Should I replace tank with Tankless

Discussion in 'Tankless Water Heater Forum' started by dsbeck, Sep 20, 2013.

  1. dsbeck

    dsbeck New Member

    Messages:
    4
    Location:
    NE Ohio
    We are remodeling a bath for our son who is disabled. As a part of the remodel, we are adding a 6' whirlpool/air spa tub, so that he can "soak" in the tub at times.

    We currently have a 50 gallon condensing hot water heater (about 10 years old - put in during construction of the house).

    I am thinking about replacing the water heater with a tankless. We have a Takagi Jr. on the other side of the house (guest bath, laundry, kitchen) and it works great, with the typical problems of cold water sandwich and long start up times to get hot water. Not a big deal on that side of the house.

    The 50 Gallon supplies 3 bathrooms, which is what the tankless would replace. My concern is that to fill the tub, 50 gallons would not be enough, and kind of defeats the purpose of having the tub !

    I am looking at the Eternal (GU145) but have not found a lot of good reviews lately on these. It looks like it eliminates the "cold water sandwich" with it's onboard mini storage, which is what draws me to it. I CAN'T have the sandwich, or my wife will KILL ME !!

    I would love to hear suggestions/recommendations. Efficiency is important, but I want something that will work and last!

    Thanks,
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    NE Ohio can get pretty cold in the winter. How cold does the incoming water get in the middle of the winter? One of the big things with a tankless is that the amount of water it can heat depends on how far it has to raise the incoming water's temperature or you have to live with the water out the tap getting colder and colder as more people demand hot water. IOW, tankless tends to work best when there isn't a huge load. FIlling a large soaking tub demands as much of the heater's output as possible, or it just takes forever to fill the tub! Getting one big enough may require upgrading your gas service all the way back to the street, which can be quite expensive. THis is where a tank works better...you have likely lots of time to reheat it, but you can dump hot water as fast as the valve can deliver it to anyone that needs it.
  3. dsbeck

    dsbeck New Member

    Messages:
    4
    Location:
    NE Ohio
    Jim:

    With our other tankless, we don't have any issues with water flow, so I don't anticipate any with a new one.

    Most of the water heaters have flow regulators that keep the output temp the same, and modulate the flow, so the water won't get "colder and colder".

    My biggest concern is running out of hot water with a tank, before the tub is filled? That defeats the purpose of a soaking hot bathtub if you run out of hot water before the tub is filled ?!?!
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    It all depends on your flow rate. In the middle of winter, if you're trying to fill the tub and someone else is trying to use hot water, to maintain the desired temperature, the flow may get down to a dribble at the multiple outlets. A single showerhead uses at most 2.5gpm...a big tub filler could reach 12gpm or more. How long are you willing to wait to fill the tub? The alternative is to get a decent sized tank. You can make the tank appear bigger by running it hotter (more standby losses) and using a tempering valve on the outlet (where I live, regardless of the WH thermostat setting, you are required to have a tempering valve). As a tank gets bigger, the volume to external surface area ratio gets smaller, IOW, you get more volume with a smaller gain in surface area, which helps on the standby losses. Better tanks have lower standby losses as well as more efficient burners. Depending on the outlet temp of the WH, keep in mind you probably arent' filling the tub with warmer than about 104-degree water, so it isn't all hot, and if you're using say 140-150 degrees in the tank, then tempering it down to 120, then mixing it down to 104 at the point of delivery, not all of the water going into the tub is hot. So, check out the capacity of the tub you want, and then decide if you prefer to use a tank, or go with a tankless. WHen you're figuring costs, count in the potential of upgrading your gas service and annual scale cleaning on the tankless. Fail to do that, and after a few years, your output will diminish radically. IOW, lots more maintenance on a tankless system than an almost set it and forget it on a tank. WHen it does need repair, finding someone who can fix it right the first time is lots harder than replacing a tank.

    If you size the tank properly, you should never run out of hot water, and you can fill the tub as fast as the valve will let you. Highly unlikely to happen with a tankless in the middle of an Ohio winter with the ground water temps approaching freezing. Most tankless systems are spec'ed to raise the water 70-degrees, assuming your supply is 50-degrees or so, that's fine since it can reach 120, which is the 'normal' supply temp in a home. You have to read the fine print to determine how much rise you can get with your supply, and then decide if it is enough. But, when it's down to near freezing, either the flow drops way off, or the temperature does. Some of the tankless units are designed to be installed in series, so it raises the water in stages, when required. But, now you've more than doubled the install cost and the maintenance issues, too. But, it will be able to produce LOTS of water on demand.
  5. Dana

    Dana In the trades

    Messages:
    2,804
    Location:
    01609
    It matters what the volume of the spa is. If it's under 100 gallons you can fill it in a reasonable amount of time with a 199KBTU/hr condensing tankless, but above that could be painful. I don't know of any tankless units designed to work in series, but many can be run in parallel, with an extra controller or a cable between them.

    But you may already be looking at running out of BTU capacity at the gas regulator at the meter if you already have a tankless, and are looking at adding another.

    So...

    How big is the spa, and how much burner (furnace/boiler, TK-Jr, cooking range, etc) total is already hooked up to gas plumbing?

    Is your heating system hydronic (pumped hot water), and if yes, do you have space next to it for tank big enough to fill the spa?
  6. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    I have a tankless and found the cold water sandwich problem to not be significant at all. The only way it is even noticeable is if somebody bleeds the hot water out of the pipe at a very low flow so as to not fire the heater. That is a pretty low flow so is a rare event. Even then, it is more of a lukewarm sandwich as I suspect some mixing occurs in the 1" pipe.

    I have only tried one brand, of course. After much research I went with the Noritz NRC1111. If you are comparing specs, look not only at temperature rise vs flow rate, but also pressure drop vs flow rate.

    I really like my tankless. However, I would suggest you consider a water softener if you have hard water, or commit to descaling annually using the Noritz service valves and vinegar.
    Last edited: Sep 24, 2013
  7. lipton80205

    lipton80205 New Member

    Messages:
    5
    Location:
    Denver
    I don't have any experience with Eternal brand, but I did look at the spec sheet. Guessing you are going to need around 70 degree temperature rise to bring the water to 105 degree. The stated flow rate for the GU145 is a little over 5 GPM on 70 degree temperature rise. So if the tub has a flow rate of less then 5 GPM you should be ok as long as there are no other calls for hot water. However, depending on the size of the tub, it might be take a long time to fill!

    Also, Jim and Dana bring up a good point. You may be close to maxing out your gas meter capacity since you already have 1 tankless on the meter.
  8. Dana

    Dana In the trades

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    2,804
    Location:
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    To fill a 120 gallon spa at 5 gpm is requires the meditative patience of the Buddha.

    Volume matters.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    You really want to be able to achieve a higher temp at the outlet of the tankless than 105-degrees...there will be some loss in the piping and some things really want higher temperatures (like a dishwasher, but some have built-in heaters). Throw in someone turning on a faucet to wash their hands or some other flow, and your output will drop. If you're running all hot (and some valves won't let you), there's no way to adjust it warmer should the supply temp drop some. Personally, I've measured 33-degree incoming water in my second story bathroom's vanity after a cold spell...this is after running through over 100' of supply pipe (I live in the middle of a row of townhouses). Supply water temps in the upper part of the USA can be quite cold in the winter...when taking a shower or a bath, a couple of degrees can make the difference between comfortable and uncomfortable. A tank's output (at least for most of its output) will be pretty consistent, and likely start out way higher than 105-degrees (typical indirect is around 140-degrees or so, and could be higher).
  10. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    you're ignoring the mixing valve and cold volume

    So let's suppose a 70 degree rise is needed to go from 50 degree water to 120 out of the heater. One is not going to fill the tub with 120 degree water, so some of the flow will come from the cold side of the mixing valve. Besides won't the spout be plumbed in 1/2" pipe? I know in my new bathroom I feed the 1/2" thermostatic valve with 3/4" copper, then 1/2" to the tub filler. I can get 6 - 7 GPM out of it, which seems quite high and fills my (admittedly standard-sized) tub pretty quickly.

    7 GPM would fill a 100 gallon tub in 14 minutes. Which might seem a while, but that is a big tub!. Unless you're willing to install a 100 gallon high-BTU tank with 3/4" valve and tub filler, the fill time does not seem totally unreasonable. And that assumes you have the upstream plumbing (1") to support the flow rate. And even then you'll get the gradually cooling effect on the hot water side.

    So I do think this is a reasonable application for a tankless. But do check about the gas meter capacity. I had the gas company upgrade mine for free when I repiped.

    Here is the spec sheet for the big Noritz.
  11. Dana

    Dana In the trades

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    Location:
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    50F incoming water? Yer kiddin', right?

    The AVERAGE annualized incoming water temp in NE Ohio is around 50F, but average mid-winter incoming water temps there are about 40F (and sometimes even colder.)

    Tub fills are typically at ~108-110F water out of the spout- that's a 70F delta.

    5 gpm x 70F= ~175,000BTU/hr, which is about what you get out of a 199,000 BTU/hr condensing tankless running at max fire. It might do a little bit better than 5 gpm, but not 6.

    Whether it's 110F straight out of the tankless or 120F out of the tankless mixed down to 110F, the BTU rate requirements are the same.

    The description of the tub was "...a 6' whirlpool/air spa...". Maybe those come as small as 100 gallons, but it won't be a lot less than that, and 120 gallons or more isn't out of the question.
  12. dsbeck

    dsbeck New Member

    Messages:
    4
    Location:
    NE Ohio
    OK, first Thanks to all for the replies!

    1) My wife has finalized the SPA, and it looks like total volume is around 65-70 gallons, not 100, so that makes filling the tub a little more palatable.
    2) I have no idea what incoming water temperature is during the dead of winter here - never measured it.
    3) We have 2 gas (forced hot air) furnaces, a tank water heater, tankless water heater, and a gas fireplace currently on our gas service. I'll have to check the meter capacity if we add a 200K BTU Tankless in place of the current tank heater (figure out the BTU difference between existing and potential new). I remember when we built the house, plumbers made the comment that the gas main inside the house was oversized due to all the BTU load (I think it is 1 1/2" pipe?)
    4) During tub fills, I doubt we will have any other demand on the tankless. It is always possible, but unlikely on how and when our son would use it.

    Hope this answers some of everyones questions.

    Dave
  13. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Gas pipe size sounds good as long as the meter supports it. You'll likely need to enlarge the pipe to the tankless to 1". Unless you have another tub or shower on the proposed tankless I would not worry too much about 5-6 GPM max in the dead of winter. A good thermostatic tub valve will just reduce the cold and the heater will reduce the flow to get your desired temp.

    I noticed a significant decrease in my gas bill due to increased efficiency and no standby losses.
    Last edited: Sep 27, 2013
  14. Dana

    Dana In the trades

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    2,804
    Location:
    01609
    Deep subsoil temps will more or less track the 50 year outside weather temperatures, but with surface reservoirs and water towers and water mains buried at less than super-deep the average annual incoming water temp will track the average outdoor temp, and the winter/summer extremes will be within 10-15F of the annual average temp. In NE OH the deep subsoil temps are about 50F:

    [​IMG]

    Your incoming water temps will probably dip a just a bit below 40F on incoming water temps in mid-late winter, and maybe hit between 60-65F in mid-late summer, much as it does in my central MA location. (For system testing reasons I sampled the incoming water temp at my house last week at 63F, which is about the temperature of the soil about 5' down at the end of summer. By March it'll be 38-40F if history is any guide.)

    Look at the BTU input ratings to all of those gas burners. I'm guessing you're riding close to the limit as is. Most residential gas meters are only good for ~450K BTU/hr, and that's the max size regulator they can put on them. With some guesstimatin' on furnace sizing I come up with:

    Furnace 1: 75K

    Furnace 2: 75K

    Takagi Jr: 140K

    50gallon tank: 35K

    fireplace: 30K

    That adds up to 355K

    You probably have 40-50K of cooking appliances too, which brings you to about 400K.

    If you swap the 35K water heater for a 199K tankless you've blown past what the meter itself can handle.

    But look at the nameplate input ratings on all of it and report back.

    Even with 1.5" gas plumbing you don't want to install a 199K modulating burner somewhere along the chain with a bunch of other burners drawing from it or you'll have distruptive interactions messing with it's operation. It needs a dedicated line (probably 1-1/4" unless it's really close) teed off as close to the regulator as possible.

    A 75-80 gallon tank with a ~75K burner would fill that tub. You get about 2/3 of the volume out at near the full storage temp before it starts to fall off fast, and at a 140F storage temp you'd have enough.

    lifespeed: I can't see how significant the differences in the gas bill could possibly be unless you are using a TON of hot water every day or you had the worlds worst standing-pilot ignition low-insulation atmospheric drafted pig with half the insulation gone. With 3 people in the house showring daily my non-space heating fuel use from the beginning of May to the end of September is less than 20 therms/month, and that's heating the water with a 48 gallon indirect & ~82% efficiency low-mass modulating boiler (Takagi KD20.) Swapping that system out for a condensing tankless dedicated to water heating would cut at most 3-4 therms/month of household gas use during the summer, and less during the heating season. Even at the summertime buck-fifty/therm usury from the gas utility I'd have a hard time feeling the $5-6/month difference.

    This has been studied to death by utilities in CA, with careful in-situ testing as well as bench testing in engineering labs under a variety of load profiles. Real world EF (as opposed to EF test data) on a condensing tankless it about 0.82-0.85 for higher volume (75-120 gallons/day) users, about 0.75 for low volume (25-30 gallons/day) users. Real world EF for high volume users on a bargain-basement 0.55-0.60 EF non-condensing tank is in the 0.65 range, but extremely low volume users might see only 0.40. Swapping in a condensing tankless makes a significant cut on a percentage basis, but low use means it's already a near-nuthin' bill, and cutting that in half yields about half-nuthin' in absolute dollar terms. The economic rationale for tankless is only there for high fuel prices (like $2+ propane) or extremely high volume hot water usage on natural gas, usually commercial applications, not residential. And that's why subsidies for tankless units are so low.

    The rational reasons for buying a tankless are space savings, and zero recovery time, not the fuel savings economics. Rare is the house where a condensing tankless is the next-most-cost-effective way to save 75-90 therms/year on fuel use. Yes, it's more efficient than a bottom of the line tank, but the present value of the difference in installed cost of the future fuel savings are negative within the normal lifecyle of the unit, even using a 1% discount rate in your NPV calc.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    FWIW, I have measured my incoming cold water temp at 33-degrees, with a decent, accurate meter in southern NH after a cold spell in winter. Ohio probably doesn't get that cold - it is maybe 100-miles further south (depending on exactly where you are). True, it doesn't get that cold all the time, and then not usually for long, but a week or so, those extra degrees a tankless needs to raise the water can make the difference between comfortable and uncomfortable. I had an embedded coil WH for awhile, and during those cold snaps, even just a showerhead running it had trouble keeping up. Regardless of the system, it can only raise the water temp so much as it flows by/through the heat exchanger. Start out colder, unless you slow it WAY down, it comes out colder. Trashed that system, and added an indirect, and now I've never run out of hot water, and it's more efficient, too.
  16. DonL

    DonL Out of the Trades

    Messages:
    3,901
    Location:
    Houston, TX
    "Should-I-replace-tank-with-Tankless"

    My short answer.

    NO.


    Have fun everyone.
  17. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    True, the space and zero recovery are the main benefits, and appealing enough at least to some. And they do save fuel. So, you are analyzing the cost/benefit of a tankless vs one of those new tank water heaters you can get for nothing? Last I checked the prices for the heaters were not that different, although there can be some installation costs depending on the specifics (gas piping).
  18. DonL

    DonL Out of the Trades

    Messages:
    3,901
    Location:
    Houston, TX
    Anything that has a fast large change in temperature is under a lot of stress.

    Using the wrong material is a disaster.

    You should look into what the heater is made of, and figure the cost of maintaining it.


    There is more to the formula than fuel savings.
    Last edited: Sep 27, 2013
  19. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,888
    Location:
    New England
    If you can't perform the annual demineralization on the tankless, the cost savings (if there are any) over a traditional tank will be negative, especially when you consider the rework of the utilities...that service will take at least an hour of labor. If you wish to do it yourself, you'd have the first-year equipment costs, and learning curve.
  20. lifespeed

    lifespeed Member

    Messages:
    321
    Location:
    California
    Of course there is more to it than fuel. I did look into the heat exchanger, and bought what I believe to be one of the better units on the market. "100% stainless steel corrosion resistant heat exchanger fused with a commercial grade copper heat exchanger, 25% thicker piping than standard models"

    Apparently there are lots of people who don't like these things. Everyone is entitled to their opinion, but opinion is not a substitute for proven performance. After two years I don't see anything but great performance, economy and convenience from my heater. I guess we'll see how it's working in 10 years.

    Because I live in an area with very hard water I installed a water softener. This has been great for ALL the plumbing in the house, especially the water heater. So I expect to do little to no maintenance on the heater, although I will flush it with vinegar every few years despite it not being strictly necessary.
    Last edited: Sep 29, 2013
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