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Thread: tank or tankless?

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    DIY Junior Member jwcarpenter's Avatar
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    Default tank or tankless?

    I am in a partnership with 3 family members who jointly own a vacation cottage in Maine. We are about to do some badly needed renovations and are in a debate over tank vs tankless ELECTRIC water heaters. Space is at a premium but there are currently 3 showers plus a tub, and there can be as many as 11 people in the cottage at once. 3 partners want to eliminate one cramped downstairs shower and put new 80 gal water heater in the space, since we will be adding a new shower upstairs. One owner is emphatic we need to retain old shower bringing us to total of 4 showers and a tub, and wants us to go with a tankless electric water heater to save space. Concerns are the amount of hot water output from a tankless vs 80 gal rapid recovery tank, as well as cost to upgrade electrical to support tankless, plus the utility bills of operating tankless vs tank. Welcome thoughts and experience others have!

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    Test, Don't Guess! cacher_chick's Avatar
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    First thing to consider is that there is no such thing as an electric tankless unit that will keep up with the demand of multiple bathrooms, even moreso considering your location, where the incoming water will rarely if ever exceed 50 degrees.

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    Forum Admin, Expert Plumber Terry's Avatar
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    With electric, I would consider using a tempering valve on a tank, setting the tank on high, and blending down the water down to 120 at the taps.
    Done that way, an 80 gallon tank set to 180 puts out 119 gallons at 120

    Installing tankless with electric would require much more power than a tank heater.
    Even when I'm installing a gas tankless, the piping to the unit makes a difference too.
    Last edited by Terry; 11-22-2013 at 09:27 AM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Most people do not really understand how any tankless system works...but, it's pretty simple. Think of how much water needs to be heated if you're running say those four showers at the same time - a nominal 10gpm. One BTU will raise one pound of water one degree. Ten gallons is (for rounding figures) 80#. So, 80 BTU for each degree. Typical well water may start out at close to freezing there with a well deep in the winter but we'll say 40-degrees. The water will cool off some getting from the heater to the showers, so say it needs to be 110 coming out to get comfortable at the shower. So, that's 70-degrees * 80#, or 5600 BTU in one minute, or 336,000 in an hour. To convert from BTU to Kw, you get about 99KwHr. With a 220vac input, that's about 450A. This is all assuming you could find an electric tankless that big, it was 100% efficient, and you could get the power company to upgrade your service to that level (may be doubtful).

    Somebody should check my math...it's been awhile. But to get 10gpm of shower temp water 'instantly' takes a lot of energy! It's much more reasonable to build it up over time in a tank.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Just getting 10 gpm through a tankless heater of any kind would be challenge. Math aside, any whole house electric heater needs a lot of amps for a short time period.
    Licensed residential and commercial plumber

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    DIY Senior Member kcodyjr's Avatar
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    DIYer here. Just went through a similar analysis deciding what to do in my MH, albeit at a much smaller scale.

    That many showers, with Maine water temps, makes a single electric tankless a complete non-starter. You'd need several of them, so you might as well go with a point-of-use configuration. Each unit would have to be a smaller whole-house unit to provide enough temperature rise. You could maybe get away with 15kw units if you stuck with ultra low-flow shower heads, but I'd run each bathroom off its own 24kW unit in a series-hybrid setup with a tiny tiny (2 gal) tank on the sink. You will indeed need a *HUGE* electrical service. That's 100A actual peak draw on two 60A double-pole circuits, per bathroom, plus a 120V circuit for the under-counter minitank.

    If you're really preferring a whole house single-install tankless solution, you'll want gas, either natural or propane. It would have to be a fairly large unit at that; you might prefer two 200Kbtu/hr units, in series; the first heats water to 70-90, the second to the target temperature. If natural gas isn't available, you'd need a pretty big honkin propane tank.

    Four bathrooms, or even three, is to the point that some households consider independent *tanks*. I'm not sure 80 gallons really cuts it for 11 people; that's just 7 or so gallons each. I've read that temperatures above 120F cause accelerated aging in tank and tankless alike, so I'm not sure how fond I am of the tempering valve approach.
    Last edited by kcodyjr; 11-24-2013 at 07:23 PM. Reason: volts, amps, bah...

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    Forum Admin, Expert Plumber Terry's Avatar
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    I've read that temperatures above 120F cause accelerated aging in tank and tankless alike, so I'm not sure how fond I am of the tempering valve approach.
    Well...........you can always use two tanks. That's what many people do. More money then tempering a tank, but at least your two tanks will last longer?

    Water in Maine to get 10 gallons per minute would require two 199,000 btu has tankless heaters.
    Trying to get 10 gallons per minute with electric is going to be much harder than that.
    Putting point of use heaters at each location would be a large electrical and plumbing expense.

    Quite a few of the new and improved models "do" use tempering valves.

    When you're trying to get eleven people showers, and it's for occasional use, I don't think turning on the water heaters a few weeks a year is a lot of wear and tear.
    Last edited by Terry; 11-24-2013 at 08:51 PM.

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    DIY Senior Member kcodyjr's Avatar
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    Quote Originally Posted by Terry View Post
    I don't think turning on the water heaters a few weeks a year is a lot of wear and tear.
    Relies on human involvement to turn them off. In my field (I.T.) we avoid that wherever possible.

    From my layman's perspective, the human factor is an even bigger problem with plumbing than with computers. A rack full of servers isn't going to leak all over the floor.

    Speaking of leaks, that's a dimension that hasn't gotten much attention. If this place is going to be unattended much of the time, freeze protection is going to be an issue, right? I wouldn't feel too warm and fuzzy about turning the heaters off altogether, but a vacation mode would be a good middle ground...

    ... but I'd also be worried about unintended shut-off due to utility glitches, when nobody's home to notice and relight the pilot.

    We've all been talking about flow rates and BTU/hr and kilowatts, but maybe we should be focusing on idiot-proofing?

    EDIT: I've done a little more digging. Seems like the going recommendation is to run tanks at 140 anyway due to concerns about Legionnaire's disease. This doesn't negate the fact that higher temperatures accelerate corrosion, but some jurisdictions actually mandate this usage, so the proverbial bridge is crossed.

    I still think an 80 gallon tank is cutting it close, even if you figure it for an effective 104 gallons. I'd go bigger.

    Overall analysis stands. To make tankless electric work, you'll need to bring in huge service, 500A minimum, with the attendant wiring and plumbing costs.

    It comes down to, how bad do you want that 4th bath?
    Last edited by kcodyjr; 11-25-2013 at 01:45 PM. Reason: More research.

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