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Community and Q&A

Sizing a Gas-Fired Combi Boiler

Dlauffenburger | Posted in General Questions on

My house is 5 year old single story 2250 SQFT home with a Superior Wall insulated basement located in Warren Pa Zip Code 16365.

Currently my Heat/AC is provided with a Closed Loop Geothermal Heat pump (5 ton) connected to a mix of radiant tube and Forced Air Central Air system  (See Attached Diagram).  I have recently been given access to Lease Gas and want to install a Gas fired Combi unit (into my existing system) to provide my DHW and home heating, which will significantly reduce my electric bill.

Initially I was told to just install a Weil McLain Aqua Balance 150 and it would do everything I will need, but after reading a lot of the posts on here about sizing boilers and combi units I think an AB 150 is oversized and would have short cycle issues.

I do not have a Manual J for the home and have not been able to find anyone to perform one, but I did find a heat loss calculator on the web and was able to generate a Heat loss calculation (See Attached Document).  Unfortunately my blower door test did not include the garage so theACH50 infiltration rate does not include the garage so that skews the Infiltration data on the Heat Loss calculation.

Blower Door Test results for living area are:  CFM50 1658, ACH@50 = 2.38, ACH Nat = .1575  (I know I need to install an HRV to provide ventilation, which I am currently researching)

As I sifted through all of the data provided and reviewed all of the boiler sizing posts I am confused on a couple of points:

When sizing a combi unit how do you select a the proper size? Is it the Heating load or the DHW requirement that dictates the size?  (An AB150 seem oversized)

Given my need for 5 GPM of DHW and the BTUh of heating given in the Heat loss calculations there doesn’t seem to be a combi unit that fits the requirement.

How does a units Turn Down capability fit into the equation?

How does the outdoor reset effect the decision?

Would it make more sense to install a dual use hot water heater (like an HTP Versa Hydro)?

Thank You for any info and guidance.

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  1. Expert Member
    Dana Dorsett | | #1

    >" When sizing a combi unit how do you select a the proper size? Is it the Heating load or the DHW requirement that dictates the size?"

    It's a function of both. The high-fire output needs to be able to cover the domestic hot water load, which usually several times the space heating load. The min-fire output has to be low enough to not short-cycle on the available zone radiation. If the output is being buffered in a buffer tank of sufficient size (the Versa is inherently self buffered) the latter requirement goes away.

    A temperature rise of 70F (mid-winter 35F incoming, 105F at the shower heads) and 5 gpm (= 2500lb/hr) flow takes 70F x 2500lb/hr= 175,000 BTU/hr of burner output. That is clearly outside the max capacity of the W-M Aqua Balance 150, but within the range of the HTP UFTC-199 or Navien NCB-240E combi boilers.

    1. Dlauffenburger | | #8


      Thanks for the info. Attached is a copy of the heat loss calcs. I have been fighting with the system trying to get it add to my OP but I wouldn't allow 2 attachments.

  2. joshdurston | | #2

    Just out of curiosity, what temps are you maintaining your buffer tank at, it looks like everything is hydronic?
    Fan coils and geothermal don't always make for an efficient pairing unless the coil was carefully selected.
    But with properly sized and installed infloor heating, you should be able to get excellent efficiency.

    I've seen some pretty poor performing geothermal system because they were set to maintain high temps all the time, but I've seen great performing systems with infloor running in the 75-95degF range.

    1. Dlauffenburger | | #9


      The buffer tank is a 40 gallon water heater, not connected to electric.

      The Set Point on the Geo unit is 110F, but the system normally satisfies the heat call with the batch tank between 84 and 95F. I have only seen the batch tank get above 105 a couple of times.

      The Air Handler is a MultiQqua 60CWA2.

      1. Expert Member
        Dana Dorsett | | #12

        Running an EWT of ~90F water into a 5 ton air handler must deliver some pretty tepid air if it's used for heating. Can I assume that is only for cooling?

        With a 4o gallon buffer configured as the hydraulic separator for a combi-boiler you can run the boiler fixed-temperature and still get there. If the buffer temp never exceeds 105F it will always be running in condensing mode- no point to using outdoor reset. There is enough mass in the system to not really worry about modulating the burner with load too.

        The Build-it-Solar load calculator isn't really very good as online load calculators go. Try if you really care about what the load number really are, not that it will change the system by very much.

  3. user-2890856 | | #3

    Versa Hydro ( PHE130-55 ) would make the most sense considering your criteria . Low end modulation on most any combi will be too high for your loads for a VAST portion of the season . If you want to avoid short cycling as stated you will require a buffer tank , probably ample sized , associated plumbing and cost .

    REMEMBER : You will have to find that individual that can install either system properly . Your odds of that are much better with a package unit like the Versa Hydro

    1. Expert Member
      Dana Dorsett | | #5

      The system currently has a 40 gallon buffer in the middle which could be put to good use here, and reasonable amounts of thermal mass in the zones to boot.

      The performance issue would be the domestic hot water flow limitations, which are the same with any tankless. A Versa would fix that allowing MUCH higher flow rates, but it's an expensive fix if not really needed. A 199KBTU tankless or combi can deliver about 5 gpm tub fills or showers even at the coldest PA incoming water temps.

  4. Expert Member
    Dana Dorsett | | #4

    Looking at your zone radiation and system block schematic there is at least 75lbs of water in the smaller zone probably 80-90lbs interacting with every burn on that zone, which should be enough to keep a ~17-20K min-fire combi from short cycling.

    If the combi is set up to be heating the 40 gallon (333 lbs) buffer tank in the middle there is effectively zero possibility of short-cycling the combi on zone calls. Just about anybody's 199K combi boiler would be fine.

    1. user-2890856 | | #6

      Have an idea that none of these zones are anywhere near low end modulation even at or below design Dana .

      Dlauffenberger . Could you tell us what type buffer/ batch tank that is , who is the manufacturer ?

      1. Expert Member
        Dana Dorsett | | #7

        True, neither zone will likely emit the min-fire output of a combi boiler, but there is also a reasonable amount of thermal mass to work with even in the zone radiation, enough to be able to suppress truly egregious short cycling by opening up the differential swing.

        Combi boilers are rarely a great fit, and that's true here too, but with some thermal mass to work with they don't have to be TERRIBLE (although evidence in the field might indicate otherwise, given the weak grasp of hydronic design most installers installing them seem to have.)

        To be sure, I wouldn't look at this system and immediatly think "combi boiler- great solution", quite the contrary! But could you make it work? Sure!

        1. Dlauffenburger | | #11


          Your hesitation on using a Combi unit for this configuration are enlightening, what would be a better and best option?


          1. Expert Member
            Dana Dorsett | | #13

            A small modulating condensing boiler with a big turn-down ratio plus an indirect water heater operated as the "primary zone" is tried and true, easy to design for. There are several mod-cons that can modulate down to almost 1/3 the minimum BTU rate of a 199K combi boiler.

            With an indirect water heater the domestic hot water flow rates are not burner-limited- if the indirect is sized for the biggest tub you need to fill, the tub can be filled 10, 12 gpm or higher- whatever your water pressure and plumbing can deliver. And when showering multiple other high rate hot water draws can come on/off without major impact on "family harmony" with the person singing / screeching in the shower.

            Looking at your BIS load calculation output your 99% design heat load is about 46K, which could be covered by a Lochinvar KHB055 with some margin, with a max-fire output of about 52K, and a min-fire output of 7.9K (less than half that of a 199K combi). If the confidence isn't high on your load numbers and you want faster recovery on the indirect the KHB085 has a bigger, 10:1 turn down ratio, with about 81K out at high-fire, 8.1K out at low fire, which is STILL less than half the min-fire output of a combi boiler.

            For less money the somewhat simpler HTP UFT-080W modulates between 7600 BTU/hr @ low fire to about 76K at high fire. The UFT boilers are even simpler to install, since they come pre-plumbed with a secondary output port and controls to support an indirect water heater. (They are almost DIY-able, but it's best to study up on hydronic system design first.)

            There are others. Find one that has reasonable local & regional support, call the distributor for contractor recommendations, since the distributor knows who is constantly turning in bogus warranty claims on mis-installed systems or tying up the tech support with questions clearly answered in the manual, and who installs dozens per year with minimal need of support. HTP's headquarters are less than a 90 minute drive from my house, which makes it an easier call for me, but I wouldn't turn down a Burnham ASPN-085 or Lochinvar KHB085, for an application like this, both vendors have decent support in my area.

            With a min-fire output less than 1/4 your design load it's possible to set it up under outdoor reset control where it will run nearly continuously through the entire heating system with maximally stable room temperatures and high efficiency/low maintenance. Even with the 40 gallon buffer you can't quite do that with a combi boiler putting out nearly half the design day load at min-fire.

      2. Dlauffenburger | | #10


        The batch tank is a Bradford White electric hot water heater that is not connected to electric. The boiler would be connected directly to the batch tank, and the hydronic circuits hook to the batch tank via different connections.

  5. Expert Member
    Peter Engle | | #14

    FWIW, I've got the HTP boiler that Dana mentions above, with a 30 gallon DHW indirect fired tank. Plenty of heat for a 2000+ sf 120 year old house in CZ5, and no hot water problems ever.

    1. Dlauffenburger | | #15

      Thank You for the reply and information. I think the indirect route will be a much better solution, just have to start the pricing comparison for the boiler and indirect tank setup.

      Thank you for the info, it is always good to hear real world results.

  6. rhl_ | | #16


    I haven’t had a chance to read existing responses, but, let me warn you against the aquabalance. I own the AB 155c. I’m rather disappointed with it. It’s made quite cheaply, I’ve had it now for two heat seasons, and I’ve had two fittings fail on it.

    Also, more importantly, it doesn’t support external controls. A smart design is having the boiler be asked to output water at different _design_ temps based on both outside air and which zones call for heat. So for example if it’s radiant floors only, don’t output more than 120 deg water.

    I would look at Buderus as another option.

  7. qofmiwok | | #17

    This is so confusing. We will have 2 people in a fairly large, highly efficient house, and were thinking about doing some radiant heat loops in the garage, maybe a 5000 BTU max load. A Navien combi-boiler keeps being recommended but it sure seems like overkill with the smallest one being 60kBTU heating and 160k DHW. Someone on another GBA post suggested an ATP Versa Hydro, and the built in buffer tank handles the modulation, but the smallest one (55 gal) seems to be over $6k. Is that what I should expect to pay for what I need? Then I see other combi-boilers that are $2000 and claim to be just as efficient. Is the problem with these just that the turndown ratio is less?

    1. Expert Member
      Dana Dorsett | | #18

      Combi boilers basically suck even for regular code-min type houses. They're a better match for homes with big heat loads and small to moderate hot water needs. They suck even worse for highly efficient houses.

      That said, with high mass radiators such as concrete slabs they can be made to work without short-cycling.

      BTW: Navien has some serious QC or design issues with heat exchangers on their tankless water heaters & combi-boilers leaking exhaust into the inside of the cabinet, which mixes the combustion air causing lower efficiency & longevity. There may even be class-action suits coming. (Do an internet search on the terms [mikey pipes] [navien] [heat exchanger] ) Despite being easier than most to size correctly and easy to install it's worth waiting until those issues get resolved before buying a water-tube heat exchanger type Navien. But the NFC series fire tube versions are completely different, and simply cannot leak in the same way.

      With it's 11: 1 turn down ratio and slab radiation an NFC-175 or NFC-199 should work just fine.

      1. qofmiwok | | #19

        Thanks for the heads up. I don't want to give myself a headache for something I don't need. And the NFC-175 is just crazy capacity for water for 2 people and an optional 5000 BTU of garage heat. What would you recommend for a stand alone low load hot water system in a highly efficient house? Just a standard high efficiency NG water heater?

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