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Thread: Tankless in a cabin?

  1. #1
    DIY Junior Member Hermosa's Avatar
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    Default Tankless in a cabin?

    Hi,
    New here, been reading a lot, good info. I'm thinking of replacing the water heater in our cabin with a tankless unit. We are running propane, and we only use the cabin one weekend a month, so I don't want to be heating water 24/7 and using expensive propane if I don't need to. The climate is lake tahoe, so we have some cold weather, the water heater closet is accessable from a door on the outside of the cabin, vented from the outside also. The water heater is 20' from the shower, its a 1 bathroom place 700sf, really small.

    I would like to replace the current tank heater with a tankless in the same spot. I'm just worried it won't be able to heat the water nice hand hot given the incoming water is probably 30 degrees or cooler in the winter. I assume the shower load is 3.4gpm or less.

    This would save me a ton of money on propane. What do you guys think, am I looking at this correctly? any help appreciated! A tankless solution has really caught my attention for the cabin.
    Last edited by Hermosa; 09-19-2010 at 06:34 PM.

  2. #2
    In the Trades Gary Swart's Avatar
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    You need a tremendous amount of power, either gas or electricity to operate a tankless. In addition, they are extremely expensive to buy and maintain. I think the ton of money you think you will save on propane will be more than eaten up with the purchase, upkeep, and operation costs of a tankless. I'd consider turning the propane off and draining the tank at the end of the weekend of use.

  3. #3
    DIY Senior Member zl700's Avatar
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    LP tankless makes even more sense to you, but not so much to save on LP, if you turn the existing tank off due to non use when leaving. If you leave the tank on you use a bit on pilot (if one) and to fire to maintain temp.

    Problem with turning tank off is the water cools, condensation starts and the tank rots out prematurely. With a tankless, all stops and no more worries, in addition it is really easy to drain the 3/4 to 1 gallon from it stopping any condensation while idle and gone, along with any energy use caused by freeze protection kicking, not to mention eliminating any freeze concerns.
    If Payback is so important to you, why are you not driving a Toyota Corolla?

  4. #4
    In the trades Dana's Avatar
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    For a cabin off-grid going with something like the Bosch 1600H that doesn't require electricity for ignition, nor a standing pilot is about right. It pulls a respectable 0.80 in an EF test, and can be vented with cheap B-vent (unlike their only slightly more efficiency 0.82EF+ bretheren). It's enough burner to run a full-flow shower even in coldwater country, but would be a bit underpowered for a 2 bathroom + laundry situation (some cabin!) Since it requires no electricity and uses cheap vent pipe, it'll be a much cheaper to install than bigger-deal units.

    The standing-pilot versions of mid-efficiency tankless wouldn't be a disaster either. Just turn off the propane when you leave, re-light when you return.

    Just be sure the propane regulator & fuel plumbing can handle the full BTU rating of whatever you install- even the 1600H is 3-4x the burner of a typical tank heater, and more than many heating systems.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, I doubt your incoming water is 30-degrees, but that's an aside! Low 30's, very common. In the fine print, the output temp or volume will start to go down as the inlet water temperature goes down. Most can do about 70-degree rise at the stated (big print) spec, and it drops almost in a line either in temperature or volume as the inlet temp drops. You might end up with a lukewarm shower on a really cold day, depending on the head, distance, and insulation. So, with near freezing water coming in, the volume you may have could be barely adequate for a shower, depending on the unit you select. There are some that would have no problems, but they tend to cost more.

    Do you maintain any heat in the cabin when you leave? Any water left in the tank OR tankless unit, if it is turned off with no other heat sources, is subject to freezing. If you drain the lines, and turn off the power, you have a chance. On the tankless units that are designed for freeze protection, depending on the space, they may not freeze internally, but there may not be enough heat to keep the pipes nearby from freezing, depends on the construction of the area and how cold and drafty it is.
    Jim DeBruycker
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  6. #6
    In the trades Dana's Avatar
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    To take a standard 2.5gpm shower with 32F incoming water and a fairly scalding 112F showering temp takes 100KBTU of output. A 117K burner with an 82-84% raw combustion efficiency (such as the Bosch 1600 series) can deliver that shower. (I lived with a similar sized E.L.M. with ~40F late-winter incoming water for over a decade- it kept up with a 3gpm flow, but not 3.5.)

    Most 2.5gpm shower heads deliver around 2gpm unless the water pressure is unusually high.

    Which is another thing to consider with any tankless: If your water pressure drops below 20psi it may lose efficiency due to insulating micro-bubbles on the water side of the heat exchanger. It'll even sizzle & bang a bit audibly if it goes as low as 10-12psi. Be sure the water system pressure is up to snuff before installing any propane-fired tankless.

    Freeze up of the heat exchanger can happen in very-cold climates even if the place is heated if the flue vent is short or side-vented. Draining the thing is probably a good idea if you're planning to leave it unattended during bitter cold weather.

  7. #7
    DIY Junior Member Hermosa's Avatar
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    All good points, thank you for the replies!! I want to gather some more data before i decide what to do. I may look at an internal spot for it also. Once I do some more homework, i'll chime back in with some questions.

    thanks again!!

    Hermosa

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The one guy responded he could go one volume, but not the other with 40-degree incoming water...drop that water temp down another 8-degrees, to just over freezing, and you'll get a similar, but smaller volume. Most of the things are designed for 50-degree incoming water. ANything below that, and you need to read the fine print carefully to understand what you can get out of it at your desired temperature and volume. So, one rated in big print for 5gpm, might only be 2gpm at just above freezing. Some restrict the flow (less water, same heat, same temp), or just let the temp drop (same water, same heat, cooler output). Then, your water may be quite hard, and if you don't service the thing periodically to remove the mineral deposits, it's the equivalent of the unit gradually getting smaller. While adding the valves and peripheral equipment needed to flush the system with a mild acid isn't hard, it does take time and prior planning (and more costs).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    In the trades Dana's Avatar
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    It's pretty easy math: 2 gpm is about 1000lbs of water per hour, and 1kbtu is 1000lbs x 1 degree F. In other words, for every 2gpm times degree F rise you need 1KBTUs of output from the tankless.

    Combustion efficiency on any non-condensing tankless is between 80-86%, so figure on getting at least 80% of the fuel-input BTU specs into the water. A 117KBTU burner will deliver at least ~94K out at full flame. A smaller 100K burner will deliver at least 80K out.

    If you have 32F incoming water (unlikely, unless you're literally taking water directly from the outflow of a glacier right where face is calving) and run the shower at a typical 105F, that 's a 73F rise in temp.

    A 117K burner (like the Bosch 1600 series) will support at least 2*(94/73)=2.6 gpm of flow at those temps, about the specified flow of a 2.5gpm showerhead.

    A 100K burner (like the Bosch 1000 series or those inexpensive Marey units sans safety agency approval) will support only 2*(80/73)=2.2 gpm, which is still more than at the true typical flow of a 2.5gpm showerhead.

    Then figure your real input water temp will likely be at least 3-5F higher than that (measure it.) It doesn't take anything like a 150-200k burner to run a cabin shower unless you insist on a real gusher of a shower head, or truly scalding 115F showers, and you're taking your water out of a glacier.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    I have measured 33-degree input water at my townhouse in NH. I can envision similar water temps elsewhere depending on the location of the water supply, and the depth and length of the supply line. Also, depending on the length of the line from the heater to the shower, and where it is run, it is likely you'll cool that water off some, and maybe a lot. Ideally, the tankless unit is near the point of use, and that loss is minimized. Also, you'll need to adjust your anti-scald shower valve season to season, as if adjusted properly for the summer with warmer incoming water, you'll never be able to get the 'all hot' required to get a sufficiently warm shower in the winter. Some thermostatically controlled valves might still work without adjustment. Also, if the tankless is setup with a tempering valve on the outlet, I think you'd always be leaking a little cold into the outlet (probably less when new, and more as it ages), so this also could compromise the available hot water temperature.

    Yes, it can be made to work. Yes, equipment costs will be higher as will maintenance. Will it save enough in energy costs, depends. If you ever wanted to fill a tub or a washing machine with hot, in the winter, a tankless would need to be sized considerably larger than for just a shower. Even to fill a washing machine with what might be defined as warm might be impossible (since the hot would be barely warm at full volume). So, if you know the limitations, you can figure the costs, then decide.

    If you wander up there on the weekend, and the WH is broken, you've got a chance of fixing or replacing it that day. Highly unlikely in the boonies to be able to do that with a tankless on a weekend. Simpler is often better.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    George the Plumber Gsalet's Avatar
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    I wouldn't call Tahoe the boonies anymore, a rinnai tankless 9.4 heats 9.4 gpm at 45 degree rise that's 4.7 gpm at 90 degree rise, yes it has freeze protection, but only when there is electricity. As for maintenance you only use the cabin about a month a year so maintenance would be about once every 12 years. I am not a big fan of tankless but for small homes or seldom used cabins it,s perfect, No waiting for the tank heater to warm up after driving for 4 hours in the snow to get there. Just be sure to drain it down if the cabin is going to be unheated in the winter.

  12. #12
    In the trades Dana's Avatar
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    In cabin situations the SPACE SAVINGS alone can be worth it!

    Unless you really need more than a single shower flow you don't need a monster-burner. The installed cost of something like a Rinnai is 2x what it would take to put in a ~120KBTU Bosch. Sure, it's a lot nicer unit, but the math (and experience) sez the smaller unit is still enough burner. It's a cabin, not a winter-palace with a large spa to fill. Under 100KBTU/hr there could be capacity issues though. At the anticipated price of propane, getting rid of the standing pilot & standby load might indeed pay off the difference between a tank and a cheap tankless within the lifecycle of the unit, but probably never within the lifecyle of a larger Rinnai.

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