Effect of windows on Man J?

[old yeller]

Hello all-

I will be replacing all of my older windows soon, and was curious from an HVAC standpoint what the effect might be.

How would the MAn J calculate heat load for single pane aluminum windows circa 1949, and compare that to new Andersen double pain low-E wood/vinyl frame.

There are 8 windows total at an average size of 3' by 4.5'

Thanks in advacne for any info-

[Gunnery Sergeant Hartman]

Have a Man J run with your existing windows vs. your new ones.
Size your system for the needs with the new windows...
It can make a big difference sometimes...

[Irishmist]

Old Yeller: I for one, recognize and appreiciate your foresight in taking into account the effect new windows will have on the overall efficiency of your home, including the HVAC system. Good for you! I agree to have the load calc performed with existing and new criteria and see what the difference is.

[docholiday]

Contact your window guy and see if there is a "fenistration" number avaialbe for the new windows. Most new windows have this, I would imagine Anderson plays along like everyone else. Its a pretty accurate number used in load calcs. You may also want to figure what typ of window treatment you expect.

The old ones will not have this number but single pane windows from back then werent much better than a garbage bag.

[pstu]

Using HVAC-CALC with my own S.Texas house (large window area but well shaded), calculated cooling load 87,500 BTUH and heating load 59,100 BTUH. This is with a summer outdoor design temperature of 99, indoor 75. Winter outdoor design temperature 32, indoor 72. This house has large single pane windows but pretty good shading.

Fibbing to HVAC-CALC and telling it I had the "best" windows with double pane low-E glass. Calculated summer cooling load 62,200 BTUH which is 21.6% better. Winter heating load 46,300 BTUH which is 29.0% better.

Your house will probably have less glass area, but may be in a colder climate where the winter improvement will be bigger.

Hope this helps -- P.Student

[dan sw fl]

Originally posted by old yeller
How would the Manual J calculate heat load for single pane aluminum windows circa 1949, and compare that to new Andersen double pain low-E wood/vinyl frame.

8 windows total at an average size of 3' by 4.5'

Heat/ cooling loads would be reduced
>50% due to the the windows alone.

108 Square Foot of windows:
Cooling load would be reduced ~ 4,000 BTUh due to tinted glass which translates to nearly 1/2 Ton.

IF your current unit is 2.5 ton, a 2.0 ton may be sufficient ... or ~20% reduction
( similar to previous post by perpetual student).

[Xavier]

Old Yeller, what percentage are the current windows on the heating and cooling load?

If you completely reduce the entire energy loss through the windows, that would be your approximate maximum savings!

Make sure you rerun the entire calculations, because in most cases, there is no ROI for window replacement. Adding window treatment will have a big payback.

[old yeller]

I hear you on the no ROI- but I think in this case there would definitely be an ROI at some point.

I live in Dallas. I have already replaced my HVAC systems, so I can't fix what I've already had done. I don't have the Man J program to do the calcs myself.

My current windows are single pane, aluminum frame, they leak like sieves, and weren't put in very well to start with. Garbage bags was pretty darn close to their effective R value.

The new ones will be darn near bulletproof installations (done correctly by yours truly), double pane, wood/vinyl frame, low-E glass. The are from Andersen at about 260 per window (wow). (I hope that doesn't break the no prices rule- it's not HVAC related)

By knocking off a half ton of AC, and the ongoing savings, I could see ROI in about 3-4 years. And do it comfortably.

Somebody mentioned that the windows could be >50% of the heat load? wow- that seems high.

Thank for all the great input-

[pstu]

>>Somebody mentioned that the windows could be >50% of the heat load?

My Manual J run was for a 32F outside winter design temperature (99F summer), certainly somewhat milder than Dallas would need. If I remember right, window area is approx. 35% of the wall area -- it's a lot of windows in any case. And my windows are responsible for 38% of the heat load (and 42% of the cooling load). So I believe it would only take a little more extreme outdoor temperatures to get to 50% and over.

I have often heard that double pane windows are less relevant in a hot-humid climate, because of the small delta-T (temperature difference) between outdoors and indoors in the summer. Likely 15 degrees or less quite a large percentage of the time. But the numbers according to HVAC-CALC surprised me at how much difference good windows make.

I would certainly lean toward the better windows with any new construction, however for many existing homes the economic problem is how to cope with what we've got. What a whopping big project it would be to replace them all!

Hope this helps -- P.Student

[Edited by perpetual_student on 07-25-2005 at 05:46 PM]

[sweets]

What are the SHGC and U-factor numbers on the glazing for the Anderson windows are that you have chosen.
There are different strengths I guess you could say of "low-e". The common range can be anywhere from .29 to .42. The lower the SHGC (solar heat gain co-effecient)number, the better the glazing keeps out the heat. This lower number helps in a climate with high cooling loads. But you also have to check the % of Visible light transmittance . Typically the less heat that the glazing lets in (the lower the SHGC #), the lower the percentage of light that it lets in also. The trick is to get the amount of "low-e" you want with the highest Percent of Visible light transmittance, or add more s.f. of fenestration to make up for the loss of light.
The U-factor is the ability of the glazing (glass) to keep the generated indoor heat in winter from transmitting outside through the glazing, keeping more heat inside.
So depending on whether your are in a high cooling load climate or large heating load climate, should help you determine the glazing. Especially with how expensive those windows are. Windows have NFRC labels that give these numbers. Also on Window company websites. Frames and cladding very important too for those numbers.
A very informative book: "Residential Windows" by Carmody,Selkowitz and Heschong.
If a salesman ever tells you the windows will pay for themselves with the energy you'll save, that is incorrect. With doing nothing else but replacing windows (especially with the cost of these, but Anderson are the Mercedes of windows) you could never live long enough to recoup those $$. Energy savings isn't a reason to replace windows. A low SHGC will reduce a cooling load.
I believe in California it is even illegal for Window Replacement Companies to make a claim of energy savings.
Just thought you might like the info.

[Xavier]

Sweets I agree completely.

If a homeowner adds up all the "POTENTIAL" savings quoted by salespersons from: New furnace, New AC, Windows, Insulation, Zone system, Booster fans, ERV & HRV etc., the utilities would have to pay the homeowner each month$$$$$$$$!

[Panama]

You can approximate the Manual J(7) HTM for Andersen High Performance Glass windows by specifying Dual Pane, Tinted, Roller Shades Half Drawn. Add Low e to this and you will approximate the same windows but with curtains or blinds drawn.

[old yeller]

OK- NFRC rating for an Anderson 200 series Narrowline:

Low-E Glass without grilles:

U-Factor- .35
SHGC- .34
VT- .53

Heat Gain- 113 (356)
UV Xmission rate 16%
Krochmann Damage function 24%
Certified Air infiltration .10
OITC 21

How does this stack up?

Oh well, if I don't recoup most of the money, then at least I will have bought some comfort and some peace and quiet. These windows I have now let in ALL noise from the outside. If someone it mowing the yard, it sounds like the window is raised because it is so loud coming through into the house.

I have replaced one window already that is closest to the new Trane 14Xli compressor- you can't hear it inside the house now.

Thanks to all for the great information-

[dan sw fl]

Originally posted by old yeller

NFRC rating for an Anderson 200 series Narrowline:
Low-E Glass without grilles:
U-Factor- .35
SHGC- .34
VT- .53

Heat Gain- 113 (356) UV Xmission rate 16%
Krochmann Damage function 24%
Certified Air infiltration .10
OITC 21
How does this stack up?
-

Window Rating doesn't get ANY better than that ! !! !!!