A/C - dehu for shop

My shop stays pretty cool with the doors shut up tight, but the humidity
gets very high.....up to 75% at times.
Things get drippy when it's worst possible conditions because there is no or not much vapor barrier. Minimal insulation......

The one room is about 1750 sq ft.......
My question is, would a small 5000BTU window type AC unit dehumidify
that area to ...say.... 65 or 60% RH ?
Not really looking to cool it, but make it dryer.
If it would cool it a bit, that'd be good too.

Isn't an AC unit a 'more efficient' dehumidifier than a purpose built -dehumidifier-....??

What is the formula for calculating dehumidification ?

dk

ANY small AC is not going to do much cooling until it can get the humidity down, but it will do some. So it depends on whether you want the heat back, or not. Up here, PNW, it's been VERY cool. I would want the heat, for now. The only real difference in function between an AC and a dehumidifier is that using a dehumidifier, instead of placing the condenser coil OUTdoors, you place it INdoors, which simply -- dumps the heat back into the building.

So if you want the heat, a dehumidifier is really MORE efficient, because it keeps the heat energy that was in the air/ humidity inside the building, plus adds the heat due to heat of compression, heat due to compressor friction and the inefficiency of the compressor motor.

Go to Walmart and buy a dehumidifier, it will do wonders in drying out your shop.

Lord Byron

Ok, well, I'm learning something new already.

2 things tho:

I don't want the heat......it was 90 here today.
I already have 2 5000 BTU window units that are like new.

I had a dehumidifier running in a 600 sq/ft. shop, sealed up as tight as could reasonably be done. I always felt the comfort of the dry air, not the discomfort of the minor heat generated by the dehumidifier. It was always cooler with the doors closed and the air dry than with the doors open to the humidity of summer in Boston.

QB

Window units are the way to go for what you're trying to accomplish.

The heat a dehumidifier will add to the space is easy to figure. Just look at the nameplate on the unit and see how many watts it uses. All that wattage is converted to heat, just the same as if you had an electric heater of the same wattage. Everything else in the cycle is a wash since the entire unit is within the space and there's no way to dump the heat outside the space.

The typical cheap 5000 btu A/C unit made within the last few years is going to draw about 450 watts, more or less. Because of the way the units are built, nearly 100% of that 450 watts will be dumped outside the space. In addition, all the latent heat extracted from the moisture it condenses is dumped outside the space, as is whatever sensible heat the unit removes from the air.

The cooling capacity of the window unit is a statement of its net capacity to remove heat from the space, via a combination of condensing humidity and cooling the air mass. As roadrunner said, there's not much cooling happening until the humidity is lowered. The cooling cycle balances all that out without external input. IOW, the unit doesn't really care if 90% of its capacity is going to dehumidification, and 10% to cooling, or vice versa. It just eats 450 watts and does whatever happens to add up to 5000 btu of heat removal per hour.

When you compare the net effect of the window unit vs the dehumidifier on summertime space conditions, there's more difference than you might first think. For simplicity, assume you have a dehumidifier that uses the same 450 watts as the window A/C. To further simplify, we'll keep everything in watts.

The dehumidifier is incapable of removing any heat from the space, so its net effect is the addition of 450 watts. The window unit is rated to remove 5000 btu, or 1465 watts of heat per hour. The net difference is 1465 + 450, or about 2000 watts per hour. The dehumidifier WILL make the space warmer. The window unit won't warm the space, and, depending on humidity conditions, CAN cool the space as well as dehumidifying it.

One further thing worth mentioning...... Most window units have a slinger ring on the condenser fan to pick up condensate and throw it onto the condenser coil. They also have a rubber plug that can be removed to allow the condensate to drip outside in liquid form. Leave the plug in place, and if its been removed, replace it with something suitable to plug the hole. Throwing the condensate onto the condenser makes the unit more efficient because the water carries heat as it is evaporated from the coil, effectively increasing the heat rejection capacity of the condenser.

The difference between the two is simple......

an air conditioner is rated to COOL the area, while the dehumidifier is rated to remove water.

This matters because the A/C only removes water when running, and the closer to the "proper" size (for cooling) it is, the less it is running. Typically it cools the air MORE, and does not re-heat it, which can have the effect of RAISING the relative humidity.

The dehumidifier cools the air less, the air only needs to be below the dew point to condense water, so it is often lower power, and it runs more of the time. it actually removes water without increasing relative humidity.

You don't need insulation, just a vapor barrier, to keep the good effects of it contained. With A/C, you need to insulate in order to avoid trying to cool the world.

I find the added heat is essentially un-noticed.

With low humidity, the sweat evaporates right away, actually cooling you, so you sweat much less and feel much cooler.

But I sure do hate that "damp cold" that often is a part of air-conditioned environments.

dkmc said:

Ok, well, I'm learning something new already.

2 things tho:

I don't want the heat......it was 90 here today.
I already have 2 5000 BTU window units that are like new.

Click to expand...

Dehumidifier and air conditioning --- you get cool dry air.
Do both.

Be sure to set your controls so the fan turns off when the compressor on your A/C is off. On many A/C units the moving air holds the condensate in the coils. When the fan turns off there is a rush of water coming off the coils. If you run the fan continually the condensed water returns to your cooled envirement.

FredC said:

Be sure to set your controls so the fan turns off when the compressor on your A/C is off. On many A/C units the moving air holds the condensate in the coils. When the fan turns off there is a rush of water coming off the coils. If you run the fan continually the condensed water returns to your cooled envirement.

Click to expand...

It seems to me that most all the units sold today have no provision to
shut off the fan with the compressor! Seems like that is defeating the whole purpose? I have one that keeps the fan on for 5 minutes after the compressor shuts down. You can feel the humidity come back in the room.
I can't believe the people that engineer this stuff don't seem to have enough background knowledge of how it should operate.

I repeat, a Walmart dehumidifier is a lot more efficient at removing moisture out of the air than a small window type air conditioner. The added heat that it supplies is not oppressive in the dry air that it produces. I have basement living area (500 sq. ft.) that is air-conditioned with a whole house air conditioner, and I still have to use a dehumidifier in the basement area. It makes the difference that makes this area comfortable. My unit will remove 5 gallons of water a day when the weather is humid. I don't think a small window air conditioner could remove as much moisture out of the air. Cold, clammy air is objectionable to me, and is hell on machine tools, but the choice is up to you.

Lord Byron

I don't think a small window air conditioner could remove as much moisture out of the air

Click to expand...

I was wonner'in that my own self as well.

I already have a dehumidifier also.....
I'll experiment with both and see what happens.

BTW.....
Are you Bruce Nelson or "Lord" Byron??
What kind of first name is Lord?
Do they have "Lords" and Dukes in Wisconsin USA??
Is the Duke of Earl a chum of yours?
Or are all the Dukes in Hazard county?

I'm just a tad curious 'ma Lord...

FredC said:

Be sure to set your controls so the fan turns off when the compressor on your A/C is off. On many A/C units the moving air holds the condensate in the coils. When the fan turns off there is a rush of water coming off the coils. If you run the fan continually the condensed water returns to your cooled envirement.

Click to expand...

Not correct by any stretch of the imagination. Air moving thru the coil tends to move the condensate toward the leaving face of the coil, thus causing it to collect in larger droplets and drain off the coil faster than it would with zero air velocity.

Most residential size equipment, window units or otherwise, have the cooling coil located in a draw thru configuration. IOW, the fan is on the leaving side of the coil, and air is drawn thru the coil, into the fan, and discharged from the unit. This causes everything before the fan to be under negative pressure. The "rush" of water when the fan stops is due to the return to zero pressure. 1/2" of negative static pressure will cause water to stand 1/2" deep in the condensate drain pan even though the drain is clear. Stop the fan, and that 1/2" of water suddenly drains from the pan. The point being that this condensate was being held in the pan, and not on the coil itself.

The extreme of this occurs with stopped up filters and a fan that's capable of significant static pressure. The negative pressure exceeds the depth of the drain pan, so the condensate overflows into the unit housing. While the fan is running, the negative pressure holds the bulk of the water within the unit housing. Fan stops, and a good size unit may dump 25 gallons or more of water out thru every available orifice.

Cliff,
Is there some practical reason why they have the fan run-on after the compressor stops??

Bruce Nelson said:

I repeat, a Walmart dehumidifier is a lot more efficient at removing moisture out of the air than a small window type air conditioner.

Click to expand...

Exactly what scientific or engineering principle would you use to support that statement? If you actually understood how an air conditioner works, you'd know it makes no sense, because an air conditioner is a dehumidifier by the very nature of its operation.

I have basement living area (500 sq. ft.) that is air-conditioned with a whole house air conditioner, and I still have to use a dehumidifier in the basement area. It makes the difference that makes this area comfortable. My unit will remove 5 gallons of water a day when the weather is humid.

Click to expand...

Blowing some cooled air into a basement that's cool and damp isn't air conditioning, and it isn't remotely similar to the situation Dan described in his shop. A cool damp basement is the perfect application for a dehumidifier because the space, by definition, needs dehumidification and not cooling. Making the (fairly reasonable) assumption that Dan's shop isn't subterranean, he will have a combination of sensible and latent load while a damp basement will have a load that's pretty much exclusively latent.

I don't think a small window air conditioner could remove as much moisture out of the air.

Click to expand...

Once again, why not? To put some numbers to the question, a 5000 btu air conditioner could remove a max of 14.75 gallons of water per 24 hours if there's sufficient latent load to keep it operating in pure dehumidification mode.

Cold, clammy air is objectionable to me, and is hell on machine tools, but the choice is up to you.

Click to expand...

And just how would you propose to create cold and clammy conditions in an 1800 sq ft shop space using only a couple small window units? Impossible is the word that comes to my mind.

dkmc said:

Cliff,
Is there some practical reason why they have the fan run-on after the compressor stops??

Click to expand...

Yeah there is a good reason Dan. There's a temperature sensing bulb mounted just off the face of the cooling coil that controls the compressor via return air temperature. If you stop the fan, the mass of the cold coil (typically about 40*F) in close proximity to the sensing bulb is going to hold the compressor off until the coil warms to the point where the bulb decides its warm enough to cycle the compressor back on.

By running the fan continuously, the bulb is sensing the temperature of the air in the vicinity of the unit rather than the temperature of the cooling coil itself, so the compressor will come back on based on a more realistic measurement of space temperature.

Rsidential A/C systems are about the only ones where its common to shut the fan off when there's no call for cooling from the stat. Larger systems always run the fan continuously in an attempt to keep space temps even and to keep the air moving somewhat.

Since you've got a dehumidifier, there's nothng wrong with running it too, particularly in the initial phase where you'll be drying out not only the air but everything in the shop that has the capability of holding moisture, such as sheet rock, bare block walls, and of course all that furniture and carpet in your executive office suite. Personally I find my molded plastic chair is super efficient in that it won't hold any moisture at all

I've found when running window units in an application like yours, its a big help to also run a box fan to keep the air moving around the space in general. You don't get much air throw from a window unit, so the space near it tends to be cooler than the rest of the space. This colder air has a decreased moisture holding capability, so you tend to get more cooling and less dehumidification as a portion of the air is recirculating. Humidity migrates thru the air by nature, but less of the moisture will migrate into air in the vicinity of the unit if that air is colder than most of the rest of the shop. By using the box fan, you can keep a better supply of "fresh" air in the vicinity of the unit, and get increased dehumidification as a result. The fan doesn't have to be near the unit. All it needs to do is keep the air stirring. I bought one at Wal Mart a couple years ago, actually made in the US, for about 20 bucks. IIRC, its a Fasco brand. Probably made in Outer Mongolia by now though It has a trunnion type mount so it can be tilted to any angle, which works better in a shop than one that's fixed and blows straight across the floor.

As always, I appreciate you sharing your vast technical knowledge.

My office is more sparsely elegant that you imagine.
Cheap 1970's 'look' paneling installed in the late 80's
And bare naturally weathered plywood on the floor.....

Experiments here with my dehumidifier have shown it gets most moisture in the evening as the temp drops and the humidity there fore climbs higher, being high humidity its easier to get the water out and the colder air helps too. Come the following day as the air warms the relative humidity drops really low!

Other thing is circulation, need to stir the air through out the whole shop. Can be as simple as where the dehumidifier - ac is to having a fan.

My ceilings are low and because of that I have 'low overhead'...
Like 9 feet.

I'm thinking a couple ceiling fans would mix-it-up pretty well...
but maybe I need a fan that stirs it in a horizontal loop...?

Ceiling fans are fine, with one caveat. If the ceiling stays hot in mid day due to lack of insulation above, then you don't want to run ceiling fans because the air moving across the ceiling will turn it into a large plate heater.

If the ceiling isn't hot, then ceiling fans are ideal IMO because they're quiet and they can move large quantities of air at low velocities which is what you want when the goal is to keep the air mixed up.