Tankless water heater - turn off ok?


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Old 09-03-08, 04:57 PM
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Tankless water heater - turn off ok?

Good day folks.

I currently have a Weil-McClain oil fired boiler, Carlin burner, with tankless domestic water heating.

System has served my purposes without problems for nearly 20 years. I leave it on 24/7 during winter to keep the house at 60, and use a wood stove when I need more heat in the living room.

In summer, I only turn on the boiler for hot water. As I now live alone, I just use hot water for a daily shower and dish washing. This means the boiler fires for about 10 minutes to heat the water, then shuts off. When it shuts off, I turn off the furnace power. That one firing gives me plenty of hot water for the day, and of course it refills with cold water as the hot water is used up during the day.

Today I had my annual boiler maintenance (cleaning and checking) by the oil company. Tech told me that it's not good to turn off the furnace. Says letting the furnace alternate between hot and cold is bad for just about every part in the system, and leads to corrosion and leaks.

Is this true? Should the furnace power be left on 24/7, even in summer when I only need a single "hot water" dose?

Or is it just a way to sell more oil?

Thanks.
Tom
 
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Old 09-03-08, 05:30 PM
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There is some truth in what you were told. I personally think that a tankless coil in the boiler is a terribly inefficient way to heat domestic water. If I were in your position I would seriously consider an electric water heater. Even with relatively high electricity rates it would probably be better than using your boiler for a short period of time once or twice a day.

Once you have an alternate source of hot water you can shut down the boiler during the entire summer.

[I know that many will disagree with that last statement.]
 
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Old 09-03-08, 06:02 PM
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Furd - thanks for the info.

I actually had considered an electric water heater, 'til I realized that I'd be heating a "whopping" 40 gallons, which would just sit most of the day, with the electric element cycling on/off night and day.

As summer use of my tankless (10 min/day) costs about 2 for electricity (burner motor) (at 18/kwh) and about 4 for oil (at $3.50/gal), seems I'm spending under 10 per day for hot water.

I don't really know how to calculate electricity use for hot water, but I can't imagine a 220v electric water heater keeping 40 gallons "hot" would be less expensive than my current "method".

But certainly open to hearing a way to compare the two!

Tom
 
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Old 09-03-08, 06:48 PM
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Do you like arithmetic? It takes one BTU (British Thermal Unit) to heat one pound of water one degree Fahrenheit. There are 3,414 BTUs in one kilowatt of electricity. Take the temperature of the hot water desired and subtract from that the temperature of the incoming cold water to determine the temperature rise. There are 8.3 pounds to a gallon of water. Multiply the number of gallons you use by 8.3 and then multiply that times the temperature rise to determine how many BTUs are needed to heat the water.

Assuming 40 degree incoming water and 120 degree outgoing water is an 80 degree temperature rise. That would require 664 BTUs for every gallon of water heated.

Assume you take a 5 minute shower and your shower flows at 2-1/2 gallons-per-minute (gpm) and you use straight hot water with no cold blended you would use 12-1/2 gallons of water. It would take 8,300 BTUs to heat the water for your shower. Divide that figure by 3414 (BTUs per kilowatt) and you get 2.43 kilowatts to heat that water. An electric water heater operates at about 95 to 98 percent efficiency so multiply by 1.05 (for 95%) and get 2.55 kilowatts. Multiply that by the rate per kilowatt ($0.18) and get 46 cents to heat the water.

Now let's do it for the oil. Firing the boiler just for domestic hot water you will be lucky to attain an overall efficiency of 60%. Since #2 fuel oil has an ultimate BTU rating of 142,000 BTUs per gallon we have to multiply that by 60% (0.60) and get a usable figure of 85,200 BTUs per gallon. To heat the water for your shower, assuming 100% efficiency in transferring the heat from the boiler water to the domestic water (which you won't get) would take 0.10 gallons of fuel oil. If the fuel oil cost is $4.00 a gallon then multiply 0.10 times $4.00 and get 40 cents to heat the water by the boiler. However, throwing in a "fudge factor" of 75% heat transfer from the boiler water to the domestic water mean that it would take an additional 25% of the 8,300 BTUs to heat the domestic water or 10,375 BTUs of fuel oil burnt. That would be 0.12 gallons of fuel oil and at a price of $4.00 per gallon comes to 48 cents to heat your shower water not counting the additional electricity to operate the burner.

So you see, even at the outrageous price of 18 cents per kilowatt hour it is likely still cheaper to heat your domestic water with electricity than using the tankless coil in the boiler.

Now for the issue of the water heater cycling throughout the day...modern electric water heaters have very efficient insulation and they lose maybe one or two degrees an hour under worst case scenarios when no water is being used. In other words, your electric water heater will likely use NO electricity between uses of the hot water.

Play with the numbers and see for yourself.
 
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Old 09-03-08, 06:59 PM
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Furd, don't forget to add the mass of the boiler into the equation.

Nutmeg, efficiencies aside, you may be losing far more BTUs out of the boiler / tankless than you'd have leak through the jacket of an electric tank.

Is there an option for Smart metering there? That's probably the ideal situation. Most utilities actually want some electrical load over night.
 
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Old 09-04-08, 03:58 AM
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Originally Posted by furd View Post
So you see, even at the outrageous price of 18 cents per kilowatt hour it is likely still cheaper to heat your domestic water with electricity than using the tankless coil in the boiler.
Ha, 18 cents/KWH, that'd be like a dream here (Guam). We pay just a hair over 30 cents/KWH. Hafa Adai
 

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Old 09-04-08, 04:07 AM
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This is *really* getting interesting. There are enough variables in the equation to satisfy Einstein ... or at least a government accountant! Believe me, I really appreciate you folks giving me those numbers - I never would have thought in the BTU terms. Being a frugal Yankee (Connecticut) I tend to think only in "dollars per use" terms.

In summer, my oil burner only runs 7 minutes to heat enough water for a full day's domestic use; regardless of winter or summer, the incoming well water is always very close to 55 degrees. That's why I figured it was only around 10 to heat my "daily supply". I'm just counting the amount of oil burned (using .8 gph) and electricity used (motor draw and transformer for spark).

But what really surprises me is how similar your calculation results are for tankless vs. electric water heater costs. 46 and 48

I had pretty much discounted an electric water heater, as I can now use my tankless coil during a power failure, with just my 4000 watt gas generator in the garage. Can't use that generator for an electric system! That's one reason I wanted to stick with my 120v tankless system.

I'm still a concerned that even if I do move to electric for hot water, I'd be spending more to keep that 40 gallons "hot" 24 hours, than running the oil burner for 10 minutes.

And of course, several boiler salesmen promote the "external" hot water storage tank systems, where the hot water is heated by the oil burner but not "stored" in a tankless coil.

This is fun!
Tom
 
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Old 09-04-08, 12:52 PM
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Furd, don't forget to add the mass of the boiler into the equation.
I did. That's why I used the 60% efficiency factor for the boiler water. Of course that figure is nothing more than a guess and it could be even worse but likely no better. Likewise the 75% efficiency figure I used for the transfer to the domestic water is just a guess and it could be higher or lower depending upon the condition of the tankless coil. I tried to use numbers that were fairly realistic yet still slanted in favor of the boiler/tankless coil option.

Ha, 18 cents/KWH, that'd be like a dream here (Guam). We pay just a hair over 30 cents/KWH.
I pay a bit over 8 cents per kilowatt hour on the winter rate schedule and my power company has one of the highest rates in Washington state thanks to the Enron fiasco. I guess our low electric rates kind of make up for our higher-than-average gasoline prices. Strange about the gasoline prices since we even have refineries in the state.

Nutmeg, if you can supply all your domestic hot water needs by only running your boiler for ten minutes (you wrote seven minutes) in the morning then I would continue doing just that. I couldn't possibly do that for several reasons that are unimportant to you. If your boiler stays hot enough to supply you with enough hot water to wash your dishes in the evening and your hands, face, neck and ears before you go to bed then you can't really ask for anything more. I would be darn sure to have the boiler thoroughly cleaned EVERY year after the heating season and a "quickie" cleaning at the beginning of the heating season.

As for keeping the 40 gallon water heat hot all day I can almost guarantee that if you do not draw any water from that tank it will not use any electricity in a 24 hour time frame. I think it was the day the President Clinton was inaugurated for his first term that we had a sever windstorm and I was without power for almost three days. I had an 80 gallon electric water heater and I took my shower and did all the regular things during that time (except cook meals since I had an electric range) and I still had hot water for my shower on the third day. And I tend to take long showers. And Who made an excellent point. If you have the option for off-peak electrical usage at a lower rate then you could install a time clock to limit the water heater operation to the off-peak hours.
 
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Old 09-04-08, 03:27 PM
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I run a 40 gall electric ... at 140F ... costs me about 35-40 a month ... spouse likes long hot showers, we wash whites in hot water, etc ... we're at almost 20 / kwh here ... and also on a private well so our water is also 55-58 ...

However, when we go on vacation, I shut off the well and water heater ... when I returned from a week long vaca, the water at the tap closest to the heater was still 105 ... and enough that I could shower ...

numbers for comparison to think about ...
 
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Old 09-04-08, 04:40 PM
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Smart metering is indeed available here (as of July 1, 2008). But it doesn't seem to save much at all, and the power company warns that the peak and off-peak rates can be changed by them with no notice.

With our dandy new de-regulated electric rates, I pay 18/kwh. I could switch to "variable" (clock hour) rates (I believe that's what you're calling Smart metering), where I would save a whopping 1.2/kwh on the power used between 8pm and noon.

Yikes - I just checked my latest electric bill. Silly me - I pay 19.2/kwh - I must have blinked.

Gentlemen - it really looks that I'd be better off staying with a tankless coil on the boiler. In summer I run the boiler only once a day, for 7-10 minutes to heat my "daily hot water supply". Seems that is very close to the same cost as that 40 gallon electric water heater.

I'm about to replace my 25 year old boiler, and will likely go for a Peerless WBV-3. Heating oil here is around $3.75 per gallon.

It might be nearly the same cost to heat water using electricity, but I'd have to run a new domestic line, purchase a heater, install it, and upgrade my residential electric service to 200 amps. I don't see a big advantage there, considering the purchase, upgrade and installation costs.

But again I'm probably missing something.

Tom
 
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Old 09-04-08, 06:11 PM
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Originally Posted by NutmegCT View Post
But again I'm probably missing something.
No you're not. Particularly under those circumstances.

When you replace the boiler, get an indirect tank and ditch the thankless coil.
 
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Old 09-04-08, 06:46 PM
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"When you replace the boiler, get an indirect tank and ditch the thankless coil."

I can get that Peerless WBV-3 system with tankless coil installed for $5000.

The quotes I got show an added $1500 (or so) for a Peerless Partner 40 gal indirect tank, plus another $500 labor. Sure seems it's gonna cost a small fortune to save that 50 a day!

Does that sound right?

Thanks.
Tom
 
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Old 09-04-08, 07:39 PM
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If you think that number ($0.50/day) is a realistic number; and sufficient to project into the future of your time in the house; and includes the cost of future service/cleaning/replacement of the coil over the lifetime of the boiler, then go for the coil.

IMHO, if this is a long-term investment, then treat it like one and figure out the true cost of ownership, so to speak.

In truth, I know very little about tankless coils, other than once heating season starts the number one question on this forum is "why isn't my tankless coil working right?" And that in most situations (but perhaps not yours), it's a very inefficient way to heat domestic water.

My impression is that a decent 40 gal indirect like the Partner can be had for about $1k. Labor might be another 5 hours, depending on circumstances of the installation. Maybe more, maybe less.
 
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Old 09-05-08, 03:49 PM
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It might be nearly the same cost to heat water using electricity, but I'd have to run a new domestic line, purchase a heater, install it, and upgrade my residential electric service to 200 amps.
You could install the electric water heater adjacent to the boiler and that way the additional plumbing would be minimal. And unless your existing electrical service is maxed out it is highly unlikely you would need to upgrade the service to 200 amperes. An electric water heater with 4500 watt elements requires a 30 ampere circuit but it only draws 18.75 amperes.

Here's what I suggest that you do. Determine exactly (more-or-less) how much hot water you may actually use in the course of your day. If it is all used at one time, i.e. you do a Navy shower and shave in the morning and then wash the dinner dishes from the night before along with your breakfast dishes and thereafter only wash your hands with cold water, you could probably get by with a much smaller water heater than a "normal" ( ) person would. Size the water heater with a gallon rating 30% larger than this figure because you can only draw about 70% of the volume from a tank-type heater before the water goes too cold. You might even be able to get away with a 20 gallon model, possibly even less if you set the temperature high enough and blend in a lot of cold water to a usable temperature. You could set this up on a time switch so that it only ran from about an hour before you get out of bed in the morning until you are done with the hot water usage.

I once had a co-worker that was so thrifty that he turn the water heater in his apartment OFF, replaced all the light bulbs with 40 watt ones and set the heating thermostats at 60. He claimed the heat never came on because he had a middle floor apartment and he received cast off heat from the units all around him. He bathed at work and likely ate his only hot meal of the day at work as well. He would cook up turkey legs he paid thirty-nine cents a pound for (in the 1990's) and he had a fit when the price went to forty-two cents a pound. I could NEVER live the way that he did but to each his (or her) own.
 
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Old 09-09-08, 02:09 PM
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I think your turkey-eating buddy even went farther than I do! Washing dishes in cold water is a bit of a stretch, even for me, the "Frugal Zealot".

Considering that:

1. I can run the boiler/tankless on 120v with my 4000w standby generator even during a power failure (New England blizzards are not rare!), and

2. I've *never* in nearly 20 years run out of (summertime) domestic hot water on a single boiler "cycle", and

3. Even an indirect tank served by a boiler would be trying to keep 40 gallons warm, and

4. Even the indirect tank would re-fill with cold water when I do my "daily shower and shave", so (I assume) would require the boiler to run again ...

Still seems I'm better off staying with my tankless. I *want* to accept the indirect tank idea, but if I draw off x gallons for that daily shower, it tops up with incoming cold water, which lowers the overall tank temp and requires re-heating.

My brain gets hung up on finding an advantage to the indirect tank, as it would need re-heating by the boiler after each use. With the tankless I have now, it certainly needs re-heating ... but it's a smaller amount, and can be done *without* immediate heat loss if I just run the boiler for 10 minutes before I need the water.

I do really appreciate the explanations. It's just that I can't see the advantage to the extra $2000 for an indirect tank, or even a less-expensive and smaller electric water heater. My addled brain keeps telling me the tankless is re-heating a *smaller* amount of water.

Tom (master of Analysis Paralysis)
 
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Old 09-10-08, 02:51 PM
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Oh, I could tell some stories about Hans, but I won't.

1. Keeping warm and having hot water during a power outage is not lost on me. I have a gas-fired water heater and after the last multi-day power outage I invested in a small genset to power my furnace, refrigerator, microwave oven and a few lights. You are correct that your 4kW gennie won't supply an electric water heater unless you get a really tiny one.

2. If it works for you, then that is all that matters.

3. The insulation on indirect water heaters is even better than what an electric water heater has. The standby losses on the indirect would not be so high as to require boiler operation in 24 hours of no hot water usage.

4. Quite honestly, I wouldn't be surprised that you could go two days without boiler operation and have hot water for your morning ablutions. In fact, if you ran the indirect at 140 degrees and either manually blended in cold water or had an automatic tempering valve at the outlet of the heater to automatically blend the temperature down to 120 max you might be able to go three or four days based upon your frugal usage.

Your tankless coil probably doesn't hold more than a gallon or two at the most. The mass of boiler water is what enables it to give you the amount of water you use at an acceptable temperature. Just as an electric water heater does not energize the elements for every time some little bit of hot water is used neither does an indirect heater require boiler operation every time the hot water faucet is opened.

As I stated before, you have a unique lifestyle that not a lot of men and almost no women, at least in this country, would willingly adopt. The short ten-minute burn cycles on your boiler are not conducive to either economy of operation or longevity. You are probably correct that given your lifestyle choices the tankless coil IS the least cost option.
 
 

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