# Help with my new home HVAC sizing

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• 05-05-2013, 12:42 PM
udarrell
Quote:

Originally Posted by dan sw fl
I understand ACH=
_____TIGHT __: 0.15
SEMI-TIGHT __: 0.26
_ AVERAGE __: 0.39
SEMI-Loose __: 0.61
___ Loose ___: 0.82

based on my use of Manual J8 abridged.

Some residences with foam products ought to be < ~0.2

Right Dan; I was just using .4 ACH for his situation because his home needs to be above the code minimum home air-changes of 0.35 ACH, when it is properly ventilated.

Of course, the proper way is to do the blower door test & use the actual lower ACH number so the load-calc would estimate the needed makeup air...then he should use 0.2 or less ACH.
• 05-05-2013, 01:50 PM
Cwatts15
Ok I tried to figure out the Manual J. This doesn't make sense to me. Did I do this right?
Attachment 380341
• 05-05-2013, 02:28 PM
dan sw fl
LOOKS like you put in about 900 sq feet of window which seems to be nearly 3 X too much.

Ceiling is ok.
Walls are not enough.
• 05-05-2013, 03:22 PM
dan sw fl
A GUESSED 12-Minute analysis Version of _Whole House Load Calculator_ is attached
simply
knowing 2,900 Square Feet total and walls are foamed.
Ceiling R-38
_Walls R-18
Total Window Area 380 Square Feet

I would say windows may be ~6,000 BTU/Hr too much for Sensible Load (Heat Gain) if you actually have SHGC < 0.36.

However, I don't even know how many windows, total glass area, exact orientation, U-value nor SHGC.
• 05-05-2013, 04:40 PM
Cwatts15
I tried to open you calc, but it says I don't have permission. I tried to redo the calculations. See if this is better. I do have a lot of windows on our southeast wall, and all of them are under a cover patio. I really appreciate all your help.
Attachment 380381
• 05-05-2013, 04:53 PM
Cwatts15
Ok, I was able to open it, and it is really different from mine. So bottom line, what does calc recommend for HVAC size and ventilation needs. I know I am stupid when it comes to understanding all this.
• 05-05-2013, 05:18 PM
mark beiser
Quote:

Originally Posted by Cwatts15
3 bids- all very different, 1: 2ton & 3ton 80% 13seer Lennox- gas unit with optional ERV, 2: 2ton & 5ton 93% 13seer Armstrong with option fresh air damper. 3: 5ton & mini-split Armstrong.

Pure speculation without doing a load calculation, but I think it would probably be best if you toss all 3 of those bids in the trash.
#1 and 3 are the kind of sizing you would see in a stick built house with a questionable thermal envelope in North Texas.
#2 is just ridiculous.

In a tight, well insulated 2900 sq ft house, unless you have some really big unshaded west windows,I'd be skeptical of anything that came out to more than 3 tons + ERV for the main part of the house, and a minisplit for the master bedroom.
If you do have really big unshaded west facing windows, a conventional system will always struggle to evenly heat and cool the home due to its load diversity.
Applications where there is high load diversity is where systems like a Daikin VRVIII-S really shine.
• 05-05-2013, 06:53 PM
dan sw fl
Quote:

Originally Posted by Cwatts15
Ok, I was able to open it, and it is really different from mine.

So bottom line, what does calc recommend for HVAC size and ventilation needs.

I don't even know how many windows, total glass area, exact orientation, U-value nor SHGC.

I wouldn't make a Specific design recommendation without drawings, specs and
frequent field checks on adherence to building thermal envelope specs.

I would use an ERV at about 150 CFM with intermittent timed operation.
... 1/3 to 1/2 the time.
• 05-05-2013, 07:44 PM
dan sw fl
Quote:

Originally Posted by Cwatts15
I tried to open you calc, but it says I don't have permission.
I tried to redo the calculations. See if this is better.
I do have a lot of windows on our southeast wall, and all of them are under a cover patio.

620 Square Feet of Glass would be typical of a 6,000 Square Foot McMansion in FL or ...
• 05-05-2013, 08:10 PM
Cwatts15
Ohhh, this is crazy. I have 21 windows 7 of which are little square ones and 2 are 1x5 in bathrooms. I thought my windows were about 325sqft or so. I do have 3 large sliding doors on the SE (backside of house) that are under covered patio. Only 3 - 4x5 windows, and 4 - 2 x 1.6 windows facing SW or NW.
We are will have foam walls, but not foaming the roof. That will be blown cellulose with radiate barrier sheathing. We are getting pretty good windows-energy-star approved. I have tried my best to give you the correct info. I guess that's why I am a nurse practitioner and not into HVAC. But I am sure trying to understand it all.
• 05-05-2013, 08:18 PM
mark beiser
Quote:

Originally Posted by Cwatts15
We are will have foam walls, but not foaming the roof.

Why not?
• 05-05-2013, 08:23 PM
jtrammel
It'd probably be more bang for your buck to spray foam roof deck and use traditional insulation in walls, especially if any ductwork will be in the attic.
• 05-05-2013, 11:12 PM
dan sw fl
Quote:

Originally Posted by Cwatts15

Ohhh, this is crazy.
I have 21 windows

7 of which are little square ones and
2 are 1x5 in bathrooms.

I thought my windows were about 325 sq ft or so.

I do have 3 large sliding doors on the SE (backside of house) that are under covered patio.
Only
3 - 4x5 windows, and
4 - 2 x 1.6 windows facing SW or NW.

We are will have foam walls, but not foaming the roof. That will be blown cellulose with radiate barrier sheathing.
We are getting pretty good windows-energy-star approved. I have tried my best to give you the correct info.

I guess that's why I am a nurse practitioner and not into HVAC.
But I am sure trying to understand it all.

How will the house be built without drawings?

Window Identification
7 little ones _____ 20 sq feet
2 1 x 5 _________ 10
3 4 x 5 _________ 60
4 2x1.6_________ 13

16 Windows _____ 103
5 unknown

3 Large Sliding Doors
10 x 8 ________ 240
--------------------------------------------------------------------------------
I guess _large_ is somewhere between 6 x 8 up to 22 x 10, and I have seen that
and anywhere in this range.
Including 20 x 10 retractable wall at right angle to another 20 x 10
forming a " disappearing double wall'
opening to about a 4,000 square foot deck.

So, crazy is all relative.
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