Help Sizing a Tankless Water Heater

Discussion in 'Tankless Water Heater Forum' started by schmede, Jan 27, 2009.

  1. schmede

    schmede New Member

    Messages:
    4
    I h ave decided to go with a tankless (that is not my question) but I need help sizing it.

    I live in Salt Lake City. The winter water temp averages 46 degrees but can be as cold as 34. I like HOT showers, and will have two showers going at once every day. So probably a max of 5gpm most of the time.

    I decided on the Noritz 931 since it can raise the water temps about 75 degrees when 5.6 gpm are flowing. It is 84% efficient

    BUT...that model does not qualify for the $300 tax credit. Do you think I will be happy with the Noritz 841 which will raise the temps 75 degrees when 5.0 gmp are flowing? It is 93% efficient. Do you think that will raise the temp enough for a HOT shower when two showers are going? Also ss the efficiency rating between these two units (84 vs 93) enough to notice a difference in my gas bill?

    Thanks in Advance. I have to decide this week so any help would be great!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,053
    Location:
    New England
    Well, let's see...34+75 is 109, now run that through maybe 20-30' of pipe or more, and you'd be running straight out all hot water that is likely at least a few degrees less than that 109 ...maybe a lot. You might want to set the shower temp where you like it and run it into a large glass and then stick a thermometer in it to see what temp you really like. My guess is it may be a bit more than 109-degrees if you like a hot shower. I don't think you'll like either one if you like a hot shower...and, if anyone opens another fixture - say to wash their hands, the temperature will either drop OR it will decrease the flow (can you say anemic shower?) to try to maintain the desired outlet temperature. Is there a way to make this work? Yes, even if you want to stay with tankless...put two in series and you should be able to not only take two showers, but maybe run the dishwasher or run a load of clothes at the same time...it won't be cheap. Easier to use a tank, though, and probably cheaper in the long run.

    As the thing ages, you need to delime it or the temperature rise goes down (the minerals act as insulators). how often you have to do that would depend on your water and use.
  3. NO it wont...

    No it wont suffice..
    if you take one shower at a time you might be ok with the temp of the water as low as you claim it gets...

    you will not be able to take two showers at the same time with temps that low...

    it has got to do with flow rates...

    good luck with the whole experience



    people will walk through the fires of hell for
    that 300 dollar tax credit..

    and spend a fortune for it...

    they will drag their bodies across burning sands towards
    a mirage....telling themselves its all worth it in the long run....




    grab that brass ring.....
  4. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    I think that after spending close to 3 grand to have it installed you will try very hard to convince yourself and everyone else that it is the most wonderfull thing in the world while secretly wanting to bash the thing to hell and put in a decent water heater. :eek:

    Did I say that?

    Why yes, yes I did. And I'm stickin by it:rolleyes:
  5. around here its more like5 grand

    I have seen quotes as high as 5 grand for the tankless water heater in our area...

    they will drag themsleves across burning sands
    just for that 300 dollar rebate,
    just to say to their freinds that they have gone green.


    then next year
    you could shove bamboo shoots under their fingernails
    and they still would not admit that they screwed up.......





    .
    Last edited: Jan 27, 2009
  6. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Install two units. The second unit will handle the higher flow rates as well as the really low water temps you are talking about. Now that you spent the $$$ on the install of two tankless units and have to maintain them both on a regular basis, I think the wonderful $300 tax credit doesn't look so wonderful.
    Last edited by a moderator: Feb 19, 2009
  7. schmede

    schmede New Member

    Messages:
    4
    The 841 is out but what about the 931?

    Guys...point taken. But I am not trying to be trendy and green I am just trying to add some space to my little bungalow and a tankless will allow me to do so, plus I already have a 3/4 gas line and I only have like a 2 ft run to the outside wall. Basically I have the ideal set up for going tankless. SO...

    If I run the numbers on the Noritz 0931... 34 degrees (yes, I called the water district and they told me since it is run-off water it can get that cold...although the average in winter is 46, I want a HOT shower everyday in the winter)

    plus, based on the Noritz chart an 80 degree rise at 5.3 and an 85 degree rise at 4.9 means my shower should be over 110. I just want to know from a practical standpoint if those charts are to be trusted? Also, the run to the master is only a few feet.

    P.S. Based on a little more research, I believe the Noritz 0931 will qualify for the tax credit. I will make a phone call tomorrow to find out for sure.
  8. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Another thing to remember after you install the tankless. Some tub and shower valves(if its with in 10 years old or newer) have a temp limit stop installed behind the handle. They are set to ensure your max water temp will not go over 110 or 115 degrees. Since your old heater was making 120 degree or hotter, that limit stop will need to be readjusted to your newly supplied 110 degree water supply from the hot side.

    Just remember you have been warned. I have installed over a 100 of these not because I sold the people on the idea. They where already sold on it, and I tried to show them all the pros and cons of owning one of these. Some of the people did end up having me install the second unit due to the high flow rates and low temps. Others like to call me and complain they spent all this money and it works like crap. I just tell them I forewarned them.
  9. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,351
    Location:
    Yakima WA
    Let's not start another tankless water heater debate! The gentleman is obviously convinced the tankless is the solution to his problem and really isn't interested in hear us preach the facts of tankless life. He's going to do it anyway, so let's back off and do what he will.
  10. Go big....

    Last edited: Jan 28, 2009
  11. schmede

    schmede New Member

    Messages:
    4
    Ok. I have Decided...

    I have decided to go with the Noritz 0931. In a worst case scenario situation I think it will serve its purpose well.

    34 degree water
    2 showers (5gpm)
    rise up to 80 degrees

    should mean a hot shower and a super hot shower when the conditions are less harsh.

    My take away from this thread is...if you go tankless, don't half ass. By the way guys I am a woman not a gentleman and that is probably why I never half ass anything and why I have been more than happy with all the remodeling decisions I have made over the last year.

    P.S. My dishwasher and washer both have internal water heaters so should not draw much from the tankless.

    Last edited by a moderator: Feb 19, 2009
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,053
    Location:
    New England
    On either the washing machine or dishwasher, (at least with the dishwasher), why you are running them, they pull all hot - if, and only if, the temp is not high enough, they'll raise the temp. Now, some washing machines I've seen (mostly overseas) only have cold coming in, and if you want it hot, it heats it; but, I've not seen any of those here - they may exist, though. A washing machine has nearly unlimited flow, and could likely try to draw 12-15 gallons per minute. A DW often has a smaller line, and would be limited, but not that much. Either could easily overcome the flow available from a tankless in the winter-time with cold supply water.
    Last edited: Jan 28, 2009
  13. schmede

    schmede New Member

    Messages:
    4
    There are a few models that have internal heaters on the market in the US. I bought the samsung silvercare.

    http://www.samsung.com/us/consumer/...type=washers&model_cd=WF218ANW/XAA&fullspec=F

    I don't know how much hot water the washer initially draws from the house water, but I know it can raise the temp significantly for sanitizing

    I really don't use the silvercare function, but I bought it because it is very quiet and works well in a laundry space just off the kitchen

    Last edited by a moderator: Feb 19, 2009
  14. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
  15. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    Since you seem educated enough about the OP to suggest he purchase a unit...can you tell me if he has hard or soft water...and if hard what his TDS are and his GPG is.
    Last edited: Jan 28, 2009
  16. Bigtruck

    Bigtruck New Member

    Messages:
    5
    tankless


    I installed an electric Tankless and it works perfect. I've had it for about 7yrs.

    I can take showers in two showers and use the dishwasher at the same time. No change in temp.

    My sister installed a gas tankless. Her water temp fluctuates dramatically.

    Ray
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,053
    Location:
    New England
    A 3/4" rough-in valve I looked at had a spec of 18.5gpm at 45psi - this was for a tub. Some washing machines have small internal solonoids, so that limits the volume as can the hose you use and the shutoff valve you have. But, it could be the equivalent to a hose you water your yard with.

    Depending on your pressure, a 1/2" pipe can free-flow probably in the area of 8-9 gpm, 6-7 more common.

    On a tankless, some of them just restrict the flow rate to maintain the temperature rise, so yes, if yours worked that way, you'd be able to fill up the washing machine fine, but it would take longer to fill.

    If the tankless doesn't have a flow restrictor built-in or one added on, then the faster you flow the water through it, the colder it gets. Most have some means of slowing the flow.

    Attached Files:

  18. ingeborgdot

    ingeborgdot New Member

    Messages:
    119
    Location:
    Kansas
    I love mine. I would never own a big tank heater again. The only thing for me is you have to wait a little longer for the hot water. After that nice long showers. We went with a nice big size, not even the biggest but almost. It is a rinnai.
  19. sjsmithjr

    sjsmithjr Geologist

    Messages:
    295
    Location:
    Knoxville, Tennessee
    Sorry, I thought you were in Austin. Does Texas still use the 2000 IPC? Do some counties in Texas, like Bowie, not have a permitting or inspection process for gas, electric, and plumbing work? Some of the very rural counties in Tennessee only have electrical and septic permits and inspections.

    At any rate, for anyone under the IPC that would like to know how to properly size their water distribution system I believe the correct table to reference is 604.3, Water Distribution Design Criteria Required Capacities At Fixture Supply Pipe Outlets.

    If I'm mistaken I'd appreciate a correct reference; we're under the IPC where I live as well.

    Bradford White's Tankless sizing tool is great if you just want to play with various configurations: http://rightspec.bradfordwhite.com/Sizing/EverhotRes.aspx

    I tried it out myself. Plugging in an overly optimistic water temp of 60F with and output temp of 130F, my house's configuration (one shower, dishwasher, clotheswasher), took the default for fixture flow rates (who's to say I'll always want that 1.5 gpm shower head), checked simultaneous use and was spec'd two 180000 BTU units with a maximum hot water delivery rate of 8.6 GPM.:eek:

    If I change my evil ways, ignored code or got a variance, put a lockout on the washing machine, and told my wife we couldn't add that second bathroom she's been wanting (or plan on using them both at the same time) I could get by with one unit with a maximum hot water delivery rate of 4.3 GPM.
    Last edited: Feb 11, 2009
  20. sjsmithjr

    sjsmithjr Geologist

    Messages:
    295
    Location:
    Knoxville, Tennessee
    I can't remember if it was on this thread or another related thread but the observation was made that plumbing codes didn't address tankless sizing. Another poster observed that 8 gpm (Bradford White calculator) was unrealistic (I'm assuming too they thought it was too high) if I wanted to supply a shower, a dishwasher or kitchen sink, and a clothes washer simultaneously (peak demand). I understand that the total gpm given by the BW calculater is a function of the units spec'd and temp rise required as opposed to the calculated demand. Given that these are off the shelf items, it is almost certain some over demand capability will available.

    I then remembered that we had done some work in Florida a few years back. I humbly submit to you for your use a Residential Tankless Water Heater worksheet, 2001 Florida plumbing code. Adjust as neccessary for your minimum incoming water temp and required temperature rise; the physics and math will work across state lines. I think the worksheet would make a great sticky.

    ...and before anyone says it, it is not inferred by this form that you could get by with sizing the system to not provide simultaneous use of some fixtures. The minimum gpm allowed is 3 GPM @50 degree rise. Why 3 GPM minimum you ask? I'll leave it to you to figure out.
    Last edited: Feb 12, 2009
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