(206) 949-5683, Top Rated Plumber, Seattle
Page 1 of 3 123 LastLast
Results 1 to 15 of 37

Thread: Help Sizing a Tankless Water Heater

  1. #1

    Question Help Sizing a Tankless Water Heater

    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. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,412

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Master Plumber master plumber mark's Avatar
    Join Date
    Feb 2005
    Location
    indianapolis indiana - land of the free, home of the brave....
    Posts
    4,243
    Blog Entries
    1

    Talking NO it wont...

    Quote Originally Posted by schmede View Post
    . 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!
    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. #4
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    Location
    S. Maine
    Posts
    2,039

    Default

    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.

    Did I say that?

    Why yes, yes I did. And I'm stickin by it

  5. #5
    Master Plumber master plumber mark's Avatar
    Join Date
    Feb 2005
    Location
    indianapolis indiana - land of the free, home of the brave....
    Posts
    4,243
    Blog Entries
    1

    Talking around here its more like5 grand

    Quote Originally Posted by nhmaster View Post
    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.

    Did I say that?

    Why yes, yes I did. And I'm stickin by it
    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.......





    .

  6. #6
    Illinois Licensed Plumber SewerRatz's Avatar
    Join Date
    Nov 2008
    Location
    Chicago, IL
    Posts
    1,705

    Default

    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 Terry; 02-19-2009 at 04:37 PM.

  7. #7

    Default 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. #8
    Illinois Licensed Plumber SewerRatz's Avatar
    Join Date
    Nov 2008
    Location
    Chicago, IL
    Posts
    1,705

    Default

    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. #9
    In the Trades Gary Swart's Avatar
    Join Date
    Sep 2004
    Location
    Yakima WA
    Posts
    7,246

    Default

    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. #10
    Master Plumber master plumber mark's Avatar
    Join Date
    Feb 2005
    Location
    indianapolis indiana - land of the free, home of the brave....
    Posts
    4,243
    Blog Entries
    1

    Talking Go big....

    [quote=schmede;181456]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...

    I am just having a little sport, no big deal and no offfence meant..

    If you absolutely must have one, I would suggest that you dont skimp on the size, its simply better to over size one rather than get out the graphs and flow charts and try to cut it down to the bare min...


    it tis certainly better to oversize the unit to do a three- four bathroom home and be fairley content with the outcome, especially if you wish to have two showers going at once


    rather than having luke-warm pisswater raining down on you for a shower next winter...becasue you undersized or put in what you thought should work..

  11. #11

    Default 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.

    The dishwasher and the washer will still draw hot water from the tankless when running.
    Terry Love
    Last edited by Terry; 02-19-2009 at 04:41 PM.

  12. #12
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,412

    Default

    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 by jadnashua; 01-28-2009 at 09:24 AM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  13. #13

    Default

    Quote Originally Posted by jadnashua View Post
    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.
    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/d...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

    All plumbing in the US uses hot water for the dishwasher. That is how the plumber will connect them. So when the dishwasher fills, it will pull water through the tankless heater. Terry Love
    Last edited by Terry; 02-19-2009 at 04:43 PM.

  14. #14

  15. #15
    Plumber Cass's Avatar
    Join Date
    Nov 2005
    Location
    Ohio
    Posts
    5,984

    Default

    Quote Originally Posted by gregsauls View Post
    To the original poster on this topic.

    A properly sized and installed tankless water heater will serve you well. Look at what your max hot water flow rate needs are, your lowest inlet temp historically and the proper sizing of your water and gas lines (3/4" typically needed). Then look at your typical needs & average inlet temp and base the size of the tankless on something in between or... nearer the max serviced needs if you don't want to "think about it" during worse conditions.

    Do descale your tankless once a year.... just like you would drain a tank type unit every year or so due to sediment.

    Getting a "fair" answer from some on this board will not happen here. There are a few that simply hate tankless systems for their own reasons. There are also some of us here who have them, installed them ourselves and are quite happy with our purchases. Those of us who have them will continue to advocate them despite others "noise".
    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 by Cass; 01-28-2009 at 12:20 PM.

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •