Tankless to supplement existing conventional w.h.

Discussion in 'Tankless Water Heater Forum' started by BusaNoob, Feb 6, 2011.

  1. BusaNoob

    BusaNoob New Member

    Messages:
    4
    Location:
    Maumelle, Arkansas
    Hi all. I have a question question concerning Tankless water heaters. I moved into new home a while back. The back bathrooms have had troubles getting hot water supply because they are over 70ft from the Gas Tank Style Water Heater in the garage.
    This weekend I climbed under the house and put pipe insulation on all the piping. That improved but there is still a 3 minute wait / waste before the water gets hot. I am wanting to add a tankless before the "T" that supplies the two bathrooms.

    I am hoping to find an Electric Tankless to supplement the existing heater. Preferably 120 because I only have one open slot in my subpanel. Is there a quality brand that you suggest ? I am a newb to the plumbing world. I am confident that I can get it installed. I really need your help / suggestions in choosing the right model. Also if there is an inline filter to help keep it clean and prolong its life, Im all ears.

    Thanks in advance
  2. Terry

    Terry Administrator Staff Member

    Messages:
    15,121
    Location:
    Bothell, Washington
    There aren't any tankless electric units that only run on 120
    To fry or heat water on demand, you need a lot more power then that.

    I would add a recirc line back to the heater. Then there will be almost no wait, and your volume of heated water will increase.
  3. BusaNoob

    BusaNoob New Member

    Messages:
    4
    Location:
    Maumelle, Arkansas
    Thanks for your response. Ive seen 120v tankless models from chronomite and stiebel just searching on the net. Dont know how great of brands they are. I can install a 240 subpanel, but if I can go the easy route with 120, I will.

    I also read about installing a recirc. But the recirc would much harder to install running a line all the way back to the tank. I would have to drill through walls, floor, in a tight spaces. Just too much work.
  4. Dana

    Dana In the trades

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    An 8-10gallon electric tank heater in series at the T to the bathrooms would solve the wait, and probably increase the overall efficiency of the system on short -draws (less water drawn to get hot water for a low-volume use such as hand-washing => less heat abandoned in the 70' of distribution plumbing.) On bigger volume draws the main tank is still providing the bulk of the HW. Standby/distribution losses would be lower than with a recirculation solution.

    The small tank local to the bathrooms need only be as large as as it takes to stay acceptably hot when diluted with the < 2 gallons of water in the 70' run from the main HW heater. It's primary function is maintaining the volume at temp to handle the dilution, with no need to heat the water quickly, so even a 1-1.5kw single-element version would be fine. On longer draws with 120F+ water coming into the tank it's recovery time is already very short. On short draws the temp doesn't drop enough to matter. A 10 gallon side-connection unit (eg: Rheem 81VP10S, Reliance 6 10 SOMSK ) will fit inside a 20" cube of space, and would be more than enough volume of dilution-buffer for the cold slug of water in the 70' of distribution plumbing. If you have the space, somewhat larger 12-20 versions can be similar or less money- you probabaly won't have to spend more than $250 if you shop around.
  5. BusaNoob

    BusaNoob New Member

    Messages:
    4
    Location:
    Maumelle, Arkansas
    Many thanks. I will have to check the size of these in person. But that is a good option. Hopefully it will fit in the crawl space under the house. Is there a downside to installing an electric tank unit under the house (venting) ? I see you guys are shy about the tankless water heaters. Are they not as good as they appear to be ?
  6. Dana

    Dana In the trades

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    An electric tankless of sufficient power to deliver the temps you want while waiting for the water from the gas heater to arrive would require a new breaker panel (and maybe even a service-drop upgrade). Sure, it would work but at a high upfront cost for what you're trying to get out of it. The efficiency of the mini-tank approach would be slightly less, but even if you were paying a buck a kilowatt-hour it would take a very long time to recover the difference in cost since it's duty-cycle would be quite low, with the bulk of the hot-water heating coming from the gas-fired tank in the garage. (A mini-tank's standby loss would 25-40kwh/month what you'd pay to run a tankless to fix this issue- less, if you extra-insulate the tank and near-tank plumbing beyond.) Even on a shower or bathtub draw you'd only be heating up the first couple of gallons with the tankless, after which the hot water from the garage tank is supplying the heat. Factoring out installation costs, heating water with a gas will be cheaper than with electricity in most utility markets, so you still want to run it in series with the garage tank so that the gas burner carries the bulk of the water heating load.

    The heat output of electric tankless HW heaters are pretty limited compared to their fire-breathing gas-fueled cousins, making them pretty marginal for anything but point-of-use heaters in cold-water areas. They're probably OK most of the time in Arkansas, where subsoil temps are in the mid 60s F and mid-winter incoming water temps can still be north of 50F, but to run multiple showers you need the tepid incoming water temps of southern Florida or southern Texas to really make them work well. As finish heat to a solar tank they can still be pretty effective in coldwater areas, but that's a more specialized application. Gas fired tankless can be sized for use almost anywhere, but there isn't always an economic rationale for them on efficiency, etc. Primary reasons for going that route are high gallons per hour needs (large families of serial-showerers, or one water-hog teenager ;-) ) and buying back some floor area. They have their quirks, but most are a tolerable tradeoff for the other benefits. Getting sufficient gallons per minute for multiple simultaneous draws is still a limitiation with a gas fired tankless, just not as significant a limitation. With tank type heaters the instantaneous flows can be quite high, and the limitations are gallons per hour rather than per minute.
  7. BusaNoob

    BusaNoob New Member

    Messages:
    4
    Location:
    Maumelle, Arkansas
    Okay, so lets eliminate gas heating from the mix because I will be doing the install myself. I will not be dealing with the gas lines at all, dont want to burn the place down ! If I read what you are saying correctly, a small electric tank model (around 10 gallons) would be my best option and cheapest to run, because it will cost less to run and less upfront costs. The electric tankless would do the job, but cost more to run because it will have to heat a high volume of water in a short amount of time ?

    Also, what will this do to my water pressure. It seems that dumping water into a reservoir (2nd tank) would lower my pressure unless I have some sort of pump to bring it back up ? Sorry for the weird questions, but I am a newb to this.
    Last edited: Feb 9, 2011
  8. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
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    You have a couple of things backwards- the electric tankless would be ever so slightly cheaper to run, but not sufficiently cheaper than the mini-tank to ever make up for the installed-cost difference.

    The mini-tank will likely be cheaper to run than a recirculation loop, due to the higher standby and abandonment losses of that approach. (It could be a wash, depending on the cost of gas heating that standby loss vs. the cost of the smaller electric standby loss, which less total energy loss, but at a typically higher price per unit energy in most markets.)

    The second tank doesn't restrict the flow, causing lower pressure at the faucet- think of it as just a very FAT section in that 70' of distribution plumbing. Fatter pipe== less restriction, higher flow. An electric tankless DOES restrict flow (another reason you don't want to go that route) as a method of keeping the temperature up when it can't deliver enough heat to run at full flow, like when the incoming water is too cold.
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