Tank or Tankless for new log home?

Discussion in 'Tankless Water Heater Forum' started by k2ham, Aug 28, 2009.

  1. k2ham

    k2ham New Member

    Aug 27, 2009
    Good afternoon,

    We are building a 5000 sq ft home that will become our retirement home. It is located in the Great Smoky Mountains/Knoxville/Gatlinburg region of Tennessee.

    Specifics on the house... a log home with five bedrooms in all... 2 in the basement (1 with private bath), 1 on the main floor (master with private bath), and 2 in the loft (each with private bath). There is a kitchen on the main and an efficiency/in-law kitchen in the basement. There is a half-bath on the main floor for common use and another full bath in the basement for common use and the bedroom without the private bath.

    As for usage... the home will be used intermittently until I fully retire in about 6 years. We'll probably be spending at least 3-4 nights a week there though until then. The house will primarily be occupied by me and my wife. However, our two daughters may spend some time there also. While most folks downsize when they retire, we've taken the plunge to try to afford and build a home that will be a gathering place for our immediate and extended families... thus the large size and number of bedrooms. So, while the house may be fairly empty at times, at other times we may have a really full house.

    With that background, what would you recommend that we consider for water heaters? We've installed high efficiency windows and extra insulation in the ceiling of the home. Three, 3-ton electric heat pumps will provide the heating and cooling and we will also have an EPA II fireplace on the main floor and (at some point in the future) an efficient wood stove in the basement to assist in the heating.

    Any thoughts and advice would be greatly appreciated!

  2. k2ham

    k2ham New Member

    Aug 27, 2009
    I failed to mention that our energy sources are limited to electric and propane...
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  4. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    I suspect that propane is going to limit your options. It might be good to analyze the average cost of propane and electricity in the area over several years. Then use that to determine which fuel is normally most cost efficient in your area. You will need to look at the Energy Guide assumptions and adjust for the relative utility prices to compare.

    You will probably want a lot of capacity for such a large home. Some of this depends on your intended lifestyle. Are you planning to have a shower (or showers) with multiple 2.5 gpm heads? Are you going to fill a large spa bath periodically? Or are you more in the efficiency mode with plans for something like single 1.5 gpm heads for each shower? The storage or instantaneous capacity could differ a lot depending on the answers to these questions.

    Electric water heaters appear more efficient on paper, 90+% vs. 60+% for gas. However, electricity generation itself is thermally inefficient (electricity is a higher form of energy than heat.) So the net effect is that burning gas directly for heat is more efficient than converting it or coal to electricity first, then using electricity to heat the water. This is reflected in the pricing of electricity vs. gas.

    Sometime this year we should be seeing the first Energy Star ratings for the gas condensing water heaters. Their thermal efficiency will be 90 - 96% but their efficiency factors (Energy Star which include standby losses) will be 0.82 and higher. A.O. Smith has a pricey power vented one that could provide nearly the same continuous capacity as a tankless because it has a high capacity burner. Being an early adopter has its negatives though including risk and cost.

    With standard tank water heaters, the recovery rate of an electric water heater is about half that of gas.

    In a large home it makes sense to insulate the hot water lines during the build too. This will slightly improve the warm up time at the taps and result in slightly higher temps at the taps on the long runs. It improves both convenience and energy efficiency.

    That's my take on it, but I'm a homeowner, not a plumber or builder.
  5. k2ham

    k2ham New Member

    Aug 27, 2009
    Hi Runs with bison... thanks for the reply.

    Yes, propane in the area we are building appears to be high... I haven't gotten specific information on it but do know that several other homeowners in the area regret having installed propane gas furnaces. At least one family has already swapped it out for an electric heat pump. We are planning on only using propane for the cooktop in the kitchen.

    As for usage we won't have a large spa bath and most of us are strictly "shower" folks. I'll get as efficient shower heads as possible (i.e., as I'm allowed to by the three women in my family)!
  6. Master Plumber Mark

    Master Plumber Mark Master Plumber

    Feb 6, 2005
    Sensitivity trainer.. plumber of mens souls
    indianapolis indiana - land of the free, home of
    do the electric route

    propane tankless scares me....

    with an electric water heater you can also just shut it
    down when you are not there,,, and flip the breker back on
    when you arrive for the weekend..

    electric is a much simpler way to go....
  7. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    There are some decent ~1.5 gpm shower heads now. As you say, it is a matter of personal taste with women being the pickiest about them. The Roadrunner is my first pick based on reviews, followed by the High Sierra, and the Oxygenics. I've not yet gotten to compare them all in the same shower environment. I like the High Sierra but some in my family are not as crazy about it--it does work better for showering than the old 2.5 gpm head it replaced. I have high hopes for the Roadrunner because of the full spray pattern, but my order was delayed, so I don't yet know if it will pass muster with the spouse.

    I don't know much about the tankless systems as I've never had one. Some people swear by them, some swear at them. For me a change like that with the higher burner rates and different venting would require a careful retrofit. With new construction that's not a factor.

    Are you going to be on some sort of municipal water/sewer or will it be well and septic?
  8. Gary Swart

    Gary Swart In the Trades

    Sep 1, 2004
    Yakima WA
    Tankless are very expensive to buy and install. The electric service will have to be very large. Cold incoming water will limit the hot water available. Considerable delay in getting hot water especially to distance fixtures. Does not provide hot water in very small volumes. Frequent tank cleaning required. Qualified service people often hard to find. Cost over the life of the unit somewhere around twice that of conventional heaters.
  9. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    Correct me if I'm wrong, but tankless would only make sense for gas/propane or sizing for long continuous supply. With electric the whole tank can be better insulated than with gas (since gas requires substantial heat exchange surface and a flue that produce some losses.) That's one of the two reasons that electric water heater tanks can have such high energy factors--the other reason being the non-condensing mode burner operation of current gas water heater tank systems.

    So I see no compelling reason for tankless with electric other than long continuous demand. I wouldn't anticipate any energy savings compared to a well insulated tank and surrounding piping.

    When gas condensing tank water heaters become established I doubt there will be any compelling reason for tankless gas water heaters--except possibly the smaller footprint. The A.O. Smith one has a burner rate like that of a small tankless. It could be some years though before these become mainstream choices, the kinks are worked out, and prices drop.
  10. zl700

    zl700 DIY Senior Member

    Aug 26, 2009
    Western NY
    Runs with Bisons

    While tanks have come a long ways, they have a long way to go to catch the tankless.

    I assume you are refferring to the A.O. Smith Vertex which is limited to 100,000 BTU, 127 gal first hour delivery and a mere 6 year tank warranty.

    Versus 240 gal first and every hour at same rise, 15 year warranty, 95% EF, such as the Navien Condensing for instance.
  11. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    Sounds like a better unit than any of the standard non-condensing units. I do wonder about how I would arrange combustion air and venting for that high of a gas load...which is well above both my current furnace and water heater capacity combined.

    The Vertex would do far more than I need. I'm down to about 144 ccf/year for water heating with a .58 EF gas tank heater, and am looking to cut that to about 120 ccf/year in the next few weeks. Tankless might eliminate 30-40 ccf/year of standby losses for my current low efficiency tank, but that won't buy much. I would rather have the 50 gallons of hot water in the tank ready to go and a more even supply temp.
  12. Dana

    Dana In the trades

    Jan 14, 2009
    100K condensing burner like the Vertex could deliver a continuous hot water stream until the propane ran out at the incoming water temps found in the Smokies (and 127 first-hour gallons in a tank that size is enough to serve up 4 simultaneous showers- just how much showering d'ya figure a retired couple needs? :) Methinks a Vertex, like a Navien is way overkill in this application too, eh? It's a retirement nest man, not a commercial laundry! 240 gallons/hour!?! They might need 240gallons per WEEK! If they were filling 100 gallon spas and the like, then they could use it. Otherswise, it's a waste.

    A heat-pump-top electric tank may make sense though- half the year it'll be reducing the air-conditioning load, and it's overall operating cost would be well under any propane-fired unit. See: http://www.energy.wsu.edu/documents/building/res/ht_pmp_water_htrs.pdf

    If you DO go with a propane fired tankless, even a cheapo ~125KBTU non-condensing unit would be more than enough unless you expect to have simultaneous showers going on during the cool-water winter months. (My 3-person family has done fine with a 125K gas-fired tankless for over a decade in New England with much cooler incoming water.) Standby costs for a tankless are zero, but propane costs are not. In the TVA region electricity is cheap, and heat-pumps can magnify that. The only reason to go with a propane fired tankless is for the continuous stream (10 showers in succession while filling the spa? No problem!). Since you probably don't need that kind of capacity often- a Japanese-style heat-pump-top tank with a COP of 2 or better is probably a better, lowest lifecycle cost & highest efficiency option.

    But there are ways of boosting the capacity of a tank to near-tankless proportions for showering purposes if you're still on the decision edge...

    Whatever source you use for hot water, since it's new construction and you're the showering type, designing it to be able to take advantage of a drainwater heat recovery system on the main shower with a drainwater heat exchanger is a good idea, and WILL pay off (even if it takes 10 years, if you live in 5cents/kwh land). It'll more than double the first-hour showering capacity of any tank, so you can get by with a smaller, less lossy tank, and cut the energy use for each shower in half or more(!). It won't do squat for tub-filling or other batch draws, since the drain and HW have to flow simultaneously to get the heat exchange. For shower taking families the average effective-efficiency boost will be more than going from a a bottom of the line 0.80EF tankless to a condensing tankless or Vertex for less money out. It's the equivalent of adding 25-30K of burner to the heater during showers- a 25-30k burner that doesn't use any fuel. It just dumps cooler water into the septic than it would have otherwise, pre-heating the incoming water to both the water heater and the cold mix on the shower to 60-80F with heat that would have otherwise gone out with the flow.

    See: http://www.renewability.com/uploads/documents/en/home_retrofit.pdf


    Canadian gummint testing reports:



    DOE short-report:


    Basically, get the longest & fattest version that fits- the heat recovery efficiency is all about maximizing the surface area between the drain piping and potable-piping-wrap. (A 4" x 48-incher performs about as well as a 3" x 60-incher, etc.) It'll be a large fraction of a grand, but it's worth it for capacity alone (what does marriage counseling cost, eh? ;-) ) Even if you went with a standard 40 gallon electric tank, with a drainwater heat exchanger it'd be good for 4 consecutive showers before anybody felt the chill- more if you go with minimal flow shower heads and/or short showers.
  13. flamefix

    flamefix New Member

    Sep 4, 2009
    Heating engineer in the UK licensed to fit service
    Exeter, England
    You could look at an indirect solar installation coupled to a 500litre -1000litre store. The 1000litre store or larger would be good if you plan to go ahead with the wood fired boiler as it will give somewhere to cycle the heat through. Same with heatpumps you'll want a store to accumulate the heat in from where it is distributed to your heating circuit and your hot water.

    If you're away a lot you'll get some stagnation in the solar but if adequately sized you can minimise that. It depends on what your solar availability is in that region, but the solar will at least raise the water temperature sufficient to reduce the load on any back up heat input source you utilise.

    Have you looked into the cost of Wood Pellets and wood pellet stoves, boilers. The cost for wood pellets here are about 25% higher than oil but cheaper than propane/LPG. They can modulate and you can programme them to come on and off much like an oil boiler, coupled to a bulk pellet hopper you'll have a lot less work to do than running a wood burner. Pellets require processing so they won't be as cheap or available or carbon neutral as wood itself.

    Here we talk about efficiencies on air source heatpumps as being much lower than ground source could you fit a ground source bore hole driven heat pump rather than air source? Again I'm not expert on these so I'll bow out on saying more.

    ahh my bad I assumed a wet system and you're using warm air/cold air ventilation for heating and cooling right?
    Last edited: Sep 6, 2009
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