I'm starting to design a home for my wife and I to retire to. I'm early in my design and just getting started with estimating heating and cooling loads. I started using the calculator provided by Borst Engineering and Construction (thanks Borst for providing these tools). The house will be oriented with walls facing straight north/south/west/east. To get a better estimate of cooling load impact of the windows, I ran numbers for each face separately. I plugged in 100 sq ft on North with 0 sq ft on S,W,E. I repeated for S,W,E faces. I used SHGC of .5, R3, and window type 2 for all windows. The results for window solar sensible heat gain don't seem right. Here's what the calculator shows: North 2012 BTU/Hour East 2760 BTU/Hour South 3622 BTU/Hour West 7992 BTU/Hour I expected the south windows to produce the greatest heat gain. I expected east and west to be the same as each other and less than south. I expected north to be nearly zero. I also input Internal Lighting Sensible Heat Gain of 1000 watts. The calculator shows Internal Lighting Sensible Heat gain of 3753 BTU/Hour. Shouldn't this be 3412 BTU/Hour? Can someone explain the discrepancy between what I expect and the results? Am I using the calculator incorrectly? Are my assumptions wrong? Thanks, Larry Lunsford

Sorry, I posted this in wrong forum.

In general, cooling load analysis is much more complicated and far less accurate than heating loss analysis. The cooling load analysis software on our website is based on the 2013 American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook Cooling Load Temperature Difference (CLTD)/Cooling Load Factor (CLF) methodology and recommendations. We have validated the software against ASHRAE examples for this CLTD/CLF methodology. However, there are many inputs that you need to enter correctly in order to obtain valid outputs. It also helps to have the ASHRAE handbook so you can select the appropriate ceiling and wall types as classified by ASHRAE for your building design.

Frankly, the ASHRAE CLTD/CLF approach has become somewhat obsolete. So you may want to consider running some other software as a comparison. What exactly do you expect? We can’t critique your results without first seeing you actual inputs/outputs. Please feel free to send them to us and we would be happy to provide additional guidance/suggestions.

I plugged in all my numbers and run the calculation. Not too surprisingly, a large percentage of the heat gain was from the windows. I then tried to determine how much heat was coming in each side of the house. That's when numbers didn't seem right and I tried 100 sq ft each side. How do you get Solar Sensible Heat Gain from windows that face straight north? I was especially surprised to see the SSHG per square foot from the north face windows was over half the SSHG for the south windows. What am I missing?

Okay, it sounds like you want me to speculate about this without having the benefit of seeing your actual inputs/outputs. The software only outputs the total SSHG of all the windows (i.e., north, east, south and west). So you must have made all the window areas zero except for the side that you tabulated your values. You observed that the north side had a SSHG value and you observed that the north side SSHG was greater than 50% of the south side SSHG (and perhaps you expected the north side would be zero or at least very small). Assuming that you entered zero for the South Wall Deviation from True South input parameter, I can understand how you would expect the north side to receive significantly less solar exposure and associated less heat gain. As I indicated previously, window Cooling Load Temperature Difference (CLTD) and window Solar Cooling Load (SCL) is quite complicated.  The cooling load varies significantly depending on both the orientation and the time of the day as quantified and tabulated by ASHRAE.

Here’s an excerpt of the code for the mid-morning north windows SCL:

'Window Solar Cooling Load (SCL)'
nsclbtupsfh=0;
esclbtupsfh=0;
ssclbtupsfh=0;
wsclbtupsfh=0;
if (tod==0){
'For Mid-Morning & North Windows'
if (windnztype==1){
nsclbtupsfh=35;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
nsclbtupsfh=75;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
nsclbtupsfh=35;
}
}
if (windnztype==2){
nsclbtupsfh=32;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
nsclbtupsfh=73;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
nsclbtupsfh=31;
}
}
if (windnztype==3){
nsclbtupsfh=30;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
nsclbtupsfh=61;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
nsclbtupsfh=32;
}
}
if (windnztype==4){
nsclbtupsfh=27;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
nsclbtupsfh=58;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
nsclbtupsfh=30;
}
}

Here’s an excerpt of the code for the mid-morning south windows SCL:

'For Mid-Morning & South Windows'
if (windsztype==1){
ssclbtupsfh=64;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
ssclbtupsfh=35;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
ssclbtupsfh=150;
}
}
if (windsztype==2){
ssclbtupsfh=56;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
ssclbtupsfh=31;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
ssclbtupsfh=134;
}
}
if (windsztype==3){
ssclbtupsfh=54;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
ssclbtupsfh=33;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
ssclbtupsfh=121;
}
}
if (windsztype==4){
ssclbtupsfh=46;
if ((swdftsdeg<-22.5) && (swdftsdeg>-67.5)){
ssclbtupsfh=31;
}
if ((swdftsdeg>22.5) && (swdftsdeg<67.5)){
ssclbtupsfh=104;
}
}

So it would appear that the ASHRAE SCL tabulated values (in units of BTU/SF-H) for the north windows are indeed greater than 50% of the south side values. I say "appear" because I tend to share your expectations at the moment and I will now need to have our software guru check that we have indeed coded these values correctly. Since we at one time validated this code against the associated ASHRAE example data, I would be very surprised if we coded these values incorrectly. However, this is certainly possible as this software has about 2500 lines of ASHRAE tabulated data.

Here’s the solar radiation heat gain code for each window side and the total SSHG that is output by the software:

'Window Solar Radiation Heat Gain'
wnsrhgbtuph=windnasf*(1.15*windnshgc)*nsclbtupsfh;
wesrhgbtuph=windeasf*(1.15*windeshgc)*esclbtupsfh;
wssrhgbtuph=windsasf*(1.15*windsshgc)*ssclbtupsfh;
wwsrhgbtuph=windwasf*(1.15*windwshgc)*wsclbtupsfh;
twsrhgbtuph=wnsrhgbtuph+wesrhgbtuph+wssrhgbtuph+wwsrhgbtuph;

So you can see that the ASHRAE methodology for determining the SSHG is the window area times 1.15 times the window SHGC times the aforementioned and complicated SCL values. The four window sides are then summed to determine the total SSHG that is output by the software.

I will certainly report back our findings as soon as possible. Again, the ASHRAE CLTD/CLF approach has become somewhat obsolete. These days we use the ASHRAE Radiant Time Series (RTS) approach as shown in the consultation section of our website for performing our building heat transfer analysis. While it is not any less complicated than the CLTD/CLF approach, it has proven to be far more accurate than anything else currently available and it can provide by-the-minute heat transfer information which can be useful for some of the applications we get involved.

Hi. Thanks for looking deeper into this.

I thought I gave you all the pertinent values in my first post. Here's ALL the values I used with your calculator.

Design Outdoor Dry Bulb Temp 90, Design Outdoor Wet Bulb Temp 55, Design Outdoor Daily Temp Range 40, Design Indoor Dry Bulb Temp 70, Design Indoor Relative Hum 10, Mid-Morning or Mid-Afternoon 1, Ele 7700, Lat 37, S Wall Deviation 0, Window Zone 2 (N,E,S,W), N Win Area 100, E Win Area 0, S Win Area 0, W Win Area 0, Win R 3 (N,E,S,W), Win SHGC .5 (N,E,S,W),Wall Type 4 (N,E,S,W), N Wall area 840, E Wall area 600, S Wall area 1200, W Wall area 840, Wall R 45 (N,E,S,W), Door Area 0 (N,E,S,W), Door R 5 (N,E,S,W), Ceiling type 5, Ceiling area 2376, Ceiling R 55, Bldg Vol 25000, Infiltration Rate .05, N People 2, Internal Appliance Sens Heat 2000, Internal Appliance Latent Heat 500, Internal Lighting Sens Heat 500.

As I noted in first post, after getting results that did not seem logical, I ran the calc 4 separate times. Each time I kept everything same except window area. Plugging in 100 sq ft windows one side at a time I got the Window Solar Sensible Heat Gain for each direction.

Can you explain how 1000W of Internal Lighting Sensible Heat results in a calculated heat gain of 3754BTU? 1000W should be 3412BTU.

Since my first post, I played with some other values. I ran calc for morning and afternoon. The WSSHG for East and West switches when I toggle time - that makes sense. The high gain for E/W vs S suggest the calc is using very early morning and very late afternoon sun positions. If the calc is using sun positions of SW (sun at a bearing of 225) and SE (sun at bearing of 135) then I would expect to see same gain per sq ft of window on south and west (or south and east).

I'm building in Pagosa Springs, Colorado. The summers are mild and even in mid summer, overnight temps drop into upper 40's. The Design Outdoor Daily Temp Range did not seem to have much affect on the load calculations.

I guess all of this begs the question - what is a good calculator for a DIYer?

Thanks, Larry

Yes, we will be looking at the SCL tables as mentioned previously at our earliest opportunity. BTW, we always appreciate feedback on our software and services, and we continually strive to improve both...so thank you!

With regard to Internal Lighting Sensible Heat Gain, it is normally factored higher than a straight wattage to BTU/Hour conversion. I believe ASHRAE factors it from 5 to about 20% depending on the type of lighting. Unfortunately, I am not at a location where I can check and provide you more info about this right now. Why factor it at all you might ask?  Because when you turn lights off, they still generate heat for some period of time.  There is also the situation that a lights wattage rating is not always consistent with its actual heat output.  Since this is just a simple DIY calculator, we just factored it 10% instead of asking you to define whether you are using incandescent, ballast florescent, compact florescent, LED, etc. Here’s the code used in our software:

'Internal Lighting Heat Gain'
ilhgbtuph=1.1*3.412*ilhgwatts;

With regard to your earlier question about people heat gain, here’s the code:

'People Sensible Heat Gain'
pshgbtuph=245*npeople;

'People Latent Heat Gain'
plhgbtuph=155*npeople;

People heat gain varies significantly based on their activity level.  Again, everything in our free DIY cooling load analysis software is based on ASHRAE methodology and recommendations for a residential buildings (i.e., NOT commercial buildings). BTW, the reason I cut and paste the code into my responses is because we always make our code viewable to users of our software.  How to view our software is explained in first page of our software instructions.  So one can quickly take a look at the code and sort out these sort of questions if they are willing and able.

Frankly, I don’t know how many DIYers actually do cooling load analysis. It seems like the vast majority of folks we deal with just do a heating loss analysis. We have heat loss analysis software on our website too. Build it Solar also has heat loss analysis software on their website.

With regard to other DIY software, I assume you mean free or low cost. You might want to try HVAC-Calc. I believe they charge something like $50 for their home owner version. I believe HeatLoad.com and Slant/Fin still have free online calculators too. I recently read that Slant/Fin also has a new Android/iPhone app.

If you are willing to spend big $$$, you can purchase software from Uponor or Wrightsoft like the “pros” do. I believe this software is still largely based on 1960's data and ACCA Manual J methodology, and generates very pretty analysis reports even though the accuracy of the actual analysis is typically marginal compared to ASHRAE RTS. The "pros" like the pretty reports because it impresses customers.

And finally, you may want to consider doing building energy modelling using software such as DOE2.

We compared the ASHRAE SCL tabulated data to what is in our software. We were not able to find any discrepancies.

Typically, buildings experience the largest window cooling load at either mid-morning or mid-afternoon. This is largely a function of the window area and the orientation of the window placement. Solar heat gain that enters the windows results from beam irradiance, diffuse irradiance, and ground reflected irradiance. We typically only think about beam irradiance, i.e., the irradiance that enters the windows directly from where the sun is located in the sky. Beam irradiance is typically the largest of these three heat gain sources, however, the other two are not insignificant and I suspect that is what we are observing here.

With regard to the SCL code previously presented and as can be seen from the attached data, at 10 AM (i.e., mid-morning) the North SCL is 35 BTU/SF-Hour and the South SCL is 64 BTU/SF-Hour. This agrees with what is in the code and indicates that ASHRAE believes that the North SCL is 54.7% of the South SCL at this time of day. You will note that the largest SCL at this time of the day is East having a SCL of 154 BTU/SF-Hour.

At 3 PM (i.e., mid-afternoon) the North SCL is 36 BTU/SF-Hour and the South SCL is 63 BTU/SF-Hour. This indicates that ASHRAE believes that the North SCL is 57.1% of the South SCL at this time of day. You will note that the largest SCL at this time of the day is West having a SCL of 158 BTU/SF-Hour.

At this point, I have no reason to disagree with the ASHRAE SCL data or their methodology in applying it for cooling load analysis. Based on our experience and from reports by DIYers that have successfully used our software, it is adequate for determining a suitable air conditioning system capacity for a new building. If you have an existing building, you may get more accurate results by using our existing building energy usage software.

Heat loss and cooling load analysis can be tricky business. We have seen examples of badly done analyses by “pros” using expensive software, so doing a good job with this does require a certain level of competence and time/patience.