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Thread: water heaters....tank or on-demand?

  1. #1
    DIY Senior Member Master Brian's Avatar
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    Default water heaters....tank or on-demand?

    I have heard pros and cons of both, but am curious what everyone thinks.

    I currently have a 40gal tank, whirlpool. I hate it, but it was in when house was purchased and I have already upgraded it to the new thermal coupler for what it is worth. There are 3 of us living in the house, with a fourth due soon and I have 2 bathrooms.

    The 40gal tank just doesn't seem to supply enough hot water for back to back showers and this morning, my 3y/o was taking a bath, while I took my shower and my water got cold. She had maybe 1/4 tub of water and I take, maybe a 5min shower. This tank isn't but maybe 3-4 years old, if that.

    I have also thought about adding some radiant heat and I see where many recommend a hot water tank for that, which brings me to my questions.

    1) Will a wall mounted, gas, on-demand unit supply hot water for multiple functions at once or is that not what they are designed for? For example, can I take a shower, my daughter a bath. What about a shower and running the washing machine at the same time? Currently, that doesn't work well!
    My understanding is that the on-demand units aren't great at providing hot water to several items at once.

    2) If the on-demand units aren't good at several items at once, can they be plumbed to work in conjunction with a hot water tank? Or is that like I figure not real efficient and a waste of money.

    3) While researching radiant floor setups, I came across a hot-water tank that was about $3500, forget the brand, but what makes one of those worth $3500 vs. the $400 units? It was 90% or 95% efficient, but even if it cost nothing it would take 8 years to recoop the cost over my current unit.

    4) Is 40gal tank style big enough for 4 people or should I look at a larger unit? Or maybe even just a 2nd unit and plumb the showers to the 2nd one. It wouldn't be hard, as everything is being converted to pex.

    My thoughts are....get either a bigger tank style or an on-demand unit. I have thought about trying to use the current tank for a radiant system, as I don't think it's bad, just that it might be having a hard time keeping up.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Master Brian View Post
    My thoughts are....get either a bigger tank style or an on-demand unit.
    You might first check your heater's thermostat setting and achieved water temperature. There are nine of us here in our house, and we all do fine with a 40-gallon electric water heater ... and I sometimes take long showers to warm up after spending all day in a 60-degree shop, and even if a couple of grandchildren are just getting into a tub when I get home. If you turn your heater's thermostat up, however, you might need a tempering valve to assure nobody ever gets scalded.

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    Plumber in Previous Life sixlashes's Avatar
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    You have a few options.

    1. Purchase a high efficiency 60-80 gallon tank with inlet/outlet for your radiant heat. Pro - may use existing gas service and flue, can use for small amount of radiant heat. Con - May not be enough during combined domestic/radiant use. (If a high number of Btus will be needed for heat, a boiler may be a better choice for efficency)

    2. On-demand. Pro - theroretically, unlimited hot water Con - moderately high installed price for a unit large enough for your requirements, gas service would need to accomodate 200,000+ Btu (this may be a show stopper). Also, the upkeep on the tankless unit is beyond what most want to routinely do. (do a search on this forum). The existing flue may not be usable; most (not all) units use a category III vent and it is expensive. Electric power available?

    3. Add another 40 gallon high efficiency tank water heater. Pro - possible lower cost, minimal maintenance. Con - existring gas supply aqequate for another unit? Flue? Will have issues hooking up radiant heat to either unit.

    Some guys swear by the tankless units. I was originally very interested in installing one in my house, a very similar scenario you describe. The efficiency was a hook, but decided against it because of complexity and higher routine maintenance.

    The tank unit you questioned the price on was probably a very high end multi-use unit with a stainless steel tank and burner. Only you can decide if it is worth the premium. I settled on a small tank type with 90+ efficiency and a higher capacity (100,000 Btu+) burner. It also has the hookups for the radiant heat. Bradford has one as well as the Vertex from A.O. Smith. The guys around here swear by the Bradfords; you would be very happy with it. If you have the gas capacity where you want to install this, it would be a good choice.

    Whatever direction you go, combining domestic hot water with radiant heat could present health issues, ie. stagnant water in summer months getting into domestic side. If you are not a bonified plumber, definitely find an experienced plumber for this install.

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    One who lurks Basement_Lurker's Avatar
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    Hmm, the amount of hot water you are getting doesn't sound right to me. Perhaps one of the elements is dead or corroded badly.

    A tankless unit is not recommended for simultaneous draw by multiple fixtures. If you want that kind of luxury, then you need to either look at multiple point of use tankless units, or you need to go with a larger tank unit, like a 60 or 80 gal unit. Gas units are more expensive than electric ones, but they will give you better recovery times if you have multiple people trying to take showers one after the other. Also, bottom entry tank units will actually give you like a 1/4 to a 1/3 more hot water than a standard top entry unit.

    The expensive tank you saw was probably a Marathon unit. That tank is an all plastic unit with alot of insulation. The tank will never corrode, but apparently there have been problems where the tank gets tapped.

    For me, a 40 gal unit is fine for two roughly back to back 10 min showers. I would suggest that you follow leejosepho's suggestion of looking at your thermostat setting. Whirlpool water heaters are supposed to be the worst..I've never installed one so I can't say if they are for sure. Basically it all comes down to your usage, if you can't live with your current tank's output, then it's time to move up to a larger tank, and I do suggest you spend the extra money on a bottom entry unit.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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    Illinois Licensed Plumber SewerRatz's Avatar
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    I never been a fan of tankless heaters, Most people around here that have 4 or 5 baths think one tankless heater will supply them all the hot water in the world while 3 or 4 people are showering and doing laundry. This is far from true. A tankless heater can only raise the temperature of the water if it has time to do it. In other words if the flow is 4 gpm a Single tankless unit can provide good temperate water. But at 11 gpm it can not raise the temperature high enough so a second unit is needed and in some extreme cases you might need 3 units.

    Now Bradford White addressed the troubles people have with the big hot tubes and multiple shower issues with this water heater. http://www.bradfordwhite.com/images/shared/pdfs/specsheets/115-B.pdf

    This 25 gallon heater produces 155 gallons of hot water in the first hour, this is a whole lot better than the 90 some gallons of hot water a standard 50 gallon heater does.

  6. #6
    In the Trades Gary Swart's Avatar
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    Here is a recent article quoted by another forum user.

    This is out of Consumer Reports.

    Heating water accounts for up to 30 percent of the average home's energy budget. Some makers of gas-fired tankless water heaters claim their products can cut your energy costs up to half over regular storage heaters. So is it time to switch?

    Probably not. Gas tankless water heaters, which use high-powered burners to quickly heat water as it runs through a heat exchanger, were 22 percent more energy efficient on average than the gas-fired storage-tank models in our tests. That translates into a savings of around $70 to $80 per year, based on 2008 national energy costs. But because they cost much more than storage water heaters, it can take up to 22 years to break evenólonger than the 20-year life of many models. Moreover, our online poll of 1,200 readers revealed wide variations in installation costs, energy savings, and satisfaction.

    With the help of an outside lab, we pitted Takagi and Noritz gas-fired tankless water heaters against three storage water heaters. EvenWe didn't test electric tankless heaters because many can't deliver hot water fast enough to replace a conventional water heater if ground*water is cold. in areas with warm groundwater, most homeowners would need to upgrade their electrical service to power a whole-house tankless model.

    Our tests simulated daily use of 76 to 78 gallons of hot water. That's the equivalent of taking three showers, washing one laun*dry load, running the dishwasher once (six cycles), and turning on the faucet nine times, for a total of 19 draws. While that's considered heavy use compared with the standard Department of Energy test, we think it more accurately represents an average family's habits. We also ran more than 45,000 gallons of very hard water through a tanked model and a Rinnai tankless model to simulate about 11 years of regular use.

    Here's what else we found:

    Water runs hot and cold
    Manufacturers of tankless water heaters are fond of touting their products' ability to provide an endless amount of hot water. But inconsistent water temperatures were a common complaint among our poll respondents. When you turn on the faucet, tankless models feed in some cold water to gauge how big a temperature rise is needed. If there's cool water lingering in your pipes, you'll receive a momentary "cold-water sandwich" between the old and new hot water. And a tankless water heater's burner might not ignite when you try to get just a trickle of hot water for, say, shaving.

    Nor do tankless water heaters deliver hot water instantaneously. It takes time to heat the water to the target temperature, and just like storage water heaters, any cold water in the pipes needs to be pushed out. And tankless models' electric controls mean you'll also lose hot water during a power outage.

    Up-front costs are high
    The tankless water heaters we tested cost $800 to $1,150, compared with $300 to $480 for the regular storage-tank types. Tankless models need electrical outlets for their fan and electronics, upgraded gas pipes, and a new ventilation system. That can bring average installation costs to $1,200, compared with $300 for storage-tank models.

    Tankless units might need more care
    During our long-term testing, an indicator on the tankless model warned of scale buildup. We paid $334 for special valves and a plumber to flush out the water heater with vinegar. Many industry pros recommend that tankless models be serviced once a year by a qualified technician. Calcium buildup can decrease efficiency, restrict water flow, and damage tankless models. Experts suggest installing a water softener if your water hardness is above 11 grains per gallon. Ignoring this advice can shorten your warranty.

    Efficient storage models are pricey
    We also tested the $1,400 Vertex, a high-efficiency storage water heater by A.O. Smith. The manufacturer claims its installation costs are similar to a regular storage model. But its high cost offsets much of the roughly $70 per year the Vertex will save you. Instead, we recommend buying a conventional storage water heater with a 9- or 12-year warranty. In previous tests, we found that those models generally had thicker insulation, bigger burners or larger heating elements, and better corrosion-fighting metal rods called anodes.

    Posted: September 2008 ó Consumer Reports Magazine issue: October 2008

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default 40 gallons

    If you had a 1/4 tub of "hot" water, you should not have had ANY for the shower, depending on the heater's thermostat setting.

  8. #8
    DIY Senior Member Master Brian's Avatar
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    Quote Originally Posted by hj View Post
    If you had a 1/4 tub of "hot" water, you should not have had ANY for the shower, depending on the heater's thermostat setting.

    Being as the 1/4 tub of water was for a 3y/o, the water was probably only lukewarm at best. Water that you or I would probably deam "cold".

    Judging from what I have always understood and what has been posted, I will forget about the tankless heaters and focus on finding a good bottom fed tank heater. I'll probably go with larger than 40gal and hope I can use it for radiant heat. The system I saw that used the tank for the radiant system, would sense when a shower, sink, washing machine, etc was being used and diverted the water to that item. Then kept the radiant system from going stagnent during the summer by somehow bypassing cold water through the radiant pipes during the summer, which they say gave a very small residual cooling effect. The cooling was not the prime thing, it was keeping the water from being stagnated.

  9. #9

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    One thing about using a heater that does both radiant heat and heating water is when it goes out on you. If you have cold winters and it fails, you have no hot water or heat. It's ok if you use it to supplement another system or keep another as a backup, but I wouldn't use it exclusively.

    I've been in supply houses when other plumbers come in complaining they have customers with no heat or hot water conveniently in December. No one wants to pay the upfront cost of putting in a new heater while waiting for warranty service or authorization.

  10. #10
    In the Trades Gary Swart's Avatar
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    A conventional water heater is "bottom fed" by the dip tube. A 40 gallon gas heater is big enough for a family of 4 if use just for house water. I would never go with a heater for both radiant and household use for the reasons already mentioned by others.

  11. #11
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Master Brian View Post
    I'll probably go with larger than 40gal and hope I can use it for radiant heat. The system I saw that used the tank for the radiant system, would sense when a shower, sink, washing machine, etc was being used and diverted the water to that item. Then kept the radiant system from going stagnent during the summer by somehow bypassing cold water through the radiant pipes during the summer, which they say gave a very small residual cooling effect. The cooling was not the prime thing, it was keeping the water from being stagnated.
    Here is a drawing of that kind of system, and I like the idea of having just one high-quality heater for everything.

    I presently have a small kickspace heater being fed through a regular recirculation line connected to our electric, 40-gallon water heater, and thereby keeping the water in the tank circulated actually gives us more hot water with a consistent temperature for showers.
    Attached Images Attached Images  

  12. #12
    DIY Senior Member Master Brian's Avatar
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    The radiant would just be supplemental heat for the house, trying to get some heat the a 2nd floor, etc.

  13. #13

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    Quote Originally Posted by leejosepho View Post
    Here is a drawing of that kind of system, and I like the idea of having just one high-quality heater for everything.

    I presently have a small kickspace heater being fed through a regular recirculation line connected to our electric, 40-gallon water heater, and thereby keeping the water in the tank circulated actually gives us more hot water with a consistent temperature for showers.


    Is that drawing a setup that is legal to use? I thought that the domestic hot water and heating water had to be kept seperate.

  14. #14
    Plunger/TurdPuncher kingsotall's Avatar
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    I don't see why unless there was some type of anti-freeze in it like glycol that required a closed sysytem.

  15. #15
    Moderator & Master Plumber hj's Avatar
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    Default heater

    Going back to a previous postings, gas heaters do not have either upper or lower elements, and the flue for a 40 gallon heater would be completely inadequate for anything larger than a 50 gallon one or a tankless heater.

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