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.

Views: 1804

Replies to This Discussion

Ok could you clarify -- 96% will take years to pay for itself in energy savings?  Are you saying a 96% isn't worth it unless rebated or high gas price -- period --  OR just if it's over-sized?  As an energy auditor -- I'd really like to see sealed combustion for safety in air - sealing and my customer is looking for the noise reduction of a small sound-proofed mechanical room.

Taking 20K off because I'm not running the duct work in the attic is sounding more reasonable but sheesh -- I'm not looking forward to having this conversation with the heating tech who told me I need a 100K unit!

He's not going to want to "break the norm" of the 50btu per sqft he's been installing for years. The "rule of thumb" has been passed down for generations. Oversizing by 2-3X is the norm around here so much that wide temperature fluctuations are considered a
"characteristic of gas heat".  HVAC contractors would much rather CYA on meeting setpoint at extreme temps than worry about uneven temps between cycles. I have NEVER seen an undersized gas furnace.

Don't try to sell the 96% based on energy savings, you will loose every time. The payback simply isn't there. Sell the 96% based on quiet operation and not using conditioned air for combustion. Look into any local rebates from utility companies, may offset some of the cost.

Hmmm...maybe he'll be reasonable and admit an 80K is over-sized enough.

Selling a 96% unit on not using air from inside the building is my goal  & noise reduction is an issue here because it's one big room and she's doing sound editing.

I appreciate your input.

Monday AM -- he was willing to reduce the size to 80K w/o a fight -- it must have been 3X down to 2X --- only a $50 difference in price -- but I think it will just work better.

It did reach -10 F here this winter for a few days but it might be another 3 years before that happens again.

All the soft ware I use to do heat loss and gain I can put in blower door #  this will take out the " I think its tight" .   On this steel building is in the way you set it up.    I tend to under size the heat more so it takes longer to heat up the building but the unit runs longer with out the on off on off.  If there is no plumbing you could just turn off the system at night.   With the big door a heat/cool vent would have to be above the doors to "WASH" the door with hot/cool air - I feel this is bigger than the size of unit

There is one bathroom -- toilet and sink in the far corner of the front of the building.  The furnace is going in the other front corner in the picture posted.  The overhead doors are used to remove larger pieces of scenery from the building which doesn't happen often and may not be happening at all with the new different direction her business is going in.

Its the gaps in the doors that makes very big holes.    Its very hard to make overhead doors tight -  most are just a hole in the wall

the BD is 6275 CFM@50  & the bottoms of the doors were tight just not the sides -- there is weatherstripping on them but it's old or not on right anyway -- 6275 is too high for just overhead doors -- that's like a window open -- it has to be the tops of the bays.  the owner is going to stuff them with inso and caulk the ceiling and I'm going to take another test to confirm the reduction.

I just did a Whole Building heat loss/gain  on this 2 big door steel shop -  YES I did a lot of guessing not knowing the state or zip or weather, so I used my address.  as is heat need is 218,654 BTU/Hr  un sealed nor add "R"  If you go back and seal and add "R' to walls and attic( very big) footing, seal doors( best your can) down to  61,000 BTU.    AC is the same way 20 people   some working  38,000 BTU or 3.5 ton  if not sealed you  can b e as high as 90,000 BTU's or 8 tons.  If you want a copy look at my web sight and leave a word.  www.ericsenergy.com  I will be at ACI next week talk to me in the hall way.    I run a load for each job I do and give a before and after its sealed and add "R"   I have gotten 92% lower heat use.



Lots of good info here.  Keep in mind, much of the load calculation details are buried in the software. I have a contractor using Elite.  He has 3 choices buried in the preferences Tight, Normal and Leaky for BD numbers. Tight is defined 'sort of'. I read it at 7 ACH @ 50.  The defaults are .5 U factor for windows etc.

Among other things, tons of AC is also used for air flow.  To get that right,  you have to do a Manual D after the Manual J. 

The manual J is a number; getting there is not rocket science. You can download a free Spread Sheet from ACCA to do them. Most guys in the field cut their teeth on existing homes.  These have problem duct systems, 60% furnaces and no energy efficiency.  They bring their experience along.  Many times it doesn't get updated as other things change, like insulation, windows, and equipment technology.

They also need to use Manual S.  Many times I've seen Man J calcs the Contractor disagrees with. To use that number ACCA designed the process to include Man S.  My home has a 39K Man J load. I have a 60K furnace. Man J + Man S arrived at that number. As a homeowner,  I'm glad I don't have a straight Man J sized furnace.

In the end, the Mechanical Contractor has a license and liability insurance. He is on the hook, legally, for the install.

Hi John --

I looked up the U-values for the windows -- listed as .67.  Quite a bit of the "Component Description" is not accurate.  I ran through the envelope specs with the tech that came to the site.  He wasn't there very long.  So I'm not surprised that some things aren't accurate.  There is no ACH number listed -- at least under that title.

There is a manual J calc with the report which I also haven't seen one of before so when the "System Main Trunk Size" is listed as 12x20 in --- and the proposal says they want to do a 14" dia round for the main trunk -- that's not making too much sense either.  I want the ducts run inside the envelope at the ceiling(which is 9.25' high).  It would be great for efficiency and the owner is not concerned at all about appearances just function.

There is nothing about Manual S -- unless it's included in some other number.

The liability is probably what makes them over size?  He was willing to back off the size to a 80K without a objection which to me means it was oversized -- by a lot.


Discussion Forum

HVAC integration – the past, the present, and the future

Started by Igor Mitbarg. Last reply by Lucifer Prescott Apr 11. 1 Reply

Target superheat formula

Started by Miguel Orduno. Last reply by John Proctor Feb 18, 2015. 6 Replies

Now for Something Different

Started by John Porterfield. Last reply by John Porterfield Jan 12, 2015. 2 Replies

Latest Activity

Profile IconSonny Roncancio and Chester Energy & Policy joined Home Energy Pros Forum
19 hours ago
Ryan Moore updated an event

LEED for Homes Green Rater Training at Fort Collins, CO

December 7, 2017 to December 8, 2017
Green Insight will host this 2-day training in Fort Collins, CO December 7-8,…See More
Rodney Bailey liked Rodney Bailey's discussion Energy audit equipment for sale.
tedkidd liked Mike Rogers's blog post Focusing on payback to motivate customers?
Home Energy Magazine's blog post was featured
Home Energy Magazine posted a blog post
Quinn Korzeniecki added a discussion to the group Building Performance Institute (BPI)

Rebates and Incentives: Illinois

Curious about home performance incentives and rebates available in your area? Check out this BPI…See More
Profile IconStacey Cordeiro and Melissa Ames joined Home Energy Pros Forum


  • Add Photos
  • View All

© 2017   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service