GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Hydro-Air vs. heat pumps

lostinHVAC | Posted in General Questions on

Hi all,  what a great site..I read some advice from Dana Dorsett and was really impressed so figured I would ask.. So I am in the middle of a gut renovation and I am really having trouble deciding on an HVAC system. I live in Eastern Mass (Zone 5A). and have a 2000 sq ft bungalow. 2 beds and a bath upstairs … a soon to be large open floor plan downstairs with 1 bed 1 bath , + kitchen and living room which are all 1 area…

Im really intrigued by the hyper heat systems and am really leaning that way.. Im exploring installing the system myself if I can get the sizing / zones correct etc.. I have another HVAC guy ( actually 2) who say I should be installing a hydro air combo unit with 2 zones which will act as hotwater and heat with a condenser for AC.. both systems installed are about the same cost..

I have attached the quotes for each, just to show you sizes/ plans… Im looking for advice on which way to go.. Thank you in advance!!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    At MA pricing (Eversource/Nat'l Grid) for gas and electricity it will be marginally cheaper to heat with the condensing Lochinvar than a right-sized multi-split. But the difference isn't huge, and having more zones is likely to be more comfortable.

    Three tons of cooling is most likely oversized, and oversized enough that it's latent cooling won't be all that great, but they may have sized the FirstCo hydro-air more for the heating, and the model comes with a 3 ton cooling coil (?). Without a Manual-J it's hard to say just how oversized it is, but a typical cooling load for a 2000' house in MA is about 1.5 tons, 2 if it has a lot of west facing window unobstructed by buildings/trees.

    Six tons of multisplit is extreme overkill for this house. Four one ton heads and a 3 ton head? REALLY? For about the same money some friends of mine in Vineyard Haven (99% outside design temp +12F, comparable to Boston or the south shore) with a ~3200' house went with a pair of TWO ton Fujitsu multisplits (done for zoning- it's oversized for sure), with five HALF ton heads and a a single 1 ton head for their large open living/dining/kitchen zone. (I pushed hard for doing it with one 2 ton multi and a 1.5 ton ducted soluion, but they couldn't find a contractor willing to install the ducts in the unfinished basement- they all insisted on ducts in the attic above the insulation.)

    Get room by room Manual-J on this place- the one ton wall coils are good f0r 13,500 BTU/hr each in heating mode when married to a multi-split, so you're looking at over 65,000 BTU/hr even before factoring in the 3 ton head, all for a house with a heat load that is likely to be under 40,000 BTU/hr @ 0F.

    It's curious that neither quote gives model numbers for any of the equipment. There is a significant difference in low temp efficiency & capacity between the -4C36NA and the -4C36NAHZ. The NAHZ is literally twice the heat pump at temperatures that matter. I could heat & cool my ~2400' bungalow +1600' of basement house in Worcester with just ONE MXZ-4C36NAHZ with enough margin to be fully covered at -8F to -12F or so- it delivers 45,000 BTU/hr @ +5F:

    The 4C36NA is alreay crapping out with only about 22KBTU/hr capacity @ +17F, and even a pair of them might not cut it during a Polar Vortex disturbance.

    It might take a mini-duct cassette or two to make it work for all rooms, but I suspect your house is a pretty good fit for one -4C36NAHZ

    There are something like a dozen FirstCo hydro air handlers with 3 ton cooling coils. It's likely that you could heat/cool this house with a 2 ton hydro-air more comfortably than with a 3 tonner, and there are at least a half dozen condensing Lochinvars that could work well with a 2 or 3 ton FirstCo.

    Don't take any of them until you've performed the Manual-J.

    1. this_page_left_blank | | #2


      Is it weird for there to be such a mismatch between exterior and interior units? 36kBTU condenser connected to a combined 84kBTU of heads? Unless half are expected to be dedicated to cooling and half to heating, it seems pretty odd.

      1. Expert Member
        Dana Dorsett | | #3

        The quote was for TWO "...brand new..." 36K BTU/hr compressors (as if they would give a price break for used equipment? :-) ), with 84 KBTU/hr of ductless head.

        I don't know where they intended to stick all these ridiculously oversized heads. Even though a pair of 4C36NAs might not be oversized in capacity for the design load, the 12K heads are crazy-oversized for the average room loads, and likely to short cycle wildly 90% of the time, and would only calm down when the -NA is running out of capacity.

        Given the efficient rectangular shape of the house it might be within range of a single 3C30NAHZ, but without running the load calculations that would be a risky call.

  2. kjmass1 | | #4

    My average insulated home in Boston is about the same size 1200sf first floor, 800sf second floor. (1) 12K head on the second floor can cool the home most of summer. I'll turn on the 7K in the master during a heatwave. We have a wide open stairway though so with your second floor layout you'll definitely want a head on the main floor in addition to ducts for the bedrooms.

    1. Expert Member
      Dana Dorsett | | #5

      From the layout of the floor it looks like the upstairs could be served by a single 3/4 ton or 1-ton mini-duct cassette, but how those ducts can be routed staying within the insulation & pressure boundary of the house, and where the cassette would be mounted can't be determined from just a floor plan. It may be possible to use the floor joist bays for routing ducts across partition walls and hallway, etc. TBD. In head-banger ceiling 1.5 story bungalows there may not be sufficient headroom to run ducts soffited below the ceiling level.

      When married to a multi-split compressor the SEZ-KD09 is good for almost 11,000 BTU/hr of heating and over 8000 BTU/hr of cooling, which is probably enough. The -KD12 is good for 13,600 BTU/hr heting, 11,500 BTU/hr cooling which is almost certainly enough. But a KD15 or KD18 delivers substantially more if it's really warranted. The load numbers matter.

      The first floor might be amenable to one or at most two uni-directional MLZ series ceiling cassettes (designed to fit between 16" o.c. joist framing) for the open area, and MAYBE a half ton wall coil for the first floor bedroom, depending on where the load numbers fall, and which way the joists run. The KP12NA is good for 15,000 BTU/hr heating, 12,000 BTU/hr cooling. KP15 is good for 21K heating/ 18K cooling, and the KP09 is good for 12K heating/9K cooling.

      Is there a full basement under the first floor?

      Is the exterior-chimney fireplace going to stay, installing an air tight woodburning insert, or...???

      I suspect a cold-climate 4C36NAHZ with a ducted cassette for the upstairs, 1-2 ceiling cassettes down stairs and an FH06 for the downstairs bedroom (oversized, but what can you do, other than duct the whole first floor?) for well under $10K in competitive bidding. But it's up to YOU to do the legwork to get the room by room load numbers and spec what heads/cassettes to use. The typical mini-split installer doesn't usually have the complete set of skills to figure it all that out, even if they're great at installing and maintaining the equipment. If you leave it up to the contractors to just guess when coming up with proposals they will oversize it all.

      My own inclination would be to go with a multi-split or a couple of mini-splits rather than condensing gas and hydro-air + split AC. As the RGGI states ratchet up the emissions targets even condensing gas might get expensive, and incentives for rooftop solar (which are already pretty good) may be ramped up. That house in Vineyard Haven with the pair of 2-ton Fujitsus had something like 8-10 kw of rooftop PV installed last July, and is nearly Net Zero, despite being below code-min on the building envelope for on at least half the house. If you can stay off the gas-grid from the get-go you won't have to make that adjustment later.

      1. kjmass1 | | #6

        How loud is the air handler for those mini ducts? My brother had his new AC equipment installed in the knee walls of an attic bedroom and i could never live with it (guess that’s why it was a guest room).

        1. Expert Member
          Dana Dorsett | | #7

          Mini-duct cassettes are very low cfm, nearly-silent compared to the typical scorched-air 80,000 BTU/hr beast married to a 3 ton split system compressor. The KD12 is rated 33db (A) at max blower speed, 23 db(A) at minimum speed:

          For reference, a typical refrigerator is 50 db(A). At high speed a mini-duct cassette would be like hearing the refrigerator in an adjacent room through a closed door. Making the final connections to the register via a flex duct minimizes the total amount of noise, as does vibration-mounting the cassette rather than bolting it hard to the framing with no dampening materials.

  3. lostinHVAC | | #8

    Thanks for the replies guys.. so the update is I had a "Diamond" Mitsubishi dealer in the house today and saw how big the house was and basically told me its not worth it for the mini splits and to just go with the hydro air system.. he said hes been doing it for 25 years and for most of the year the units are great but for the really cold days I would have issues.. Since I have this house just about gutted, there isnt any going back once I make a decision so I am ruling the mini splits out for now if I can run a single Lochinvar with 2 zones. I have to get boned up on these things though.. I very much appreciate the responses. I will get the manual J and report back

    1. joshdurston | | #9

      I wouldn't make any decisions until you get a proper manual J done. I think the results may surprise you. It will affect things whether you go hydro-air or Split.
      I would be very surprised if a single 30, 36 or 48 ([email protected]) MXZ wouldn't carry the house with 2 MVZ appropriately size ducted air handlers. If the house won't work with ductless heads, then ducted MVZ airhandlers, or low static KD heads would work great.

      Too many contractors view the manual J as just something you do to get your permit and will work the numbers to fit whatever equipment they've already selected for the job. You need to go third party to get one you can trust.

      Personally I think hydro air gives you most of the expense of a hydronic system, without most of the benefits. If I was doing forced air natural gas, I would rather get a tightly sized high efficiency gas furnace ECM fan (or two), and a dedicated hot water appliance (tank/tankless/point of use, Heat Pump). I actually have a hydro air system serving part of my house, but it only runs as a last resort (when my woodstove is out), since I have panel rads in the rooms that matter.

    2. Expert Member
      Dana Dorsett | | #10

      The heat pumps that were available 25 years ago bear very little resemblance to cold-climate minisplits.

      A "Diamond" installer just means the contractor took the certification training and has installed some number of units. That makes them an expert at installing the stuff, but not much else. The statement that it won't keep up on really cold days is an indication that despite his 25 years of experience he still hasn't figured out how to size them correctly. People are heating with ductless heat pumps and doing just fine in much colder locations than eastern MA.

      There is absolutely no advantage to going with hydro-air + split system AC over going with a much cheaper condensing gas furnace + split AC. A hydro air system with condensing boiler can make sense if there are other types of hydronic heat emitter on either separate zones, or if the hydro-air is a necessary second-stage to a zone with a radiant floor or something, but it doesn't really make sense in this house relative to an inexpensive 2-stage condensing furnace.

      If hydro-air, a 76,000 BTU/hr modulating stainless condensing water heater such as the Phoenix Light Duty (or one size bigger burner, if anticipating two "endless showers" simultaneously) commercial water heater makes more sense for your loads than a fire-tube Lochinvar with a 10:1 turn down ratio than never makes use of it's modulation function.

      The three ton AC also makes no sense at all for almost any 2000' house in MA unless it has a 20' tall mostly-glass "great room" facing west.

      Josh has it right- don't solicit any more proposals until the Manual-J is in, or you will be wasting their time and yours.

      A dual MVZ solution on an MXZ-xCxxNAHZ compressor can work efficiently if sized correctly for the zone loads. With an MVZ air handler there is the option of auxiliary strip heaters to cover any shortfall in the unlikely event that it hits -20F or something. I suspect even the smallest (12,000 BTU/hr cooling, 13,500 BTU/hr heating) 1-ton MVZ is somewhat oversized for the 99% heating load upstairs, but the Manual-J would tell.

  4. keithhoffman22 | | #11

    At the risk of sounding like a fanboy, I'm loving (I'm just a DIY not a pro)

    Jump on there for free, build your house from your plans, and get a better sizing than most hvac guys will give you. Hopefully, you are exceeding the R values that coolcalc (and WrightSoft) seem to support so remember that your actual heating load will be lower than the sizing says. (cooling is affected less than heating by wall insulation R value, which is where you might not find adequate support for high R value walls).

    If you need a manual J for a permit, you should be able to provide the (small fee) manual J off coolcalc but I think the real value is understanding your load better yourself.

    NB: while the map/satellite builder on coolcalc is neat and works fine, if you actually know room dimensions, I think it's faster and more accurate to build off actual dimensions.

    1. this_page_left_blank | | #12

      I was pretty interested to try this, but got disappointed very quickly. If you have anything other than a code-built house, you can pretty much forget it. There was no way for me to enter the actual wall construction of my house, or even a whole wall assembly R-value.

    2. Expert Member
      Dana Dorsett | | #13

      I ran into a situation on an uninsulated quadruple wythe brick house in Kentucky where CoolCalc came in about 3 times what another low-end Manual-J tool did, and about 4x the fuel-use load calc. I didn't run the coolcalc- the homeowner did, nor did I check how he did it, but he wasn't an idiot.

      Or maybe his 105 year old coal boiler with the gas-burner retrofit really WAS running more than 300% efficiency! :-)

  5. keithhoffman22 | | #14

    Interesting Dana, of my sizings (loadcalc, pro wrightsoft, old coolcalc), the new coolcalc is the lowest.

    Yes, it doesn't account for the lower U of non-standard construction details but it's a fantastic hedge against the btu happy HVAC guy for folks with less odd details.

    I did just recently ask on another GBA thread here about tricks to account for double stud walls. Perhaps it's as simple as halving the exterior perimeter of any double stud wall.

    BTW, I think the mapping tends to encourage overestimating the SF of each room due to soffits and such. Better to enter the dimensions.

    1. Expert Member
      Dana Dorsett | | #15

      It's possible the guy didn't really figure out how to use coolcalc, but he managed to get numbers out of loadcalc that were at least somewhat consistent with fuel-use calculations using optimistic assumptions about the combustion efficiency of the antique boiler.

      He had originally run the numbers using the SlantFin app, which spit out numbers similar to what he got out of coolcalc, but it wasn't tough to convince him that gravity feed museum-piece he was heating with couldn't possibly be operating at 300-400% efficiency. ;-) (Even the 75% efficiency used in the fuel use calc to establish an upper-bound was probably a stretch for the 105 year old cast iron boiler with a retrofit burner.)

      1. keithhoffman22 | | #16

        I have always wondered, when using past usage data, is how you account for the adaptive behavior of people paying the bills on the old equipment. For 3 years, I lived in a wonderful 1950s house with floor to ceiling south facing plate glass and the original ~55% efficient furnace. Our thermostat settings on design winter nights was in the 50s. Presumably knocking 12F+ off the ∆T has a big impact on design load. Even today, our thermostat is not set to 70F for heating.

        1. Expert Member
          Dana Dorsett | | #18

          Thermostat settings matter. A 2x4 framed house kept at 65F should use base 60F HDD weather data. In the case of the guy in Kentucky they keep the place 70F 24/7 so I used base 65F.

          If your average indoor temperature was ACTUALLY 58- 60F (not just the thermostat setting- did it actually get down to the 50s indoors every night?) you would have to use even lower base temperature to get the BTU/hr per degree constant, but add the requisite number of degrees to come up with what the load would be at 68F or 70F.

          In more typical houses the solar gain error are pretty much offset by the hot water use error. Most houses have a 15% or lower window/floor area distributed on multiple sides of the house, but many mid-century modern highly glazed houses have huge solar gains, and similarly huge night time losses skewing the numbers. There are the outlier houses with massive wintertime solar gains for which a fuel-use calculator is inappropriate and will under-report the actual peak load by quite a bit. I'd put a "...1950s house with floor to ceiling south facing plate glass..." in the category. But even on those houses it won't be a 50% error, under reporting the load by half.

          A fuel use load calculation is not a high precision measurement, but it IS a measurement, and with some care and adjustments for the known error factors (like very low thermostat settings, etc) it's usually within 15% of what an aggressive Manual-J would put out (usually to the low side of Manual-J)

          In milder climates the solar gains and daily temperatures can really screw this type of load calculation up. Anywhere in zone 3 or lower it's a pretty lousy measurement fraught with error, even worse in dry sunny areas like southern CA/AZ. But for zone 4 and higher it puts a pretty firm stake in the ground without a lot of effort. Reality might be 25% higher than indicated in a fuel-use calc, but not 100-200% higher, and that's usually close enough to spec a boiler or furnace model, at least as close as a typical Manual-J (which usually overshoots reality by a bit.)

  6. RMaglad | | #17

    2000sq ft (4000sq ft conditioned with basement) home here in Ontario.

    Heating with 3 ductless mitsubishi minisplits (6K in master, 6K in basement rec room, 9K in kitchen/living/dining) and 1 fujitsu mini duct (9K for kids rooms, spare room in basement, upstairs main bath, powder, mudroom, basement bath). theoretical capacity at design conditions is 33Kbtu/hr. Heat loss calcs were 22.5Kbtu/hr. I am oversized, but wanted zones and deep modulation. Couldn't really go much smaller. Total install cost was 14K canadian.

    Yes, i have 4 outdoor units, and 4 indoor units, each is dedicated.

    Units have performed flawlessly in this brutally cold winter. We've seen -20F a couple times and the house has remained at 70.

  7. Expert Member
    Dana Dorsett | | #19

    Clearly you weren't listening to the Mitsubishi Diamond installer in Massachusetts with the theory that "...its not worth it for the mini splits ...", eh? :-)

    What kind of a sales pitch is that, anyway?

    I just hope Bill Pierce takes the notion of running load calculations to heart, even if he opts for hydro-air. Sure, the typical 2-3x oversized systems will heat and cool the place, but comfort isn't all about what the thermometer on the thermostat says. Oversized ducted air heating runs very low duty cycles during cold weather, the houses feel drafty, oscillating between the warm blast and the extended chill, whereas running at a 65-75% duty cycle at the 99% outside design temp that effect goes away. The oversized air handlers that come with oversized systems are noisier and induce more wind-chill too.

    Similarly, ridiculously oversized cooling systems tend to have lousy latent load control in a Massachusetts climate due to the low duty cycle, and it oscillates between sweaty & clammy on those cycles. Cooling load per square foot on houses that size in MA are usually about a ton per 1500' , sometimes a ton per 2000', and the three ton proposal would be a ton per 667 feet- not too likely. A 3 ton multi-split is also oversized for the likely cooling load but even though the individual heads are cycling the average duty cycle on the compressor overall is enough to manage latent loads inside the house. Even though the drying isn't coming from all heads equally or at the same time it does track the sensible loads, and unless all interior doors are closed drawing the humidity down in one zone draws the humidity down in other zones.

  8. FluxCapacitor | | #20

    I hope the OP tries a few more Installers before giving up
    On the multi-split option.

    I went through 5 Diamond dealers before I found two I was truly happy with.

    I’m having a Mitsubishi multi split installed here in New Jersey in a couple of weeks.

    I feel confident all the Diamond dealers know HOW to to install the systems but many (like Dana said) aren’t necessarily in tune with the design/performance needs of everyone’s home.

    The Diamond installer I have settled on was extremely knowledgeable about the the Mitsubishi systems but also cared enough to point out a few things I can easily do myself to tighten up my house.

    My installer also quickly pointed out that my not-so-well-thought-out idea of individual wall units in small rooms may result in performance and comfort issues...just as others here on GBA (Dana in particular) have suggested.

    I’ll follow up on my original post once installed.

    Wish me luck.

    1. Expert Member
      Dana Dorsett | | #21

      The installers already contacted are probably perfectly good installers, but perhaps lousy system designers prone to oversizing everything. That's why it's better to have a disinterested third party who is NOT in the businenss of installing and maintaining the equipment running the load numbers.

      With a professional Manual-J in hand performed by someone who makes their reputation & living on the accuracy of their numbers the homeowner can go to the installers and have them propose solutions to cover those loads that are not more than 1.5x the load for any given zone.

    2. Mark_Be | | #23

      I am in NJ as well and in the process of getting quotes for a Mitsubishi mini split installation. Would you mind sharing the names of the dealers you were comfortable with? Thanks and good luck.

      1. FluxCapacitor | | #24

        Hi Mark,
        I’m not sure if mentioning contractor name is OK here?

        Also, my install is scheduled for March 4 so I wouldn't be able to review the contractor until after the job was complete.

  9. Barry_E | | #22

    I asked a friend of mine who owned an hvac business, how many homes he had properly sized and put the most efficient system in. He answered one the entire time he was in business. He stated he was always asked to give the best price and his competitors the same. He had one person who was a contractor building his own home to ask for the (best) system, not the best price.

    When I spoke with him about the mini split he said they were hurting profits as they were a quick install and money wasn't being made doing the duct work as with a heat pump.

    The above fits after reading this thread.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |