Advice on Electric backup for Oil DHW -- custom setup.

[tman]

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 times.

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!!! T

[hallerb]

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 insulation?

[RBM]

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

[John Grabowski]

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?

[Pete C.]

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.

[hallerb]

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

[Paul M. Eldridge]

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:

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

[KJPRO]

Go figure!

[Telstra]

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.

[Mark]

ask the electric company about "off peak" metering

Mark

[tman]

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

[tman]

it really doesn't save much -- like 5% off your off-peak, in exchange for vastly higher on-peak... out here in CT..

[KJPRO]

Cause Engineer and Simple don't mix... DUH!