Instant on Water heater? or 80 gallon or both?

Discussion in 'Remodel Forum & Blog' started by mikki499, Aug 7, 2008.

  1. mikki499

    mikki499 New Member

    Aug 7, 2008
    Hello, I currently have a natural gas 40 gallon hot water tank running my kitchen (dishwasher and sink) as well as one full bath. I’m in the process of doing an addition which will add a second story with 2 additional full bathrooms (giving a total of 3 full baths). One of the added bathrooms is a kid’s bathroom which will have a standard shower and vanity. The added second full bath is a master bath with a custom walk in shower and a jet air tub. The master shower with body jets can use up to 8GPM and the high flow fill for the tub can use up to 12-17GPM. I’ve personally been leaning towards the instant on water heaters to potentially handle the whole house, but I’ve been warned that the flow as well as the capacity could present a problem in this setup. It’s been recommended to me to use a 80 gallon water heater to be able to fulfill the 50 gallon reserve requirement for the tub but this to me seems like a waste of energy as well as maybe needing a second 40 for the rest of the house?!?! And this recommendation has come from a plumber that is a friend of the family. In my recent research should I maybe consider one, two or three instant on gas water heaters (maybe piggie back two together for the flow potential for the master and one to handle the other 2 full baths). Or might it be in my interest to use a storage tank of some sort associated with them?!? I’m open to all recommendations as I’m not as familiar with the instant on water heating systems. For heat I have forced hot air in case any of that matters.

    Odds are there will only be 2 showers on at the same time ever given a family of 4 with 2 kids. Thank you in advance for your replies! This site looks great and very useful!!!

    Also forgot to mention that I'm located in Massachusetts for weather reasoning in case this matters.
    Last edited: Aug 7, 2008
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    To get enough power to heat at flow of water you'll need almost a commercial system if you go tankless. You almost certainly don't have a big enough gas line, and would likely pay a demand fee that is significantly higher than what you have now, even though you wouldn't be using it all that often. You might need a gas line upgrade a couple of sizes larger.

    A WH is normally good for about 70% of its volume from a high flow situation, which filling the tub would be. With a tank, you can store the heat rather than dumping it all in at once while there is demand.

    A WH doesn't lose huge amounts of energy, especially if it is in an area where you would be heating anyways. In MA, your heating season is much longer than your cooling one.

    In MA, it is likely that the incoming water temp in the winter could approach freezing on a long cold spell. A tankless, especially at the flows you are talking about would need a huge amount of heat. A gallon is about 8#, so at 17gpm, that's 136# of water per minute. Say it needs to raise that water from 33 to 110, that's 10,472BTU/minute or 628,320BTU/hour. If you assume maybe 85% efficiency, your input energy would be 739,200BTU/hr. Increase the flow by someone else running a shower or the washing machine, or dishwasher and you could be looking at a one MILLION BTU system. Most residential units peak around 200K, so that'd be 5. You could probably by a nice couple of cars for the kids for what it would cost to install it, if you had room.
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  4. sixlashes

    sixlashes Plumber in Previous Life

    Aug 6, 2008
    Retired USAF Aircraft Maintenance Superintendent,
    Pensacola. FL
    I am in a similar situation; a guy building my own home trying to figure out what water heater to use. I was a licensed plumber over a decade ago until my knees said no more.

    I was initially drawn to the tankless water heaters with their promise of unsurpassed efficiency, but upon much research have turned away.

    Reason #1 - Efficiency. There are condensing storage gas water heaters that are as efficient and some that are more efficient than standard tankless. One I looked at was AO Smith's Vertex at 90% thermal efficiency. With every day use, factoring in the standby loss of approx 548 Btu/h, it beats out the tankless units I was looking at. It is a 50 gallon - 76,000 btu unit that your family would be hard pressed to drain. It has a 127 gallon first hour delivery. Look at This is a power-vented unit which you vent with PVC pipe. The low temperature of the exhaust is a clue to the efficiency. It does not have the heat loss up the flue that standard vented units have.

    AO Smith also has a direct-vent model that brings combustion air in through PVC and exhausts out through PVC. The direct vent is more money, but is 5% more efficient, 24,000 Btus more powerful and does not use inside air for combustion and create all of the problems accociated with it.

    Reason #2 - Venting. Many tankless water heaters use cat III vents that get expensive pretty quick. These units are not economically viable when the unit is not close to an exterior wall or a suitable roof location to vent through. Both of the Vertex's are very flexible with venting requirements.

    Reason #3 - Gas supply. Tankless unit(s) to even come close to your requirements would be 350,000+ Btus. Do you have a gas line big enough to produce this much volume? Supplying your master bath shower + any other shower (or fixture for that matter) in the house, you are correct that you will need more than one unit.

    As Jim posted, with your water temps and demand, I cannot see tankless being viable. I have three teens and a pre-teen and am going with the 100K Btu direct-vent and am confident it will serve me well.

    This is just my 2 cents, but it may give you a starting point to research and questions to discuss with your plumber.
  5. Master Plumber Mark

    Master Plumber Mark Master Plumber

    Feb 6, 2005
    Sensitivity trainer.. plumber of mens souls
    indianapolis indiana - land of the free, home of
    you say you have a Nat gas. 40 .???

    with natrual gas ,
    why dont you just install a 75 gallon???

    or better yet install a 40 and a 50 gal series

    or do 2 50 gallons in series........

    normally, a 75 gallon gas should be able to take care of a
    whirlpool and three bathrooms.... we see it all the time....

    stay away from all the on demand heaters...
    Last edited: Aug 8, 2008
  6. mcconnellplumbing

    mcconnellplumbing Scotsman

    Dec 31, 2010
    plumbing contractor, 25 years, high end residentia
    Wellston, OK
    People, people, people. A 199,000 BTU tankless water heater requires a good 3/4" gas supply of natural gas. that's it. If two units are installed in tandem, with communication cable, they operate together for a flow rate of over 10 gpm with a delta temp rise of 60 degrees. That's incoming at 60 and outgoing at 120 degrees. Alloting for cutting the hot water temp with cold, it allows for a continuous 14 gpm of shower temp water. If the incoming water is colder, say 45 degrees, the 60 degree rise results in 105 degree shower water at a sustainable 10 gpm. Gas supply is critical though. A short run of 1" off of a 1 1/4" gas trunk line would work. The 1" with a cross section of .78 sq. in is almost twice the sq. in of 3/4" , .44 sq. in. We install the Navien tankless and they vent with PVC at a 98% efficiency. We can also pull combustion air through the roof or side of the house or mount the unit outside on the exterior wall. With two units in tandem, one is the designated master, the other a slave, but they alternate after so many calls for heat cycles so neither is "on call" more than the other. When hot water is called for, only one fires and the other unit fires up only after a certain flow rate is called for then it pitches in and helps the other. The 98% efficiency, endless flow within heating parameters, PVC venting, etc. are all real good, but, I could go on for an hour about the drawbacks too. gotta go. Will love to argue good and bad about tankless later!!
  7. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    One large tank type heater, 75 gas or 80 electric, is all you need. In most cases the gas one will be less expensive to operate and will provide more hot water than the electric one.
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