methods for determining heat/cooling(?) loads

[pmover]

all,

this question is directed to skilled engrs whom practice hvac. i've studied hvac (college) and extensively worked in the hvac industry, but have not done any hvac design work. so i am inquisitive!

this question is realted to arch5599 post on 05-01-03.

i heard of engineers determining heat loads (or cooling loads?) based on a Btu/hr-sq.ft and by ASHRAE standards (fundamentals handbook) - determining overall room/bldg U based on design of bldg. i understand that there is a difference in construction (residential and commercial), and considering what tools are available,...

Please clarify which method is actually practiced most often and for what reasons (client specified?)

for what reasons is the Btu/hr-sq.ft method used?

the Btu/hr-sq.ft method seems too simple and can provide results which lead to over/undersized equipment (example of air conditioners running 100% of time during summer months in houston, tx and not keeping apt. rooms adequately cooled).

which method provides results that are realistic - i understand that in the hcav world, results can vary >10%, depending upon method used.

it seems to me that determining heating and cooling loads as stated in the ahsrae fundamental handbook )or by other recognized industry stds) is the more appropriate and professional method.

thanks!
-pmover

 

[DouginMB]

There are those that might use "rule-of -thumb" methods to estimate cooling/heating loads for buildings, but in my opinion they are asking for trouble. Rule-of-thumb guidelines work well for initial estimates, but a cooling (or heating) load calculation using ASHRAE or DOE methedologies should be performed for every building before selecting final equipment is selected.

When we get estimates from building owners prior to our design, they are almost always suprised to find that their estimates are low. They are usually based on a contractor's rule-of-thumb estimate. These estimates almost never account for the impact of ventilation on the cooling load.

ASHRAE's published methods essentially require you to employ a computer program. If you want to run a cooling/heating load for a building, and don't want to shell-out the big money, I recommend getting Carrier's "System Design Manual Part 1: Load Estimating". It guides you through a hand calculation procedure and would be good for simple jobs. I still use it and have never had a problem with over/under-sizing equipment. In the same vein, ASHRAE's method for Residential load analysis (Fundamentals Ch. 27) works well too.

 

[anandHVAC]

I vote DouginMB.Rule of thumb is only for initial estimation
i.e to determine the project cost. But it should never be used for selecting the equipment for actual installation.
There are many Heatload software available in the market the best is carrier's E-20 II, which gives you energy analysis also.

 

[pmover]

thank you gentlemen!

your responses confirms what i suspected.
-pmover

 

[pmover]

any suggestions as to where i can obtain Carrier's "System Design Manual Part 1: Load Estimating and Psychrometrics"?
i investigated carriers web site and was not able to find it. is the manual discontinued?
-pmover

 

[DouginMB]

Contact your local Carrier commercial sales rep.

 

[cjw81]

The de-facto standard is the Manual J inspection which is in it's eighth printing and is the one that you should get. It cna be purchased at ASHRAE, RSES,there is an online shortform that will perform some of the simpler calcs at Mr.Hvac.com, you can also purchase the full software which complies with Manual J standard. Also a good residential and commercial version is available at

http://www.webworks.com.

To comply in New York State for any rebates you must use manual J speciifications. This was instituted to reduce the tendency for installers to oversize units for a "safety" factor. Oversizing tends to utilize too much power, and due to shortcycling, is not able to properly dehumidify the space.

 

[grnblr]

Dear Pmover,

could u get the carrier systems design manual.

I too need a copy can u help
reg
grnblr

 

[chicopee]

Use rule of thumb for bidding purposes.