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Community and Q&A

Indirect domestic hot water options

pjwst6 | Posted in Mechanicals on

A couple of questions about indirect hot water systems.

1) What savings do folks see over common direct fired water heaters? I’m especially interested in if anyone has experience or can point to a case study for multifamily buildings. I can’t find any research online on this, and I’d really like a decent reference.

2) Any thoughts on the relative longevity and efficiency of 1) package indirect systems (i.e. Rheem Spider) vs. 2) stand alone boiler with indirect tank with integral HX, vs. 3) stand alone boiler, stand alone heat exchanger, and storage tank? I like the modular approach of #3 given that tanks oxidize and wear out, but wonder if anyone has any experience. For multifamily use, common indirect tanks only have 5 yr warranty. Tradeoff for #3 is the second pump.


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  1. kevin_in_denver | | #1

    You will get some pretty good answers at

    In the meantime, I can give you my experience with 20-40 unit apartment buildings. The answer is a little different for an older building with a steam boiler.

    Assuming you have a postwar building with hot water baseboard heat, the most common arrangement was originally a "sidearm" water heater. Many of those all-copper water heaters (like Everhot) are still in service. Most energy-savvy owners would have replaced the sidearm by now because it just doesn't make sense to fire up a 1 mmbtu boiler just for hot water in the summer. In comparison, a 120 100,000 btu gallon commercial water heater costs much less to operate.

    (I saved over $1200/yr in 20 unit buildings where I made this change in Denver)

    But commercial tank style heaters only last 8-16 years, and have a energy factor of under 0.60. (you can increase the life of a tank style heater with proper maintenance: )

    The best overall solution is to replace the main heating boiler with a modulating condensing boiler and an indirect water heater. If you have added attic insulation and dual pane windows, the actual design heat loss of the building is probably closer to 400 kbtu than the original 1 mmbtu. ( I did this and had savings of over $3000/yr in a 20 unit building.)

    If replacement of the main heating boiler is off the table for some reason, then it is such a tough decision that I would find out who the best contractors are, interview and get quotes from several, and go with the one that makes the most sense for your building.

    Keep in mind that most installation contractors can't give a good answer to the question "how much will it cost to operate and maintain?"

  2. pjwst6 | | #2

    Kevin, Thanks for your post. If I understand, you had $3,000 per year in savings from replacing the main HW boiler and adding an indirect, in a 20 unit building. Did you ever try to separate the space heating savings from the DHW savings?

    Is there really no research around tracking savings from indirect over direct fired DHW? This is one of the most basic efficiency upgrades.

    Seems that using a typical setup, indirect DHW is not going to operate at a setpoint that would allow for condensing operation, so thermal efficiency will be in the mid 80s at best, and savings are going to be from the reduced standby losses. I think mostly these systems are designed for 160+F setpoints. But, if a large stand alone HX were used, maybe system could be optimized for low setpoint and condensing operation. Is that ever done?

    Also, an integral condensing packaged DHW system, like the Rheem Spider, where there are no pumps, and where flue gas directly heats the DHW directly, on paper seems like a nice alternative. They claim 93-97% thermal efficiency, but I wonder what DHW temp this efficiency claim is for. I also question the longevity and maintenance costs, as I think this is relatively new, and once the tank goes, you may have to throw away a large investment. Anyone have experience with these direct fired condensing DHW systems? Here is a link to a brochure:

  3. kevin_in_denver | | #3

    I like the AO Smith prices much better for similar efficiency:

    I can't speak to longevity.

    I have also recently tried a very inexpensive modular combination like you describe, with two pumps, a 119kbtu tankless water heater, an $80 flat plate heat exchanger and a couple of electric water heaters for storage. Material cost was around $1000, but alas, it hasn't worked reliably, and the electrical usage was more significant than I expected.

    My boiler contractor is very happy with a Weil McLain 85% setup, they are lasting really well.

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