Dehumidification with DX Split

[mech5]

I'm trying to control humidity in a space using a conventional 5 ton DX split system. The system will have an electric heating coil and a wall mounted thermostat.
My question is, will dehumidification be possible? Do they make a thermostat that has a dehumidification cycle built in?

 

[sprinkler1000]

Dehumidification is possible, in the cooling cycle.

Control of humidity is not possible with a DX conventional split system.

 

[mech5]

Someone said that as long as I specify a thermostat that has a dehumidification cycle built in, it will work. The heating coil will be for re-heat of the sub-cooled air. Does that make sense?

 

[sprinkler1000]

I assume that in your case you have a máximum level of relative humidity that can not be passed.

Basically it depends on the rate of water vapor generation in the space. The latent load capacity of the DX system is limited, so in certain cases you will not be able to dehumidify.

Maybe it will be good to provide more info on the use of the space and why they want to control humidity.

 

[mech5]

Yes, the maximum humidity that cannot be exceeded is 55%. If I do a load analysis on the space and enter the target setpoint and humidity, then as long as I select a DX split that meets the sensible and latent loads, I should be ok. With regards to bringing the humidity down to acceptable levels. Right?

The space is not a latent load generating space. It's just a parts assembly 2000 sf space with about 6 people in it.

 

[sprinkler1000]

Yes,in that case it is posible to put a limit on humidity, based that the application is not critical and the occupant load is low.

 

[MintJulep]

Thermostat.

Therm being the root for heat, that is to say temperature.

If you want to actively control cooling and reheat to maintain a specific space RH then you need to measure RH and act accordingly.

 

[scottOUR12]

You might be best advised to include hot gas bypass to the evap coil. You can retard the system capacity with this feature and by doing so, cause the evap coil to stay below or near dew point. This is important to prevent the compressor from cycling off, once off, any moisture on or in the coil will be sent down stream back into the facility.

Basys makes a line of thermostats that can be configured to maintain humidity as well as sensible heat that have a broad selection of outputs for the devices you need. You may want to consider using a Sporlan Electronic Hot Gas Bypass valve to modulate the hot gas feature based on an output from the Basys device.

I have done this as well as an EXV based on the actual load requirements.

For the condensing side, look at Danfoss and their VFD and related compressors. These compressors can deliver service from 30 Hz to 90Hz to help maintain your specifications.

 

[DRWeig]

Thermostats with humidity control

Choose carefully. First, read all the specs. Second, download the manuals for the ones that pique your interest and and read them. Third, sketch how you'll apply the one you think is best. Fourth, purchase one.



Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[DRWeig]

Just plain humidistats

There are these too. and many others if you put this in Google: humidistat

Disclosure: I consult with Kele

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[NegativeZero]

I would not use a standard 5 ton split with electric heat to control humidity. the electric heat in a split is not designed to be a reheat unless you rewire it to do so. but now your inventing.

need to figure what your sensible cooling load and latent cooling load will be. And size the unit accordingly.

make sure your looking at two stage compressors or variable-speed compressor and fan.

there are companies who make units to control Humidity.

 

[mech5]

The contractor wants to use a conventional DX split. That's why we're going in that direction. I'm specifying a Honeywell 8321 Thermostat. It has control of AC for dehumidification cycles and reheat. We have a wide range for the humidity, so it will be easy to curb if it gets out of hand. In addition, there is a humidifier system that we will specify to increase humidity during colder months.
I understand that I won't be able to control the humidity down to a narrow window, but I think what I have specified will get us in the range as long as the latent and sensible loads are satisfied.

 

[EnergyProfessional]

Not sure if you are the designer. But if you design to de-humidify, why is the contractor in charge of deciding what is used?

if the contractor gets to pick equipment, he has to make sure it works, which in this case won't.

 

[mech5]

We were hired by the contractor to do the design. They wanted us to see if a 5 ton unit would handle the load. In this case, it does. The space will not much generate humidity. The only humidity that the space will see is from 6 people breathing and the OA we will be bringing in to satisfy code requirements. Plus any OA that leaks in, but there is only one exterior wall. The rest are shared between conditioned spaces. In addition, the space humidity requirements are not narrow, but are much closer to typical office conditions.

 

[EnergyProfessional]

It depends on your climate, OA can bring a lot of humidity and split-systems can't handle that. typically you need DOAS. unless you live in Arizona or so...

If the contractor hires you to do the design, but tries to overrule you anyway, let him and tell the costumer to call the contractor if they have humidity issues.
Write your concerns in a memo that they have to sign that they acknowledge you warned them of humidity issues.

 

[Drazen]

as far as i know dx - dehumidification is rough solution used for non-critical applications, and if your parts assembly room has so tuff requirement, lack of preciseness can cause some costly waste.

your space is small. if contractor is stubborn, put dx in, but add dedicated space dehumidifier to do the cleverest part of job.

 

[willard3]

Unitary DX machines come with two-row coils which are bad at controlling wet bulb temperature, no matter what kind of stat you use.

 

[317069]

Can you tell us what is your load characteristics?

 

[NegativeZero]

5 ton split, will run until it makes temp or cuts out on low pressure. I will assume the fan runs during occupancy? while compressor is off, OA is coming in. If you have a high humid day your in for a fun day. Reheats will be popping on and off. Because you need to have them cut in on a supply air set point. Now you need a supply air sensor and a SA sensor on a DX system trying to dehumidify, will play havoc with the reheats. I'm really trying to visualize this. Sounds like the contractor is trying to just get by. I hope you keep all your e-mails.

^^^^ agree

 

[cry22]

One: it is a code violation to have simultaneous cooling and heating (de-humidification and reheat)- see ASHRAE 90.1
Two: you need to downsize your AC and let it run all day to insure dehumidification.
three: relying on thermostat to dehumidify is a mistake. downsizing equipment is key. If still afraid to downsize, suggest to use two small units - one controlled with a thermostat, one controlled with a humidistat.
Four: enclosure design is important, assuming you have an air tight enclosure here, if not, you will have substantial infiltration. verify you climate and properly locate your vapor barrier
Five: Any time a contractor drives a design and issues a one directive to keep engineering fees low is a disaster.