New tankless boiler coil question

I just recently installed a new 117,000 btu output utica boiler that had a tankless coil. I went with the unit that had the tank less coil because my boiler broke on a 0 degree day and that was all my supplier had in stock that was the correct size. so now i have the boiler installed i have been listening to how often it runs and it seems to run for three minutes every 20 minutes with the high limit set at 180 and low limit set at 120 with the diff at 25. my boiler has a max g.p.h. of 1.00. i ran some quick numbers and came up with 3.6 gallons of fuel oil a day. At 3.60 a gallon i come up with 12.96 a day. I don't know if my math is wrong but that is incredibly expensive if i'm right. i had electric hot water before and my total electric bill was around 110 dollars a month. My electric hot water tank is shut off but still hooked up. It would just be a matter of disabling the low limit on aquastat and closing the valve going into the coil to switch back to electric hot water. i was just looking for some advice on what would be a cheaper way to make hot water. I was also wondering if adding an indirect would be even cheaper than electric. I live in upstate ny and generally have the heat on 7 months out of the year. thanks for your help

travis

During the heating season the average temp of the boiler is going to be higher anyway, so not all of the $12.96/day is rightly applied to the hot water bill- it's a fraction thereof. (but during the summer it sure is.)

Either look it up to see if it's in the manual, or call Utica to find out what the operating low-limit is on the boiler, as well as whether it's a model that would be damaged by cold-starting it, and explain that you want to use it at the highest efficiency without using the HW heating coil. Most gas fired boilers have no issues at all with cold starts, but with oil-fired boilers it's model-specific. If it's not a cold-start type, backing it off to the lowest low-limit that doesn't violate the warranty would result in the highest efficiency. If that "legal" temp is below 140F and you have a tile-lined flue rather than a stainless-steel liner you may have to bump it up to avoid corrosive flue condensation though. (Your current as-operated low-limit setting of 120F is too low for a terra cotta liner.)

An indirect may or may not be cheaper than running with electric- it depends on your per-kwh electricity rates and your anticipated costs of oil. An indirect on an oil-fired boiler typically runs at 50-60% net efficiency in "hot water only" mode (with big error bars, depending on volume of use, etc. see this document), and no space heating calls, so during the summer out of the 138,000BTU/gallon figure you get something like (0.6 x 138,000= ) 83,000BTU into the water, best case. At $3.60 gallon every that's (360/83=) 4.3 cents per 1000BTU.

At standard EF test volume usage an electric tank runs ~90% efficiency. Every kwh is good for 3412BTU at 100% efficiency, or (0.9 x 3412=) 3071BTU at 90% efficiency. So if your electricity is something like the national average of 12 cents/kwh, the cost of heating water in the summer is (12/3.071=) 3.9 cents/kwh.

During the heating season it'll be cheaper to run the tankelss coil, especially if the boiler is inside of conditioned space (like an insulated basement), since the standby loss accrues to the space, lowering the space heating load, but during the summer it's going to be cheaper to run the electric tank. Some people plumb an electric tank in series with the output of a tankless coil so they can just turn off the boiler and not have to switch on the electric tank. With a tempering valve limiting the output temp of the electric tank and setting the tank to 120F it'll have hotter than 120F water entering the tank when the boiler is hot, but won't drop out when the boiler is cold.

[edited to add]

If the boiler is cold-start tolerant, setting up the boiler to only fire when there is a call for heat from space heating zones would be the most efficient, since that would lower the average temperature, which lowers the standby loss. "Smart" economizer controls that "learn" the system behavior to anticipate when the end of calls for heat that cut the burners early to let the boiler's heat be purged into the zone, thus lowering standby temp.loss are built into many newer boilers, but are also available as retrofit form, bypassing some of the aquastat controls (eg Intellicon 3250 HW+). These controls also purge heat at the beginning of a call for heat, by waiting until the boiler drops to the programmed low-limit temperature before firing the burner. With this type of control the boiler room is no longer the warmest place in the house since there's less standby heating up the boiler room, more of it being sent into the zone that was calling for heat at both the beginning and end of the call.