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Old 08-13-15, 02:43 PM   #1
Elcam84
Apprentice EcoRenovator
 
Join Date: Jun 2013
Location: TX
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Default Manual j design temps

I don't have access to the software anymore as the computer it was on went kaput. I'm figuring out our house again...

Does anyone have an updated list of the design temps to use for the man j calc. I just got done going over the temps by the hour from the closest weather station to us and that was allot of figuring going from year to year.... The numbers in the end gave me a summer design temp of 105. Which doesn't surprise me but the only references I have found online show 99* and 100*. The man j figures dated from 1997. I hope that isn't the currently used and just ignoring nearly 2 decades of data, as its way out of date.

In that time our summers have gotten hotter and hotter. We go for weeks on end with temps of 105* and it's 100* by 11am and then doesn't dip below 100* till usually exactly 7pm. That means 8 hrs over 100* every day and summer is almost 4 months long with those types of days making up an easy 60+ days. And in a really hot summer those highs will be 115*+ for weeks.



A friend of mine still does hvac and they like others have been using the 99* outside temp and sizing systems according to it as well as duct work and get so many service calls to check their systems cause the house isn't staying cool. We both agree looking at the real data for here that the man j numbers are just way off.
After talking to a few building inspectors in city's that require a man j they no longer fight with the contractor if they use numbers around 105 for the calc because the city has gotten complaints from homeowners that ended up with a hot house due to 99* in the formula. Also the man j assumes 74 inside which is higher than what many customers want. I would use 72* for me but some keep their houses at 68 as well.

The last time I did our house with the stock numbers as it sits it came up with 31000 btu. Our ac is 36000 and I have to close off the rooms we don't use and push that air to the rest of the house cause 3 tons is not enough.

Anyway rant over. Not bashing the man j but their design temps aren't accurate to real weather data for here and also for interior temps. He more I think about it it looks to be designed by an engineer but they swayed from the comfort side to the save a buck and be uncomfortable side. I hope they have updated those numbers to include the last decade plus and I can quit complaining about it...

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