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Are these calculations wrong?

user-5631279 | Posted in Mechanicals on

Hi all; we’re building a 2300 sqft ranch with a basement for a total of 4600 sqft of conditioned space in climate zone 4-A. The slab is 4″ concrete with R15 under it. The foundation walls are 8″ concrete with R15 against them. The rest of the walls are aerated concrete with insulation for a minimum of R21.5 – 11.5 for the block and another 10 for the foam. The windows have a low .20s U-factor and a SHGC of .26. Most of the windows face East with a few facing West and North. There is only one small South facing window. We will be using an ERV for ventilation. We are doing our best to make the construction very air tight and I think we’ll do pretty well in that regard though not spectacular.

We paid a company to produce load calculations for us (at a lot higher price than I’d expect) and this is what they delivered:

Total Heating Load: 57,564
Total Cooling (Sensible) Load: 54,838
Total Cooling (Latent) Load: 17,108
Total Cooling Load: 71,945

The problem is that pretty much everyone involved in efficient construction from the the county plans inspector to a GC specializing in AAC thinks this seems too high for our house. This works out to effectively a 6 ton system – it’s not huge but it’s pretty good sized system for sure.

Ideally, I’d be able to use a single outdoor condenser and four interior units – two horizontal 15k heads in the basement and two 15k ceiling cassettes upstairs. That would let me use something like Mitsubishi’s MXZ-8C60NA (60k BTW 2-8 zone unit). If I need to be all the way to 72k BTU then I’ll probably need two exterior units and the heads get trickier.

Anyways, that’s my question – do the numbers feel reasonable to you experts or do they seem high to you as well? Thanks in advance!


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  1. user-5631279 | | #1

    Sorry, I forgot to add that the attic will be R70 blown insulation.

  2. user-2310254 | | #2

    Six tons sounds like a huge oversize. The main level isn’t that large and it shouldn’t take much capacity to heat and cool the basement. What does the HVAC engineer say to justify his/her recommendation?

  3. user-5631279 | | #3

    The number came right out of a room by room manual J calculations I paid a local company to do. They farmed it out to an engineering firm to do the actual work so I've never been able to speak with them directly. I've attached the redacted version they gave me.

    Looking at it in detail, I see a few things that have become or are inaccurate. They have a whopping 15k BTU heat gain associated with lights though I'm using LEDs everywhere. I suspect that's a big reduction off of incandescent bulbs. They also have the windows at .3 and that's too high. Does anything stand out to others? Thanks!

  4. GBA Editor
    Martin Holladay | | #4

    The cooling load is much too high.

    It looks like the heat gain assumptions for glazing are based on double-pane clear glass. That type of glazing is illegal. Your windows must have a lower solar heat gain coefficient, so that number is clearly wrong.

    The design conditions for heating and cooling are unusual. The assumptions are backwards. It's typical to assume an indoor temperature of 75°F in summer and 72°F in winter. Your engineer got it backwards, increasing the loads.

    Heat gain for people seems to have been added twice. And I'll bet the assumptions for air leakage are too high.

  5. Expert Member
    Dana Dorsett | | #5

    You correctly spotted that even the very top of the first page has double-digit percentages errors, starting with

    "Window U-Value: 0.3"

    ...(= code minimum) when your worst windows are U0.025 (if it's in the low 20s).

    Then they magically changed windows for the cooling load from U0.30 (argon filled single low-E double pane) to a clear-glass double pane for the cooling load? What's with that?

    Your stated SHGC of 0.26 cuts the window gain numbers by at least 55-60%, assuming they even calculated the clear-glass windows correctly (not a good assumption, given the rest of it.)

    The line "Roof R-Value: 0.020408 (Per county Standards)" is dead wrong- that's probably a U-factor, and is the pro-forma county code requirement, not the construction & R-values you specified (though that doesn't have nearly as much effect on the numbers as the mis-statement of the window U- factor.)

    As Martin already noted, on the second page there's another huge thumb on the scale"

    "Winter Design Conditions
    Elevation: 1100
    Outside DB: 4
    Inside DB: 75 "

    That is a temperature difference of 71F.

    Code minimum indoor design temp is 68F, and I sincerely doubt that the 99% outside design temperature is +4F (extremely rare in zone 4A locations.) See if you can't get a better estimate of the 99% outside design temperature here:

    Most 4A locations have double-digit 99th percentile temperature bins, so let's assume it's really +10F outside design temp, and use +68F for an indoor design temp. That's a 58F temperature difference.

    The effects of the overstated design temperature alone would overstate the heating load by (71F/58F=) 1.22x , a 22% overstatement even if they had gotten the rest of it right (which they clearly did NOT get right!)

    When there are double-digit errors obvious on a first glance I'm a bit disinclined to read the rest of it- it's a hack job.

    A couple years ago I saw a Manual-J on a superinsulated house where U-factors & R values were at code minimum, that in no way reflected the house design that had been submitted. About the only thing they got right on that one was the inside and outside design temperatures. This one didn't even get THAT right.

    A properly aggressive Manual-J on an IRC code minimum 2300' house (at code-max air leakage) for zone 4 with heat recovery ventilation would come in with a heat load between 25-30,000 BTU/hr give or take, (a bit more than half the load calculated here) depending on the actual design temperatures, and a cooling load of 1.5-2.5 tons. With minimal SHGC 0.26 west facing window area and all high efficiency lighting it would be on the low end of that range on the cooling load.

    With a better-than code house and properly accounting for dynamic thermal mass effects of an AAC wall the true peak loads would be lower still.

    garbage in= garbage out

    Hopefully what you paid for the garbage can be refunded. That type of load calculation is an embarrassment to the industry.

  6. user-5631279 | | #6

    Thank you all for the input, I really appreciate it! Sadly, no refund is available. I'm pretty disgusted with HVAC contractors around here in general. I've had a heck of a time getting someone to even talk mini-splits. As soon as they hear "efficient house" they start pushing extremely hard for a "geothermal" ground source system with a series of wells. When I tell them I want mini-splits, the price shoots through the roof and becomes just plain exorbitant - or they simple never get back to me with a quote. How can they charge more for a mini-split system install when there is no ducting to be done and just a handful of lines to be run? I get the equipment is more expensive but I'm getting quotes that are significantly higher than seems reasonable - on the order of several times the equipment price or more. Assuming a several day install, that puts their hourly rate somewhere between a neurosurgeon's salary and a flight to the International Space Station. Finding that I wasted $700 for my "approved" load calculation is a bitter pill to swallow. Sorry, rant over.

    As to your question Dana - apparently 4 degrees is the correct outdoor dry bulb temperature according to that site. I found some docs from our county that call out that 4 degrees as well as the heating temperature difference of 68 degrees and the cooling temperature difference of 21 degrees. You and Martin are correct, the 72 and 75 were flipped too.

    Based on all this, my hope for a single compressor outside at 60k BTU seems quite reasonable. The Mitsubishi I listed is capable of scaling all the way down to 7k BTU. I figure four heads for it should give a good spread of air. That seem reasonable? Thanks again!


  7. Expert Member
    Peter Engle | | #7

    60kBtu is not reasonable, if your maximum load is only 40kBtu. Oversizing equipment will cost you money for the life of the house in decreased efficiency & increased energy use, even with variable capacity systems. It seems like two 15kBtu heads in the basement is significant overkill, though you probably need 2 of something to circulate your heating and cooling through a space that size. You've got to start with an accurate manual J and go from there. There are a number of online services that will provide Manual J calculations based on your plans and specifications, at a far lower cost than the $700 you spent on a lousy one. Most of them also provide other HVAC design services. If your local level of expertise is that low, you might be able to outsource the entire HVAC design, and then get competitive bids for the predesigned system. You will probably save enough on right-sized equipment to pay for the design and support services.

  8. GBA Editor
    Martin Holladay | | #8

    If you follow Peter Engle's, advice, you might use this article to guide you: Who Can Perform My Load Calculations?

  9. Expert Member
    Dana Dorsett | | #9

    For the record, got a ZIP code? The only 4A locations I can think of with 99th percentile temperature bins that cool are the cold edge of 4A in eastern KS or western MO, say Kansas City or thereabouts(?).

    A 72F indoor design temp for heating isn't usually a code requirement either, but find out if that's actually a requirement. But even if it is, at a 68F temperature difference a tight IRC 2015 code min house that size with an insulated basement and heat recovery ventilation would usually come in well under 35K for a heat load, not 50K+.

    The Mitsubishi MXZ-4C36NAHZ hyper-heating multi-split will deliver a max 45,000 BTU/hr @ +5F outdoors, 70F indoors (a 65F spread) and still more than 40,000 BTU/hr net after defrost. It also gives you the 4 zones you're looking for:

    After a properly aggressive & accurate heat load calc that's probably going to be your best Mitsubishi multi-split pick.

    In my area 3 tons of geothermal runs north of USD $25,000, often north of $30K but cold climate multi-split solutions come in at around USD $4000/ton, or less than $3000 per zone. Non cold-climate multi-splits run about $3500/ton in competitive bidding. That's roughly half the cost of ground source heat pump solutions.

    Without running the numbers myself, the loads in the basement are going to be tiny- less than 10K, probably less than 6K. At most you'd be looking at pair of half-ton heads, or a single 3/4 tonner (maybe a mini-duct cassette, depending on how it's layed out.) A pair of 15K heads on a hyper-heating compressor at +4F could heat my whole sub-code circa 1920s house & basement, which is slightly is bigger than yours, with more above grade square footage, and nowhere near IRC 2015 code levels. For $700 you should have received the room-by-room load numbers, not just the summary. If you have that, what do all of the basement rooms add up to?

    Your whole-house cooling load would be under 3 tons too, possibly under 2, definitely not 4.5.

    Assuming a 68F delta is a code requirement, the calculated 57,574 load & 71F temperature difference can be instantly adjusted to 57,574 x (68 / 71) = 55,141 BTU/hr. The calculated 7717 BTY/hr of window losses would then be adjusted to 7717 x (68 / 71) = 7391 BTU/hr, but since your real U-factors are less than U0.25 the max window losses would then be 7391 x ( 0.25/0.30 )= 6159 BTU/hr. The difference then peels another (7391 - 6159 =) 1232 BTU/hr off the adjusted whole house load for a re-adjusted (55,141- 1232 =) 53,909 BTU/hr .

    That is a (57,574 - 53,909 = ) 3665 BTU lower design load, more than 6% reduction). This is just the tip of the iceberg, the instantly obvious errors that SHOULD be readily accepted by an inspector with your own 1 pager showing the arithmetic. Is a Manual-J & equipment capacity info required to get a sign off on the heating system to get a certificate of occupancy?

  10. WPmichael | | #10


    On that link you provided ( ) On the first page of the specs, @ 17 degrees F there is a rated capacity and a maximum capacity? Do you know the distinction between these numbers?

    Also at 5 degrees it only mentions a maximum capacity. Would you know at 5 degrees what would be real world expectation of the amount of heat produced?

    I use a similar m-series unit and I am baffled at what amount of heat I could expect depending on the temperature. I seen some graphs which show 100% capacity until 5 degrees at which point the line begins to slope down.


  11. user-7297437 | | #11

    I know this was all posted last year. Hope you don't mind a bit of followup.

    Michael Grundvig, I'm sorry you had to deal with those ridiculous contractors. I'm glad you questioned the results and reached out. I'm hoping you were able to connect with some actual professionals to work on your home. As far as costs, many contractors are still pricing based on product markup and charging extra for their lack of knowledge. Or worse. But I digress.

    Dana Dorsett points out the Mitsubishi MXZ-4C36NAHZ. This may indeed be a good match for the project. However altitude, temperature, and lineset derating must be applied to verify capacity of the system operating at those conditions. Every HVAC system is subject to various derate factors, but most are not actually calculated. For Mitsubishi equipment, professionals can use Diamond System Builder ( to verify equipment matchups, apply derates, calculate required refrigerant charge, and generate outputs. It isn't the most amazing software. But it works and helps to avoid some critical design mistakes.

    I'd like to address WPmichael's questions about the rated capacity vs maximum capacity of the MXZ-4C36NAHZ.
    - The "rated capacity" is the output of the unit at specified AHRI test conditions. For example, rated capacity for heating at 47°F outdoor dry bulb (DB) is tested at 43°F outdoor wet bulb (WB) and 70°F indoor DB. This test also locks the frequency of the inverter at 60 Hz. This doesn't help the functionality and efficiency of the system. But it keeps the test consistent with typical 60 Hz systems.
    - The "maximum capacity" is the approximate output tested by the manufacturer when the system is allowed to operate as engineered, modulating as needed for performance.
    - Because this is a Hyper Heat cold climate unit, the output at 5°F is effectively 100% capacity as stated in the spec sheet and unadjusted for altitude. The capacity then tapers off to about 34 kBTU at -13°F. This is a very strong unit. And as a branch-box type hyper heat model, it continues to operate as the temperature drops, with no cut-out temp. I got to see these operate at -25°F in Chicago this past January.

    But remember to have your HVAC professional properly size and derate the equipment for the climate and location. And if they don't know what that means, find a real professional.

    Shawn LeMons
    Performance Construction Manager (Colorado)
    Mitsubishi Electric Trane US, LLC

    1. Expert Member
      Dana Dorsett | | #13

      Shawn- Thanks for the link to the System Builder tool!

      >" And as a branch-box type hyper heat model, it continues to operate as the temperature drops, with no cut-out temp. I got to see these operate at -25°F in Chicago this past January."

      Can you spell out in more detail how & why the branch-box gets around the thermistor error shut down seen on other Mitsubishi cold climate systems? (Or why that shut down is ever necessary?)

      1. user-7297437 | | #15

        Dana, sorry, I just saw your follow up question. As I understand it, the branch-box types as well as P-series, and S-series outdoor units use pressure limits instead of temperature limits for system protection. Details such as charge, line set lengths, indoor temps, airflow, etc can effect system pressures. Here we are back to the importance of quality and following mfr installation guidelines. :-) And of course good building shells that allow the house to coast longer in the event of extreme weather conditions.

        That said, our published specs stick to the conservative -13°F tested condition data for these units. Thus the anecdotal extreme polar vortex conditions in Chicago last winter. Perhaps case studies may be helpful for some readers.

        Also, I believe diagnostics for the limit pressure switches can be found in the various outdoor unit service manuals on After opening the PDFs, search for "low pressure" or "high pressure". Hope this helps.

        Shawn LeMons
        Performance Construction Manager (Colorado)
        Mitsubishi Electric Trane US, LLC

  12. moose_head27 | | #12

    I passed through the calculation quickly and noticed your air leakage seems way too high. Your homes ACH is 0.5 which is very low. I did a rough calculation from the information you passed along and your heat loss due to air leakage should be a fraction of whats calculated. I'd ask your engineer to double check that figure as half of your heat loss is coming from air leakage.

    He must have inputted 5 ACH instead of 0.5 thats my best guess and used some very conservative assumptions throughout the calc.

  13. matt9923 | | #14

    Its a shame how the contractors are. They come out and berate me when I tell them what my manual j numbers are. Then they make up there own oversized system and send me $21,000 quotes.. I ended up doing all my own work.
    I haven't found one person in the industry who has come by the house I'm rebuilding and understood or agreed with my design. You can't be scared to try, if I got it wrong Ill fix it and still be in less then they would have charged.

    I spent the time and read through Martins “ how to do a heat loss”, I bet its more accurate then what you got for $700. Also download be opt for free. With those two calculations and advice here id be confident in sizing equipment.

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