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Opinions on Tank Combi Boilers

cbut8995 | Posted in General Questions on

Any have recommendations or have opinions on tank combi boilers?

I saw this one by LAARS and have a friend who used this brand for his new 30 unit development in NY, NY. He doesnt know how they are yet either but they were European according to him and very expensive. But he did use the mascot FT instead of the one I am looking into.

I was planning on using this tank combi waterheater/boiler for my 1 bedroom apts around 550 sqft (45 gallon option) and my the other larger 1 bedroom (1200 sqft) using the 72 gallon option.

Any opinions if these would short cycle at all given they are tanks?

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  1. Expert Member
    Akos | | #1

    That is a good match for a low load application, they aren't chep though. Since it is a tank, it won't short cycle.

    The size of the tank mostly depends on your hot water use not the heat load since the heat exhanger for space heat is the same size in both. You would want a larger tank for something that can have 2 to 3 simultaneous showers or a large tub.

    I'm guessing the heat load of the largest unit is well under 20kBTU, the unit can supply 140F water for space heat, let you use much shorter baseboard runs.

  2. cbut8995 | | #2

    I called LAARS today but even the rep on the phone doesnt seem to know as much as you as they didnt seem confident when I was asking them questions. They said it will most likley short cycle at times as my heating load for the smaller tank is still under 20k and the heat exchange value minimum is still greater than what I would need. For example at my small apts of about 20 feet of baseboard at 600BTUH/ft I would only be at 12k BTU which is just over 50% of their minimum for the smallest boiler. Does this seem right to you from what the rep said that this combi heat tank boiler will still short cycle?

    I was looking into the WM Aqualogic system combined with WM97. Would this sytem work better for my scenario since it is technically 2 separate systems that are wall hung but can be connected and combined. I am hoping this wont affect the short cycling issue and can work for me completely even if this is more expensive.

    Also do you have an opinion on WM products? I have currently use there floor standing models for just space heating on a lot of older rental units and they work perfectly with baseboard but this technology from them seems new to me.

    Another option I thought could work would be the HTP Elite Ultra COmbi Boiler:

    It seems the lowest combi ELU-120WCN has a minimum heating output of 11,200 and minimum DHW 11,200 BTU. I am hoping this is a winner for us. What are your thoughts? I also never heard of this brand HTP so I am not sure on its reliability.

    Other similar low BTU i found were:

    AO Smith:,000-BTU-Natural-Gas-Combi-Boiler/


    1. Yupster | | #3

      The "short cycling" through the heat exchanger in a combi hot water TANK would just be the circulator pump turning on and off, if set up to be controlled that way. There are plenty of ways to fix that, pump with a lower flow rate through the heat exchanger, or put a mixing valve on the heating side of the heat exchanger and don't worry about what temperature the water comes though at.

      And to repeat a commonly heard phrase on this need a manual J before making any decisions on heating equipment design.

  3. Expert Member
    Akos | | #4

    The rep doesn't know what they are talking about. Tank units won't short cycle because it takes a lot of time to heat up that much water.

    I would get an HVAC designer go through your design, no need to guess at details. At the water temperature the Laars combi provides, you'll only get 300 to 400 BTU/ft out of baseboards. If you really need 12k btu, you'll have to add more length or use the taller ones.

  4. cbut8995 | | #5

    Thanks all. I think we are sticking with either:

    1) Weil M AquaLogic with WM97 Boiler lowest BTU heating 14K BTU
    2) HTP Elite Ultra combi lowest BTU heating 11.2k BTU
    3) lochinvar noble combi lowest BTU heating 11K BTU
    4) Burnham US Boiler K2 Combi lowest BTU heating 12k BTU

    Do you have any opinions on them and their ability to not short cycle. Is the Weil M option better since its 2 separate systems technically so it wont short cycle even if the minimum heating BTUs are not met as it is significantly more for the Weil M option around 5K for each while HTP and Lochnivar are each about 2.5k? Akos, I know you are somewhat familiar with the apt designs.

    This is due to the direct vent option that my architect urged me to choose given the LAARS combi heat is not a direct vent.

    THank you all for your input!

    1. Yupster | | #6

      Did you consider something like this:

      Direct vent, much better quality than the LAARS, won't short cycle, 95% efficiency.

      Consider that adding a buffer tank to a boiler can resolve short cycling issues. Also makes for a good hydraulic separator and air/dirt separator.

      1. qofmiwok | | #9

        Any idea the price range on these?

        1. Mark_Nagel | | #10

          Many folks in this forum are great, but this site is dedicated to heating with boilers:

          Per your posting in another thread, if it's only 5k BTU then you're unlikely going to be able to serve that low of a load with a gas boiler: and I don't even think electric boilers go that low. Unless, that is, you're also heating up a lot of DHW (read "low space heating requirement but high water heating requirement [LOTS of showers etc.]).

          NOTE: I am NOT an expert in any of this. Nothing to sell. Have bought nothing pertaining to any of this. I'm just learning as I research for possible solutions for my own self-build.

    2. Expert Member
      Akos | | #7

      There is not much point about talking about short cycling till you have real heat load numbers for each apartment.

      If you can get a tankless combi to work, it saves a lot of space and simplifies the install.

      The Laars combi is power vented, not sure what the issue is, it doesn't need a chimney. You do have to provide make up air for it, but that should be no problem for most installs.

  5. cbut8995 | | #8

    Hi Yupster hydro versa is too large for the 1 bedroom/1 bath units in the building but I am going with the idea of having a combi boiler and then adding a buffer tank below it.

    Agreed Akos, but just seems that all combi boilers out there are too large no matter the case and we are just going to add a buffer tank to resolve that issue.

    Hoping it works out.

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