Heating, Air Conditioning, Fridge, HVAC/Supply/return ducts

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QUESTION: Craig,

First of all, thank you for providing your time and valuable service!
I am going to install a new Goodman 2 ton split system HVAC that I have already purchased in a newly built residential work shop in Texas (hot climate) that I built by myself, no subs or helpers. I use the shop for restoring an antique car. Here is the scoop on the building construction
1. 22 x 38 rectangle building, exterior was wrapped in 1/2 OSB, 2 layers of building wrap, with 2 stone VENEER
2. 8' ceilings with concrete floor
3. 7/12 hip roof
4. bottom joist (2x12's) are decked with 5/8 plywood for storage in attic
5. there are no interior walls that subdivide any area with no plans to do such
6. building is  standard IRC constructed with a hip roof
7. 4-3040  windows
8. 1 3068 service door
9. 2 - 7'x8' overhead insulated doors (don't know r factor) with weather stripping
10. 3 ' wall insulation (faced) r13 and 10 ceiling(faced) insulation r30
11. 1/2 sheet rock walls
12. eave vents every 6' and a 16' ridge vent is installed

HVAC EQUIPMENT
2 Ton 13 Seer Goodman Heat Pump System - GSZ130241 ARUF24B14
1. Air handler/coil is placed as horizontal unit  LEFT in the attic with secondary condensation pan
2. all electric system with heat pump
3. Unit is NOT equipped with heat strips nor will it be. I can supplement with space heater if needed. I love the cold but hate the summer heat.
4. Air handler is capable of  1150, 890 or 640 cfm depending on wire tap you use
5. 24000 cooling btu
6. R-401A system
7. Attic is wide open for duct installation
8. Will be using flex duct
9. system is pre-charged for 15' of line

Now for my question/s

I am looking for optimal vent/register sizing and placement for both return and supply air. I have yet to construct the plenums but plan on using start collars and cement to seal all joints, seams, etc. I have replaced several units over the years and am very familiar with the process of evacuating the system, brazing copper, etc.. and electrical is no problem as well. What I have never done is design any of the duct work and really don't want to learn that side of the system. So placing and sizing of duct is what I need to know with system being optimally sized as opposed to undersized. Noise from high speed fan and/or air from supply or return vents is no problem, I just want to stay cool. I will call a good local HVAC person to come and check the system after I finish installation to see if it needs to be tweeked. I prefer the vents to blow hard, no matter the noise, if that helps to assist you in the size and location of ducts/registers.

Thank you for any guidance you can give me in the duct work. If any more info is needed I am all to happy to supply it.

Ron

ANSWER: First things first.  I am sorry for the delay.  For some reason this site is not sending me emails when I get new questions.  

The new unit should have heat strips because you have a heatpump. They are needed when the unit goes into defrost.  I would have not bought a heatpump if you never plan on using the heat from it. There are just more things to go wrong for something not being used.

Did you complete a manual J?  You can find programs on the net to help you do that.    That will give you the unit size for the space.  Too big of a unit is going to give you poor humidity control.  A rule of thumb is 400-600 sq ft per ton. The colder you plan on keeping it, the harder the humidity will be to control.

The last thing is I would have suggested you use a mini-split type system.  With just a big open space it will work just as good, if not better then a ducted system.  It will also be cheaper to install and maintain in the long run.

Craig    



---------- FOLLOW-UP ----------

QUESTION: Craig,
I plan on using the heat pump but if it maintains the shop at around 60 - 65 degrees I'm good with that. I understand in defrost it is going to pump cold air for a while but I have a low energy oil/electric space heater to subsidize that will keep up with anything central Texas can dish out in the winter. So I can't imagine needing heat strips as I very much prefer the cold side, i've really grown to hate the heat. I did complete a manual J load calculation and from what I came up with is a single 20x20 return and 6-9" supply. Does this sound close to you? 400 RA vs 381 supply, Going on 2cfm per sq inch delivering 800cfm? Thanks again for any thoughts or comments you may have.

Answer
You need to use a ductulator to determine size at the duct.  You also need to size it for the max air flow.  Both supply and return will be the same, as what goes in must come out. The link is a good source for a free online ductulator.

http://efficientcomfort.net/asp/ResDuct_Web/ResDuct_Web.asp  

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Craig HVAC Expert

Expertise

I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.

Experience

I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

Education/Credentials
5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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