Temp change with HVAC 2

[eric8]

Okay guys...so I know for a residential application you want your HVAC appropriately sized to dehumidify and provide adequate cooling, and to prolong equipment life by preventing compressor short cycling. All the papers I have read show that a heat pump for a home can provide about a 20 degree temperature drop from the return air to the vent outlet.

With that being said, I'm wanting to get a mini split unit for my garage. I don't care about dehumidification, SEER, etc. I do, however, want a unit that will quickly cool my garage on super hot days with the door opening and closing, pulling in cars with hot engines, etc. I also want a unit where it can be a hundred degrees outside that can cool it down to 65 degrees in the garage should I feel the need (or if that's even possible). In other words, I'm wanting to oversize my unit if it will meet my goals. My question is, assuming a ridiculously oversized unit, are you still going to have a point of diminishing returns with BTUs? Assuming an infinite number of BTUs, is the best one can still expect a 20 degree temperature drop? Is an obversized unit simply going to get me cooled down 20 degrees quicker, but never get it cooler than the 20 degree temp change?

 

[LittleInch]

An oversized unit or units for normal steady state heat load will be diminishing returns in terms of cost, but there is no real limit on temperature drop.

you refer to heat pumps and then split units? 20F drop is just a reasonable balance between size of unit versus normal domestic requirement. 20F without knowing the air flow or BTU is meaningless.

However the faster you want your garage cooled to 65 F, then the larger the unit is required, but once you achieve your 65F then it will become over sized.

Two or more units which can be brought on line to lower the temperature then off line when not needed will help in the efficiency stakes, but cost more.

Your money, your choice.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Drazen]

What you want doesn't belong in HVAC field.

Such requirements are only stated in industrial cooling, for quick-freezing.

Not to mention that such approach for living beings is very unhealthy, so forget about that.

 

[MintJulep]

The practical limit to air temperature leaving the cooling coil is the freezing point of water.

 

[chicopee]

Install an AC unit that would take care of the temperature drop and install ceiling and/or sidewall fans to circulate the air quickly. I would also have vinyl strips acting as curtains at each garage door openings similar to installations seen at for example Costco walking refrigerated rooms used for storing vegetables.

 

[GT-EGR]

The more oversized your unit is, the quicker it is going to cool the room to as cold as you want it (within reason, but should be able to get to 60s unless you have a large an constant load).

The 20 degree number you speak of is pretty arbitrary, if you grab a cutsheet of an AC unit, any good catalogue will let you pick an entering air temperatures, and then show what the corresponding leaving air temperatures of the unit will be. So it will be less than 20 if you are bringing colder air to the AC unit, and better than 20 if you bring warmer air to the same AC unit. It will also vary if you bring less airflow through it you'll get colder temperature coming out, and more air will end up at a higher temperature coming out.

So if your garage door opens, and lets 100 deg air in, your unit will actually give you a much higher delta in the beginning, because when the AC unit sees a higher entering air temp on its same cold surface, it will draw more heat out of it. Then as the room temp starts leveling off, you'll hit this 20 deg assumption (for example - air in at 70 air out at 50) that you speak of. Eventually once you start bringing in 60 deg air, there isn't enough temperature difference between that and the coil temp, so your BTU/h drops off, and likely doesn't keep up with the BTU/h your space is adding on at that point.

Without knowing your exact operation or climate, I would say get two AC units to do the job - and can operate one at a time and run the second at a higher temperature setpoint just to catch up when the doors are open and you get a blast of hot air. You could also try evaporative cooling on those very hot days since it sounds like you aren't worried about humidity levels.

 

[BronYrAur]

The size of course depends on the load. You mention opening and closing the garage doors. How frequently? Hot engines will add acute loads, but they will not always be present. Is the garage insulated? Are the doors insulated? 65 deg might be tough to hold. Being residential (unless we are talking about a mansion) I would assume you only have 220V, 1-phase. That will limit your equipment size. So, multiple smaller units may be a better option.

 

[IRstuff]

Assuming a standard two-car garage at 400 sf and normal heat loads, you'd need around 0.6 to 1 ton of cooling. Bear in mind that grossly oversizing may result in poor humidity control.

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