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HVAC sizing for cooling: Manual J vs. HVAC contractor?

Nplus | Posted in Mechanicals on

We are still in the planning stages of a 2 story single family whole house remodel in the San Francisco Bay area.  The HVAC system is going to be a Mitsubishi multi-split.  My question lies in the 2nd floor which will use a fully ducted system (ducts buried in attic insulation) with one high static pressure air handler for the entire 2nd floor (PVFy-P12NAMU-E1).  A manual J was performed that I have attached, which calls for a cooling load of 12892 BTU.  The capacity of the specified air handler is 15998 BTU (11999 Btu sensible, 4000 Btu latent).  Despite the HVAC design specifying 1 ton air handler, the HVAC subcontractor my general contractor works has quoted us a 2.5 ton air handler, probably based on rule of thumb.  When asked about the discrepancy, the HVAC subcontractor said that he thinks putting a 1 ton air handler is a big mistake since the Manual J, D, S calculations do not allow for extremely warm days or if there are several people at the house.

Looking at the Manual J, the design temp is listed at 85F.   Our climate occasionally gets into the mid 90s.  Despite reading through all the HVAC posts I can find on this fantastic website and trying educating myself, the HVAC subcontractor has made me question whether a 1 ton air handler will be enough to keep the house comfortable at temps outside of the design range.  Obviously I would prefer a right sized system, and not fall into the trap of a grossly oversized system that I have read about and the associated short cycilng.  Should I insist on a 1 ton air handler (and risk annoying the HVAC sub), or compromise by going with a 1.5 ton air handler?  Or is there something else that I am not thinking of?

Thanks in advance and for all the great advice on this website.


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  1. GBA Editor
    Martin Holladay | | #1

    You are right. Trust the Manual J calculation. Oversizing by a factor of 2.5 "just to be sure" isn't good HVAC design -- it's ignorance.

  2. this_page_left_blank | | #2

    If I recall correctly, the manual j has extra capacity factored in already. So the contractor is mistaken with his warm days and many people comment.

  3. Expert Member
    Dana Dorsett | | #3

    Insist adamantly on going with the 1-ton, which matches the calculated sensible load, and has extra margin on the calculated latent load, matching almost to perfection.

    I'm not sure if "idiot" is too strong a word to describe someone who would quote a 2.5 tonner for a Manual-J calculated 1 ton load. Go ahead and annoy them- their opinion doesn't count. You might even insist on finding another sub, since annoyed possible resentful people tend to be a bit less diligent in their work.

  4. Jon_R | | #4

    At the same compressor output, a moderately larger (say 25-50% > compressor rating) air handler will be more efficient (less delta-T) and not short cycle (both sizes run when the compressor runs, so basically the same). It will reduce latent output, but your numbers indicate that this is a benefit, not a problem (required SHR = .93, which suggests more like 500+ CFM/ton = 500-600 CFM, not the too low <= 400 CFM you are considering). It's humid climates where AH up-sizing (more than typical CFM/ton) will cause humidity problems. Don't confuse AC system or compressor advice with AH advice.

    Not enough information to say which is best in your case (probably the similar "1.5 ton" AH @ 410-585 CFM), but for air handlers, the best option is more complicated than "don't oversize BTU". As Manual S (*not* J) dictates, CFM and SHR are important to AH sizing for maximizing comfort and efficiency.

    edit: the manual S procedure below comes up with 642 CFM on medium speed. Ie their overly optimistic "2 ton" AH for 1 ton of sensible load in your dry climate is right-sized, not over-sized.

  5. Expert Member
    Dana Dorsett | | #5

    At the low/very-low (often negative) latent loads of the SF Bay area it's almost that simple.

  6. walta100 | | #6

    I agree trust your calculations insist on a 1 ton unit.

    Do yourself a favor and make room inside the conditioned space for your HVAC equipment and ductwork.

    Attic duct work will increase your operating cost by about 30%. All ducts leak no matter how carefully installed. That means the supply ducts are leaking conditioned air outside of the conditioned space, depressurizing your house forcing unconditioned air to find to find the gaps and leak into your home.

    Buried Attic ductwork is also risky in some climates, the warm moist air in the attic will want to find cold AC ductwork and condense into liquid water in your attic.

    Take the time to look at the links below


  7. Nplus | | #7

    Thank you to everyone for their expert advice.

    How much extra capacity is already built into a typical Manual J and S? It is not infrequent that our climate here exceeds the design temp of 85F. In just the past 4 months, we had 24 days that exceeded 85F with several days in the 90s, and a peak of 97. The calculated sensible load of 11998 Btu almost perfectly matches the sensible capacity of the 1 ton air handler at 11999 Btu. Will the system be able to provide adequate cooling for the days significantly outside the design temp?

    I want to arm myself with enough knowledge that when I insist on 1 ton to the HVAC sub, it doesn't come across as "I read this on the internet and so I know better than you [the expert]." As a physician, I see the parallels but from the other side, where patients self-diagnose or self-treat based on what they find on the internet, which I find frustrating.

    Walter, thanks for the links to the articles. As for putting equipment and ducting inside of the envelope, this is something that my architect, our HVAC designer, and I wrestled with for quite some time. We tried and tried to make it work, but as with any build or remodel, there are trade offs, and this is one that we decided to make. At least we are going with buried ducts (which I learned about on this website), and humidity here is not much of a problem.

  8. Expert Member
    Dana Dorsett | | #8

    C0oling loads of insulated buildings aren't dominated by the outdoor temperatures. Solar gain through windows has a bigger effect on cooling loads than heat conducted through the walls or roof, and that solar gain is about the same when it's 70F outside as when it's 100F outside. The fact that it sometimes hits the mid-90s or even 100F doesn't change the peak load by nearly as much as you might think- doubling the temperature difference won't come anywhere close to doubling the load. If you had access to the Manual-J tool you could tweak the 1% design temp or indoor temp to see just how little that seemingly large 10F jump in temperature to 95F makes on the peak load numbers.

    On those days when it nudges on 100F you may lose ground by a few degrees- it might even hit close to 80F for an hour or so but nothing like 85F. When running at a 100% duty cycle for a handful of hours the air will will be dry indoors, dry enough that 80F would not be a comfort issue for most people. On days where it's predicted to hit the high 90s one can even pre-cool the place by setting the thermostats to 72F a handful of hours prior to the anticipated peak temperature, which would utilize the thermal mass of the building to offset the peak load.

    It wouldn't be insane to spec a 1.5 ton air handler here, but it IS insane to install something that has twice the the capacity of the calculated 1% outside design temp. If they made a 1.25 tonner that would be the "right" choice, but if it comes down to the 1-ton vs. the 1.5 ton, stick with the 1-ton.

  9. brad_rh | | #9

    You are right to be skeptical of internet knowledge, but some of the folks here are right most of the time :) Especially DD. It sounds like you have an HVAC designer who did the manual J? If you want to trust a local person instead of the virtual friends here, trust the guy who did the manual J. Talk to him about the inputs, how many humans are in the input (100 W each if I remember correctly). Maybe bump up the design temp, the old temperature data may not be conservative enough.
    As DD says the indoor temps will not get too bad if the outdoor is above the design temp. How picky are you and the rest of the household about temperature? The other thing to consider, which might point in the direction of the 1.5 ton (not 2.5) is if the house is empty all day, you can save by setting back the thermostat, which would mean you would need a little extra to cool down after work.

  10. Nplus | | #10

    Thank you everyone for weighing in and providing invaluable information. The HVAC designer who performed the Manual J is Andy Pell of Fresh Air HVAC sizing who seems quite knowledgeable and helpful. He is not local, but just another "virtual friend." I am much less picky about household temperatures than others in my family, especially hot temperatures, which has me vacillating between 1 ton and the 1.5 ton.

    It sounds like 1 ton is the experts choice but we may have to tolerate some rare higher indoor temps when the outdoor temp is high and there is a lot of solar gain. With that I may also have to tolerate some household complaints. 1.5 ton wouldn't be a terrible decision, but would be less efficient and we would have to tolerate some short(er) cycling, but there wouldn't be the household complaint issue when hot outside with a lot of solar gain. No doubt that 2.5 ton = insane.

  11. Expert Member
    Dana Dorsett | | #11

    Exterior shades or using small fans blowing at sweaty humans in the warmer spots in the house on days where it's going to hit the mid-90s buys a lot of comfort at the sub-1% outside temp extremes.

    Most of the heat-sensitive people would just opt to sit in the cooler rooms anyway, even without those additional measures.

    Ice cream & popsicles work too! :-)

    As dumb as dumb rules of thumb are, what's the square feet of space we're cooling here? A typical ratio for a reasonably tight not-super performance house would come in around a ton per 1000-1500' for smaller houses, but there are exceptions well to both sides of that number. Old school hacks use a ton per 500' and begrudgingly have moved on to a ton per 750' and oversize most houses by 2x.

    Take a look at Allison Bailes graphic plotting real Manual-J feet per ton ratios against house size in this bit o' bloggery:

    Mind you, most of those houses are also in the southeast, where the latent loads are quite substantial, unlike your location.

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