HVAC Question

[Alan Schell]

I am in the process of adding HVAC to my new shop in North Mississippi. It is a 24x48x8 metal building with a 3:12 roof pitch(peak of roof is 11'). The entire building has 4" of Icynene insulation.

I am thinking about installing a Heat pump window unit rated at 16,000 btu's heating/ 18,500 btu's cooling, with 9.7 EER. I do have a window big enough for the unit.

I currently have a dust collection in place. The tools i am currently using most include chop saw, jointer, planer, table saw, bandsaw, and lathe.
My questions are:

1. Should this unit be sufficient? I can get a unit from the big box store during a season close out on sale.

2. Can I add to the life of the unit by putting a pre-filter in front of the unit or will this cause more problems than it would solve?

3. With the roof already having blown insulation, is energy efficient to add an insutated ceiling? Since this building is off of a second meter from the house I am charged at a higher commercial rate from the local power association.

Any additional suggestions would be greatly appreciated.

Thanks.

[Jim O'Dell]

Not a pro here, but common sense would tell me the extra insulation and lower ceiling (not cooling the vaulted part) would be more efficient. Would it be worth the cost? Maybe, maybe not. I'll leave that for the people with more expertise to answer. But my biggest question is: Is there outside ventilation for the area you would close off? If not, I'd leave it alone.
Congrats on the nice sized shop. That's almost double what I have. And there are times I wish I had more. Ok, all the time I wish I had more. Jim.

[steve reeves]

That's close to 1,200sq ft.. that window unit isn't going to even come close to being sufficient.

Depending on the heat load from whatever machinery you'll be running... and the number of people in the shop (I'd assume just you)...

I think you're going to need 24,000 BTU as a bare minimum. And window units are not efficient at moving air... the shop would be "cool" or warm within maybe 15 ft of the unit and considerably warmer or cooler further away from it.

I'd opt for a 2 or 2.5 ton central unit and run some ductwork.

[David Freed]

I think you're going to need 24,000 BTU as a bare minimum. And window units are not efficient at moving air... the shop would be "cool" or warm within maybe 15 ft of the unit and considerably warmer or cooler further away from it.

I'd opt for a 2 or 2.5 ton central unit and run some ductwork.

I think 24000 BTU would be a better choice also. I have an 18000 BTU window a/c mounted in the center of a 16x48x10 shop. I have a high volume fan blowing along the shop wall that circulates the cool air to the rest of the shop. Not as good as ductwork, but a lot cheaper.

[David Smith79]

Alan,


There are many things to consider when sizing a cooling system for a building.....

Are you planning on keeping the building cooled at all times?


1. temperature difference between outside and inside of bldg.
2. HEat transfer through the structure
3. Occupancy loads and equipment loads placed on the cooling/ heating system
4. Building code requirements
5. etc.

------Simple rule of thumb is 400-500 SF per ton of cooling capacity (apartment). 12,000 BTU/HR = 1 ton of cooling capacity

You seem to have done well with the insulation. I believe Icynene has an r value of 4 per inch thickness. Someone may tell me different, but I believe that would give you an r value of 16 in areas where it is a full 4 inches. Many things are also factored when calculating your overall building energy efficiency. Windows and doors are obviously a weak point. Your roof/ceiling insulation is very important is it takes the most radiation from the sun. you may save some cooling dollares by dropping in a ceiling, but would have to consider the initial cost of the ceiling and insulation. I would say long term it pays off if you keep the builiding cool at all times. Another plus would be storage above the dropped ceiling!

Sorry for the incoherent rambling..... THe long and short is you would be constantly running your cooling system (depending on temp difference between outside/inside) if a 18000 btu were installed at 1150sf. I would suggest a minimum of 24000 btu~2ton unit... Max 3 tons.

If I had the money to spend.....Save your money on installing the drop ceiling and run some flex duct in the building to DX system. THe higher the SEER rating the better. You'll get a more completely conditioned bldg and be much happier in August.

David