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Indirect Water Heater Efficiency

RS_2 Member Posts: 13
Does anyone have any numbers to support the efficiency of indirect water heaters? Yes, they boast about being well insulated but that's like boasting that your hot brewed coffee in a Thermos will stay hotter longer instead of being in a coffee mug, but that says nothing about the energy efficiency of converting the hot water from the boiler into hot water supply. In the summer when a boiler would normally be turned off it seems that they would be especially inefficient. Why? Because you're heating water in the boiler which you then use to heat up water in the heater. And as anyone who's even vaguely familiar with the laws of thermodynamics (no exception to these laws have ever been found) there is no free energy lunch so energy is lost in this process. This is why it is impossible for a heat exchanger to be 100% efficient. Consider brewing a cup of tea using two methods to get the necessary hot water:
Method One: Put water in kettle and bring to boil
Method Two: Method one then put hot water in a tube then emerse in cold water to heat up this water. Who is willing to bet that Method Two even has an outside shot of being more efficient? Unless you like to lose you won't take that bet. Can anyone prove that Method Two is even within 20% of Method One? I have contacted manufacturers and professional plumbers and after their talking about tangetial topics none have been able to say that their unit is x% efficient at converting hot water from the boiler into hot water in their water heater but they all assert that it's more efficient! I respond as Reagan said, "Trust but verify". Can anyone provide verified numbers? By the way, for those who have paid attention so far there are also other energy (efficiency) losses: electricity to circulate the water from the boiler to the heater, heat loss in the pipes and I'm sure the reader can think of others. I seems that these Thanks for your input on the efficiency of making not storing the hot water.


  • Troy_3
    Troy_3 Member Posts: 479

    Heres one for ya- How about the stand by losses of putting a 3" or 4" steel pipe through the center of your stored water and allowing air to vent through the roof 24 hrs. a day. Seems a little inefficient to me. Kinda like insulating your wall and leaving your window open. A high efficient low mass boiler comes up to temp very fast. With insulated connecting pipe I have to conclude that even the worst indirect is hands down better than the best standard direct fired water heater. Is there really any question?
  • DaveGateway
    DaveGateway Member Posts: 568
    Everyone wants numbers

    But like AFUE and Federal MPG numbers on cars, your milage may vary. What's wrong with the thermos analogy? Juat like Troy said, why is allowing 40 gal of hot water with a vent pipe running down the middle any better? We all know that durring the heating season, the boiler is likely in a hot state already, so it's not like we're cold starting to heat 20 gal of boiler water from 80 to 180°. Now I agree that a high mass system might not be too efficent durring the summer. But a low mass boiler with 4 gal in the boiler and maybe another 4 gals in the indirect HX and piping can come up to max heat fast. I'd rather see the 8 gal of boiler water cool off rather then 40 gal in a gas HW tank. Now if oil is your only fuel, I'd think it crazy to spend the money and future service costs on an oil fired HW heater that only lasts 10 years if you're lucky.
  • dconnors
    dconnors Member Posts: 215
    instantaneous hot water

    this could be the answer if you need a lil hot water. i will stay with my indirect thankyou
  • Jack_21
    Jack_21 Member Posts: 99
    Replaced my indirect

    with on demand and my summer gas consumption was reduced by over 65%. I'll keep my on demand thank you!
  • Constantin
    Constantin Member Posts: 3,796

    It's hard to separate all your points, but I'll try.

    Re Heat exchangers: The water-water heat exchanger does not have to be 100% efficient. It simply has to be much more efficient than any other means of heat-loss. Then you simply run the water through it a couple of times and most, if not all, the heat transfer that could have taken place, will have. BTW, I once held a 99.98% HX in my hands... built for a NASA/DoD space application, NEAT!

    Next, you ought to consider your analogies a bit more carefully. For example, your kettle and tube example could hold true, but not necessarily. A condensing boiler will have its water up to temperature with 90-98% thermal efficiency. Assuming a 90%+ HX transfer, and you have about 90% thermal efficiency at the tap (assuming 120°F water). Now, how many kettles do you know that run cold enough to cause all of the gasses impinging on them to condense continuously?

    Granted, at very low water quantities, using any kind of tank appliance makes no sense. This is where tankless units make the most sense... but try comparing the amount of gas you'd have to go through to fill a bathtub with warm water from a kettle vs. the consumption of your water heater. I can virtually guarantee that the water heater will come out ahead due to its much larger HX and the fact that the bathtub won't go cold by the time your next shot of hot water from the kettle is ready.

    This brings us to the next critical juncture, AFUE. Your standard gas water heater will have an AFUE of about 50-65%. The minimum allowable standard for boilers is 80%. Thus, you'd have to hook up a very inefficient IDWH to not come out ahead. Highly efficienct gas water heaters may make it into the 90%+ AFUE range as well, but why have two very efficient heating appliances when one can do the job of two perfectly well?

    So how much energy do you lose by hooking up a IDWH to a boiler in a given year? Like many things in life, it all depends.
    • A low-mass, condensing boiler won't care one bit to briefly come on to keep the IDWH at temperature, then quickly cool down again. This minimizes stack losses and maximizes the efficiency of turning potential chemical energy into useable heat.
    • On the other hand, a old-school, high-mass system will burn through a lot of fuel before the boiler, IDWH are satisified, then continue to experience stack and standby losses for a long time afterwards. Some manufacturers like Viessmann seem to have developed systems with very low stack and standby losses despite falling into the high-mass category.
    • Don't get me started on electric water heaters. Yes, they are 100% efficient at turning electricity into heat. But first, some resource is 30-60% wasted before being turned into electricity. Otherwise, you wouldn't see that the operational costs of electric units are much higher than even low-efficiency gas units.
    So can I verify the claims of others? No, but utilizing the very laws of thermodynamics you cited earlier, I have come to the conclusion that my IDWH ought to be very efficient once it has been installed. That efficiency will be ratcheted up even further by the VitoSol solar water heater on the roof.
  • hr
    hr Member Posts: 6,106
    I do question the standby loss claims

    for indirects. Most claim less than 1° per hour. My tank has about a 12° differintal, yet I hear my boiler kick on at least once during the night with no DHW drain or heat call. Double check off the primary loop and located below the primary piping, haven't noticed any migration during off cycles.

    Something doesn't add up. And it's in a very warm utility room with solar and other heat gain equipment. I'd guess 80° or more as there is no AC vent in that small mechanical room.

    So with 80° ambient and 130 tank set point the loss should be within that less than 1° per hour claim??

    I have seen plenty of IDWHs installed without check protection and they transfer that energy back to the boiler when it cools below the ID temperature. I've watched and measured that happening.

    Use a good check pump on the supply and throw a second check, or zone valve, on the return would be my advise for ID installations.

    hot rod

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  • Mellow_2
    Mellow_2 Member Posts: 204
    sounds like trouble

    What would you do? How would you make hot water? Any flame under a tank of water will not last as long as indirects in what I have seen. The extreeeeeeeem heat from the flame burns out the bottom of SOME tanks. soooooo...... how would you do it.......??????efficency ..........I have to look at the big picture, I do not want to replace this in ten years. I want to clean it and get another 20yrs. If I replace it in 5 or 10 years, no savings in eff. will pay for the unit. The indirects I sell only lose 1/4 degree every hour and can stay warm for about 10 hours. Every system is differant and you can NOT say how much you WILL save(with out all the info and alot of time).....but we try to estimate.... EFFICENCY is only important if the unit runs long enough to get the savings....... Most indirects are built to last and made to store any energy generated by the boiler, not waste it.....Heaters with flames under the tanks are good for places you need really fast recovery and can work well and save if done right........
  • David_5
    David_5 Member Posts: 250
    Stand by loss

    I have a 17 year old super stor 40 gallon. I've serviced my boiler and foregot to turn it back on. 12 hours later I took a shower and didn't notice any problems. I see heat traveling up the water line on the tank outlet. If I ever replace I'll trap it to prevent it.

  • jerry scharf_2
    jerry scharf_2 Member Posts: 414
    my take on this


    Here the thinking that led me to where I have ended up on this, which is a boiler and well insulated indirect. (Mine happens to be a dual coil with solar, but the decision would have been the same without that.)

    The first is your regular old water heater. It has a 80% efficiency burner, and as Troy pointed out, it has horrid standby loss because of the flue running right through the tank. Hands down loser, the one everyone likes to compare against. No need to discuss it further.

    Second you have a boiler and an indirect. The HX efficeincy is quite good, as you loop it through the boiler rather than using a one pass system. The standby loss is better than the standard, but it still exists. You have pipe connections slowly pulling heat out and I doubt that accounted for in the manufacturers numbers, so I don't have any way to give exact numbers. My Stiebel-Eltron tank has 3" of polyurethane, so it's quite well insulated and a larger tank increses the volume to surface ratio (a good thing.) More on condensing boilers and indirects below.

    Third, you have the instant gas heater. This unit has no standby loss, but none are available with condensing HX (they have them in Japan...) So the question is do you gain more from having no standby loss or from having a more efficient burner/HX. You have to estimate the tank temperature and water use to see where the crossover is. At low use, the demand will win, as use goes up the boiler will win. If you could get a condensing demand gas unit, it would be the hands down efficiency winner. You might even get the exhaust air out cooler than the combustion mix went in.

    The problem with condensing boilers is that many of the boilers are set up for maximum recovery rate rather than maximum efficiency. You would really want to have the supply water be just hot enough to get the water to the temperature you want and get the return from the inderect below 130F. I don't know how all of them do it, but at least some don't give you this choice.

    The rub for me and why I didn't go the demand route was the finicky function on the demand units. Everyone talks about the fact that there's a max flow rate, and that you can often go below the listed flow rate in the winter. Also, if you go below some lower flow rate, the unit won't heat at all. The burner has a limited modulation range, and when you go below the bottom it has to turn off. This is often in the 1 GPM range. So if I'm washing things in the sink and I turn on the water at under half of the full flow pure hot water rate (facuets are 2GPM max) you get no hot water at all. With the tank, you get hot water from a drip to a flood (till the tank runs out.)

  • DaveGateway
    DaveGateway Member Posts: 568
    Stand by loss claims

    are probably done is a lab without any piping on the indirect. But I would expect migration through piping on both electric and gas HW heaters also. As you said, good piping and flow checks are part of a quality install.
  • S Ebels
    S Ebels Member Posts: 2,322

    For Steve Rockwell, a customer who likes to be able to make an informed decision.

  • S Ebels
    S Ebels Member Posts: 2,322
    Boiler VS Tank temp observation

    The Vitodens is capable of being programmed to maintain a preset temp difference (ex. 20*) between boiler temp and water temp in the indirect. This allows for maximum efficiency if you wish. It can also be programmed for max recovery, in which case it climbs to max setpoint and recovers tank temp AFAP. It's neat to listen to the burner ramping up and down while maintaining the 20* (or whatever) differential as the DHW temp rises.
  • Steve Rockwell_3
    Steve Rockwell_3 Member Posts: 18
    And Ask Too Many Obscure Questions

    Thanks Steve.
This discussion has been closed.