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Thread: Tankless WH basics

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  1. #1
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default Tankless WH basics

    There is a lot of hype about the abilities of tankless water heater systems. They can work well, but you must understand some simple facts. The first major effort is to determine your environment:

    How much hot water do you want to be able to use at one time? If you have a big tub, it could take a looonnnggg time to fill it with a tankless system, long enough where you may not be able to keep it warm enough.

    How cold is your incoming water…pick the middle of winter to measure. If you have a deep well, it could be cold all year round. In some places, it could be just over freezing in the winter. This will make using a tankless much harder to meet your expectations. If you live where things are more moderate, it can work out well.

    The things take a lot of heat to produce hot water nearly instantly…you may need to upgrade your gas or (unless you’ve got really inexpensive electric rates avoid electric) electrical supply. Is your supply large enough to handle the new appliance?

    How hard is your water? The harder the water, the more frequently you’ll need to de-lime the thing or you’ll end up with decreased capacity.

    Do you often use warm water? (i.e., a low-rate of hot)
    A tankless will likely cost significantly more than a tank to install and will require more periodic maintenance. Also, the expertise to diagnose and repair the thing is not as prevalent as for a typical tank, so you may be without hot water for awhile.
    Another thing to consider is that to work, nearly all tankless systems have either internally, or externally, a flow restrictor. This is required so that you can reliably get some temperature rise in your water. I like to relate it to the hand through the candle analogy. There’s only so much heat available, and if you move the thing you want to heat past it fast, it doesn’t heat up much. The more heat you have available, the more water you can heat, and the higher the flow rate you can achieve.

    There are some simple physical laws that apply here. It takes energy to heat water. So, let’s talk about that for a moment.
    The amount of energy it takes to raise one gallon of water per minute, one degree continuously takes the following amount of energy: 500 BTU or 0.147KwHr.
    Let’s take the situation where you want to draw 5 gallons per minute, your incoming water temperature is 40-degrees and the water needs to be 120-degrees. The heater needs to raise the temperature 80-degrees. So, take the 500 BTU and multiply it by 80 and then by 5, and you get 200,000 BTU. That’s the amount of heat applied to the water, and no heater is 100% efficient at it. So, let’s say that your heater is 85% efficient, that means only 85% of the heat is actually making it into the water, and 15% is going up the flue or into the room. So, to have 200,000 BTU in the water, you need to put in 200,000/0.85, or 235,294 BTU, or 68.958KwHr (assuming the same efficiency – electric should be more efficient but still isn’t 100% - some goes into the room). At a 220vac input, that’s 313.45 amps. We’re talking about some serious energy use here. Electrical tankless systems are less common and usually only available or applicable in low-flow or small temperature rise situations. Gas is more common because it is easier to achieve the concentrated heat volume.

    There is a minimum turn-on volume required before a tankless starts producing heat. A common one is -gallon/minute. Take a typical bathroom faucet that has a flow-restriction itself of a little over 1-gallon/minute. To get hot to work at all, you’d need the valve at about volume. Less, and the heater won’t turn on and you’d end up with all cold. So, there’d be no possibility of running a trickle at warm, and modulating it to a low-flow warm would likely be impossible. How often this is needed, is an open question, but you need to be aware that it is a functional reality.

    Most electronic or even mechanical systems tend to fail the more they are used. Ever notice a light-bulb almost never dies after you turn it on, only in the action of turning it on? A tankless turns on every time you turn a hot tap on (assuming you achieve enough flow to activate it – otherwise, you’ll get no hot water at all). So, while electronics are pretty reliable, all of those cycles mean the system has a long-term reliability issue just from this. The tankless won’t work when you don’t have power. A tank system could hold hot water for days, if not longer. A tankless system would be stressed to provide a quantity of showerheads going at the same time or filling a large tub in a reasonable amount of time if your winter water temperature is low.

    Bottom line, tankless systems have their place, and it is not everywhere. They have trouble with large flows and low incoming water temperatures, and require more maintenance than tanks. They are more expensive to install and while becoming more popular, not everyone stocks parts or is trained to be able to diagnose them. They can require huge peak energy inputs depending on your use requirements. Assuming you go with gas, the supply and exhaust requirements often may mean an upgrade in your service, and for electric almost certainly.

    Evaluate the specs and compare with the pure physics of the situation…the temperature rise and volume available isn’t magic, you need a certain amount of energy to make water hot. A tank system has the advantage of time to store the energy in the water. The disadvantage is it can’t replenish it immediately. You need to determine your peak volume needs or you’ll be dissatisfied.
    Last edited by Cass; 02-02-2009 at 03:29 AM.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #2
    Master Plumber master plumber mark's Avatar
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    Talking I might as well add my web site link

    JIMBO

    I might as well add my web site link to all your information


    http://www.weilhammerplumbing.com/houseofhorrors/



    this 1915 humphrey worked better than the mordern tankless do today....


  3. #3

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    Quote Originally Posted by master plumber mark View Post
    JIMBO

    I might as well add my web site link to all your information


    http://www.weilhammerplumbing.com/houseofhorrors/


    More FUD factors.... Fear, uncertainty, doubt.....
    Last edited by gregsauls; 02-06-2009 at 07:35 AM.
    "Dude, we can fix that. My old man is a TV repairman, he's got the ultimate set of tools!" --Jeff Spicoli


    http://web.me.com/greg.saulsbury/ChosatongaSpeaks

  4. #4
    Illinois Licensed Plumber SewerRatz's Avatar
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    Quote Originally Posted by gregsauls View Post
    More FUD factors.... Fear, uncertainty, doubt.....
    The biggest issue I see with most people and tankless is they do not realize you have to size the unit properly. Many times I go out on a estimate look things over ask the questions about their normal usage, then give them a price for a properly sized system. Then with shock in their voice they say "But the guy at (insert name of tankless pusher here) told me I only needed the unit that costs half that. After i sit down with them and show them the spec sheets and show them that the unit they where so in love with would only give them 70 to 80 degree water at their normal usage and colder at peak times. A tankless heater can only heat water so fast in its heating chambers. If the water does not have enough time to sit in the chamber it will not get hot enough. In a few cases people need to have more than one tankless heater hooked up.

    This is not uncommon. These units have the ability to link up their electronics and fire at the needed rate for the flow rate of water. So if you are pulling 3 gpm or less only one unit fires up, but once you exceed the flow rate for the first heater to give you the 120 degree water, the second heater will fire up. If you exceed the flow rate of both heaters then you have to add a third and link it up.Only time I seen 3 or more tankless heaters in a home hooked up to each other was where their was a large family (12 people) with 5 baths and they all where taking showers at the same time and also tried to do other things like they used to when they had the 100 gal heater in the basement.

    So in short they do need to be properly sized to meet your daily demands.

  5. #5
    Master Plumber master plumber mark's Avatar
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    Talking killing their dreams,

    [quote=SewerRatz;182346] Then with shock in their voice they say "But the guy at (insert name of tankless pusher here) told me I only needed the unit that costs half that.
    quote]

    that is about what happens here also....

    and they usually dont hear a word you told them.....and mark you off their list as a crook,
    (you got to be the felow that is lieing to them)

    and then go with the unit for half the price from a hardware store in town.....

    and live happily ever after,
    at least until they realize
    that they screwed up

  6. #6

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    Quote Originally Posted by SewerRatz View Post
    The biggest issue I see with most people and tankless is they do not realize you have to size the unit properly. Many times I go out on a estimate look things over ask the questions about their normal usage, then give them a price for a properly sized system. Then with shock in their voice they say "But the guy at (insert name of tankless pusher here) told me I only needed the unit that costs half that. After i sit down with them and show them the spec sheets and show them that the unit they where so in love with would only give them 70 to 80 degree water at their normal usage and colder at peak times. A tankless heater can only heat water so fast in its heating chambers. If the water does not have enough time to sit in the chamber it will not get hot enough. In a few cases people need to have more than one tankless heater hooked up.

    This is not uncommon. These units have the ability to link up their electronics and fire at the needed rate for the flow rate of water. So if you are pulling 3 gpm or less only one unit fires up, but once you exceed the flow rate for the first heater to give you the 120 degree water, the second heater will fire up. If you exceed the flow rate of both heaters then you have to add a third and link it up.Only time I seen 3 or more tankless heaters in a home hooked up to each other was where their was a large family (12 people) with 5 baths and they all where taking showers at the same time and also tried to do other things like they used to when they had the 100 gal heater in the basement.

    So in short they do need to be properly sized to meet your daily demands.
    I couldn't agree more with your comment about proper sizing of the tankless unit based on customer HW needs. The failure of the big box stores is to NOT have people trained to make the sale on a proper size unit. If a person is presented with a choice of "small, medium, or large", they will tend to undersize to save a dollar. On a tankless unit this will come back to haunt them in bad performance.

    Education of sales people, installers and users can only help this situation.
    "Dude, we can fix that. My old man is a TV repairman, he's got the ultimate set of tools!" --Jeff Spicoli


    http://web.me.com/greg.saulsbury/ChosatongaSpeaks

  7. #7

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    Quote Originally Posted by SewerRatz View Post

    Only time I seen 3 or more tankless heaters in a home hooked up to each other was where their was a large family (12 people) with 5 baths and they all where taking showers at the same time and also tried to do other things like they used to when they had the 100 gal heater in the basement.

    So in short they do need to be properly sized to meet your daily demands.


    How was that single 100 gallon tank water heater working out for them? I mean, 5 simultaneous showers plus other things would drain that tank in how much time - say 7 or so minutes? Then what did they do?

    Was you example based on an electric or gas system?



    Tutorial on tankless water heaters - click here!


    .
    Last edited by Ladiesman271; 02-23-2009 at 07:26 PM.

  8. #8
    Illinois Licensed Plumber SewerRatz's Avatar
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    Default

    Was a gas system. They claimed the single 100 Gal tank was working fine for their needs, when it went out they talked to a guy they new that talked them into going tankless. I could not tell you if it worked, or maybe they felt that now they have the "endless supply" of hot water from the tankless system they where able to push it further than they did with the tanked system. Heck I wish I was the guy that sold them the set up they are running. Only reason I got to see it was when they called me to power rod their main sewer. I would of asked more questions about the system but it was very hard to understand their very broken English.

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