Hello to everyone again -- This is an update on a building I'm working on for a NG furnace.  It's been about a month (a very busy month) since I last posted questions and I have been to the building -- I have some answers and another question about reading LOAD CALCULATIONS. 

The 2052 sq. ft. building has only 1.5" insulated garage door panels for the ceiling as the only insulation (in the attic floor along with 2 part foam on the perimeter walls averaging 2.5" to 4" thick) so I'm looking at blowing cellulose to a depth of 1' in the attic after some air-sealing to bring the heating loss down.  The blower door number was 6275 CFM@50 -- there is a construction flaw where 2 part foam applied to the perimeter's stud bays didn't completely reach up behind a top 2 x 6 band joist (is that what it's called if it's at the ceiling and not at floor level?)  Most bays are pulling air during the BD test.  Also the ceiling garage door panel joints are not caulked.  So I'm hoping to bring the CFM down to some reasonable level by caulking the cracks and using bagged inso stuffed into the stud bays.

I found an HVAC company that gets load calc's done and they gave a bid for a Bryant preferred two stage and ductwork installed inside the envelope (ceiling is 9'3").  They're sizing it at 100,000 BTUs.  

They did two load calc's -- one with minimal insulation (present situation) and one if 1' of cellulose is installed in 2052 sq. ft. of attic floor.

This is the first time I've read/seen an actual load calc so I have a question -- (the HVAC co. has a call into the guy in another city who did the calc and I'm still waiting for his answer) --- The software is "Elite RHVAC Residential HVAC Loads" and approved Manual J and Manual D -- 8th Edition, Version 2

On the first page, the "Project Report" down in the "Check Figures" section the number for square foot room area is correct at 2052 --- the "Volume (ft cubed) of Cond. Space" concerns me.  I would get this number by taking the sq. ft of 2052 and multiplying it by the ceiling height of 9.25' (9'3") for a total of  18,981 cu. ft --- The reports however had two different numbers -- minimal insulation calc being 32,832 and the insualtion added calc's number being 20,520.

Can anybody who knows this software tell me why these numbers are not the basic length x width x height number?

Is there a place on this report where I can see whether they considered all the ductwork is run inside the envelope not up in the attic?

And I don't see where the Blower Door number was addressed at all -- or isn't this the type of calc that uses the BD number?

According to both load calc's (minimal insulation) and ($2,000 more insulation added) I need a 100,000 BTU furnace.

Well -- thanks to everybody in advance for input.

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If you have these types of issues, perhaps you should consider having the load done by another contractor.

How would I find one in my area?  This firm is the first I've come across.  They do commercial and industrial HVAC along with solar installs and some residential.  I was thinking this week of contacting the companies that sell the load calc software to see if they have any purchasers in my area but if there is a better way I'd like your input.  Thanks

If you supply me with the data, I'd be glad to perform the calculations for you.

Isaac - I appreciate the offer but from the whole discussion here & what I've learned about the load calc's I got copies of I think a 80K two stage unit would be right.  Anything smaller and I'm going to have trouble finding an HVAC co to put it in for me and I really don't want to have to sign off on a lower BTU unit until I know more about sizing.  Thanks again for the offer.

This is exactly why oversizing continues, nobody is willing to stick their neck out. The load calcs show 60k once you take out the "fudge factors". If you wanted to experiment you could leave stage 2 unhooked and see how well it keeps up on stage 1. This would give some real world feedback on how much heat loss there actually is in the structure vs. calculated. If the system can't keep up it would just be a matter of hooking stage 2 up. Yes that means they overpaid by selecting a 80k high/60k low instead of a 60k single stage, but it is what it is.

I do "experiments" on my own house that I wouldn't do in a customers house. I installed a 44k furnace in my own 1600sqft house. Runs 20 out of 30 minutes at 68f indoors/5f outdoors. I feel totally comfortable installing small furnaces on NEWER homes, but with older homes there are too many variables.

I also installed a 2 ton AC which is 1/3 smaller than the norm for our area, guys at the shop thought I lost my mind. Yesterday it got to 93 outdoors and the AC ran about 12 minutes out of 30 with an indoor setpoint of 75f. Last summer my highest electric bill was $89. Again, wouldn't do this at a customers home w/o knowing exactly how much heat gain they have.

60k is INPUT of your furnace right? If it's a typical 80% that would give you a 48k output.

Hi Kari,

I gave a presentation at RESNET on this topic. Here's a link to the presentation. It contains some samples of Elite reports, as well as a chart that shows where specific data shows up on various reports in the software. Hope it's helpful...


Thank you!  very interesting -- I printed it and will read it over thoroughly.  I figured when they put front door orientation to the north they were using a maximum condition but now I see how much higher it is than the west orientation of the front doors to this building.

Question -- is there a market for selling the load calculations or do HVAC companies like to do it in house?  I wouldn't mind learning how to do them.  I have BPI-BA and PA State WAP auditor certificates and have been helping my husband in our contracting business for over 9 years now.

90% of residential installs are done by the WAG or rule of thumb methods. 500sqft per ton for AC/ 25BTUper sqft are some common ones. It's been "working for contractors" for years, why change now? The rules never got adjusted for better insulated homes, now the extra capacity helps compensate for crappy installs. Thermostat will always meet setpoint and no customer complaints. Gas/electric bills aren’t their problem, and decently insulated homes have reasonable utility costs to begin with.

Even when "load calcs" are done, fudge factors like 20,000BTU loss for ductwork (PLAN to loose than much heat in ductwork on a new install?) "pad the bill". Contractors will adjust the input to make the output spit out what they are comfortable installing.

In reality a 60k 96% furnace is probably plenty for your building. Good luck getting a contractor to "bet on it", especially since the installed cost is only $200 or so less. Would you be willing to stick your neck out on the off chance that 60k isn't quite enough?

My grand pa would start with a temp of -40 in Kansas City  but now I use +2 using Write soft ACCA MJ +D.   Gramps would put a 5" supply each 10' in the floor  Main trunk was only 6"X 14" but the heat only system was 1.6" in water SP.   I now see the same duct work with a AC system on it with a max of .5" in water SP.    Each year the compresses blows or the heat exchanger has a hole.   The same thinking is used for 80 yr  with the start of motor on blower.  We need to math and science to what we do

No, I wouldn't be willing to stick my neck out that far.  It got down to -15F this last Jan.  That is not a regular occurrence in these parts but I wouldn't want to hear from a customer who just paid $9K for a high eff. unit + ducts, tell me the furnace isn't keeping up when the weather does that again in 3 years.  I just don't want it so over-sized it short cycles continually.

Yes. It's not uncommon for contractors to hire out the design. We perform design work for contractors all over the US. We also train folks on how to do it, if you're interested in learning. Check out our site and let me know if you have any questions: https://homeenergypartners.com/product-category/online-training-cla...


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