Should I use my oil or electric water heater when burning wood

NoGoodAtScreenNames

Burning Hunk
Sep 16, 2015
222
Massachusetts
Hi Everyone. I have a somewhat unique set up in my house. I have an oil burner as my primary heat (baseboard water) which also heats my hot water with a separate reserve tank. Nothing too crazy about that. However, I also have an electric water heater. The prior owner installed it because he didn't want to burn oil in the summer. So you can switch between which system you want to run at any point.

The theory is that oil is more efficient if you are already burning oil to heat the house but not when using the boiler just to heat the water. The oil burner is not high efficiency - probably around 15 years old.

The old owner didn't burn wood for heat. I'll burn on nights and weekends. Basically if we are home we burn.

If the stove prevents the oil heat from coming on or coming on as much then I think it's like running the oil in the summer for the hot water.

I'm curious what you think would be a good way to think about which is better. I'd like to save money but also interested in the option that's better for the environment. How would you try to figure out which way to go?
 

legrandice

Burning Hunk
Oct 5, 2006
215
South Hadley, MA
I have the same setup at my house. I installed a heat pump electric water heater a few years ago. The difference for me is that I heat my basement office with the oil boiler so it is on for a while during the winter. I used electric for the hot water last winter, but I think I will use oil this year as it's pretty cheap. The heat pump also made my basement pretty cold, which I then had to burn oil to heat back up.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,150
SE PA
Ok. This is simple. The oil will be cheaper to run than the electric for HW, this season. Figure it saves you $20-30/month in HW.

Then there is standby losses, which might be 2-3x larger than the HW usage, if you never run oil for space heat. The losses in the winter will offset your wood consumption. Each gallon of oil you burn is worth probably between 100,000 and 110,000 BTUs (with the rest lost up the stack). IF you figure your wood burning gets you 5000 BTU/lb, then each gallon you burn in standby is like 20-22 pounds of wood. Like a small load, or half a large load. If you burn a gallon per day, costing you, what $2/day, it will offset that much burning...like one small burn per day in the shoulder season, half a load per day in the winter.

Combine the two numbers, and you are prob net going to cost a $1 more per day, but will save ~2000 lbs of wood per year.

Which would you rather? have $100 or not burn another 2000 lbs of wood?
 

Circus

Feeling the Heat
Jan 11, 2013
280
EC Wisconsin
Heating or not heating, whenever the boiler is hot, waste heat is going up the flue. You might as well tap off some hot water. Occasional wood burning isn't a factor. Use the electric water heater when home heat is not needed and the boiler can be shut down. Yes, using a boiler just to heat water is wasteful.
 

maple1

Minister of Fire
Sep 15, 2011
9,879
Nova Scotia
Sounds like you don't have a cold start boiler (15 years old) - I think I would run the electric all the time.

But it might come down to exactly how much of the time your boiler will be running to heat the house - sounds like not much with the stove going most of the time.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,150
SE PA
I agree with the above ^^^^ when oil is $4/gallon. At $2, you can make the case for oil, as much as I hate to admit it.
 

maple1

Minister of Fire
Sep 15, 2011
9,879
Nova Scotia
I can't make the case for oil under any circumstances, with my past experience.

Right now we are heating our DHW for $20-25/mo, with $0.18/kwh electricity.

Back when we did it with oil in the summer, it was taking 3/4 US gallon per day. That was with a tankless coil so likely almost a worst case scenario. But even with oil at a very cheap (for here) $0.80/l cost, and our current electric cost, using anything more than 1 litre of oil per day would make oil more expensive than electricity. And I don't think there is any oil boiler setup that could heat our DHW with 1 litre of oil per day, especially one that isn't cold-start.
 

velvetfoot

Minister of Fire
Dec 5, 2005
9,973
Sand Lake, NY
Then, there's the wear and tear on the boiler and the necessary annual maintenance charges.
 

NoGoodAtScreenNames

Burning Hunk
Sep 16, 2015
222
Massachusetts
Thanks everyone. I knew I was asking a question that wouldn't have a clear answer and just wanted to hear some opinions on how to think of it. Now I know better how to think of it and it's clear as mud :confused:. It's hard for a homeowner to make fine tuned efficiency decisions since there's so many variables we can't measure ourselves.

I looked at the yellow efficiency cards on each appliance. My oil burner is 83% and both the fully electric dhw and the electric reserve tank tied to the oil are about the same kW/hr. I assume the rating on the reserve tank assumes it's only running electric or else it wouldn't seem to ever make sense running it???

My current thought is that $ wise there's probably not much difference especially with low oil prices. But all things being equal I'd rather get my btus first from tree companies and Forrest managers removing dangerous and diseased trees, second the electric company because in theory there are some renewables in the mix and lastly the oil companies.

So I'll probably run the oil option in the coldest 3 months when my "occasional"* wood burning can't prevent the oil heat from turning on. But keep the electric going in the shoulder season.

Thanks again.

* only on this forum would burning wood 100 hours a week count as occasional.
 
  • Like
Reactions: woodgeek

NoGoodAtScreenNames

Burning Hunk
Sep 16, 2015
222
Massachusetts
So the past few years I’ve been running the oil from the start of January until sometime in March. After tracing the plumbing a little better than when I first moved in, I realized that I have a direct heater that dumps into a tank as opposed to an indirect system that steals some heat from the boiler. By this I mean every time I use hot water, the same volume of cold water goes through the boiler then into the tank. Anyway, that’s not the reason for my post...

The electric recently sprung a leak. I was happy to have a parallel system that I could cut over to instantly while I figure out what do with the electric. I could have decided that fate has made the decision that I will burn oil for the water heater.

However, I’ve decided to replace it with a Rheem hybrid heater (heat pump and / or resistance options). Going by the yellow sticker it will cost about $10 a month to run which is about as low as I think I can expect. I’m going to try to run it through this winter and see. My basement is pretty big and stays warm in the winter so plenty of standby losses from the boiler to absorb from the air.

For now I’m planning to leave the oil isolated and off full time. If it works out I may just have it disconnected. I don’t love the idea of having a tank with uncirculated water in it for long periods / forever. But that’s what I’ve got now anyways.

Another option would be to use the oil as a pre heat into the new tank, probably tempered with cold water. But that will be something to think about for next year after I see the performance in the winter. I’ll be able to monitor the energy usage directly using an app so it should be a fun new toy.


Sent from my iPhone using Tapatalk
 

woodgeek

Minister of Fire
Jan 27, 2008
4,150
SE PA
With hybrids, it is easy/cheap to heat cold water, and takes progressively more electrical energy per BTU added as the water heats up. So using an oil as a pre-heater or tempering tank is not a good idea.

Just run them separate.