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Combi boiler for new construction

cbut8995 | Posted in General Questions on

I am building a new building in Brooklyn, NY. Its going to be a rental building all 1 bedroom apartments except for 1 large 1 bedroom/2.5 bath apt with basement apt. I am looking for the best most efficient combi boiler for the 1 bedroom/1 bath (approx 516 sqft). The large 1st floor apt with basement is about 1200 sqft.

Any recommendations on brand/models as I don’t mind paying for a premium if they dont cause any problems for a really long term given this is rental building is part of my retirement plan.

Building Layout attached.

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  1. Expert Member
    Dana Dorsett | | #1

    Combi boilers generally suck for apartments, but they can be made to work. They are better suited to homes with large heat loads and moderate hot water use, whereas zpartments have very low heat loads and moderate hot water use.

    The 99% design heat loads of apartments in Brooklyn are usually well below the minimum fire output of a 199KBTU /hr combi boiler that would be needed to support 2.5 baths. Most 1-bath apartments would need at least a 130- 150KBTU/hr combi boiler.

    The higher the minimum fire rate, the more radiation/baseboard is needed to keep the thing from short-cycling itself into low efficiency and an early grave. Read this for a more detailed explanation:

    Navien's NCB series is probably going to be the best fit. The NCB-150E or -180E would be fine for most single-bath apartments, and can throttle back to about 11,000 BTU/hr-out or 13,000 BTU/hr out, respectively. To run in condensing mode without excessive cycling takes between 50-60 feet of typical fin-tube baseboard for the -150E, 60-70' for the -180E, all operated as a single zone.

    The NCB-21oE or NCB-240E would be necessary for a 2 or 2.5 bath apartment. Both of those have a min-fire output of about 17,000 BTU/hr, and would need 80-90' of baseboard to run in condensing mode.

    The actual design heat load of a 516 square foot apartment is likely to be less than half the 11 KBTU min-fire output of the -150E (which is SFAIK the lowest minimum fire output combi currently on the US market). The design heat load of a 1200' first floor above a code minimum insulated 1200' basement might be more than half the 17K output of the -210E. These combi-boilers will essentially NEVER modulate with heat load, but given sufficient heat-emitter can still operate efficiently.

    Don't let an HVAC contractor talk you in to installing less baseboard (or equivalent radiation) that what works out to more than the 250 BTU/hr per running foot, at the minimum fire input of the unit. Many old school installers accustomed to dealing with cast iron boilers would do one of two things:

    1: Calculate the heat load, divide by 600 = the number of feet of baseboard.

    2; The max-fire output of the boiler divided by 600- the number of feet of baseboard.

    Neither of these are going to be correct.

    It gets even fuzzier if they are using crummy BTU per square foot rules of thumb for calculating the heat load rather than a formal Manual-J or I=B=R type load calculation. In my area hacks tend to use 25 BTU/hr per square foot, or 35 BTU/hr per square foot as a load "calculation" (reliably oversizing by 2x or more) for option # 1.

    That would make the 516' apartment either:

    516' x 25 BTU/hr = 12,900 BTU/hr...

    ...divided by 600 BTU/hr per running foot = 21.5 feet of baseboard, (which is too small by half, even for the -150E)


    516' x 35 BTU/hr = 18,060 BTU/hr per running foot= 30 feet of baseboard, still WAY too small to run the -150E at condensing temps without short-cycling.

    1. cbut8995 | | #3

      Thank you for this info Dana. This is very helpful. We are just short on space as you can see in the basement and for other properties we developed we used a tank gas water heater from AO and separate gas boiler for the baseboard heating from WM. Given we are short on space I was hoping a combi boiler would be a good option without using too much space or having to short cycle.

      Would you think a gas water heater and a LGred ductless minisplit system would be sufficient enough if we use high enough BTU for the mini split for both heating and cooling without having any baseboard heating?

      I am always worried if we have a blackout the tenants would complain on no heat, no AC, or no hot water, etc.

    2. cbut8995 | | #12

      Hi Dana,

      Do you think the LAARS combi Heat Combi Boiler is a better alternative to fit this need then? We would use the 50 gallon for all 1 bedrooms.

      Each 1 bedroom has 1 bathroom, 1 kitchen, 1 W/D, 20 total feet of UF2 runtal baseboard heating.

      2 apts in this category have 30 feet of UF2 runtal baseboard heaters instead of the 20 due to the mezzanine. I am hoping this doesnt change much

      The 2 bedroom, 2.5 bath apartment would probably use the 75 gallon tank as it has 2 full bathrooms, 1 half bath, kitchen, larger W/D, 55 feet of UF2 runtal baseboard.

      A general question about these combi heaters, for the heat option be used for different rooms or when you turn the thermostat on it must heat all of the baseboards simultaneously and cannot be separated.

  2. Expert Member
    Akos | | #2

    Are you looking for a single combi for the whole place or a separate heating unit for each apartment?

    The common way here (Toronto) is an open system with a power vented tank water heater connected to a hydronic coil. The whole setup usually fits into a small closet. Even the smallest BTU gas water heater can easily heat a 1000sqft+ apartment.

    1. cbut8995 | | #4

      We are planning on doing separate combi for each apt as the tenants pay their own bills but I have been reading this forum a lot and keep seeing short cycling etc and do not want this headache when tenants move in.

      1. Expert Member
        Akos | | #5

        If you are going to be installing AC for each apartment, I would skip the hydronic heat completely and go with heap pumps. NYC is warm enough that any of the smaller cold climate heat pumps can easily cover the heat load of even the larger unit.

        For hot water, you can go with either a tankless water heater for each unit or skip gas completely and install an electric tank.

        Either option is cheaper than trying to horseshoe in a combi + AC into such low heat load apartments.

  3. cbut8995 | | #6

    Are electric tank water heaters more expensive in the long run compared to gas tank water heaters?

    I have another project finished and have have heard tenant complaints on how expensive it is for electric heat (mini split) and hot water (tank) compared to other apts that use gashot water and has gas heating (hydronic baseboard)

    1. Expert Member
      Akos | | #7

      Electric hot water is definately more but you do save a bit since there is no gas meter fee.

      I think the bigger issue is that it is all on one bill so it seems high. The reality is that it is only a bit extra per month.

      For example, my gas/gas apartments use around 200kWh per month, vs gas/electric DHWT is 350kWh (~$15 extra per month). Last time I did the math, heat pump would have bumped up the winter time heating bill by $30/month. Add the two together and take out the meter fee, and you are not paying all that much extra. But people like to complain....

      1. cbut8995 | | #8

        Thanks for the insight. I just worry a lot when it comes to electric and its possible inefficiencies in heating but I guess since the 1bed/1bath apts are small the heating should be fine even with 10+ feet height ceilings.

        We already will have gas due to cooking and was hoping a good combi solution that wouldnt short cycle so adding heat as gas would make it seem less of a waste to bring gas in JUST for the cooktop given the wall oven is electric anyways.

        I think our best bet would be for the 1 bed/1bath (516sqft) apts use gas water heating, gas cooktop and 2 Electric ductlesss mini splits from LGred for heating and cooling. Just want to make sure I don't have much problem or cost with them with 10 years as my returns are based on this lifespan. For the 4th floor mezzanine apts that are still 1 bed/1bath but almost 700 sqft and 18 feet ceilings probably same bet but we will end up having 3 mini split units in total.

        But for the larger apt which I plan on living in on the 1st floor and basement I was hoping for a combi unit as I plan on having more than 50 feet total of runtal baseboard heaters underneath the windows along with a ductless mini split system with heating and cooling.

        1. Expert Member
          Akos | | #10

          I guess if you are already paying for the meter fee, heat pump would be more.

          I would follow Dana's suggestions and put in enough length to limit cycling of a combi, or if you can reduce the baseboard length to under 30', you can go with an open system off a standard power vented tank. Make sure your local code allows for this and that there is a timer on the pump to keep the water in the baseboards from stagnating in the summer.

          I'm starting design work on a reno on a smaller rental, for that I'm going all electric as eliminating all gas work/plumbing/appliances takes $15k out of the build cost (plus less maintenance down the road).

    2. Expert Member
      Dana Dorsett | | #9

      >"Are electric tank water heaters more expensive in the long run compared to gas tank water heaters?"

      At Brooklyn's utility rates, yes, by quite a bit.

      Almost any 3/4 ton mini-split would cover the likely loads of the 516 square foot apartment, at a cost for heating only somewhat higher than condensing gas. The 3/4 ton LA090HYV1 should be able to handle it, but run a room-by-room Manual-J first. The resolution on the pics isn't really good enough to take a WAG at it or figure out how to optimally place a wall-unit, or if it would even have a chance of heating/cooling doored-off rooms.

      A mini-ducted 3/4 ton Fujitsu AOU/ARU9RLFCD would offer better heat/cool distribution to doored off rooms, and has enough capacity (about 16,000 BTU/hr at Brooklyn's +15F outside design temp) to cover the likely load of even the bigger apartment (if not the -18RFLCD surely would). But it's better to figure out the ducting & placement before the place is fully finished, even if the ducts are going to be in the basement.

      1. cbut8995 | | #11

        I think the best scenario after discussing with you would be to do this:

        Apt 2A,2B,3A,3B 1 bed/1bth 10 feet ceiling 516 sqft apts would use gas water heaters and 2 zone LGred mini splits:

        Apt 4A,4B: 1 bed/1bth with mezzanine ~700 sqft w/ 18 feet ceilings would use gas water heaters and 3 zone LGred mini splits:

        Apt 1 1200 sqft LArge 1 bed/1 bth 10 feet ceiling: gas water heater, gas wall hung boiler from Weil M to save space for the 50+ feet of underwindow runtal baseboard heaters and the following LGred concealed duct mini split

        I know for apt 1 is probably overkill to have gas heating with Weil and the concealed duct mini split but this is the apt I am living in and want to make sure if one of them doesnt work the other is ready to be used as backup. Not sure if this is okay if we never really use the heat option of the mini split will it work just as well in emergencies only if we never really turn it on because we are using the hydronic baseboard heating?

        1. Expert Member
          Dana Dorsett | | #13

          >"Apt 2A,2B,3A,3B 1 bed/1bth 10 feet ceiling 516 sqft apts would use gas water heaters and 2 zone LGred mini splits"
          >"Apt 4A,4B: 1 bed/1bth with mezzanine ~700 sqft w/ 18 feet ceilings would use gas water heaters and 3 zone LGred mini splits"

          The fly in the ointment with multi-zone units have a much more limited turn-down ratio, and will short cycle on zone calls if oversized for the load. The minimum output of the 2-zone LMU240HHV @ +47F is a whopping 10,248 BTU/hr, which is WAY more than the heat load of a pair of 516 foot apartments at that temperature. The single zone units can modulate down to a much more reasonable ~1000 BTU/hr (each) at 47F:


 (see page 11)

          The 3 zone unit has an even higher 13K+ minimum output @ 47F:

          It's MUCH harder to properly size a multi-split due to the limited modulation range.

          1. cbut8995 | | #14

            Hi Dana,

            I think I was discussing with Akos regarding this and came to the conclusion of having Gas Tank Combi Heat Boiler from LAARS for baseboard heating for all of the apts:


            Each 1 bedroom has 1 bathroom, 1 kitchen, 1 W/D, 20 total feet of UF2 runtal baseboard heating. and use the 50 gallon tank option. This will also have a 2 zone minisplit system with both heating/cooling from LG red and probably around 24k BTU outdoor unit with 9k and 12k BTU indoor units

            2 apts in this category have 30 feet of UF2 runtal baseboard heaters instead of the 20 due to the mezzanine. I am hoping this doesnt change much and use the 50 gallon tank option. This will also have a 3 zone minisplit system with both heating/cooling from LG red given the mezzanine and probably around 36k BTU outdoor unit with 9k and 12k and 12k BTU BTU indoor units

            The 2 bedroom, 2.5 bath apartment would probably use the 75 gallon tank as it has 2 full bathrooms, 1 half bath, kitchen, larger W/D, 55 feet of UF2 runtal baseboard. and use the 75 gallon tank option. This will also have a 3 zone minisplit system with both heating/cooling from LG red and probably around 42k BTU outdoor unit with 12k and 12k and 18k BTU indoor units

            He said it wont short cycle given its a tank so I get the best of both worlds saving space with 1 machine and getting the tank feature as well.

            What are your opinions on this building out. I rather be over compensating for heating and cooling but I just dont want any short cycle or problems in the long run etc.

            I attached pictures of the layouts to see if it helps.

  4. Expert Member
    Akos | | #15

    The UF2's do 350btu/ft at 140F water. I would check how close the 7kbtu is to your load, you might need a bit more length.

    The multi splits with such low load apartments will cycle like crazy. I'm guessing the cooling load for the apartments is probably between 5k to 10k with something with larger west facing windows, so even the smallest multi split is probably 3x oversized. This is a cooling cost and comfort issue as these don't modulate well, so the head is either on or off, they tend to overcool the space especially something like a bedroom.

    What would would better is a one to one mini split strategically located (one to one means one outdoor unit for each indoor unit). I've had no issues with cooling 2 story apartments with a head on the upper floor hallway near the ceiling. You do need to keep the bedroom doors open. I've never had a tenant complain from cooling for this setup.

    For you personal unit, I would recommend a ducted one to one unit. This is a bit more cost because of the ducting, but would let you close your bedroom doors and not overheat at night time.

  5. cbut8995 | | #16

    Hi Akos,

    Thanks for this. I had no idea this could short cycle and be that bad. I am just so confused as I have seen many developments each bedroom having their own mini split indoor unit and their bedrooms are very small. What are the downsides of short cycling and is the only way to actually adequately get the perfect size mini split indoor/outdoor unit for the apt?

    Other options I thought would work is the have all apts have a single zone duct unit and then the same temperature everywhere and just have the duct run to the bedroom and living room.

    The other option was to have the smaller 1 bedroom apts have 2 indoor units (7K BTU each) with an 18K outdoor compressor. This is the smallest one I seen and thought it could work.

    Are there any brands or options I could possibly add or select that would not short cycle even if the sizing is larger than the required size of the room as I do not mind paying for a premium either.

    Or another option do you think this two 6k BTU on a 12k BTU condenser from mitsubishi would be a better option. This is the lowest I could find from the big brands of minisplit. I know this has the hyperheating but I am not sure if it includes a basepan heater.

    My previous email shows the size of the apts if that helps.

    Thank you.

  6. Expert Member
    Akos | | #17

    Cooling load is mostly climate and window size dependent except on the top floor where you'll get some extra heat from the roof. The layout doesn't help much. Might be good to get somebody to do a cooling load calculation (not an HVAC tech but an engineer) on each apartment to figure out the exact load. Much easier to figure out which configuration works best.

    As Dana mentioned, multi splits have much smaller modulation range and the heads attached to them don't module, they are either on/off. They can be made to work, but they have to be well matched to the load. I'm pretty sure even the 18k 2 zone multi is way oversized.

    Typical bedroom load is somewhere between 1500 to 3000 btu, the smallest head (6k or 9k depending on manufacturer) is somewhere between 6x to 3x of that. You see a lot of pictures with this head in each room setup, this is how I ended up with a 9k head in my own bedroom. It just doesn't work, way oversized.

    Flows too much to be comfortable and temperature control is terrible, I usually use to pre-cool the bedroom and turn it off overnight. Makes the setup kind of useless.

    A single ducted unit is the best for each apartment, but that ads some extra install cost because of the ducting. A single wall/ceiling mount in the living space is much cheaper and does 90% of the job.

    If you are going with a ducted unit in a multi story apartment, make sure there is a return near the ceiling on the 2nd story. This is a detail you can't skip if you want good comfort in the summer time.

  7. cbut8995 | | #18

    What are the downsides of doing a 6K unit in the bedroom and 6k unit in the living room on a 20k Outdoor condenser? I could not find a smaller single zone duct mini split so I may just end up using this option even if its overkill as its significantly cheaper and takes up less room in terms of installation. I can't even use fujitsu so I have to use the premium mitsubushi because they are the only ones that carry a smaller 6k multizone.

    Besides it being too strong and the tenants will probably shut it off instead of leaving it on, I still dont understand the short cycling aspect of it or what it would do. I saw that it can shorten the life expectancy but these Ive planned to last around 10-15 years minimum so would having too large of a unit shorten it so much that the investment now would not make sense say around only half its expected life before it needs to be replaced.

    Thank you.

    1. Expert Member
      Akos | | #19

      Multi splits have a min limit on head sizing. Usually it is 70% of rated capacity, so you can't put 12k of heads on a 20k unit. You can do 9+6 though. The 6k is still way oversized for a typical bedroom.

      The cycling is not great for compressors, I can't tell you how it will effect life, since these are only used for cooling for a couple of months out of the year, I doubt will be a problem. More of a comfort and energy use issue. Multi splits are also not cheap.

      There are plenty of small ducted units, some are surprisingly cheap. I've used the wall mount units from these folks before:

      LG unit:!/product/29605

      Mitsubishi and Fujitsu also make a number of low BTU slim ducted unit:!/product/29057

      1. cbut8995 | | #20

        Ill look into the LG unit ones as I saw one that was low static and 9k btu but only problem is the outdoor condenser that is compatible doesnt come with a hyperheat option and only standard. This is the same problem with mitsubishi as they the single indoor units with 1 outdoor units dont have the extra decrease in temperature for operating I would like.

        That is surprising about the 70% unit rated capacity because they are selling a 2 indoor unit 6k each with a 22k BTU. I was seriously considering this but it seems it may be a bad idea but why would they sell this set:

        I was looking at the spec sheets for the outdoor condenser and not sure if I am reading it right but does this:

        mitsubushi hyper heat condenser lowest power at 6K BTU cooling

        LGred condenser: lowest power at 8.4K BTU cooling

        fujitsu XLTH: lowest power cooling at 6.1k BTU cooling

        1. Expert Member
          Akos | | #21

          Maybe I'm missing something, but if you are heating with a combi, why spend extra money on hyper heat units? New York is warm enough that a non hyper heat unit can do the heating without issues if you are looking for a backup.

          The indoor/outdoor sizing ratio you have to check the engineering manuals and combination tables from each manufacturer, it won't be in the two pager specs. The 70% is Mitsubishi.

          Except for efficiency and comfort, there is no problem with the over sized multi, but for your personal unit, I would do ducted.

  8. cbut8995 | | #22

    Agreed. Thank you Akos. I definitely will be doing the concealed duct mini split for the personal unit as I like the seamless look. I will def be using linear lot diffusers for a more modern look at the openings. I still cant find how the ducting works for the concealed duct mini splits but I believe the outdoor unit connects to the concealed duct unit like a normal mini split and then you need duct work for the places you want the air to come out of or if it is right at a wall it can simply be used like that without any duct work and just a grill covering I assume.

    I would prefer the hyperheat options only because I have about 30 units nearby in another building that use the Fujitsu wall mounts and they are all non hyper-heating and sometimes fail or don't heat up well according to at least 5 complaints I have.

    Are there any like articles or Q/As you think that basically judge which brand is better for concealed duct units and wall mount mini splits? Id like to stick with 1 brand from now. I haven't used LGred yet and am only familiar with Mitsubishi or Fujitsu. Mitsubishi is more expensive for me that Fujitsu and all my GCs say they're are more reliable but I've been doing a little research on LGred and they seem great and somewhat cheaper than both Fujitsu and Mitsubishi.

    1. Expert Member
      Akos | | #23

      Most brands are the same, what you should look for is support network. If the local installers mostly do one brand best to stick to that even if it is a higher cost option. Most of the cost for these is the install cost anyways, the cost delta between the different units barely budges the overall cost.

      Ducted mini splits are similar to a standard house furnace. Connect return duct with filter on one side, supply duct on the other side. Duct design takes work, not something that can be done over a forum. Linear diffusers are not easy as they are higher pressure drop. A competent HVAC designer can size the require ducting and diffusers for you. If you don't get this right, the unit will not perform well, so it is worth the money. The hvac designer can also size your baseboards, getting these right and saving say 20' to 30' of baseboard length will probably pay for their work.

      There is no way a non-hyper heat will under perform in NYC unless somebody forgets to close the windows. Most units deliver near rated BTU down to 17F, with the over-sizing you are looking at for the multi splits, they'll work. The only way they'll under perform is if the installer didn't get the refrigerant fill volume right or there is a leak.

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