Do Air Conditioners Control Humidity?

[jayhawker]

I got into a discussion the other day about this and made this statement to the other person. " That in order to control humidity the system has to have a set point thru a humidistat and be able to reheat the supply air to keep the room temperature stable via the room thermostat" . The other guy said then how come I see water running out of the condensate drain? and I said that because the system is removing moisture from the air passing over the coil in the form of latent heat. Then he said isn't that controling the humidity? and I said no thats removing humidity without any predictable control point.

[BaldLoonie]

Guess it depends upon how exact ya wanna be. To really control humidity, you are right. Just running an A/C will dehumidify but with oversized equipment or very high latent load, you may not really be controlling humidity.

[tex~n~oz]

If its sized right.

[hvacbear]

Jay you are correct in the literal sense of control, lowering humidity is not control. A regular residental unit removes humidity but it does not control it the thermostat cycles on and off due to sensible heat only. This is one of the (10,000) reasons that correct sizing is critical.

[Airmechanical]

I got into a discussion the other day about this and made this statement to the other person. " That in order to control humidity the system has to have a set point thru a humidistat and be able to reheat the supply air to keep the room temperature stable via the room thermostat"

if your talking about for example a clean room that needs a specific temperature and humidity level

yes you will need reheat, humidifier, cooling, and heating!



.

[madhat]

If this arguement is with a customer, I have them pull the Mechanical prints, and or specifications. That usually ends the arguement right there. :)

[teddy bear]

I got into a discussion the other day about this and made this statement to the other person. " That in order to control humidity the system has to have a set point thru a humidistat and be able to reheat the supply air to keep the room temperature stable via the room thermostat" . The other guy said then how come I see water running out of the condensate drain? and I said that because the system is removing moisture from the air passing over the coil in the form of latent heat. Then he said isn't that controling the humidity? and I said no thats removing humidity without any predictable control point.
Most in this group feel downsizing the a/c smaller controls humidity. Few realize that may days, the cooling load is mostly or all latent. A/cs remove 70-80% sensible/30-20% latent heat. This means drastic over-cooling. But like the man says, depends on what you call control. My choice is adding a dehumidifier that removes +5,000 btus latent while adding 8,000 btus of free reheat/ KW. Set the temp and %RH and get absolute control regardless the cooling load. Check out Ultra-Aire. The Dehu TB

[teddy bear]

If its sized right.
How? It's raining out 70-75^F. Removing 60,000 latent load per day requires 240,000 of sensible cooling per day using a/c. What's the size of the a/c and how cold is the home going to be? TB

[Shophound]

If your conditioned space always had a constant sensible and latent heat gain, and the system was removing both forms of heat at a rate consistent with this gain, then yes, you can control a space without reheat, with just a cooling coil.

In the real world, particularly residential, this is seldom done. In commercial applications reheat is more often found in environments that require tighter temperature and humidity controls beyond the demands of comfort cooling, such as clean rooms, museums, hospitals, etc.

I was reading in the ASHRAE Journal the other day about a system installed in a new high school that uses desiccant wheels for primary outdoor air dehumidification, with reactivation heat coming from, I think, the discharge gas off the compressor before it went onto the condenser. The desiccant wheel did the majority of dehumidifying, with the cooling coil seeing primarily a sensible load. This was on a VAV system, so it was much easier for the building staff to maintain proper temperature and humidity control in the classrooms by temperature setting alone. It was in this past May's ASHRAE Journal...pretty neat stuff.

[hvacbear]

Trane has some interesting package and rooftop units that can control humidity in the way you describe less the dessicant wheel I think. They are trying to get the word out because it is available but not utilized ( or advertized) very often.

A system is designed for a set of conditinos on a "design day". When a system encounters this exact set of conditions ( a very small % of the time) it will created design conditions (i.e. 72 degrees 50% RH or whatever it was designed for). During all other times it lowers humidity but controls temperature (sensible heat).

[Defrost]

If this arguement is with a customer, I have them pull the Mechanical prints, and or specifications. That usually ends the arguement right there. :)

I'm going to use that one!!!!!!!

[onetime]

No standard air conditioning unit is designed to control humidity, they control temperature, dehumidification is a by-product of the cooling process. You can put a humidistat on an a/c unit to control humidity but it has very limited capabilities without reheat, accumulator and less air flow to aid in dehumidification.

[teddy bear]

If your conditioned space always had a constant sensible and latent heat gain, and the system was removing both forms of heat at a rate consistent with this gain, then yes, you can control a space without reheat, with just a cooling coil.

In the real world, particularly residential, this is seldom done. In commercial applications reheat is more often found in environments that require tighter temperature and humidity controls beyond the demands of comfort cooling, such as clean rooms, museums, hospitals, etc.

I was reading in the ASHRAE Journal the other day about a system installed in a new high school that uses desiccant wheels for primary outdoor air dehumidification, with reactivation heat coming from, I think, the discharge gas off the compressor before it went onto the condenser. The desiccant wheel did the majority of dehumidifying, with the cooling coil seeing primarily a sensible load. This was on a VAV system, so it was much easier for the building staff to maintain proper temperature and humidity control in the classrooms by temperature setting alone. It was in this past May's ASHRAE Journal...pretty neat stuff.
This is my point. Commercial majors mfgs. offer this kind of control. Whole house dehus like Ultra-Aire provide an add-on package that provides this for residential/light commercial a/c applications for a fraction of the cost. Maintain 50%RH, provide filtered fresh air, and distribute throughout the home. Dehu TB

[onetime]

Trane has some interesting package and rooftop units that can control humidity in the way you describe less the dessicant wheel I think. They are trying to get the word out because it is available but not utilized ( or advertized) very often.

A system is designed for a set of conditinos on a "design day". When a system encounters this exact set of conditions ( a very small % of the time) it will created design conditions (i.e. 72 degrees 50% RH or whatever it was designed for). During all other times it lowers humidity but controls temperature (sensible heat).

The Trane unit is basically a poor copy of the Aaon unit and isn't even built at the factory, they build them at the field office, we've been doing this work for Trane in our area for several years. It prefer the Aaon control system and design, I've installed and service both for the last 15 years. But thats just my opinion.

[Shophound]

This is my point. Commercial majors mfgs. offer this kind of control. Whole house dehus like Ultra-Aire provide an add-on package that provides this for residential/light commercial a/c applications for a fraction of the cost. Maintain 50%RH, provide filtered fresh air, and distribute throughout the home. Dehu TB

It's a point well taken, TB. Locally we have experienced a very wet and cool spring, with daily high temperatures just now beginning to peak at or near ninety degrees, enough to put more sensible load on my home's central a/c, resulting in longer run times. During the wet and cool period, while my efforts at curtailing infiltration and duct sealing helped considerably, the system would not run long enough to really keep the moisture levels down where I'd like, as you say, less than 50% RH at 75 degrees. It's either over-cool or tolerate the higher humidity at 75 with a fan blowing on me...or wait for hotter weather to get longer run times. :p

Now if I had some extra loose change floating around I would be interested in your Ultra-Aire unit for our shoulder seasons. Something to keep in mind.

[tex~n~oz]

How? It's raining out 70-75^F. Removing 60,000 latent load per day requires 240,000 of sensible cooling per day using a/c. What's the size of the a/c and how cold is the home going to be? TB


Equipment size isn't based on "what-if" it's based on "average highs and lows". Ok, it's raining like Thor is comin home and it gets muggy in the house.
So? It gets muggy.
Now if it's ALWAYS raining then hopefully the engineers will take that into consideration and up the ante on the equipment a bit..

How cold is the home going to be? Whatever temperature you've adjusted your stat to, unless you choose a humidity set-point instead or perhaps an enthalpy control.... I dunno... That would be a bit odd but I've seen stranger things happen.

I'm not saying you'll nail a perfect 50% with a properly sized system, but you'll generally stay close enough for human comfort.
Look around and you'll see very very few humidity controls on most general purpose AC equipment. Those times that you do see them, question the heat-load calc used to size the gear or perhaps it's no longer general purpose and humidity control is a must.

[emcoasthvacr]

Reheat control is used when both room temperature & humidity need to be controlled. The humidity contoller (return air humidistat) and the supply air temperature controller controls the damper motor in the return air system.

Face-and-bypass control is used to control only dry-bulb temperature.

[refertecbd]

I got into a discussion the other day about this and made this statement to the other person. " That in order to control humidity the system has to have a set point thru a humidistat and be able to reheat the supply air to keep the room temperature stable via the room thermostat" . The other guy said then how come I see water running out of the condensate drain? and I said that because the system is removing moisture from the air passing over the coil in the form of latent heat. Then he said isn't that controling the humidity? and I said no thats removing humidity without any predictable control point.

Thats where sizeing the unit or units is important if its only controlled by temp. an over-sized unit will cool a space fine, but if the run time of the system aint that long then your not removing the humidy that you should. On the other hand larger Com. HVAC controllers are desighed to control humidity and temp. collectivly. In this case heat and cooling are operating together.