HVAC Tutorial Part II - Air Tempering vs Air Conditioning

[chuckspeed]

Alright...It's on to Part II of the basics.

Air conditioning systems:

little bit of history first...

Back (WAAY back) in the day when the first A/C systems were installed in theatres in the Midwest, they literally 'hung' blocks of ice in ductwork and blew air over them to cool the space. The term 'ton' of refrigeration stems from the weight of the block of ice required to cool a space in an hour's time - 2000 lbs of ice was equivalent to 12,000 BTU of heat rejected during the latent heat (phase change) from water to ice. Imagine 20 tons of ice hangin' in a big box with air blowin' over it - yup. That's the first A/C system - honest. Got diagrams from the Smithsonian somewheres on the subject...

Anyway, cooling a shop space is once again a little bit different than cooling a living space. Here's where we want to discuss the term 'air tempering. Air tempering differs from air conditioning in that the system is designed to reduce the temperature inside the facility (in this case, a garage) by 20 degrees relative to the outside air temperature. Tellyouwhut - if you're out in baking heat - a 20 degree drop feels darned good!

There are multiple benefits to air tempering. One is equipment sizing - here we can do more with less, as we're not trying to maintain a set temp - we're only trying to 'cut' the heat a bit. This reduces the tonnage requirement by 25-30%, depending on where y'all live.

The second bennie is not so obvious - it has to do with latent vs. sensible heat. Sensible heat is defined as the heat you can feel, or 'sense'. Latent heat is less obvious - it's the heat required to evaporate a given quantity of water. Anyone who's swam inna pool in Phoenix is REAL familiar with the latent heat of evaporation - you can be swimmin' in 80 degree water, get out and be in 106 degree sensible heat, and freeze yer ass off...simply because the humidity is in single digits.

The reason for this has to do with the quantity of water in the air. Believe it or not, but an A/C unit works much harder to wring buckets of water outta the air (dehumidification) than it does to reduce the temp of the air. Air has a really low specific heat - where water is many, many times higher. So...the average A/C system spends a good portion of its time dehumidifying, as opposed to cooling. FWIW, we feel better about heat when the humidity is low - that's where the term 'It's a dry heat' comes from. There's a scientific tool called a psychrometric chart (invented by some dude on acid, I'm sure) which clearly explains the relationship between temp and humidity - but it's a ten week course in most schools. Really bizarre, and the real world stuff is much simpler, when translated into a shop - honest.

Okay - back to air tempering.

By downsizing the equipment relative to the space, we end up with a system that runs longer. This is really important, as most installers use a 'rule of thumb' which results in an oversized system - one that will satisfy the design conditions (and then some). unless you live in the desert (phoenix, vegas, san berdoo, etc) you don't want to oversize the system - you want to undersize it! By undersizing a garage system, it will wring more buckets of water outta the air - and make the garage seem 'cooler' in the process.

Yah - I know that sounds counterintuitive, but keep in mind we try to be kinda active in our garages. When we're active, we sweat...when we sweat, a low humidity is preferable to a low temperature, as low humidity will 'feel' cooler than a higher humidity and a lower temp! When the equipment is oversized, it shortcycles - which limits its ability to wring those buckets outta the air.

So...you wanna know how to size A/C systems for air tempering , eh?

Stay tuned...Post is getting a bit long for that. I'll follow up next time!

[Tom]

So that's why my little window unit running constantly seemed to do ok.

[Lu47Dan]

That was imformative , You can hear the gears grinding on this board . But I understood what you are trying to get acrossed . the more water vapor you pull out of the air the lower the sensed tempature is . instead of standing there dripping with sweat , you are standing there in a damp t-shirt and saying it not bad in here today . Did i get it right ? Dan :beer:

[REFLEXX]

I've got more questions than answers now!?!

I'm in So Cal, where it's usually 80-90 in the summer, but once in a while we peek into the 100s. I oversized my A/C units because I figured that I would be generating more heat compared to a house. I'll have machines with electric motors and computers that put out heat. I might be welding, roll in a hot car from outside, bring in some materials that have been in a shed, etc... Not just the temps outside.

[chuckspeed]

To answer your questions...

Yup. You guys are 'getting' it. Low humidity and a not-so-cool temp is better than a big A/C unit that short cycles.

Left out something...Air movement. Air moving across skin will have an 'apparent' temp that is a good ten degrees lower than it actually is - so low humidity air at 80 degrees is gonna feel pretty darned cool if it's blowing on ya!

Anyway - to reflex's point...each kilowatt of electricity is worth 3413 BTU of heat. Don't care how you're using it - it all gets converted into heat energy somewhere in the process. It's pretty unuusal to see more than a couple of HP in use at any time inna average home shop - figure half a ton for internal heat gain. This is a pretty good figure, as most electrical stuff isn't gonna run continuously. Also - once you get all that stuff cooled down, the thermal mass (see Part 1) works in your favor - you can literally flywheel the space thru a period of heavy use with minimal temp rise.

I lived in SoCal for a few years - know what you're talkin' about.

A story:

Pop was a tool and die maker - and inveterate car guy. Back when he ran his shops (he's retired now) he'd order every door opened and every exhaust fan on as soon as the sun went down during the summer. When the sun came up the next morning, they'd shut all the doors and shut off the exhaust fans. Believe it or not, the thermal mass of the equipment and concrete kept the shop pretty cool until about 4:00 in the afternoon - without benefit of air conditioning!

I've used this sort of approach in the first house we owned - a tiny 1100 sq foot brick ranch with a single 1-ton window shaker. According to the 'experts', I needed two tons; flywheeling the house negated the need to use any more than one ton, and most warm days I didn't need to run the A/C until about 2:00 in the afternoon.

A quickie rule of thumb for A/C unit sizing is 500-650 sq ft/ton, BTW. I'd lean towards the high end of the scale for a shop - and make up the difference with fans. Much lower operating cost thataway.

[Tom]

A 'window shaker' and a ceiling fan. I'll give that a try if it ever warms up around here. :shivers:

[chuckspeed]

Here's an idea which works in larger shops and spaces with higher ceilings:

http://www.bigassfans.com/

yeah - you read that right - big ass fans. The correct scientific name is high volume, low speed (HVLS) fan technology; they move large amounts of air with a teeny motor. If you have the room for one - they're pretty slick!

[mrl05]

Thanks Chuckspeed. These two HVAC threads have been quite informative.

I have a 24 X 20 garage and last summer it seemed to hover around 85 - 90*F (even in the evenings) with excessively high humidity. I have been debating the A/C or dehumidifier question this past winter and wondered if a dehumidifier would do the trick. Could I run a dehumidifier to lower the moisture in the air and a ceiling fan to circulate the air for a comfortable garage environment? I really do not want to go A/C as I don't spend that much time out there. Plus, I am looking for a shop temp of around 80* F and thought that running an A/C to maintain that temp would not be cost efficient.

Thanks,
Mike

[chuckspeed]

Thanks Chuckspeed. These two HVAC threads have been quite informative.

I have a 24 X 20 garage and last summer it seemed to hover around 85 - 90*F (even in the evenings) with excessively high humidity. I have been debating the A/C or dehumidifier question this past winter and wondered if a dehumidifier would do the trick. Could I run a dehumidifier to lower the moisture in the air and a ceiling fan to circulate the air for a comfortable garage environment? I really do not want to go A/C as I don't spend that much time out there. Plus, I am looking for a shop temp of around 80* F and thought that running an A/C to maintain that temp would not be cost efficient.

Thanks,
Mike

Mike -

That's a hard question to answer without knowing exactly where you are. If you're close (within 15 minutes) of the coast, you're going to be hard-pressed to wring out enough water from the air as the garage isn't tight enough to be able to effectively dehumidify...Unless you keep it tight!

You see, in high humidity environments (Savannah GA is a good example) the night time low temperature is governed by the dewpoint. The psych chart effectively shows that as the air temp lowers, the humidity rises - and rises - and rises - until 100% humidity is reached. This is the dewpoint, or the point at which water condenses from the air - usually in the form of low clouds or fog. If you watch it - night time lows are usually equal to the dewpoint in these high humidity areas!

By the same token, for dehumidification to be truly effective, you'd have to lower the humidity down below 60-65% - and the amount of energy required to do this is gonna be darned near the amount required to air temper.

NetNet - you'll probably require about a ton of A/C, and about 3/4ths of a ton of dehumidifcation to be effective. The price differential (relative to operation) isn't gonna be all that much; just set the temp on the A/C unit high!

[bmwpower]

The problem I encountered when having my HVAC system put in was that for the size of the furnace, only certain size coils were available. I would have liked to have gone with a smaller sized coil, but supposedly it wouldn't fit with the rest of my system. Something to do with the air handler. At least that's what the HVAC guy told me...

[imported_banzaitoyota]

I got a quote for a BigASS Fan, got a visit from a high pressure salesman and a quote for $4500. Guess they are used to selling to the NASCAR Teams

[chuckspeed]

The problem I encountered when having my HVAC system put in was that for the size of the furnace, only certain size coils were available. I would have liked to have gone with a smaller sized coil, but supposedly it wouldn't fit with the rest of my system. Something to do with the air handler. At least that's what the HVAC guy told me...

heating/AC systems come as packages, and the installers always try to 'up-sell' you. my first house was teeny, requiring only a 60K BTU furnace and some goof sold the old lady a 125,000 BTU furnace. New house is 2-1/2 times the sice of the first house, and only has a 100,000 BTU unit.

The size of the A coil is constrained by the size of the plenum, as the coil has to have the same footprint as the plenum's cross-sectional area. The size of the coil, however, does not constrain you from installing a smaller compressor and condensor, and the only real effect it will have is a longer delta T on the evap coil.

[chuckspeed]

I got a quote for a BigASS Fan, got a visit from a high pressure salesman and a quote for $4500. Guess they are used to selling to the NASCAR Teams

I thought they made a 6' unit which was quite a bit less...

[bmwpower]

heating/AC systems come as packages, and the installers always try to 'up-sell' you. my first house was teeny, requiring only a 60K BTU furnace and some goof sold the old lady a 125,000 BTU furnace. New house is 2-1/2 times the sice of the first house, and only has a 100,000 BTU unit.

The size of the A coil is constrained by the size of the plenum, as the coil has to have the same footprint as the plenum's cross-sectional area. The size of the coil, however, does not constrain you from installing a smaller compressor and condensor, and the only real effect it will have is a longer delta T on the evap coil.

Figures. I'm probably screwed. He did bump up both the heat and AC capacity, but at no additional charge. Something about he "reworked" the numbers and this was a better fit. The furnace is 100K Btu 80% gas unit. This summer will be the first time I get to kick the AC on. If my memory serves me right, it's a 2 ton unit for a 30x30x14 garage, 2 garage doors, etc.

The good thing is the contractor lives down the street, so if it don't work properly, guess who will be on his doorstep?

[chuckspeed]

Figures. I'm probably screwed. He did bump up both the heat and AC capacity, but at no additional charge. Something about he "reworked" the numbers and this was a better fit. The furnace is 100K Btu 80% gas unit. This summer will be the first time I get to kick the AC on. If my memory serves me right, it's a 2 ton unit for a 30x30x14 garage, 2 garage doors, etc.

The good thing is the contractor lives down the street, so if it don't work properly, guess who will be on his doorstep?

The size isn't all that bad - he upsized from a 1.5 ton to a 2 ton unit, as he based his selection on 500 sq. ft./ton. If he used 650 sq ft/ton, he'd have picked a 1.5 ton unit.

There was an old engineer who often boasted, 'I've never had a problem with my equipment selections or designs! They've always worked!' I had the opportunity to work thru his calculations some years later, and discovered the reason why - he oversized by a factor of 2 on that job. Engineer-types often do that - add safety factors to insure their selections will work. In our energy-consciuous world, however, oversizing = larger electrical and fuel bills. Right-sizing, although it takes more time to grind thru the calcs, is the way to go!

[imported_banzaitoyota]

I thought they made a 6' unit which was quite a bit less...

Chuck, If they did, the salesman tounting their wares didnt seem to want to say anything about it
John

[imported_banzaitoyota]

And trust me, I am looking for a decently priced vane axial fan for exhausting my dyno room as we speak!