Should I do Manual J window bare or assume blinds

Hi,
How should I do my windows on a Manual J? As no shade, blinds or internal shade? Or do I do them all with some type of shade built in or all bare?

HVAC Calc, based on Manual J, runs its numbers with no window treatment.

I think this is wise because window treatments vary in their ability to block out solar heat gain. And some homeowners will want certain windows with an unobstructed view no matter what the weather is like outside.

Is there a code for that or is a rule of thumb?

If you will notice at the bottom of hvac-calc it says that it is an estimate only, actual loads vary due to weather & construction differences. If you are off half a ton here or there I promise you can still get into heaven.

Steve,
I'm still learning this stuff. What whould you or any other person reading this do? I'm thinking If I size an AC with blinds or other internal shade and the customer does not use any window covering, then I just undersized his AC. Correct?

Quote:

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Originally posted by elmer5
Steve,
I'm still learning this stuff. What whould you or any other person reading this do? I'm thinking If I size an AC with blinds or other internal shade and the customer does not use any window covering, then I just undersized his AC. Correct?

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You could be correct,it can easily make a 1/2 ton difference.


best way is ask the customer,blinds open or closed,during daylight hours//


Another way is to "spec. it out "when you give them the load,i.e. Load calc., assumes blinds on all windows.

Another way to look at it is,if you assume ,No blinds and they use them,now you oversized the system!

I think Lstiburek did an experiment and found out that a mediocre window with shades drawn, would have no more load for practical purposes, than the most modern, most expensive low-E windows. He attempted to prove energy savings due to those best windows and the experiment failed to show any savings. Then he drove around the test houses and noted that generally the homeowners pulled shut their drapes with the mediocre windows, and generally left them open with the best windows. I found that very interesting.

In my region a couple HVAC contractors wanted to do heat loads (in their own black-box method) assuming the drapes were all open and the trees were all destroyed by some disaster. Because that's what MIGHT happen. I'd rather not think about sizing that way.

Hope this helps -- P.Student

Around here residential calculations typically are based on the worst case. Therefore, the house is rotated to the worst orientation and blinds are assumed to be none. I usually select the none option because it is not uncommon for me to open up all of my blinds on a hot summer day. Why should I expect someone else to keep there blinds shut on July 4. As far as the building orientation goes if the calculations don't say worst case the contractor can not reuse the calculations the next time builds the same house. Not to mention the fact that north arrows are rarely on the plans.

What if the number of occupants are unknown? Or how about there were two people when you did the calc but now they had three kids and grandma moved in? Or the reverse, it was brady bunch and they all moved away to college?

How about the infiltration selection. That is pretty much an unknown right? Not in our control for sure at least how the vapor barrier is sealed for example.

Equipment capacity vaires
Solar loads vary
Infiltration vaires
Construction differences vary

This is like trying to hit a moving target. The best anyone can hope for is to get as close as possible to what is going to make the customer happy. That cannot be done by load calcs alone. They are only part of the equation. Experience and common sense are much needed to complete the project.

Blinds or not?We have used interior shading for years,near had a problem.

Shading by trees?We never count them as shading.

People?To per bedroom,no problems.

Infitration?Your judgement good,better,best constrution,this one requires your judgement.

Equip. capacity varies?Absolutly and most often you have some extra capacity.If it's less than 2000 btus excess sensible,you might want to choose poorer construction,or no blinds,we never have ,but I guess you never know.