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.
As I recall Wright Soft by default uses foot measurements & not inches / decimals though that can be changed. As for the minimal insulation, I think they counted the entire vaulted area in their calc
As for blower door number - look for something like leakiness & what they listed - tight, leaky, etc...
I can't recall on the ductwork question & without seeing the report I don't think anyone can answer why they both read 100k as there are numerous other places to cheat / bump up the size specified
The measurements are in feet -- which would make the ceiling height 9 plus -- and I don't know what they'd do with the other 3" -- maybe round it up to the next whole foot at 10'? Conditioned Space of 20,520 on the insulation added would be a 10' ceiling -- that's making some sense. And OK the larger number would include the attic cubic feet --- if you forgot to take half of it because it's a triangle (gable roof) (36W x 6Hx 57L) + the 1st floor cu.ft. would be the exact number they have at 32,832 --- sheesh!
The only thing I see for the BD # could be in the Component Description of the Summary Loads section --- "Infiltration: Winter CFM: 106, Summer CFM: 55" (for the insulation added) and "Winter CFM: 170, Summer CFM: 88" (for the minimal insulation calculation)
Something else I'm seeing in the summary -- the minimal insulation calc is based on R-11 in the ceiling which is off by maybe R-4 (can't find 1.5" insulated garage door panels higher than R6.6) And the "with insulation calc" is only R-28 (I'm thinking R6.6 plus another foot of blown cellulose would bring it to more like R 44.6)
Thank you for the input.
What design temp did they use in the software? Where is the nearest major city?
Contractors love using design temps that are "maximum extreme" that an area only sees once every 5 years. It's a CYA thing, they don't ever want a complaint of not enough heat. Energy bills and temperature fluctuations between cycles aren't of much concern to most HVAC contractors, they just want to to be able to hold setpoint.
50BTU per sqft is a '"rule of thumb" contractors have been using for years, load calc inputs are fudged to get the number they want to see. 500sqft per ton is a common "cooling rule" that contractors like to see. 100k/4ton is what that building would get in most of the country.
I'm not seeing a design temp --by that title. The "Daily Temperature Range" is listed as "Medium"
The Reference City is Correct -- it is only 10 miles or so from there -- the other two Reference cities in Central PA are more than an hour away.
There should be defined indoor and outdoor temperatures that the load calculation is based on. Something like 70f indoors/10f outdoors.
the reference city, latitude & elevation are right --- the front doors actually face west, and I don't know what "Daily Temperature Range: medium" stands for ---
Are one of the Winter/Summer numbers shown here what you're talking about?
7 degrees outdoor, 72 indoor are design temps, looks reasonable. 80% furnace would mean a 100k input/80k output.
I'm looking at a two stage 96.2% eff. unit.
the highlighted spots are errors in the envelope input -- walls are at least 2 x 6's -- foam is an average of 2.5 - 4" in the stud bays -- the ceiling is (1.5" insulated garage door panels R-6) and 12" of cellulose will be added in the attic in the scope of this job. I belive that will make it more like R-44.
80k x 96.2% = 77k output. No need for a 100k furnace. Oversizing will render the purchase of a 2 stage worthless, it will never come off stage 1. Unless there is a significant rebate involved or high gas costs, a 96% will take years to pay for itself in energy savings vs a 80%.
All ductwork should be in conditioned space, therefore the 20k ductwork heat loss should be zero. Now you are looking at a 60k furnace. But wait, that breaks the 50BTU per sqft rule he's been using for decades. We can't have that, now can we?