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Sizing a heating and cooling system

user-5574861 | Posted in Mechanicals on

We are finalizing designs for the mechanicals and I am looking for some advice. My mechanical engineer is doing all the Manual J, D, S, and HERS calculations. On my side, I am running the WUFI analysis. We are going to spec a combination of ducted, non-ducted units working off of a single VRF ASHP. Some of the questions I have are as follows:

1) Does anyone have any experience with regards to which sizing tool is more accurate in a very well insulated house, Manual J or WUFI?;
2) What size should the system be relative to the size specified above? I don’t want to spec something too small that runs all the time or something too big that cycles on and off. My gut is to spec something that is designed to operate at the size chosen in 1, but has the ability to operate at 25% greater than that.
3) All of the anecdotal information I have seen is that Fujitsu is a great product, if not the best, thoughts?


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  1. GBA Editor
    Martin Holladay | | #1

    I know that WUFI is used by researchers for hygrothermal analysis -- for modeling the flow of moisture and heat through building assemblies (floor assemblies, wall assemblies, and roof assemblies). I have never heard of using WUFI to perform load calculations.

    I know for a fact that the use of WUFI by anyone other than an academic researcher can lead to a huge number of false conclusions and misleading results. For more information on the WUFI problem, see WUFI Is Driving Me Crazy.

    So, to answer your question: The best way to do a load calculation is to use Manual J. Use it strictly and conservatively, with accurate (non-fudged) inputs.

    Your suggestion to tack on 25% to your Manual J results is a bad suggestion. Trust your Manual J. Avoid the misguided impulse to add 25% to anything. Remember: almost every load calculation made in the U.S. is wrong because of the misguided impulse to tack on fudge factors. Most equipment is 50% to 200% oversized.

    For more information on these issues, see these articles:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

  2. user-5574861 | | #2


    Thanks for your suggestions and the related articles. While WUFI does have a robust hygrothermal piece, it is also used by PHIUS to model a house for certification, much like PHI uses PHPP. So WUFI provides me with heating and cooling loads and demands. My WUFI analysis came up with an annual heating load number of number of 3.74 kBtu/sf/yr. I have not received my Manual J figures yet and I don't know how good Manual J is at designing loads for near-Passive House constructed homes. I saw this quote from Joe L in one of the articles you listed:

    "According to Lstiburek, his experience proved that high-performance Building America homes in Las Vegas could have had HVAC systems sized at about 60% of Manual J. To be conservative, his team recommended installing systems sized at 80% of Manual J."

    This kind of highlights my concern that if I design to Manual J, is it too much, especially if WUFI says the design numbers are much lower?

    My apologies for being unclear in my original post. My sizing comment was in relation to WUFI. So if WUFI says I need 36k Btu's of heating, should I install a unit that is rated at 36k Btu's at 100%, but has the ability to cycle up to 125% or greater when needed. I currently live is a 2006 code built house so it is drafty and has oversized mechanicals that cycle on and off. This is what I am trying to avoid with the new house.

  3. charlie_sullivan | | #3

    WUFI now has two product families; each family has three products. The original family is what most people talk about when they talk about WUFI, and it does hygrothermal analysis of building components--a section of a wall for example. The new family is a whole building simulation package.

    I would expect that with the same inputs a whole building simulation in WUFI would give similar results to Manual J, and my first suspicion if they gave substantially different results would be that one was based on more accurate and detailed inputs.

    My second question, however, would be the climate scenario used in WUFI's sizing recommendation, and how that compares to the 99% percentile temperature used in Manual J. Does WUFI just look at the peak requirement over a typical year? If so, that would probably lead to choosing a smaller heating system, but might leave you cold in a year that had a colder coldest week than the year you simulated.

  4. user-5574861 | | #4


    I am using WUFI Passive which is needed for PHIUS+ certification, although I am not certifying the project. You basically build a model of the house in SketchUp, import it to WUFI, build your assemblies, assign the assemblies to each part of the shell, input climate data, mechanicals, plug loads, shading, etc and it generates heating/cooling loads and demand plus site and primary energy. In my climate zone, my heating demand is limited to 5.1kBtu/sf/yr to achieve certification, but I am at 3.74. Using a standard HPHW, .6ACH and pretty good insulation levels, I was just over the limit, which makes sense that I should be right around the cutoff. However, switching to a Sanden saved me quite a bit on heating demand and brought me down to 3.74. It also brought up my cooling load slightly as it should. My heating load is 28kBtu's. It will be interesting to see how that compares to Manual J. I will have a wood burning fireplace and a couple of gas fireplaces as back-ups if needed.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for clarifying. I usually distinguish between WUFI and WUFI Passive, and so I assumed that software you were talking about was WUFI, not WUFI Passive.

    And it sounds like you never intended to tack on a 25% fudge factor. (Good.)

    Sorry for the misunderstandings.

  6. user-5574861 | | #6

    My bad for not clarifying upfront.

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