Tankless sizing

Discussion in 'Tankless Water Heater Forum' started by ducktoller, Mar 22, 2014.

  1. ducktoller

    ducktoller New Member

    Dec 7, 2013
    I have been considering replacing my 50 gallon gas direct vent water heater with a tank less. I have one large soaking tub that never gets fully filled and this has always been an annoyance. There are two showers in the house but one has 4 small Moen Body sprays with one regular shower head. I had a plumber come out and give me a bid and after I reminded him I had 4 body sprays he indicated I would mostly likely need two RU98I Rinnais to keep up with the flow. I wasn't quite ready to pay to have two of them installed so now I am on the fence.

    I did some research and some testing on pressures and actual flow rates in my shower. The pressure at the outside faucet which is 3 floors down is 77 PSI so the pressure up at the shower is probably around 66 Psi at the shower head based upon my rough calculations (feel free to correct my assumptions). I also measured the flow rates at the shower head and the body sprays the shower head was putting out about 1.7 GPM and each body spray was putting out about .6 GPM. So the total shower is using about 4.1 GPM .

    So looking at the Rinnai specs and the flow curves it seems like a single RU98I would cover it but when I went on the Rinnai website and used the Rinnai tankless sizing calculator it basically said I needed either 4 R98LSi or up to 5 smaller ones! I am assuming the different is due to using rated flow rates for typical shower head and body sprays. But even there it seems like I am missing something.

    Am I all Wet?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    What is your winter-time incoming cold water temperature? That is one of the major driving factors. You're trying to heat about 33# of water, maybe a significant amount every minute. Think about putting a pot of cold water on the stove filled with 4.1g...turn the burner up on high, how long does it take to get to showering temp? To do it within one minute, you better have one (or several in series) pretty huge burners!

    33#x60minutes=1980# of water per hour. Say your incoming water is 40-degrees, and you need to heat it to at least 110, that's 70-degrees. It takes one BTU to raise one pound of water one degree. 70*1980=138,600BTU, not counting on the efficiency of the unit, so you'd need more. Given that the water may cool off a bit on the way to the shower, you may need it warmer to start, and your incoming water may get colder. Then, that assumes you are using all hot, which may be the case if your system can't get the full temperature rise to mix in a little cold. If anyone turned on any other hot water device, your shower would get cold, which is why you want at least a little buffer (by having it hotter than needed and maybe a bit more capacity).

    Then, keep in mind that there is NO restrictions imposed (other than the valve you choose) when filling a tub. The size of the tub isn't the real big issue, it's how fast you can heat the water. A typical tub fill valve could be anywhere from 4-5 gpm, to maybe as much as 15-18gpm, if you have a 3/4" or larger supply and valve to it. Let's take an average of say 8gpm...that's nearly double the capacity you need for the shower, and exceeds the capacity of the largest residential tankless I'm aware of (they generally top out at about 199K, and that's input, not output). So, you'd need more than one.

    From what I hear, most of the water in WA area is pretty good, so you may not have as much maintenance on a tankless as those where their water is hard. Mineral deposits in the tankless can reduce the max flow and restrict the heat transfer unless you service them periodically. The acid required may become a hazardous waste issue over time...it isn't right now, at least around here.
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  4. Will Rogers Plumbing

    Will Rogers Plumbing Plumbing Contractor

    Oct 8, 2010
    Plumbing Contractor
    Bixby, Ok
    Navien makes a good tankless water heater. Eternal also make s a good one, but it is not a traditional tankless. Navien has some neat thing out now with the negative pressure gas valve and the Stainless Steel heat exchanger is a nice feature. They also have built in recirc pumps.

    Depending on your incoming water temperature will dictate your GPM output. Look at the NPE-180a, NPE-210A, or the NPE-240A units.
  5. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    In Washington, incoming water is 50 degrees. With a 199,000 tankless you can expect about 5.0 gallons of 120 degree water per minute.

    I've wondered what the Moen body sprays put out. I have a Moen ioDigital in a box that I haven't installed for that very reason. Rain shower, hand held and four body sprays. Moen hasn't been any help about the specs on the body sprays.
    How are you testing for that?

    My local supplier here is kind of down on tankless for filling large tubs. Mainly because of the time to fill them. I like to install Grohe tub fillers with 3/4" lines on those for a quick fill.

    If you replace with a direct vent or a power vent, you can add a tempering valve, bump the water heater up all the way, and blend it down to 120 on the output. With a 50 gallon tank done that way you would have 75 gallons of 120 degree water sitting in the tank, and then you have the tank heating some of that as water is being used.

    Locally in the Seattle area, we have access to Navien, Eternal, Rheem, Rinnai to name just a few.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    You'll get a lot more flow out of a high-temp tank + tempering valve than with any tankless (even a pair of tankless!). If those measures are enough to fill the tub you're all set.

    If you want to save fuel and roughly double the showering time/"apparent capacity" on the 4-sidespray shower you can install a drainwater heat exchanger for a heluva lot less cash outly than a 199KBTU/hr tankless, but that option requires at least 5' of vertical drain downstream of the shower. It won't do a thing for tub filling though, only showers.
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