Mini split sizing for hot cars

[tt396ss]

I have been searching and reading threads to learn about sizing and installing mini split systems. It has been very informative but I have found next to nothing about how to handle a hot car entering a well insulated garage. My V6 SUV produces a lot of heat and raises the garage temperature too much in the Phoenix area summers. Does this even enter into the calculations or is it treated as short term events that the system will eventually dissipate?
I am currently getting quotes. They are not in yet, but they are talking in terms of 1.5 up to 3 ton units. Based on what I have read here a 2 ton might be a good solution.
My garage is 460 sq.ft. and well insulated. It is L shaped with a single 2-car door. I just want to maintain a maximum temp of 85 degrees to prevent damage to materials in my old car and also allow me to do some work there on occasion.
The on-line calculator that I used returned 19,600 BTU. Do I need to bump that to 24,000? Thanks for your advice.

 

[justinjoyal]

If humidity is not usually a concern in your area you dont have to worry about oversizing as much.

What is the insulation R value(s) ? Windows? Appliances?

460 sqft is a small space so it all comes down to insulation/tightness.

 

[u3b3rg33k]

It should be relatively trivial for you to calculate the energy stored in a hot engine. 190-220F - desired temp = delta T, * thermal mass of engine (pretend it's a solid block of aluminum if you'd like to over-estimate).

stored energy likely isn't as many BTU as you think, and your engine bay is likely insulated if your car is new.

Sizing for the space load to your 99% design day is likely more than adequate if it's on before you park.

 

[tt396ss]

The walls are 2x4 filled with cellulose and the attic has 10". Garage door has whatever they can put in them, probably thin Styrofoam. One small window and no other outside door. Correction, my area is 640 sq ft, not 460.

Thanks.

 

[Jackfre]

I’d put in an 18 and work the hell out of it. I find that oversizing cooling is not a great idea and they tend to exceed expectations anyway.

 

[mrpizza]

put the 24000 in. And make sure you buy one that is inverter drive, they will regulate the compressor output the the proper level for conditions, meaning if you dont need the full 2 tons it will ramp down to what you need. You will be pissed if you need that 19k and don't have it available.

 

[PoorOwner]

I’d put in an 18 and work the hell out of it. I find that oversizing cooling is not a great idea and they tend to exceed expectations anyway.

This is true. I stayed overseas and they put a 27k. (8kw)
In an apartment that is 550 sq ft and it would cycle off in the middle of the night and I would be sweating. I set it to lowest possible already and it does work well if only you demand lowest temp cooling during the day. Otherwise it cycles on and off way to often.

If you are only asking for 85 while you are not there, this should be an easy job for correct sized unit. Btw a car only raise the temp 5 degeees or so in an hour... BTU could be derived based on space but I give an estimate 10,000 btu or so.

i run a 18k, 660 sq and only time I saw it seemed to take a while to pull down is when I pulled in 2 cars that day. I have a mitsubishi which has the largest 18k condenser out of any brand as it shares the same body as the 24k. And it does deliver max 22,000 btu under ideal conditions. The mitsu only takes a 15 amp breaker and is very easy on enegy usage for the BTU you get out of it.

 

[u3b3rg33k]

I’d put in an 18 and work the hell out of it. I find that oversizing cooling is not a great idea and they tend to exceed expectations anyway.

oversized hvac is almost never a good idea -
poor humidity control
lower average efficiency
shorter equipment lifespan
louder/windier operation
higher initial cost

 

[yhprum]

I knew someone in Phoenix who uses a wall mounted exhaust fan that is activated by a thermostat to blow out the hot air. The fan is near the ceiling and you need a path for fresh air to come in to replace the hot air being exhausted.

 

[Firebrick43]

Maybe a year ago I did some calc for wssix and this very issue. The btus off of a hot car are actually in the 60k range.

Used length of time to warm up, mpg, btus in an internal combustion engines wasted/rejected as heat, and btus per gallon of gas to figure it out.

However I agree that oversizing is not wise either.

https://www.garagejournal.com/forum/showthread.php?t=337783&highlight=Dehumidifier&page=2

Post 23

 

[flippin]

"More must be better" is one of the pillars of Garage Journal but I very much agree with those that have already communicated that a system which is too big is a really bad idea. The recommended size based on your volume and geographic location is the right size. In the interests of satisfying our common urge to "overpower" just spend additional funds on purchasing the best quality unit at the appropriate output.

 

[bzinsky]

Maybe a year ago I did some calc for wssix and this very issue. The btus off of a hot car are actually in the 60k range.

Used length of time to warm up, mpg, btus in an internal combustion engines wasted/rejected as heat, and btus per gallon of gas to figure it out.

However I agree that oversizing is not wise either.

https://www.garagejournal.com/forum/showthread.php?t=337783&highlight=Dehumidifier&page=2

Post 23

Come on, that is just a ridiculous calculation to figure out btu's stored in a cars thermal mass using the btu's created by the fuel it burns as the basis.

that is such a vague and inaccurate estimate that could vary so much that's it's just pointless. I think stabs at the drake equation are more accurate than saying "a cars thermal mass is around 60k btu's"

I mean it should be obvious that a 1000lbs of heated metal and fluids under the hood of a v8 truck is going to have a helluva lot more thermal mass than a honda civic. Using a rough estimate for just vehicles in general is absurd. I mean there are huge variations for things that simply are caused by say the color of the car and how it absorbs heat, or the temperature of the interior because the AC was on full blast, the outside temperature while you were driving. So much so that I don't doubt for a second the btu's variations could range from 5k btu's to 100k btu's. Hopefully nobody sizes their equipment based off your calculation.

Edit: I decided to do a little math on my own. This is for a fairly good size v8.
400kg of steel heated to 220 degrees cooled to 80 degrees (trans is a little colder so that compensate for the few components of the engine that are extremely hot)- 11,184,000 Joules
10 kg of 220 degree water cooled to 80 degrees - 2,508,780 Joules
8kg of 220 degree oil cooled to 80 degrees - 1,065,600 Joules
50kg of exhaust system cooled 500 degrees - 6,058,000
Total = 20,816,380 Joules
Btu's = 19,730

This is far from perfect, but way more accurate than yours

 

[u3b3rg33k]

Come on, that is just a ridiculous calculation to figure out btu's stored in a cars thermal mass using the btu's created by the fuel it burns as the basis.

that is such a vague and inaccurate estimate that could vary so much that's it's just pointless. I think stabs at the drake equation are more accurate than saying "a cars thermal mass is around 60k btu's"

I mean it should be obvious that a 1000lbs of heated metal and fluids under the hood of a v8 truck is going to have a helluva lot more thermal mass than a honda civic. Using a rough estimate for just vehicles in general is absurd. I mean there are huge variations for things that simply are caused by say the color of the car and how it absorbs heat, or the temperature of the interior because the AC was on full blast, the outside temperature while you were driving. So much so that I don't doubt for a second the btu's variations could range from 5k btu's to 100k btu's. Hopefully nobody sizes their equipment based off your calculation.

Edit: I decided to do a little math on my own. This is for a fairly good size v8.
400kg of steel heated to 220 degrees cooled to 80 degrees (trans is a little colder so that compensate for the few components of the engine that are extremely hot)- 11,184,000 Joules
10 kg of 220 degree water cooled to 80 degrees - 2,508,780 Joules
8kg of 220 degree oil cooled to 80 degrees - 1,065,600 Joules
50kg of exhaust system cooled 500 degrees - 6,058,000
Total = 20,816,380 Joules
Btu's = 19,730

This is far from perfect, but way more accurate than yours

50Kg of exhaust seems awful heavy. I would think mine isn't more than 25Kg...
Btw my current aluminum v8 is listed as being roughly 400lbs (wet).

Anyways...
for proper minisplit sizing, we need to know if the owner will be parking and then opening the hood, or leaving the hood closed. How fast will we be transferring that heat out of the garage?

If the goal to cool the engine as fast as possible, we need to make sure the minisplit discharge aims at the open hood, and produces air as cold as possible. figure out the maximum rate of heat transfer so we don't oversize the minisplit and short-cycle it!

wow this is stupid. leave your hood closed. Maybe put some insulation on the underside if yours is missing.

My last car had the heat-at-rest option that would run the water pump after shutdown to keep the cabin comfy. that 20k BTU (probably less on a magnesium I-6) disappears pretty fast in <15ºF weather!

 

[Firebrick43]

Come on, that is just a ridiculous calculation to figure out btu's stored in a cars thermal mass using the btu's created by the fuel it burns as the basis.

that is such a vague and inaccurate estimate that could vary so much that's it's just pointless. I think stabs at the drake equation are more accurate than saying "a cars thermal mass is around 60k btu's"

I mean it should be obvious that a 1000lbs of heated metal and fluids under the hood of a v8 truck is going to have a helluva lot more thermal mass than a honda civic. Using a rough estimate for just vehicles in general is absurd. I mean there are huge variations for things that simply are caused by say the color of the car and how it absorbs heat, or the temperature of the interior because the AC was on full blast, the outside temperature while you were driving. So much so that I don't doubt for a second the btu's variations could range from 5k btu's to 100k btu's. Hopefully nobody sizes their equipment based off your calculation.

Edit: I decided to do a little math on my own. This is for a fairly good size v8.
400kg of steel heated to 220 degrees cooled to 80 degrees (trans is a little colder so that compensate for the few components of the engine that are extremely hot)- 11,184,000 Joules
10 kg of 220 degree water cooled to 80 degrees - 2,508,780 Joules
8kg of 220 degree oil cooled to 80 degrees - 1,065,600 Joules
50kg of exhaust system cooled 500 degrees - 6,058,000
Total = 20,816,380 Joules
Btu's = 19,730

This is far from perfect, but way more accurate than yours

Thank you. I don't know the motivation but you certainly went above and beyond. I am not a thermodynamics guy, never taken a class, never knew how to do it, just taking a stab as every one else was pulling guesses out of thier butts.

Your exhaust temps are to low. Most pipes are in the 700-900 range at normal loads and cats at 1200 and higher depending on the unburned hydrocarbons. Muffler temps of 500-550 are common, only the tail pipe runs lower than 500. I would agree with your mass as even if a system doesn't actually have that much, its proximity to areas of the body allow significant heat transfer.

Also don't forget the rear axle. 250 lbs at 140f.

Also what would the btu difference be if say someone was cooling to 72degrees instead of 80

 

[bzinsky]

Your exhaust temps are to low. Most pipes are in the 700-900 range at normal loads and cats at 1200 and higher depending on the unburned hydrocarbons. Muffler temps of 500-550 are common, only the tail pipe runs lower than 500. I would agree with your mass as even if a system doesn't actually have that much, its proximity to areas of the body allow significant heat transfer.

The exhaust gas temperature can reach that high, (drove around with an egt gauge for years and I am pretty well versed in the temps) but the actual temps of a daily driven vehicle are going to be much less than the figures you gave. Those are closer to the design parameters, not the figure you'd get if you actually stuck a heat gun on your exhaust after driving to the grocery store. The CAT may be that high since it has to run at a high temp to do it's duty.

Also don't forget the rear axle. 250 lbs at 140f.

The only part that sees significant heat is the pumpkin

I mean I also excluded the brakes, which are around 300-400 degrees and probably weigh a total of 80lbs.

My numbers aren't really accurate at all either. Since if he's driving around in arizona, it's likely most of entire vehicle mass has an average temperature of over 120 degrees. I mean the body itself could easily be 150+.

Basically you'd need a laboratory to figure it out

 

[bzinsky]

50Kg of exhaust seems awful heavy. I would think mine isn't more than 25Kg...
Btw my current aluminum v8 is listed as being roughly 400lbs (wet).

I doubt your aluminum v8 is 400lbs wet, that's very light. The only motors that I know of that approach that are the GM small block, the NA amg 6.2, and the rover (3.5l?) v8. I mean there's crazy v8's like the twin hayabusa v8 on the atom 500.

Also I'm including the transmission in that figure.

50kg of exhaust is pretty fair. When you include everything from the exhaust manifolds to the tailpipe. You might have a catback that weighs 25kg

 

[u3b3rg33k]

My V8 car is 400# heavier than the AWD 6 cyl version I had before - I assume most of that is the engine. It's a lazy motor in my opinion (not high strung).

 

[LS6 Tommy]

put the 24000 in. And make sure you buy one that is inverter drive, they will regulate the compressor output the the proper level for conditions, meaning if you dont need the full 2 tons it will ramp down to what you need. You will be pissed if you need that 19k and don't have it available.

^^This^^

Tommy