Basic questions about going from tank to tankless water heater

Discussion in 'Tankless Water Heater Forum' started by Everett DeHart, Feb 17, 2020.

  1. Everett DeHart

    Everett DeHart New Member

    Joined:
    Jul 8, 2019
    Location:
    Ohio
    20 year old house, originally came with a 50 gallon electric, I changed to a 40 when the old one died. Just my wife and I, retired, occasionally another couple comes in for a weekend. Essentially, our hot water usage is a couple showers a day, clothes and dish washing appropriate to two people.

    Thinking ahead to when I replace the existing unit. Seems a waste to have 40 gallons "simmering" 24/7 for what "seems" to be light demand. Started looking into tankless. Lots of contradictory info out there. Saw a "Reem" 11 or 13KW which is supposed to handle two showers simultaneously. It was around $230. From there the price went up and up for equivalent KW units. Obviously, you get what you pay for, but, personally, don't want the cheapest or the "gold plated designer" model of anything, something in the middle. House has 200A service, so no problem there. I do have a 500 gallon propane tank for heating and cooking. Do not have a vent where the existing unit is, adding one would be very expensive. Tankless propane would require a vent and relocation to an outside wall which would end up being more of a project than the return would justify.

    Casually reading, incoming water temp is a concern with tankless units. Lots of discussion about "in Florida maybe, but not up north." etc. We're in Southern Ohio, just stuck a "taylor" wall thermometer in a glass of water and let it run for a while 46 degrees. It's February and we've had several days of below freezing temps. Does municipal water run warmer in Fla.?

    Our municipal (County) water has a fair amount of lime, also, being on one of the highest hills in the county, water pressure is around 15-18 PSI. Was going to add a tank and jockey pump but never did as everything works OK. After retirement, life gets easier. :)

    Installing an electric tankless would be very easy, load center (breaker box) and existing unit are in the utility room, add a two pole 60A breaker and run fifteen feet of cable and connect water lines.

    Have seen "repeatedly" that going to a tankless electric unit will cut electric usage in half. Also have read that with a tankless unit you have to wait longer for hot water to "appear" at the faucet? We have 3/4" copper throughout with the required energy saving faucets, etc. so we're used to waiting.

    I'm a retired EE, so feel free to get technical.

    So, has anyone gone from a tank to tankless electric? What lessons learned? From the info I presented, any "fatal errors" obvious to those who know more than I do?

    Thanks for your time,

    Ev
     
  2. SShaw

    SShaw New Member

    Joined:
    Nov 17, 2019
    Location:
    Virginia
    I'm an EE also. I looked at electric tankless recently, when I was replacing my HVAC with geothermal. If I had put one in I would have gone with the Stiebel Eltron Tempra Plus 24 for my 2-person household. The Tempra Plus modulates the power output based on the demand. It also throttles the water output volume based on the capacity, so the water never gets cold.

    The efficiency of the elements in the electric tankless is about the same as in a storage water heater. So, the savings would primarily be from eliminating standby losses in a storage water heater. This could be addressed in a storage heater with better insulation on the tank and pipes. I don't think you would save much with an electric tankless. Estimates I've seen range from $30 to $44 per year for the average household, which isn't significant to me.

    My primary interest was in space savings. I also have a jacuzzi tub that takes a lot of water to fill. I didn't like the fact there would be a huge peak current draw with the tankless though.

    If your water is 46 degrees, then you might not get more than about 1 gpm from a 13 kWh unit. You can look at the charts on the Steibel Eltron website to see the output vs temp rise for various wattage units.

    I ended up keeping my 50 gal storage heater, plus I now have a 50 gal buffer tank that the geo heats to 130 deg. If I had the space I might have put in a Marathon or maybe a heat pump water heater.
     
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  4. Everett DeHart

    Everett DeHart New Member

    Joined:
    Jul 8, 2019
    Location:
    Ohio
    Thanks for the response. Since posting this I've done more research and it does seem that the savings tend to be over sold with tankless units. Initial cost seems to be comparable tank vs tankless.-comparing a A O Smith 40 gallon to the unit you referenced. Another unknown to me is life expectancy. Given reasonable care a tank should last at least ten years in my experience unless you have bad water chemistry. No idea on tankless. Perhaps there's a reason that tanks predominate and tankless tend to be a special application or niche market?

    Something else just occurred to me, living in the country, power outages are a fact of life, usually less than 10 hours duration. I have a generator sized for critical items, water heater isn't as that would almost double generator size. The tank holds "hot" water so a fast shower(s) can be accomplished even with no power.

    Pending more responses and discussion, I think I'm going to stay with a well insulated tank when the time comes, probably down size to 30 gallon though.

    Thanks again for the response
     
  5. SShaw

    SShaw New Member

    Joined:
    Nov 17, 2019
    Location:
    Virginia
    You're welcome. The generator sizing issue was one reason I wanted to avoid the large peak current of an electric tankless.
     
  6. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    No kidding. A typical residential tank has UEF of about 0.92 (= 92% average efficiency at typical usage volumes), so there isn't much efficiency gain going to UEF-0.98 with a tankless.

    In a southern OH location there is a HUGE efficiency benefit for going to a heat pump water heater (HPWH), since the summertime outdoor dew points are high enough that all homes have a significant latent cooling load (humidity), even during the shoulder seasons. Heat pump water heaters typically have a UEF of 3.0+ (over 300%+ efficiency), taking 2/3 of the heat from the room air, largely by taking the "heat of vaporization" of the moisture it's condensing out of the air in summer. That takes some of the load off the air conditioning, and keeps the indoor air reasonably dry even when there isn't much of a sensible cooling load. If you have a basement that needs a dehumidifier to keep the summertime "musty basement" smell at bay, an HPWH will do the heavy lifting of keeping that basement dry.

    During the heating season the 2/3 of the heat going into the water has to be made up by the heating system, but it's still way more efficient annually than a plain old tank or an electric tankless. The peak draw of an HPWH in heat-pump only mode is also less than a plain old tank, not much of a strain on a backup generator.

    The only real down sides are recovery time and upfront cost. It's slower than a plain old tank and quite a bit more expensive too unless there is also a rebate subsidy available. But in most markets the net energy cost savings will pay for the HPWH a few times over during a normal lifcycle (or warranty period.) Don't be put off by online reviews dating back a decade or more when the product type was still in infancy. This technology has come a long way since- they're quieter, more efficient, and more reliable than ever, definitely ready for prime-time.
     
  7. fitter30

    fitter30 Active Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
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