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Old 08-13-15, 02:43 PM   #1
Elcam84
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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 I hope that isn't the currently used and just ignoring nearly 2 decades of data, as its way out of date.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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Old 08-13-15, 09:17 PM   #2
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Quote:
Originally Posted by Elcam84 View Post

Does anyone have an updated list of the design temps to use for the man j calc.

If the whole enchilada is too much information, try this:


This may be too involved for your needs but hewre is a dandy hydronic heating program that has the Manual J values built in.

http://www.wattsradiant.com/support/radiantworks/

I used to be able to find a Manual J pdf pretty easily. Any more, not so much.

Best,

-AC
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Old 08-13-15, 09:26 PM   #3
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Found that one a bit ago from 2012. Seems they are using chamber of commerce numbers instead of real world weather station numbers. Good formulas but it's a shame they aren't using accurate data to figure their temps. I'm not the only one to figure that out because several large hvac contractors are using 105 for their temps too. Found a reference to one of them getting grief from a city on their sizing until they showed them the data.

We exceed their 1% number in just two days. 8 hrs a day over 100* is common. Just frustrating to see it touted as the best thing since sliced bread only to find out some of their numbers are off.
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Old 08-14-15, 11:03 AM   #4
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Interesting perspective from Texas. I have a similar observation from New England.

About eight years ago, a cousin of mine was building a home (Vermont)and asked me to run a "Manual J" analysis. Did so and I noted the design temp of -35F min and a max of 90F.

I asked him when the last time he saw -35F in the winter. He said maybe back in the 1960's. How about 90F max? All the time now. In fact, the wife wants to put AC in . . . .

I did my own analysis and we got him a far better home by minimizing air infiltration, putting in an ERV for dedicated ventilation, put the dryer in a room using make up outside air and also put in basement wall closed cell foam (on outside walls). Then I did manual J (which does not have low air flow turnover rates) and did my guestimates.

The HVAC people, that did his install, were furious as my analysis showed that a 45% reduction in total BTU heating was appropriate - compared to their Manual J calculation. They were all too eager to sell him a 6 ton unit. He has a 4 ton and it works perfectly. We did put in an emergency 15 kW resistance strip and in the four years it has never been used (it is turned off in the unit).

Just as HVAC dealers are screaming in Texas that Manual J is not accounting for the now "normal" summer hotter temps, people in the north now need AC in the summer.

. . . . and educated individuals on this site don't believe in global warming . . . . .?


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Old 08-14-15, 12:11 PM   #5
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Why don't you go to degreedays dot net?

Heating & Cooling Degree Days - Free Worldwide Data Calculation

They pull from true current data.

You could pull HDD & CDD data from the last 5 years, and if you understand the Degree Days concept, you can easily derive your own design temp.

Why look any farther?

-AC
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Old 08-14-15, 12:15 PM   #6
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Quote:
Originally Posted by stevehull View Post
...and educated individuals on this site don't believe in global warming?...
You are assuming a lot here.

-AC
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Old 08-18-15, 01:45 AM   #7
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Here's an online manual j calculator
HVAC Load Calculation - Maunualj - Whole House Loadcalc
HVAC Load Calculation - Maunualj - Whole House Loadcalc
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Old 08-21-15, 03:49 PM   #8
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Quote:
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This is a very handy find!

Best,

-AC

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