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Old 11-23-2022, 08:23 PM   #1
SailinAway
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Default Cost to get hot water from tankless coil in boiler

I had the tankless coil on the boiler disabled some years ago in order to install an electric water heater. Reason: at that time oil was expensive and electricity was cheap. Now both are expensive.

The 53-gallon electric water heater is now turned off to save electricity. I'd like to calculate the cost to get hot water from the tankless coil and compare this to other ways of getting hot water. How can I calculate this?

My average hot water need: about 5 gallons per day for one shower and washing dishes. About 15 minutes a day.
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Old 11-23-2022, 08:30 PM   #2
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The tough to calculate part of this is the boiler stays hot so that you can have your 5 gallons of hot water in a reasonable time. But I would guess that maybe a gallon a day to keep the boiler hot. 5 gallons of hot water is in the noise.
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Old 11-23-2022, 08:41 PM   #3
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The tough to calculate part of this is the boiler stays hot so that you can have your 5 gallons of hot water in a reasonable time. But I would guess that maybe a gallon a day to keep the boiler hot. 5 gallons of hot water is in the noise.
A gallon of OIL a day? What does "in the noise" mean?
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Old 11-23-2022, 08:57 PM   #4
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This calculator might help: https://www.efficiencymaine.com/at-h...st-comparison/

I entered values for 5 gallons a day, price of oil $5.80 a gallon, and cost of electricity $.1658 per kwH. The result was $74 a year for a tankless coil and $55 a year for the electric water heater.

That doesn't seem right. I think both figures are too low, otherwise I would not have turned off the hot water tank. The calculator doesn't include the size of the water tank. In any case, "oil boiler with tankless coil" is the most expensive on the bar chart.
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Old 11-24-2022, 07:28 AM   #5
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A ten minute shower uses 25 gallons of water assuming the shower head has a water restriction device built in. I think you are really lowballing your daily use…
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Old 11-24-2022, 08:25 AM   #6
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Originally Posted by SailinAway View Post
This calculator might help: https://www.efficiencymaine.com/at-h...st-comparison/

I entered values for 5 gallons a day, price of oil $5.80 a gallon, and cost of electricity $.1658 per kwH. The result was $74 a year for a tankless coil and $55 a year for the electric water heater.

That doesn't seem right. I think both figures are too low, otherwise I would not have turned off the hot water tank. The calculator doesn't include the size of the water tank. In any case, "oil boiler with tankless coil" is the most expensive on the bar chart.
I think heating water with oil is cheaper than using electricity. I looked at the link you provided, it looks pretty much like I'd expect. I think the default efficiency for oil list there is too low. I changed it to .85, unless you have a really old boiler yours is probably closer to 85% than 60 % too.

For 5 gallons a day, I got $65 for oil and $86 for electric.

The thing about using your boiler is that the boiler temperature needs to be kept hot enough to give hot water in a reasonable time. This uses additional oil versus what it would take to just heat the 5 gallons a day. I'm wondering if your link takes this into account. I actually think it may based on my original guess of 1 gallon a day.

"In the noise" means that if you could instantly heat 5 gallons of water with oil and not have to keep the boiler hot, it would probably take a gallon of oil a week rather than every day.

But in the end these are just estimates and the difference in cost negligible between oil and electric. It would take far too long to break even on the cost of changing over to justify it. I would just turn the electric heater back on and forget about it until the tank fails.
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Old 11-24-2022, 06:36 PM   #7
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A ten minute shower uses 25 gallons of water assuming the shower head has a water restriction device built in. I think you are really lowballing your daily use…
Holy moly, you use 25 gallons for a shower??? Perhaps this would interest you: https://www.amazon.com/Niagara-Conse...10&sr=8-5&th=1
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Old 11-24-2022, 07:02 PM   #8
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I think heating water with oil is cheaper than using electricity. I looked at the link you provided, it looks pretty much like I'd expect. I think the default efficiency for oil list there is too low. I changed it to .85, unless you have a really old boiler yours is probably closer to 85% than 60 % too.

For 5 gallons a day, I got $65 for oil and $86 for electric.

The thing about using your boiler is that the boiler temperature needs to be kept hot enough to give hot water in a reasonable time. This uses additional oil versus what it would take to just heat the 5 gallons a day. I'm wondering if your link takes this into account. I actually think it may based on my original guess of 1 gallon a day.

"In the noise" means that if you could instantly heat 5 gallons of water with oil and not have to keep the boiler hot, it would probably take a gallon of oil a week rather than every day.

But in the end these are just estimates and the difference in cost negligible between oil and electric. It would take far too long to break even on the cost of changing over to justify it. I would just turn the electric heater back on and forget about it until the tank fails.
ITD, thank you very much for taking a look at that link and running the numbers again. My boiler was measured at 83% efficiency last year. There would not be any cost to change between oil and electric. I have both the tankless coil and a water heater installed. The aquastat on the coil is disabled.

I reread all the replies in the other thread about saving money on heating water and did some more research. I'm leaning toward putting a timer on the water heater and running it 1 hour a day. I'm thinking this might be the simplest and cheapest solution. The water heater is 4000 watts, or 4kWh per hour = 120 kWh per month x 17 cents per kWh = $20 a month.

Thoughts about that idea?

An on-demand system installed in the upstairs bathroom makes the most sense in terms of conservation of water and electricity and shortest distance to the shower, but it's the most expensive and least practical solution, involving opening the wall.
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Old 11-24-2022, 07:33 PM   #9
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Quote:
Originally Posted by SailinAway View Post
ITD, thank you very much for taking a look at that link and running the numbers again. My boiler was measured at 83% efficiency last year. There would not be any cost to change between oil and electric. I have both the tankless coil and a water heater installed. The aquastat on the coil is disabled.

I reread all the replies in the other thread about saving money on heating water and did some more research. I'm leaning toward putting a timer on the water heater and running it 1 hour a day. I'm thinking this might be the simplest and cheapest solution. The water heater is 4000 watts, or 4kWh per hour = 120 kWh per month x 17 cents per kWh = $20 a month.

Thoughts about that idea?

An on-demand system installed in the upstairs bathroom makes the most sense in terms of conservation of water and electricity and shortest distance to the shower, but it's the most expensive and least practical solution, involving opening the wall.
$.17 is only the cost of electricity—don't forget to add delivery charges.

I'd wrap your electric water heater and just keep it on. Given how little water you're using, I just don't see it making much difference turning it on for an hour to bring it up to temp from whatever it falls to the other 23 hours and keeping it at one temp—essentially, like the equation of keeping a house at one temp vs. dropping it and having to catch up later. The only difference is that a well-insulated water heater will lose very little heat.

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Old 11-24-2022, 07:41 PM   #10
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Originally Posted by SailinAway View Post
I had the tankless coil on the boiler disabled some years ago in order to install an electric water heater. Reason: at that time oil was expensive and electricity was cheap. Now both are expensive.

The 53-gallon electric water heater is now turned off to save electricity. I'd like to calculate the cost to get hot water from the tankless coil and compare this to other ways of getting hot water. How can I calculate this?

My average hot water need: about 5 gallons per day for one shower and washing dishes. About 15 minutes a day.
You could always boil a few pots of water on your new wood stove and take a bath. That wouldn't cost anything extra!
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Old 11-25-2022, 07:34 AM   #11
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Originally Posted by thinkxingu View Post
$.17 is only the cost of electricity—don't forget to add delivery charges.

I'd wrap your electric water heater and just keep it on. Given how little water you're using, I just don't see it making much difference turning it on for an hour to bring it up to temp from whatever it falls to the other 23 hours and keeping it at one temp—essentially, like the equation of keeping a house at one temp vs. dropping it and having to catch up later. The only difference is that a well-insulated water heater will lose very little heat.

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The heat loss in the winter time will most likely be toward the space heating of the house anyways.
The benefit of the tank would be that a smaller unit can be easily plumbed into the space as all the connections already exist.

Roughly a gallon per day for a tankless *coil* oil boiler to heat water for domestic use is about right. Being a one person household and restricted use... she may get down to about 300 gallons annually with the average efficiency in the boiler.

The wood heat... if longer term is going to be used, the electric with either a smaller tank... or a federally-subsidized hybrid would make the better choice.

Capital expenditure up front will need time for either of those to see a pay off.
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Old 11-25-2022, 08:07 AM   #12
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Quote:
Originally Posted by thinkxingu View Post
$.17 is only the cost of electricity—don't forget to add delivery charges.

I'd wrap your electric water heater and just keep it on. Given how little water you're using, I just don't see it making much difference turning it on for an hour to bring it up to temp from whatever it falls to the other 23 hours and keeping it at one temp—essentially, like the equation of keeping a house at one temp vs. dropping it and having to catch up later. The only difference is that a well-insulated water heater will lose very little heat.

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You know what? You are probably correct. 2 years ago $0.17 per kwh would have been an expensive delivered cost of power, at least where I was buying electricity. Now it is probably just the cost of generation. Sailing Away, make sure you are using the delivered price for electricity.
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Old 11-25-2022, 08:39 AM   #13
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Quote:
Originally Posted by thinkxingu View Post
$.17 is only the cost of electricity—don't forget to add delivery charges.

I'd wrap your electric water heater and just keep it on. Given how little water you're using, I just don't see it making much difference turning it on for an hour to bring it up to temp from whatever it falls to the other 23 hours and keeping it at one temp—essentially, like the equation of keeping a house at one temp vs. dropping it and having to catch up later. The only difference is that a well-insulated water heater will lose very little heat.
Yes, I agree about wrapping the tank. That's on my list also. I will need to compare leaving it on 24 hours and 1 hour. My last electric bill was just $55. I'm sure turning off the water heater is a large part of that low bill.
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Old 11-25-2022, 03:38 PM   #14
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Most of the energy is used increasing the temperature from the well water to the tank aquastat set level.
So the largest cost will be heating the initial 53 gallons...

After that roughly only enough electricity will be used to replace the heat lost to the space around the tank...
Except when you use the hot water... then it will have to heat the replacement water.
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