Tankless WH basics

Discussion in 'Tankless Water Heater Forum' started by jadnashua, Feb 1, 2009.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,924
    Location:
    New England
    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 ½-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 ½ 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 a moderator: Feb 2, 2009
  2. Last edited: Feb 1, 2009
  3. gregsauls

    gregsauls Homeowner

    Messages:
    56
    Location:
    Texas
    Last edited: Feb 6, 2009
  4. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,706
    Location:
    Chicago, IL
    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. killing their dreams,

  6. gregsauls

    gregsauls Homeowner

    Messages:
    56
    Location:
    Texas
    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.
  7. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221


    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: Feb 23, 2009
  8. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,706
    Location:
    Chicago, IL
    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.
  9. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221
    The use of pressure balanced shower valves together with a tankless water heater is not recommended by both Peerless and Delta. Symmons does not go that far, but they say there may be issues.

    The bottom line seems to be that pressure balanced does not mean you end up temperature balanced. It seems that the minimum cold water flow through the valve may be too great to allow for a warm shower.

    I have the old style two knob mix system, so I don't have that problem. I do know that I barely mix any cold water in with the hot when I take a shower. I will be adding a second shower in my 1/2 bath in the near future, so this could be an issue. Code requires the use of some type of an anti scald valve, and the pressure balance type seems to be a poor choice for use with a tankless water heater.


    Peerless FAQ


    Delta FAQ
    Often the outgoing water pressure from a "tankless" water heater is relatively low. As a result, these devices are not generally recommended for use with pressure balanced units due to the possible differences in water pressure from the hot and cold lines. For example, if you were to have 20 PSI on the hot supply line and 50 PSI on the cold, since pressure balancing adjusts to the low pressure, your resulting operating pressure in the shower will be reduced.

    Symmons FAQ
    Tankless (on demand) hot water heating systems are capable of producing a fixed amount of hot water per minute based on certain incoming cold-water temperatures. To provide a comfortable shower or tub/shower temperature during the colder winter months a larger amount of hot water is needed to compensate for the lower cold-water temperature being mixed in the valve. Also depending on the area, a tankless water heater must raise the cold-water temperature from 40°F to 120°F in the winter months, whereas in the warmer (summer) months the cold-water temperature might be 65°F. This increase in demand for hot water typically outstrips the ability of the water heater to produce enough hot water resulting in a luke-warm shower or tub temperature.


    .
    Last edited by a moderator: Feb 4, 2009
  10. Ladiesman271

    Ladiesman271 Homeowner

    Messages:
    221
    There has been some discussion about the cost of "proper" maintenance of tankless water heaters.


    Well, here is a discussion about the "proper" maintenance of a tank type water heater.

    http://www.nashville.gov/water/splash/h2o_heater.htm



    Water heater maintenance

    Water heaters, whether gas or electric, have become more complex and expensive. They also do not seem to last as long as they once did. This is one modern appliance that we couldn't live without, but is "out of sight, out of mind" until we have problems with it.


    Hot water heater flushing

    Maintenance books recommend that the hot water heater be drained (flushed) every six months, but few homeowners bother to do this. Often, it is put in an area that is not as accessible as other appliances. It may or may not be easy to drain, even though all heaters have a hose connection and faucet control at the bottom. This task is not only inconvenient, but often the washer and/or washer seat on the faucet must be replaced after draining. Sometimes the entire faucet assembly will have to be replaced. If this is not done correctly, the unit may leak at the faucet. Furthermore, the unit can be damaged while drained. Unless the customer is fully familiar with servicing these units, one should consult their service representatives at the gas or electric utility before undertaking these tasks.


    Odors

    A heater which is recycled off/on or left off for a period of non-heating may develop offensive odors from sulfur bacteria. The odor is hydrogen sulfide -- "rotten egg odor." This odorous water may be drawn back through cold water faucets as well as the hot water faucet.


    Temperature settings

    The heater thermostat should be set at a reasonable temperature. Scalding of infants and the elderly can be a real hazard, even when the temperature setting is within proper limits. Again, the user should consult with their energy supplier to have the unit set at a safe temperature.


    DO NOT USE HOT WATER FROM THE TAP FOR DRINKING OR COOKING!

    Because warm/hot water is much more corrosive of metals than cold water, one should not use the hot water tap as a source for drinking, cooking or dilution of infant formula. It can be high in metals such as aluminum, iron, lead, copper and zinc. The heater tank also tends to concentrate these metals and precipitate them in layers inside the heater tank. Many tanks have a metal "sacrificial anode." This is designed to wear down and can be an additional source of metals. While it may be a convenient shortcut, water from the heater is not an approved source of drinking water.


    White plastic particles

    It is not unusual for the white plastic (PVC) filler tube inside the heater to disintegrate and discharge small white particles to the faucet aerator screens. These particles can come through not only the hot water lines but the cold water lines as well. These particles may appear to be soft and crumbly, but a good test is to heat this material with a match. PVC will melt; minerals, such as calcium, will not.


    .
    Last edited: Feb 6, 2009
  11. gregsauls

    gregsauls Homeowner

    Messages:
    56
    Location:
    Texas
    Here's the link to the 2009 energy tax credits. There is a $300 tax credit on the installation of "approved" high efficiency home products. Good info...
  12. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Imagine getting a tax credit to install a piece of equipment that is marginally more efficient than regular equipment and you wonder why the economy and government are going down the crapper.

    Welcome to the great Tankless Water Heater Rip Off:cool:
Similar Threads: Tankless basics
Forum Title Date
Tankless Water Heater Forum Eternal Gas Tankless Water Heater Noise!? Jun 7, 2014
Tankless Water Heater Forum Lifetime of tankless water heaters? Jun 3, 2014
Tankless Water Heater Forum My son sprayed the water hose down my tankless vents! Jun 1, 2014
Tankless Water Heater Forum I descaled my electric tankless heater May 7, 2014
Tankless Water Heater Forum Tankless gas water heater overflow pipe? Apr 30, 2014

Share This Page