Sizing an HVAC unit for commercial building 1

[George1985]

I'm looking into sizing an HVAC unit for a gas station. Is the Manual J method the best way to properly size the unit? I hear that 'rules of thumb' methods typically oversize the system which causes excessive cycling and improper performance.

 

[Drazen]

basically you heard well, oversizing with rule of thumb method is so big problem that there are contractors associations that appeal among their members not to do so, even if they are pushed by client, who believes that it is good saving to avoid engineering costs.

the other part of story is that you would not advance much if you try to just read through manual, lot of generic knowledge and experience is needed to deal with that properly.

of course, much depends on how large your project is. manual j could be applied to small commercial buildings like gas station building likely is, but it is decision of seasoned designer. it depends on construction elements applied, occupancy, presence of other content like coffee bar, restaurant, shop. if there is much load by many people entering all the time, and if working hours have more complicated pattern, than it is likely manual j will not be appropriate.

cycling is not so big issue like it was before, because of prevalent use of inverter systems, but dehumidification on part load is. you will also need some sort of ventilation, so it is only professional design that can give you adequate warranty that you will not waste your money.

 

[jkauwale]

Use a load calculation program to properly size your HVAC system. I use Trane Trace 700 but there are others out there.



Justin K, P.E.

http://www.engproguides.com

 

[George1985]

Are there any free ones available for download that you know of?

 

[bigAlittlee]

Ha I Googled Manual J as I don't do residential and found this which I didn't realize was a joke at first. Pretty funny.

http://synergyairflowandventilation.com/docs/ManualJSDT.pdf

Air Conditioner or Heat Pump Sizing Chart
Preparing your sizing chart:
1. Print out this page.
2. Leaving this page intact carefully cut out the holes on the dotted lines.
3. For operating instructions see bottom of page

Sizing Chart Operating Instructions
1. Position yourself across the street from the house.
2. Hold the sizing chart at arm’s length from your face.
3. Look at the house through each hole.
4. Choose the opening which best fits the house.
5. Install the equipment listed under the opening or the closest size that you
have in stock.

Advantages of Using This Chart:
1. You will get to know your attorney better.
2. Some say it is much more accurate than rule of thumb sizing.
3. Local building code officials allow this to qualify as a recognized equivalent
method to the IRC requirement of sizing with Manual J. (Homeowners should
contact your local building code official and demand that the code
requirements be enforced!)
Disadvantages of Using This Chart:
1. Size will be accurate only to the nearest five(5) tons.
2. You can’t discuss this method with other contractors. They already think your
skills are poor.
3. It deprives your customer of the comfort and savings of a properly sized job.


My advice:

"Gas station" could mean quite a range of sizes. If it is large enough I would give it some effort just to be sure it is done right.

You could definitely use Trane Trace and personally, that's how I would do it because it would take me very little time. If you don't want to spend money you can find a directory here of free software that you may find useful to do a simple load calc:

http://apps1.eere.energy.gov/buildi...ng_analysis/pagename_submenu=load_calculation

I cannot endorse any of these though having not used them. You can also use Excel and do simple load calcs either by making your own using ASHRAE load calc guidelines or an HVAC textbook etc., but depending on your desire to learn and put forth effort you need to choose. I am sure there are Excel files you can find already set up out there...possibly on a professor's website or something from a college course.

Load components:
1. One way or another do a simple load calc for the envelope and determine the peak heat gain the building will see
2. Determine heat gain from lights and each piece of equipment (hot dog rollers, refrigerator compressors, etc.)
3. Determine ventilation rate which should be at least a little more than you exhaust from the restrooms. Determine the cooling rate required to bring this air from design OA conditions to your supply air temp

Add these 3 together and multiply by 1.1 for safety and you should be good.

 

[chicopee]

When you are referring to a gas station, I think that you mean to have a garage with muliple bays and an office space for transactions and a customer seating section. In the northeast, all gas stations that I have seen, only the offices and seating areas were air conditioned. The garage portions would have the bay doors wide open and no A.C.'s. Sizing an air conditioning unit for this type of set up should not be complicated.

 

[sspeare]

It sound like you don't routinely estimate HVAC loads. Some comments ......

ACCA Manual 'J' is for residential loads - not applicable. You could use the ACCA Manual 'N' procedure for commercial buildings. But, you would have to buy the Manual 'N' book from ACCA. It is an excellent reference.

If you don't want to buy any software or buy the Manual 'N' book, you could do what many Mechanical Engineers actually do every once in a while: manually calculate!

For manual calculations, I often use this book, by Faye McQuiston: "Heating, Ventilating & Air Conditioning - Analysis & Design". Study the book, and you'll be able to make a calculation that you are comfortable with.

It's dangerous to go the software route if you only occasionally do this kind of work.

 

[George1985]

sspeare,

Since I posted the original thread, I've decided to do what you suggested and manually calculate the heat gains and size the units by hand. It hasn't been too difficult so far, but I would like a good reference book for help. Your recommendation is appreciated!

 

[berkshire]

chicopee ,
The type of "Gas station" you are describing is fast dissappearing. Today the gas station is more likely to have a convenience store attached instead of a service bay.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[George1985]

Yes this type of gas station will have a fast food joint inside. No bays and/or garages. Just a convenience store / fast food joint with big windows and pumps outside.

 

[chicopee]

For the type of gas station that you are describing, then I would do the calculation long hand.

 

[George1985]

Thanks guys ive figured it out. Gonna calc it by hand. It's simple.