiler with a
storage tank to supply your domestic hot water?
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
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
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
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
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
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....
Why not feed the hot water from the electric hot water
heater into the boiler. A drop in water temperature would
indicate a need for boiler heat and a rise in temperature
would shut the boiler down and supply hot water from
the electric water heater.
So in summer or at low load the electric hot water heater
would maintain the temperature and lock the boiler out.
At high load the electric hot water heater would be unable
to maintain the temperature and the boiler would start.
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