Comparison of DHW Oil Indirect vs Electric Tank

Now, I've been thinking about this for the last couple of days and I might have had a breakthrough is proving to myself that an electric heater is cheaper to run, at least when that's the only thing that's being run (either summer, or maybe winter too if the stove is running and heating the house for the most part).

The DOE test procedure requires, in part, heating 63.4 gallons of water to 135F, and the test takes place over 24 hours.
I'm pretty sure, again, based on memory and some readings I took off my water softener, that my entire inside useage is 40 gallons per day. I figured 30 gallons of that was hot water. I was able to use that number to use the annual kwh on the Energy Guide yellow tag, which, typically seems to be 4773 for a 40 gallon tank. Electric cost was assumed $.15/kWh.

The oil consumption I got from looking at some past bills, but because of the periods and a different owner, maybe not that confident..could be lower. AVC8130's might be better, or it could be something in the middle - I put all three in the attached spreadsheet.

I couldn't believe it when those three annual oil use scenarios came out to $1037, $526, and $701, respectively, and the electric scenario came out to $334! It blew my mind. This is with 15 cent electric. Resistance, not heat pump. And, a $240 water heater.

Please tell me if I'm wrong!

I'll get working on firming up my oil and water consumption numbers, but I think they're in the ballpark.

(Also, Moderators, not sure if this is the right forum to post this, but there have been other similar threads here of late. )

I haven't put the oil on a timer yet. If my water consumption is right, I'd have to get down to .24 gallons/day of oil to break even with oil. Seems low, but maybe it's possible. Need an hourmeter.

If it works out, or it's close, it would be nice to get off the oil.

I just checked the hr meter on my electric water heater. In 69 days, 17 hr on time.
That comes out to just over $5/mo.

I was using about $100/ mo with oil.

But, as I recall, you have a solar booster of some sort, and it is summer, but, an 80 gallon tank.
Too bad, for me , you have solar, or it'd be more of an apples to apples things.

The actual number for oil used is also close to mine. Not sure what setup you have/had.

Dick Hill runs timers on everything and collects the data every Sunday night.
He found that his low mass, cold start oil boiler with a separate Amtrol tank for DHW was operating at 33% efficiency during the summer.
This is in a house with tenants, so the usage was higher, which would, in theory be better for efficiency.

I would expect a tankless coil boiler to be way under that in the summer.

Funny this is coming up now.
I have been considering trying a timer in my electric hot water heater as the only times hot water is needed is morning and night. I know its heating during the day just to keep the temp up but would it cost more only heating 2 times daily as opposed to all day? Would a modern tank lose enough heat while off to make a timer ineffective?

Wow, 33%! That's lower than I thought. I have a similar setup with BoilerMate and cold start Burnham v8 boiler (not sure if that's considered low mass, because it seems to hold 20 gallons or so.

We switched our DHW from using oil (1 gall per day, $110 / month) to a Geyser HPWH ($15 a month average).

We also use the HPWH as dehumidifier/ air conditioning in the summer and use a timer to run the HPWH at times convenient to us. In the winter, our basement stays above 55 deg F and so we can use the HPWH all year round. Run times do extend in the winter but I estimate we are still saving.
Kept the oil system intact (as its paid for !) as backup for use when we get a full house but otherwise it sits idle.

Cannot figure how oil DWH can compare to electric DHW in the summer..

Did you put the new hw heater in series downstream from the indirect.

Our basement gets into the 40's as I recall, and I insulated the walls (but not the floor). With the insert, my aim is not to run the boiler at all, but my wife has something to say about that, so it does get run. It still can get pretty cold in the basement.

I don't know how much the oil usage could go down with a timer, but 33% is quite the shocker. I was thinking 40% w/o timer, based on another thread.

Yes I have 1( soon to be 2) 4'x 8' solar collectors on the roof preheating the water. But even without the solar I estimated $20-$25/ per mo for the electric water heater. Still cheaper than oil.

I don't think a timer would save much unless you can take advantage of off peak rates.
http://www.leaningpinesoftware.com/hot_water_heater_vacation.shtml
This site makes a good case that you would not save much.

The standby loss in a quality water heater is not much. Better to just add extra insulation IMO.

Trust me, I pooped a brick when I thought of the concept of 1 gallon/day. Then I had a sigh of relief when I found out it was closer to 3/8. Then I did the math again and pooped.

Oil is NOT worth it for heating water any more.

ac

I'm glad someone said it!

Remember that some oil boilers in humid basements don't do well for a cold summer, some don't bother at all. If your going to be gone for a weekend, try turning the whole thing off and running the hot water out. Be shore to close the mackup water valve. I've seen boilers leak between sections when allowed to cool down. If all seems well, put in an electric, valve off, and drain the indirect. I've put in many electric WH so people don't hear that oil burner running in the summer. With no flue to draft heat out, most EWH have a 94% efficiency only 6% jacket loss which can be reduced more with a fiberglass blanket. I like to put a brass swing check on the hot outlet installed right off the tank's nipple to block any convection through the hot line. Those little rubber heat traps that come on the tank don't last more than a year IMO.

TS

I know, just for hot water. I can drive the 44 miles roundtrip to work in my Mini for a gallon. Wouldn't make any impression at work without washing up though.

Why wouldn't you keep the indirect hooked up to act sort of like a buffer tank? Of course it would be insulated, and not pick up ambient heat that well.

I am in the process of installing a Geospring water heater which uses about 1/3 the electricity to use in the summer if I do not feel like firing up my wood boiler w/ storage. Best part is that Ngrid is giving me a rebate for the cost of the unit itself.

Heat pump water heaters are great, especially if you already de-humidify. I'm just not sold on the longevity of them as of now...... you could duct that cooler air into the living space and pull warmer air from outside if the basement is cool and dry. Two fold A/C and cheap hot water.

TS

The equation changes with a wood boiler in the mix.

Wow, makes me really wonder about the indirect I have in my basement. I have a 40 gallon electric hot water heater sitting in my barn that was working when I pulled it out a few year ago. The thing I don't know about is the best way to hook the two up so I could easily switch back and forth.

33%, man that's bad.

K

Yeah, the 1/3 number is common but people just don't want to believe it, although there are still some tankless coil guys out there that i'm gonna guess are at 1/4 or less. Even with a cold start boiler, you have to heat up the CI, the boiler water to say 140 and now circulate that 140 water through a coil in cooler DHW. Tank up to temp, turn it all off and let the CI boiler, and it's 20 or so gallons of water cool and draft all that heat up the chimney. It's just bad all the way around, burning oil in the summer in a house that you want to be cool, seems kinda sideways. Although I do it, just not with my indirect, instantanious on a switch near the shower. Turn on, wait 20 seconds and you have endless hot water. Cheap guy like me: turn off during shower (low voltage) and shower till water rund cold, less than three minutes. Easy, and I have my wife trained and she puts up with me still......

TS

How much $ do you have in your solar setup? Is it a drainback? Does it still help in winter? I'm reading this thread thinking my 80 gal electric is probably not too bad but I know we use a lot of hot water. I want to build a sidearm that uses the 6-7 small tubes inside the bigger shell for the winter months when the wood boiler runs. I was hoping I could use the same homemade sidearm with a solar setup during the summer.

The thought of resistance electric sometimes is perceptually not cool. To repeat some prior posts of mine, our resistance electric bill is about $4.00/mo, or $50/yr. Wife and I, occasional guests mostly during the summer. We don't consciously minimize our hot water usage, although we by habit don't waste much of anything. That electric bill was about $8 month until we super-insulated our hot water heater, added insulation to hot water pipes, and added U-shaped heat traps on the hot water heater. Cost of all of that "extra" was about $50.

I haven't seriously crunched my numbers, but have come to the same conclusion: get the heck off the oil.

Even with a cold start boiler & indirect tank, electric is cheaper.

For 15 years I had a coil in a combo boiler that stayed warm all year, and estimate that I was using about 150 gallons of oil per year solely for DHW in the off-heating season. That was keeping the boiler set as low as possible, and not using that much DHW (showers are cooler in the summer), and lighting the odd wood/waste paper fire to heat the boiler now & then. I turned that boiler off around the first of August, after getting my new 80 gallon electric tank in, hooked up, and on line. Only had one power bill since then, to August 23. I think it was only up around $10 from previous months. The next bill should give a better picture. Seems pretty obvious if you think about no chimney for heat to go up, no pipes in between for heat to radiate off of, all electricity used is going right into heating the water, way better insulation - likely more factors too.

Then add in the thoughts of getting rid of the oil tank & associated liabilities & space gained in doing so....

You'd still have to have some sort of backup heat.

An update: I found some old notes and found that I use about 44 gallons/day of softened (inside house) water. Might affect numbers some, but I still don't know how much of that would be hot water (I assumed 30).

With a "hot" oil boiler with the aquastat turned down, my oil consumption dropped about 1 gallon per day for about 5 summer months. When I installed my solar hot water system and shut down the boiler for the same five months, I obviously didnt use any oil. I expect the standby loss of the boiler was far more than the hot water usage as I dotn use much hot water.

These days instead of installing solar hot water, I would install a Heat pump hot water heater and a couple of extra panels of solar electric panels. When I put in the SHW, solar electric panels were 6.50 per watt, my last batch I bought were 93 cents a watt.

Two year old electric boiler c/w circulator, found for $400 on Kijiji. My oil days are almost completely over. (I'll wait until I get this winter in before pulling the oil tank out - but the fill pipes will be disabled this coming weekend).

YMMV, of course.