Relative Humidity Vs Time

[R600a]

Thank you WAYNE3298 for starting this thread.
A lot of interesting discussion has resulted and as this is an extremely common argument in our company I am following the discussion closely.

I'd like to add my 0.02
While I disagree with the op It may have some merit in real world systems in my market because 99% of duct systems that I deal with are unsealed often finger jointed attic duct systems so when the system is running it is depressurizing the house and causing more humid air infiltration.

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[Poodle Head Mikey]

With the same logic which dictates separate AC systems for the first and second floors (which is that the applications are substantially different) - I'd like to mention that the two jobs that need doing are really better off as independent operations.

For cooling I keep my house at 75º F. and a dehumidifier set to 55%. So long as the cooling units keep the RH below 55% - the dehumidifier doesn't run. If the RH ever tries to increase - the dehumidifier brings it down. Some bear whispered that idea to me - and it really works great. <g>

I'm not sure - but I think it may cost less to operate as well.

PHM
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I keep my home thermostat at 74 F, which is comfortable, as long as humidity is low.

But on days like today, with humidity at 80 something %, then 74 F feels too hot. So if I set my upper limit of 70% RH, my system will temporarily decrease the temp setpoint. I assume this is so that a longer run-time would remove more total moisture, so that when the temperature creeps back to 74 F, WB temp and humidity would be lower. I don't know if that makes sense.

If thermostats are trying to maintain occupant comfort, they should take WB temperature into account, no? Maybe I'm overthinking this

[WAYNE3298]

Poodle when design is mentioned it is 75 F @ 50% RH. If the space is conditioned UNIFORMLY to that or any other design condition time doesn't matter. That doesn't mean that time is never a factor because to reach design the heat load not only matters it is the controlling factor.

RobertStevens you are not overthinking this because WB is what it's all about. In commercial applications around here cooling coils are designed for 55 F DB and 55 F WB which is 100% RH. RH of the space can be determined using DB Vs WB.

[WAYNE3298]

R600A you make a good point. Designs don't take into account the excess duct leakage you mentioned. That certainly makes the problems worse and if the leakage is as serious as you mentioned everything else becomes a guess.

[WAYNE3298]

OK guys I'll confess. The title of this thread even though true was formulated to generate interest in RH and it's control not to reveal anything earth shattering. In my years experience the thing I have seen most misunderstood is relative humidity even with engineers. You guys haven't let me down because this thread has really become interesting.
I hope you all will keep it going.

[RobertStephens]

I appreciate everyone illuminating this topic.

I was reading through a commercial building RTU sequence of operations. If the RH% goes over the setpoint, it will increase cooling and turn on reheat. I guess that's basically the same as a dehumidifier does. I'm not sure about the efficiency.

[WAYNE3298]

When that happens Robert the concern for comfort overrides efficiency. That is a standard way to dehumidify and works.

[behappy]

I appreciate everyone illuminating this topic.

I was reading through a commercial building RTU sequence of operations. If the RH% goes over the setpoint, it will increase cooling and turn on reheat. I guess that's basically the same as a dehumidifier does. I'm not sure about the efficiency.

At the government site there are places that efficiencies don't count. When %RH is over "X" the BAS throws the chilled water valve wide open. Then it tempers the supply air with the downstream HW coil. This will suck down %RH pretty quick. I have even seen this done with electric reheat...
SOP

[BBeerme]

Okay, I'm still a bit confused. I have a LOT of experience with controlling humidity, but not for comfort cooling. Those were environmental chambers. So as such, the amount of refrigeration for the 'conditioned space' would far exceed the amount of refrigeration for comfort cooling, at least on a cubic foot of air.

Some of these chambers would cycle between hot and cold. Or, between humid and dry. Sometimes at the same temp, other times at varying temps. Depending on what the chamber was designed for, and programming available, sometimes the temp or humidity would ramp from one to another. Other times it was as fast as possible.

There was no slowing of the air, because then there wouldn't be a uniform temp throughout. And on that note, compared to comfort cooling, based on the cubic feet being treated, the airflow in an environmental chamber would far exceed that for comfort cooling.

Some chambers would use a bare tube coil for the dehum, others would use the same finned coil, but with a regulator to keep the finned coil at 32*C. And as mentioned by others, mechanical cooling runs constant, buffered by some sort of heat to keep the temp at set point.

Had to say all of the above, to get to the part that I am still a bit confused about.

With an over driven system like an environmental chamber, temp can be achieved quickly. But the humidity lags and can take awhile. I don't think you could put time as a factor in a calculation for, as stated in the original post, "in the RH equation that relates to time".

I mean, once the coil is at 32*F, and the "room" temp is met, it can only pull out moisture so fast. I've seen it first hand. Maybe it's because of the [relative] higher air flows compared to comfort cooling. But you can definitely pull down a hot humid box to temp faster than it takes for the humidity to catch up.

Okay, I'm thinking the answer lies here.

It takes longer to remove the moisture in air than it takes to change the temp of the air.

If I'm wrong, please tell me why.

Remember, I live in an area that has none of those concerns. Me just thinks it's interesting to learn these things.

[WAYNE3298]

Cycling between heating and cooling has been used in commercial buildings for HVAC humidity control. For HVAC it's a poor excuse for humidity control but works to an extent. It is usually done when the original design didn't include humidity control and the owner needs it. I only saw a few systems like that and that was done because the cooling coil discharge temperature couldn't get below 55 F.

[BBeerme]

I've never seen that, but like I said earlier, where I live doesn't really have a humidity problem. Maybe a week a year. Maybe two at the very most; on an outlier.

I'm still confused on the time factor. How can [or could] it possibly be entered into an equation?

I mean, you can run the evap saturation a bit below freezing, but beyond that, bigger compressor won't make any difference. Removing humidity takes time. How to calculate is well beyond what I know. I just fix the stuff.

Cycling between heating and cooling has been used in commercial buildings for HVAC humidity control. For HVAC it's a poor excuse for humidity control but works to an extent. It is usually done when the original design didn't include humidity control and the owner needs it. I only saw a few systems like that and that was done because the cooling coil discharge temperature couldn't get below 55 F.

[WAYNE3298]

Time can be used to determine the approximate length of time conditioning the air will take if you know the load and capacity of the equipment. The point I was making is in an HVAC system if the entire conditioned space reaches design (75 F @ 50% RH) the time it takes to do it doesn't matter. The statement although true was made to help generate interest in this thread.

[teddy bear]

We commonly maintain ambient dew points below 32^F by passing the air through drying wheels that absorb moisture from the air and then are heated to the point were the moisture leaves the air in a regeneration cycle and that heats the wheel to the point were the wheel is dry enough to return to the 32^F dew point air.
The Munters Dry Cool is an example. This is commonly used in commercial drying of materials in the restoration industry.
Good thread. Thanks.

Regards Teddy Bear

[WAYNE3298]

The last Munters unit I balanced was on the roof of a college. I always thought they were good units they were a pain to balance.
I don't know how reliable they are but they appeared sturdy to me. What say you Teddy Bear?

[Poodle Head Mikey]

What stops the coil from removing moisture below a certain temperature? In this case 75º. If the cooling system was removing moisture until the space temperature was 75º - why wouldn't it continue to remove moisture as it continued to cool to below 75º? If the RH reduced on the way down to 75º - why would the moisture content of the air remain the same as it cooled below 75º?

PHM
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Post #32 has some excellent points. What happens also is that if a system designed to condition to 75 F @ 50% RH is allowed to cool to below 75 F the RH will increase. If the occupants know this at least some of them think the humidity is uncomfortable. They don't realize the water content of the air hasn't changed. They don't know RH is all about the DB temperature and water content of the air.

[BBeerme]

I think it is due to the RELATIVE part of RH.

Moisture in the air can remain the same, but the RH changes as the temp changes.

What stops the coil from removing moisture below a certain temperature? In this case 75º. If the cooling system was removing moisture until the space temperature was 75º - why wouldn't it continue to remove moisture as it continued to cool to below 75º? If the RH reduced on the way down to 75º - why would the moisture content of the air remain the same as it cooled below 75º?

PHM
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[Poodle Head Mikey]

Agreed - but my question is: Why would it stay the same if the cooling system is continuing to remove moisture from the air?

If the system was removing moisture as it cooled down TO 75º - wouldn't it continue to remove moisture as it cooled to Below 75º?

How could the moisture content remain the same? Which was what was stated in the post I originally questioned.

PHM
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I think it is due to the RELATIVE part of RH.

Moisture in the air can remain the same, but the RH changes as the temp changes.

[WAYNE3298]

Bingo BBeerme. The amount of moisture in the air is indicated by and depends on the temperature of the air discharging the cooling coil. If the cooling coil only discharges air at 55 F at 100% RH the amount of water in that air is fixed even though the room temperature can fall below set point because set point is DB temperature. As BBeerme said that's where the relative part comes in.

[R600a]

Bingo BBeerme. The amount of moisture in the air is indicated by and depends on the temperature of the air discharging the cooling coil. If the cooling coil only discharges air at 55 F at 100% RH the amount of water in that air is fixed even though the room temperature can fall below set point because set point is DB temperature. As BBeerme said that's where the relative part comes in.

The problem with that is that the discharge temp of the coil will change as return temp and humidity change.

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[Poodle Head Mikey]

You seem to keep including additional facts as you go along. <g>

Bingo BBeerme. The amount of moisture in the air is indicated by and depends on the temperature of the air discharging the cooling coil.

(Isn't this a pretty large IF?)

"If" the cooling coil only discharges air at 55 F at 100% RH the amount of water in that air is fixed even though the room temperature can fall below set point because set point is DB temperature.

As BBeerme said that's where the relative part comes in.

What factor of system operation would dictate that the cooling coil would perpetually continue to discharge air at 55º?