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Stay with a conventional system or go with a combi

somesuv04a
somesuv04a Member Posts: 4
edited January 2021 in Gas Heating
Hi,
My name is Andy medina, I live in Elmont NY, I need help in making a decision, please keep in mind this is my investment property and do not live there. It's a 2 family house first fl has a Well Mclain 88K BTU and a 50 gal hot water tank, 2nd fl ha a smaller Well Mclain 70K BTU and a 40 gal hot water, this system is 25 yrs old ecept for the boilers, that I did myself 10 yrs ago.

I have gotten some crazy prices and good prices to replace everything, the company I'm considering wants to give me two Well Mclain CGA 4 105 BTU and two AOL 50 gal hot water heaters or for I can get two Navien Combi NCB 240 which eliminates the hot water heater

I'm digging the combi system but as I do need this to last another 25 yrs without craziness, I'm still thinking to stay conventional.

Any info would be highly appreciated, stay safe and stay blessed

Andy

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 20,416
    First -- no prices. Would you edit them out, please?

    Then on to the actual question. There are two problems with combis. I'm presuming that the existing system heats the spaces adequately? That tells me that the existing boilers are at least big enough, if not perhaps too big. Therefore you need a maximum output for heat of 88,000 BTUh for the first floor and 70,000 BTUh for the second. However, for hot water you would need, if you use combis, an output of almost 200,000 BTUh -- each -- to keep the tenants happy with their hot water. Does not compute. Either the tenants will not have enough hot water -- which isn't a good idea -- or you will be operating the boilers for heat way down on their modulation.

    The other problem is complexity and longevity. The more sophisticated the system -- such as a combi -- the less likely it is to have a long and trouble free life.

    In my opinion I would go with separate boilers and hot water heaters, each designed and sized to do the job being asked of them. I do have a concern about the boilers you mentioned: as I noted above, it appears that your actual heating loads are no more than 88,000 BTUh and 70,000 BTUh. The boilers you quote are significantly oversized for those loads, and you could do better with smaller ones. A hot water system should be sized to the building heat loss and no more; You should ask the contractor to provide a Manual J calculation of the expected heat loss to verify the correct boiler sizing. You can double check his or her work quite easily using one of the on-line calculators. I prefer the one from Slant/Fin, here: https://www.slantfin.com/slantfin-heat-loss-calculator/ but there are others.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    somesuv04a
  • pecmsg
    pecmsg Member Posts: 3,532
    150K BTU's heating is probably WAY oversized.
    1st Get a heat loss calculation done for each unit. Slant fin is decent. https://www.slantfin.com/slantfin-heat-loss-calculator/. Gessing or replacing what was there doesn't work
    Whos paying for the fuel the Tennent or you?
    somesuv04a
  • JUGHNE
    JUGHNE Member Posts: 10,278
    Your existing boilers are not that old, why would you change them?
    They may be a little oversized for each unit already.
    What type of radiation is in the units....CI or BB?

    And if existing WH are large enough but older maybe just change them.

    As a landlord myself I am more concerned about reliability and ease of maintenance.

    New Mod Con's or Combi's will give you some grief over time with cleanings required.

    Tenants are happier with functioning units, rather than with minor, maybe 10% efficiency savings.IMO
    somesuv04akcopp
  • hot_rod
    hot_rod Member Posts: 18,190
    I like the combi option, I think you mean one for each floor? A 120 or maybe a 150K would work and you can derate the heating side down to 12 or 15,000.
    I have lived with 3 combis now the oldest is 15 years old. All mod con boilers combi or not should be cleaned and checked yearly including deliming the HW heat exchanger, plan on that cost if you go with a mod con combi. I think just the modulating function is worth consider a mod con combi, and after a load calc you might be able to run lower supply temperatures for most of the year optimizing the condensing function.

    If you already have the flue, combustion air, etc, maybe a conventional boiler and indirect are less complex and less $$ to maintain.

    A good argument could be made either way in your case. A load calc may better help make an informed decision.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • somesuv04a
    somesuv04a Member Posts: 4

    First -- no prices. Would you edit them out, please?

    Then on to the actual question. There are two problems with combis. I'm presuming that the existing system heats the spaces adequately? That tells me that the existing boilers are at least big enough, if not perhaps too big. Therefore you need a maximum output for heat of 88,000 BTUh for the first floor and 70,000 BTUh for the second. However, for hot water you would need, if you use combis, an output of almost 200,000 BTUh -- each -- to keep the tenants happy with their hot water. Does not compute. Either the tenants will not have enough hot water -- which isn't a good idea -- or you will be operating the boilers for heat way down on their modulation.

    The other problem is complexity and longevity. The more sophisticated the system -- such as a combi -- the less likely it is to have a long and trouble free life.

    In my opinion I would go with separate boilers and hot water heaters, each designed and sized to do the job being asked of them. I do have a concern about the boilers you mentioned: as I noted above, it appears that your actual heating loads are no more than 88,000 BTUh and 70,000 BTUh. The boilers you quote are significantly oversized for those loads, and you could do better with smaller ones. A hot water system should be sized to the building heat loss and no more; You should ask the contractor to provide a Manual J calculation of the expected heat loss to verify the correct boiler sizing. You can double check his or her work quite easily using one of the on-line calculators. I prefer the one from Slant/Fin, here: https://www.slantfin.com/slantfin-heat-loss-calculator/ but there are others.

    Thx man, sorry about the prices.
  • somesuv04a
    somesuv04a Member Posts: 4
    pecmsg said:

    150K BTU's heating is probably WAY oversized.
    1st Get a heat loss calculation done for each unit. Slant fin is decent. https://www.slantfin.com/slantfin-heat-loss-calculator/. Gessing or replacing what was there doesn't work
    Whos paying for the fuel the Tennent or you?

    1st fl me as the basement is attached to it, 2nd fl tenant.
  • somesuv04a
    somesuv04a Member Posts: 4
    JUGHNE said:

    Your existing boilers are not that old, why would you change them?
    They may be a little oversized for each unit already.
    What type of radiation is in the units....CI or BB?

    And if existing WH are large enough but older maybe just change them.

    As a landlord myself I am more concerned about reliability and ease of maintenance.

    New Mod Con's or Combi's will give you some grief over time with cleanings required.

    Tenants are happier with functioning units, rather than with minor, maybe 10% efficiency savings.IMO

    Hi my old units and exhaust pipe are all rusted up from water coming down the chimney which I intend to fix.
  • PerryHolzman
    PerryHolzman Member Posts: 234
    I suggest you fix the chimney and vent piping... and have unsightly boilers if they work.

    I have a 14 year old Viessmann Vitodens 200 (WB2A) in my house and have followed this forum since 2006.

    If it was my rental property. I'd put in base heavy cast iron or steel atmospheric boilers (preferably with the old style burners) without any of the fancy controls and separate water heaters if you are after reasonable cost and long term reliability. The heavier the boiler for its size = thicker cast iron or steel. Many of the most modern lowest cost cast iron or steel boilers are starting to fail by the 10 year mark because they have pushed the velocities and thinned the cast iron or steel to the point that they are now a regular consumer good type replacement item.

    The extra 5-10% in efficiency you will gain will not likely offset the cost of routine cleanings and parts on the mod/con boilers. I've dumped about 1/3 in parts alone compared to a replacement into my boiler at the 12.5 year mark; and I did the troubleshooting and labor: Likely would have been 1/2 - 2/3 the cost of a replacement boiler if I had used a heating contractor for those repairs - I just don't see any real financial payback for mod/con boilers at this point.

    I wish you well with your project,

    Perry
  • ethicalpaul
    ethicalpaul Member Posts: 4,176
    As a landlord myself I am more concerned about reliability and ease of maintenance.


    Possibly the wisest words I will read this year
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
    MilanDMaxMercy
  • Suzook
    Suzook Member Posts: 221
    Go with conventional in a rental, especially using 2 of them.