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My HVAC guy disagrees with PH Planning Package

user-2069108 | Posted in Mechanicals on

The Passive House Planning Package indicates that the heat load for my house should be 7,722 BTU/hr (4.4 BTU/hr-sqft, and the cooling should be 3,573 BTU/hr (2.0 BTU/hr-sqft).

I gave these numbers, and the entire PHPP file to my HVAC person. He was dumbfounded by the low numbers and ran our plans through a detailed Manual J calculations. Now, he admits that he cannot account for the superinsulated slab, but he came up with much higher numbers.

The quote is for various versions of a Carrier 38Mxx / 40xxxx (3 ton heat pump. So much for heating the house with a hair dryer…….)

Not that it makes much of a difference to his calculations, but we will have a BuildEquinox CERV installed.

We were thinking that a single small Japanese mini split would be sufficient for heating and cooling (open doors will have to replace multi-head outputs or ducting).

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  1. GBA Editor
    Martin Holladay | | #1

    You aren't really asking a question, so I guess you know what you want. In case you are unsure, the PHPP heating and cooling load calculations are likely to be accurate (especially if they were performed by a certified Passive House consultant).

    If the PHPP calculation were made by someone who hasn't been trained, the numbers are more suspect.

    It's quite common for HVAC contractors to doubt accurate load calculations, especially when the load calculations concern a superinsulated house.

  2. Expert Member
    Dana Dorsett | | #2

    If I'm doing the mental math right you have a ~1750' house?

    What is your outside design temperature?

    What was the contractors' whole house load number?

    The notion that even an IRC 2015 code minimum house needs 3 tons of ductless is instantly suspect (unless your outside design temp is -30F or something, in which case it's ridiculous for other reasons). The contractor probably picked the Carrier 38/40M for the zoning (the 3 tonners are 4-zone multi-splits), but a single 3/4 ton head (the series doesn't support half-ton heads) has more capacity at +17F than your PHPP calculated load, and the minimum modulated output of the compressor is most likely to be over half your calculated design load (or even the whole load.)

    If single-head wall coil solution isn't going to provide adequate distribution, a couple of half-ton Mitsubishi FH06s (separate units, not a multi-split) or 3/4 ton LG Art Cools would be a better choice, due to the lower modulation range.

    If ducts are allowed, a 3/4 ton Fujitsu 9RLFCD has plenty of capacity (12kbtu/hr @ +17F) , and modulates down to less than half your PHPP calculated load.

  3. Reid Baldwin | | #3

    Did the PHPP analysis take credit for the heating capability of the CERV? The document I have for that says it has a 5500 btu/hr capacity with a note saying that the capacity actually depends on conditions. A unit called Minotair, which functions similar to the CERV, is rated at 9400 (heat pump mode) and 12000 (air exchange mode). It would satisfy the total load by itself. You might need to improve the ventilation system ductwork to ensure that the heat gets to where it is needed.

  4. Expert Member
    RICHARD EVANS | | #4

    The CERV offers 5,500 btu's on its own (although I would check at what temperature). That may reduce your heat loads to a truly minuscule level- requiring only a couple of space heaters for the cold nights. The CERV's cooling capacity is 3,800- which could likely fit the bill. (see link below: note that GBA website has the cooling and heating numbers swapped)

    If you do end up going the mini-split route: I would choose your own equipment (or take Dana's advice) and then ask a contractor how much it would cost to install it. Don't let them figure it out for you or it will be dramatically oversized.

    One final note: I presume you are using PHI rather than PHIUS? (PHPP rather than WUFI passive?) According to a recent GBA article, it seems that PHI modeling can be a little optimistic compared to PHIUS. You might consider over sizing the equipment by a small margin to be safe. One thing I like about PHIUS is that there modeling appears to be more accurate given the climate-specific assumptions.

  5. user-2069108 | | #5

    I posted a long rezponse to Martin a while ago but it never made it to the web. I hate Android and having to use a phone to access internet.

    I am so pleased with the response to my quandary.
    I did my own Manual J calculations with the help of an blog on this web site and came up with a less than 8 kBTU/ hour heat load. The cooling load calculations were somewhat more cryptic do I did not bother. Now I trust the PHPP.
    In another article that I read on how to size a heat pump, I read that I should size the unit 1.5 times the calculated requirements to maximize the efficiency - so something in the 12kBTU seems appropriate (or two 6k units, as pointed out above) .
    In that same article, it was pointed out that one head can condition 1100 sq ft. I have 1850 - once again, two small units look to be the way to go

    I will look into getting units and having them installed - that seems far more economical.

    Dana - good guess on house size. We are in zone 5, kust outside se Columbus OH. I used design tenps appropriate for the area in my Man J calcs.

    To be continued....

  6. Expert Member
    RICHARD EVANS | | #6

    Steve, I am far from an expert on the subject so I hope Martin or Dana can chime in again. As you probably already know, ductless mini splits are most efficient when they run continuously. If they are too large then they will short cycle on and off which will reduce effiency dramatically. If you oversize by that much then I would find one that modulates way down for all but the coldest weeks.

    A 50% oversized unit may also prevent you from getting an Energy Star rating which, here in NH, is worth $4,000.

    (Typed with thumbs)

  7. GBA Editor
    Martin Holladay | | #7

    Rick Evans,
    Thanks for catching the error on the Product Guide page for the CERV. I have corrected the error.

  8. GBA Editor
    Martin Holladay | | #8

    Steve Young,
    If you are looking for an article on the GBA site that discusses all of the features of the CERV, there's a better article than the short listing in our Product Guide. Here is a link to the longer article: A Balanced Ventilation System With a Built-In Heat Pump.

  9. Reid Baldwin | | #9

    Does anyone on this forum have experience with using a CERV, Minotair, or similar device as a primary heating and/or cooling unit? I am curious about that even if the OP doesn't plan to go that way. It seems like there might be issues beyond matching the capacity to the whole house load, but I don't know what those issues are.

  10. ethant | | #10

    Reid, I have been looking into this extensively. I have found that there may be a sweet spot in terms of house sizing and using the Minotair - basically if you can eliminate other heating/cooling requirements. I have not looked further into the CERV because I believe it has some performance deficiencies as compared to the Minotair, but I am happy to be corrected on that point. When you refer to "similar device," there aren't that many out there in the North American market, but I'm happy to hear if you find an alternative. A challenge I am having is mechanical design and duct layout because most people are unfamiliar with these products and those that are are too busy to look at small projects like mine.

    1. andyfrog | | #14

      Can you go into more detail about the CERV's performance deficiencies and why you prefer the Minotair?

  11. Expert Member
    Dana Dorsett | | #11

    "In another article that I read on how to size a heat pump, I read that I should size the unit 1.5 times the calculated requirements to maximize the efficiency - so something in the 12kBTU"

    The 1.5x oversizing is really more of maximum, not a recommendation, and that would be 1.5x it's capacity at your 99% outside design temperature (in Columbus that outside design temperature is +6F: ).

    That is distinct from the "nominal" or "rated" heating capacity (which it must capable of delivering at +17F) or it's nominal COOLING capacity, which is usually how they are marketed, (not by their heating capacity.)

    Any random 12K cold climate mini-split will typically deliver on the order of 15,000 BTU/hr @ +6F, making it ~2x oversized.

    A SINGLE half-ton (6000 BTU/hr nominal cooling rating) Mitsubishi -FH06NA has 8,700 BTU/hr of HEATING capacity @ +5F, so it covers your load. Read the submittal short sheet:

    A pair of FH06 would be more than 2x oversizing.

    If heat distribution isn't an issue, a single FH06 covers it but, for a couple hundred USD more the FH09 (9000 BTU/hr nominal cooling) has the same sized coils and the same minimum modulation range and deliver about the same or slightly better as-used efficiency, and will give you a bit more coverage during Polar Vortex events, specified at 10,900 BTU/hr @ +5F.

    For a 1-head wall coil solution, the FH09 is the right size, even though it's still less than 1.5x (10,900 / 7,722 = 1.4x ).

    Almost all 12K cold-climate units have minimum modulation levels much higher than that, and will spend more time in a (comfort & efficiency robbing) on/off cycling mode. Many 9000 BTU units will have that problem too- Fujitsu's 9RLS3H cold climate mini-split has a minimum modulation of 3000, BTU/hr @ +47F, so at your calculated heating load it'll be cycling rather than modulating pretty much whenever it's above freezing out if the sun is shining.

    It can modulate down to the same 1600 BTU/hr @ +47F as the FH06, which means it will still run almost continuously throughout the heating season.

    If heat distribution IS an issue...

    A SINGLE 3/4 ton (9000 BTU/hr nominal cooling rating) Fujitsu 9RFLCD slim-duct unit in heating mode can deliver 12,000 BTU/hr @ +17F. I'd have to pull the extended temperature capacity tables on it to be more precise, but if your load is ~8K @ +6F it would still have you covered well into negative single digits before it begins to not keep up with the load. The lowest temperature at which it has a specified output is -5F, but it'll keep running and will be delivering considerable heat well into negative double-digit temperatures.

    The down side to the 9RLFCD is that (like the 9RLS3H) it's minimum modulated heating @ +47F is 3000, BTU/hr, so it would do a lot more on/off cycling during the shoulder seasons than an FH09, and may not quite hit it's HSPF efficiency numbers.

    A 1-ton Fujitsu mini-ducted would probably be somewhat overkill, probably more than 1.5x oversized for the load at +6F, (again, the extended capacity tables would tell you just how big the oversizing factor is) but since it still modulates down to the same 3000 BTU/hr @ +47F, and would be more likely to have you covered at -10F, with out a huge hit in as-used efficiency.

  12. user-2890856 | | #12

    Your HVAC guy should not be your HVAC guy . Find another

  13. user-2069108 | | #13

    Sorry for the long delay. We were preparing and pouring our slab in WarmForms that last few days.

    Reid; Thank you for the Minotair reference. I looked them up a while ago. I will look into them again.
    Reed and Rick; I do not know if the CERV inputs were added to the PHPP. I know that because it wasn't PH certified, we took a hit. My Consultant actually recommended the product (both were from the Champaign/Urbana area). I will look at the PHPP spreadsheet, but I suspect that it was included.
    Rick; I will look into the posibility of an Energy Star rating in Ohio.
    Dana: I had to read over your detailed response a few time to understand what you were telling me -
    steep learning curve for me. Some great information and even better recommendations. When I posted this question, I really hoped that you would provide your input. Whith your Manual J refenence, I had to change my 99% heating number a few degrees (We are actually just outside Lancaster, a little SE of Columbus).

    Once again, thank you to everyone that offered their advise and knowledge.

  14. andyfrog | | #15

    Did you end up installing the CERV? If so, how is it working out?

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