Load Calculation Confirmation
We’re currently building a house in Omaha, Nebraska. We’ve made some choices to increase the efficiency of the enclosure, but it seemed like the calculated needs were higher than I thought. I am not an expert, so just wanted to see if anyone could sanity check this for me. We will be using a 3 ton Mitsubishi outdoor unit (MXZ-SM36NAMHZ-U1). It will service both level of this ranch (1818 main level and the same in the basement. We will use zone dampers to provide the correct flow to each level. Here are some of the details:
Triple pane windows throughout (.18) and we were very conservative on the number of windows with most fixed.
Continuous rigid foam insulation on basement walls (R10) and there will be additional insulation in the stud bay.
2×6 exterior walls with Zip R9 on above grade walls. Filling stud cavity with blown-in cellulose.
We are going to use Aerobarrier to get to 1ACH50.
R50 insulation in the attic.
Is 3 tons reasonable to heat both levels?
Thanks,
Matt
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Replies
I suspect 3 tons is oversized by 50-100%. Devil is in the details, particularly with glazing and also matters how much of your basement is below grade vs above grade, but even with the -3F design temp in Omaha, your heating load is probably closer to 2 tons than 3.
Side note: absent a backup heat source, I'm personally more comfortable with two separate heat pump systems. While I love them, they are more complicated than most combustion or resistance heat sources which is not a great thing if they break during a cold snap.
Thanks Patrick! We are going to use heat strips as backup. We are also planning on 2 dehumidifiers to handle the latent load and we're using rigid metal ducts for the main trunk with flex pipe to the registers. I hired a very reputable HVAC design company and I'm happy with their plan, but the cost came in much higher than I thought (over 50k) and am looking for some cost savings. I've heard Steve bazcek talk about a sliding scale. Spend more on enclosure and save on HVAC.
Thanks again,
Matt
It rarely works out that way - the equipment savings are tiny and the labor stays the same. However, you might be able to downsize ductwork a tad. With strips as backup, you won’t have the lower temperature air constraints.
First step is saving cost is replace the SM with the hyper heat SUZ unit, ie 3 ton one:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_SVZ-KP36NA_SUZ-KA36NAHZ_en.pdf
The SM is a multi split and I believe it needs at least two indoor units to run. It is also much more expensive than the single zone.
As for the overall sizing, start with a proper manual J and go from there.
Agree with Akos last point here. Ask them for a copy of the Manual J they should have made and study the inputs they used. Make sure they are assuming the 1.0 ACH50 you plan to achieve. If it is such a “reputable” company they should be doing things right like using ACCA design manuals. You may also want to consider getting a second opinion or hiring a third party consultant to calculate the heating and cooling loads for you. I’m sure there are mechanical designers in Nebraska you can reach out to too but the two that are mentioned here most frequently are Corbett Lunsford and Alison Bailes I think they are both in Georgia but do design work all over the country.