Quote:
Originally Posted by gtojohn
I agree, manual j can be off quite a bit. Our design temp here is 96F however in the past ten years we have long stints of 100f+ days. One year we had over 60 days straight of triple digits. HVACs sized too close to manual J couldn't keep up. Lots of unhappy customers. Heating obviously you need to plan for those extremes as well, just because you average 40f for a low doesn't mean you won't have a freak 20f week. Perhaps a good argument for multistage equipment that could ramp up when you need it.
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Man j is seen as the best way to size and it is good but... It also has the GI-GO problem. (Garbage in garbage out).
When using the recommended numbers in our area it doesn't work for cooling. The design temp here is 99* however the last 5 years we averaged over 100 days with the temps over 100* with many of those days being 5+ hours over that temp and it will be upwards of 110* for a few hours. We had nearly a month where it was 115* to 117* every day.
The issue is that those high temps are just part of the equasion. The issue is that it will be 90* by 9 am then 100* by noon then it will still be mid 90s just before midnight with a low temp of 80-85 at around 5:30 am.
So when a unit is sized by man j much of the summer the ac will be running pretty much non stop all day and it won't cool the house to the desired temp till after midnight when the system finally can catch up.
Like mentioned many homeowners here with houses that have man j sized units were hot in their houses through the summer. Many have been up sizing their equipment because of it.
As to the devil of over sizing that is always brought up it isn't as bad as you may be led to believe. There have been DOE research that has shown that over sizing doesn't have an appreciable impact on operating cost unless it's grossly oversized and that nearly never happens. If you have a fear of over sizing you can go two stage systems or thermostats with adjustable temp settings to change from 2* to or 4* delta t (differential temp)
Allot of the same goes for heating as well and man j is still a good resource but it isn't the end all and be all and it even says that in its manual. It does need some updating for its design temps in certain regions. I researched the temp here for the 30 year period and it was a good chunk higher than the man j numbers ie their numbers aren't being updated...