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Indirect vs tankless coil

spt87 Member Posts: 13

Trying to figure out how converting to an indirect tank would save $$ vs continuing to rely on a tankless coil, especially since I've been convinced that maintaining the boiler as a hot start is better from a maintenance standpoint (so even if I put in an indirect I would keep it hot start).

Scenario: oil boiler maintained at 180F year round.  Family of 7 with the wife liking to take a hotter than average bath nightly.  The coil produces enough hot water (with the flow restricted via a globe valve on the cold inlet) to support one hot water user at a time (tub, shower, dishwasher, etc.)  We have been able to live with that limitation so far.

Going indirect would allow me to unrestrict the hot water flow which likely would allow two+ hot water users at once, but not sure where the savings in fuel would come from since once the hot water draw starts, I assume it wouldn't take long for the indirect to call for boiler water to maintain temp., so the burner needs to run roughly the same amount as it does to support the current tankless coil.

Where would savings come from?  Maybe could cut the boiler temp to 160F which might have some impact, but I wonder if it is enough to bother.  Also wonder if cutting the boiler temp 20F would be enough to offset any additional standby losses from an indirect tank (although I know they are pretty low - there is still some loss).



  • russiand
    russiand Member Posts: 73
    Indirect is more efficient

    I think that majority of the savings docome from fewer firings of the boiler during the non-demand hours. The heat is transfered to the well-insulated idirect, where the standby losses are much less than those of the boiler itself in the tankless coil case. As far as efficiency at demand time I do think that the indirect is still more efficient since you have a higher flow and more heat transfer occuring in the indirect vs. the tankless coil. Although I guess may be theoretically possible to make a super efficient tankless coil that could compete with the indirect in perfect conditions. Another point is that in most cases the tankless coil can't be set to be on priority like the indirect can and is going suffer from competing with heating zones, etc. So in practice it has too many factors against it to adequately hold it's own ground against an indirect.  
  • lchmb
    lchmb Member Posts: 2,997
    just switched

    One other thing to consider, Not that you probably notice it but if your wife like's a hot tub it probably fill's slow and loose's water temp as it's filling.. Once you go to an indirect it fill's in half the time, water is hotter, wife is happier...life is good..;) I did mine about 6 month's ago,

    Although their was some saving's, the difference in the hot water  was more than worth the cost...IMHO..
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,537

    The entire concept is to replace the very high standby loss of a hot,poorly insulated boiler with that of a well insulated indirect that unlike a boiler has no flue connection.

    You've been misinformed that  oil boilers have to kept hot,the indirect will keep the boiler warm enough with a 100-110 low limit as a floor.

    I would also use a  simple reset control such as the Beckett Aquasmart or Hydrolevel Fuelsmart to modulate water temps for even more savings.

    The best solution would be to get rid of a boiler that requires being kept warm at all,it's insanity with oil $4 plus a gallon.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • icesailor
    icesailor Member Posts: 7,265
    Indirect vs Tankless Coil:

    You have heard from the indirect lovers/Tank-less Heater haters. Now, you hear from me. With a different take.

    IMO, a tank-less coil is an indirect inside a boiler. It is heated by the water in the boiler. A water heater indirect has an indirect coil inside a water tank and has the boiler water pumped through it.

    If you called me up about a tank-less in your boiler, and you wanted to save money and not spend a lot to get a lot of hot water, I would suggest a storage tank. I use electric water heaters. Electric in name only because the only electric connection is a switch leg through the bottom thermostat. The elements are not connected. I pipe the water heater like it is a stand alone heater. 3/4" in, 3/4" out. With 1/2" copper tube, I connect the cold inlet of the tank-less to the bottom of the water heater. I put a Taco 006B bronze threaded circulator and pump out of the tank bottom and into the tank-less. On the outlet of the coil, I pipe to the top of the water heater and connect it so that the hot water enters through the top and down the dip tube. Connect a cord whip to the circulator and use the bottom thermostat as a switch. Set the thermostat to what you want. You don't need any electrical connections to the boiler controls. You can then set the low limit/operating control setting at 140 degrees. You can set the high limit at 160 degrees or higher. You can raise the operating control higher. There is no cutting in to the boiler water system.

    If you want an indirect, you need to drain the heating system, cut into the piping, install a circulator and controls, get it wired correctly, and you can't maintain a warm start. I have used this piping arrangement for over 30 years. It is much cheaper than an indirect but is an indirect without the complexity. I have never had anyone complain. If the water heater leaks, it is an easy replacement. If you install a valve on the hot water side, you don't even need to drain the water in the house. I often find when someone needs more hot water, that piping an indirect is such a giant project, the customer would be without heat and hot water for more than a day.

    Over 20 years ago, I installed a storage tank on a customers house. It was a Weil-McLain 568. I put in a 50 gallon electric as above. The house was sold years ago. It was an old house and probably at one time had a coal boiler. The iron piping behind the boiler was a nightmare. The tank was leaking. A plumber friend does the work there now. He went to try ti install an indirect. He realized that it would be a monumental project to just change a water heater and install an indirect. He asked me to show him how I did it. I went over and showed him how I do it. When the light bulb went off, he was amazed at how easy it was to do.

    I do indirects. I'm doing a couple right now on Veissmann Vitodens boilers.
  • mark schofield
    mark schofield Member Posts: 153
    Carlin Aquabank

    Some of you may remember the Carlin Aquabank. We put one in my dad's house in 1976. Fed by the tankless coil. It has a bronze B&G. Stone lined. It's still going strong with no leaks. Like Icesailor said. 
  • icesailor
    icesailor Member Posts: 7,265
    Storage vs Indirect tanks:

    Every HW dedicated storage tank I have seen is a lot more expensive than a common electric water heater.

    When I had the house I now live in, built in 2001, the plumber who the builder used insisted on an indirect. I told him no, to use a water heater. Most don't pay any attention to what I say when I am paying the bill so he went out and bought a John Wood hot water storage tank with a set of drawings on how to pipe it. The water tank was a 50 gallon electric with the elements were plugged with 1" pipe plugs. It was piped as per the manufacturers instructions. I re-piped it to my way and it works better.

    Every hot water storage tank drawing I have seen uses the same piping. Mine works better,
  • icesailor
    icesailor Member Posts: 7,265
    Carlin Aqua-Banks:

    Carlin Aqua-Banks (Actually made by Ford Products) were actually gas water heater tanks. The circulator sucked water out of the bottom (cold), pumped it through the coil and into the opening where the gas valve went. It was the same as being sent down the dip tube on an electric water heater.
  • Alan_13
    Alan_13 Member Posts: 10
    ice sailer

    I,ve installed a few using the "electric tank" storage method. Using 30 gal tanks (no space for larger tank) there was insufficient hot water, I'm guessing that when the boiler supply came down the dip tube and exited the drawoff there was not enough mixing with the tank, so ended up removeing the dip tube. Much better.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    a good indirect

    should last longer than a basic water heater, typically. If not though, any tank with a heat exchanger (internal or external) can be used as an indirect off of a boiler. I don't see "heat exchanger" in icesailor's description. If you're just using a tankless coil in a boiler to feed an indirect, you're not really gaining anything except better hot water performance than the tankless coil would offer. If you add up the tank and all components plus the coil, you'd probably have been better off simply not getting the coil in the first place and doing a regular indirect.

    Or even better, skipping the boiler for domestic and doing a stand alone water heater.

    Basic points regarding all of this:

    1. No cast iron boiler doing domestic is efficient. Both cast iron configurations run domestic at just about 40% final efficiency. Tanks perform better for delivery at the fixture and for having less problems, but efficiency isn't better.

    2. However, If you do an indirect with a purge controller and a cold start boiler, you can just about double that efficiency. then you skip the boiler standby losses, largely, and you don't pick up a massive cycle inefficiency in the mass of the boiler.

    3. You can use a stand alone water heater and gain 50% on your domestic efficiency as well. cheaper, but not as notable of an efficiency gain.

    I've attached it before but a BNL test on this kind of thing is now attached for your pleasure.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • icesailor
    icesailor Member Posts: 7,265
    Indirect Vs Tankless Coil:

    If you remove the dip tube, you destroy the function of the water heater.

    You must not have piped it properly. Did you use a circulator pumping the heated water out of the tankless and pumping into the top of the tank? In 30+ years, I have never has a storage tank not work piped as I do them. I have even used hot water tanks connected to oil fired water heaters to stop them from stacking. I set the oil fired one to run as it wishes and set the storage tank at 125 degrees. I did a  Bock 73E, 220 gallons per hour recovery connected to a 120 gallon storage tank. The place had a big load for a few hours and then no real load for the rest of the day.

    I've done it with gas water heaters where the owner was running out of hot water for his jetted tub. I added a 50 gallon electric converted to a storage tank and now have 100 gallons stored.

    By running the powered heater at a higher temperature, and running it into another tank where it goes into the load, you have theoretically increased the size of the water heater. Just like raising the temperature in a heating system theoretically increases the amount of radiation.

    If you had one that didn't work, it was piped wrong. I have been doing it for over 30 years. I can not tell you how many I have done. If I had just one that didn't work, I would have never done it again.

    A water heater that is fed at the top with both hot and cold will not ever work without a dip tube. The cold water will enter the top of the tank on a draw and go right up the hot side and no hot water. The tank will get hot when there is no draw but when the draw starts, it will short cycle. The function of the dip tube is to inject the cold water on the bottom. Stacking/layering is caused by short draws. Water tanks/heaters that have a bottom inlet don't need dip tubes because the cold water enters on the bottom.
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