High Efficiency Mini Split for High Humidity Area

[Joybob]

I've read that different units are designed for different humidity ranges. I'm in the tropics running 80% humidity. I'm wondering what the most efficient units are, (mini-split).

Paying 30 cents per Kilowatt hour so a large investment seems reasonable.

[Core4 Guy]

I've read that different units are designed for different humidity ranges. I'm in the tropics running 80% humidity. I'm wondering what the most efficient units are, (mini-split).

Paying 30 cents per Kilowatt hour so a large investment seems reasonable.

Joybob, 30 Cents a kW gives PR a hell of an incentive to do retrofits. The ROI for retrofits and new install upgrades must be great if you could actually save any energy.

The first thing I would do is for new construction make sure the vapor barrier is top of the line to stop infitration of moisture.

Other than that your not going to get much difference out of conventional units. You may want to check into different types of dehumidifiers preferably powered by a cheap source of energy (how much does natural gas cost).

I really haven't seen a unit on the market that does well with humidity. So for larger installs a built-up system with a high suction temp will help. But you still need to make sure the humidity is under control with the vapor barrier.

The latent heat of the condensate is your biggest source of waste energy. Keep that at a minimum and close the door. :)

[nike123]

You could achieve that with any mini-split unit. Slightly (around 20%) oversize installed capacity and run unit fan on low speed all the time. Where is large room, use more then one unit to help in air distribution.

[Gary]

You could achieve that with any mini-split unit. Slightly (around 20%) oversize installed capacity and run unit fan on low speed all the time. Where is large room, use more then one unit to help in air distribution.

I would say oversize the coil and undersize the compressor.

Adjust the airflow to give a supply air temp that will handle the humidity. The supply air temp needs to be about 20F/11K below the thermostat setpoint or perhaps a little lower for high humidity load.

If needed use a CPR to prevent overloading the compressor.

The idea is to get the evaporator TD as low as possible (higher SST) while extending the run time.

[The Viking]

I've read that different units are designed for different humidity ranges.

Yes they are.

But this is most apparent when you compare close control units (made for IT rooms) with A/C systems made for comfort cooling.

Us humans feels more comfortable at lower humidity levels therefore your standard comfort cooling A/C systems has been designed to remove "excessive" humidity.
Whilst close control units on the other hand needs to keep areas with high heat loads at a level temperature without dehumidifying too much, as this would allow for higher levels of static electricity build up, causing problems for the IT equipment.

However,
I'm not so sure that you would save money by finding a unit that dehumidifies less..
(I have assumed that the installation is for comfort cooling)

"Us humans feels more comfortable at lower humidity levels..."
Therefore, by allowing the humidity to drop you should be able to allow the temperature to rise....
Just a thought.

[Abby Normal]

I would say oversize the coil and undersize the compressor.

Adjust the airflow to give a supply air temp that will handle the humidity. The supply air temp needs to be about 20F/11K below the thermostat setpoint or perhaps a little lower for high humidity load.

If needed use a CPR to prevent overloading the compressor.

The idea is to get the evaporator TD as low as possible (higher SST) while extending the run time.

Over sizing a minisplit coil, as in increasing the face area, is a means of increasing sensible cooling.

Oversizing a coil, by making it more rows deep, will increase dehumidification.

PR probably has an ambient dewpoint hovering close to 80F for the rainy season.

Unless people have water features inside their dwelling units, are hang drying laundry inside the home, or running a home based catering business simmering food all day, they do not make any more moisutre than people elsewhere in the world.

The humidity in the tropics is coming from the outside air. If you can control how air moves through the building envelope, you can then effectively control humidity and prevent mold.

Ideally, you want an air conditioning system that will pressurize a dwelling unit with dry air.

Minisplits cannot do this.

[Abby Normal]

undersizing an evaporator from the nominal size with respect to the nominal compressor/condensing unit capacity, will result in a small drop in total cooling but will give you an increase in latent cooling.

The evaporator will run colder, the air making contact with the coil surface will release more moisture.

want the most dehumidification from a mini-split, remove the asian controls and jerry rig it to work from a remote thermostat, fancoil unit runs only when the compressor runs. On a specific call for dehumidifying, it runs on the lowest speed.

[Gary]

undersizing an evaporator from the nominal size with respect to the nominal compressor/condensing unit capacity, will result in a small drop in total cooling but will give you an increase in latent cooling.

The evaporator will run colder, the air making contact with the coil surface will release more moisture.


However, the condensing unit will run less efficiently due to the lowered SST.

[Gary]

want the most dehumidification from a mini-split, remove the asian controls and jerry rig it to work from a remote thermostat, fancoil unit runs only when the compressor runs. On a specific call for dehumidifying, it runs on the lowest speed.

On this we agree. The ideal would be to have a humidistat kick the fan down to low speed for dehumidification, then up to high speed to bring down the temperature.

[Abby Normal]

yes lower SST means less compressor capacity.

half the trick of high seer is to get more work out of a small compressor by elevating the SST, then increasing the condenser coil size to reduce the condensing pressure/temperature.

High seer generally is not condusive to enhanced latent removal.

If you could pressurize a place with dry air, then it would be pretty much impossible to 'oversize the AC large enough' to cause high indoor humidity.

Pressurizing takes infiltration out of the equation. A 5 ton system on an otherwise 2.5 ton load, would remove latent gains from occupants in about 5 minutes run time.

[Abby Normal]

On this we agree. The ideal would be to have a humidistat kick the fan down to low speed for dehumidification, then up to high speed to bring down the temperature.

yes on this we agree. I remember spending months trying to convince you on Auto fan in the old days :)

[Gary]

Over sizing a minisplit coil, as in increasing the face area, is a means of increasing sensible cooling.

Oversizing a coil, by making it more rows deep, will increase dehumidification.


While I agree with the second paragraph, I'm not so sure about the first. If coupled with fan speed adjustment, as I suggested, the result should be increased dehumidification along with increased efficiency.

[Gary]

yes on this we agree. I remember spending months trying to convince you on Auto fan in the old days :)

And apparently you succeeded. :)

[Abby Normal]

While I agree with the second paragraph, I'm not so sure about the first. If coupled with fan speed control, as I suggested, the result should be increased dehumidification along with increased efficiency.

Slowing the fan speed is another variable of course.

It really comes down to how much air contacts a surface, how much air bypasses a surface, and how cold the surface is.

Generally speaking a larger face area coil will run at a higher SST. The larger fac area coil will have more surface area as well as more bypass area. Slowing the air down through the coil will increase dehumidification.

I had some custom built equipment designed to remove a lot of moisture, was a combination of a low face velocity (oversized face), a lot of fins per inch, and the coil being 8 rows deep.

Another project had 12 rows of coil. Was a couple in series. 8 + 4

[Gary]

No doubt, reducing the bypass factor (more rows) is the better strategy, but it has been my experience that given fan speed adjustment the optimal balance of efficiency vs dehumidification can be achieved with todays larger coils, be it with larger face area or more rows.

Keep in mind that as a service tech it is not my job to design systems. I take what I am given and make it work in spite of the design.

[nike123]

All mini-split systems have option to slow down speed of indoor unit. Actually, they have 3 speed fan control as default. By installing slightly oversize unit in room, and lowering speed to low, we have higher temperature difference of entering and exiting air and, as consequence, higher dehumidification of air.
Need for oversizing and mounting of more then one mini-split, arises from fact that sensible capacity is lower than in normal (medium speed) operation. Also in this case, distribution of air in room is impaired because of low fan speed and that is reason to add more capacity and maybe two (or more) unit to overcome that lower air distribution problem.
Of course, all that is case if outdoor temperature is high and very humid. If outdoor temperature is mild and only humidity is high, like in rain season in some countries, than every mini-split system could perform function of dehumidification satisfactory without need to oversize anything because it has function of dehumidification, already in his control logic.

[Abby Normal]

You are far from 'just a tech Gary', but yes your goal is to make them work the best.

To the original poster, If your client has the money put in the highest seer you can get and also give him a dehumidifier.

Aprilaire and Thermastor make them that can also suck in some fresh air at the same time.

I have had good luck in the tropics with split ducted systems that draw in fresh air ehrn the compressor runs. Means put the fan in auto mode.

[Abby Normal]

All mini-split systems have option to slow down speed of indoor unit. Actually, they have 3 speed fan control as default. By installing slightly oversize unit in room, and lowering speed to low, we have higher temperature difference of entering and exiting air and, as consequence, higher dehumidification of air.
Need for oversizing and mounting of more then one mini-split, arises from fact that sensible capacity is lower than in normal (medium speed) operation. Also in this case, distribution of air in room is impaired because of low fan speed and that is reason to add more capacity and maybe two (or more) unit to overcome that lower air distribution problem.
Of course, all that is case if outdoor temperature is high and very humid. If outdoor temperature is mild and only humidity is high, like in rain season in some countries, than every mini-split system could perform function of dehumidification satisfactory without need to oversize anything because it has function of dehumidification, already in his control logic.

yes minisplits have mutliple speed settings as well as their own version of "Auto" which adjusts the fan speed setting"

With a split ducted system, "Auto" mode means the air handler fan runs with the compressor fan. Constant fan functions as an evaporative cooler and will reevaporate whatever moisture is on the coil when the comprssor cycles off. Mini-splits do this all the time.

Best for the tropics, and the hurricane belt is rendered concrete block walls, good windows that seal, not the crank open jalhousie type, and a roof that is strapped down to the house. Insulate the roof pitch, any attic gable walls and do not vent the attic.

An AC system sucking in 5 to 10% max outside air whenever the compressor runs will have good IAQ, and the pressure created will stop humid air from infiltrating in while the AC is running.

The construction techniques I described should be fairly tight and infiltration in the off cycle is greatly reduced.

You cannot pressurize with a minisplit, so perhaps consider a dehumidifier in particular a ventialting one.

If the stucture leaks like Swiss Cheese, then by all means put in the highest SEER minisplits if you want to, just have something on hand that responds to a rise in humidity like a dehumidifier.

[Abby Normal]

I deal with tropical humidity all the time, sorry if I sound anal about it

[Gary]

You are far from 'just a tech Gary', but yes your goal is to make them work the best.


And you are far from "just a design engineer". :)

Nor do I think of myself as just a tech. I take pride in being a service tech and the word "just" does not apply.

Over the years, I have shown a great many design engineers the error of their ways. They are not the infallible Gods they sometimes think they are.

My statement was by way of stating that my approach to problems is different than that of someone who designs systems, starting from the end product and working my way backwards, although we may in fact end up with the same conclusions.

[Gary]

I deal with tropical humidity all the time, sorry if I sound anal about it

It's not anal at all. Far too little emphasis is placed on humidity.

In our rush to provide higher energy efficient systems, humidity concerns were set aside or even ignored. As a result entire structures have been torn down due to problems with mold.

There is a cure, but the cure has a cost. Airflow strategies can resolve humidity problems, but part of the energy savings is sacrificed in the process. Removing moisture takes energy.

Unfortunately, a great many people in our industry place a low priority on humidity.

[Abby Normal]

My biggest point is to keep the humidity out in the first place

[Gary]

My biggest point is to keep the humidity out in the first place

A point on which I trust we all agree. :)

[Joybob]

Could I use cold water from my pool to refrigerate the rooms?

[Abby Normal]

there is no water cold enough in PR unless it is inside a chiller barrel :)

[Joybob]

No no, I meant using pool water to cool the condenser coil.

[Abby Normal]

i have seen heat pump pool heaters in the caribbean, a unit dedicated to do what you want, but to do what you have just descibed would take some engineering so that the heat could be dumped to the pool when needed, as well as dumped to trhe air. i would not advise it as I doubt the local talent is available to do the work.

[Joybob]

i have seen heat pump pool heaters in the caribbean, a unit dedicated to do what you want, but to do what you have just descibed would take some engineering so that the heat could be dumped to the pool when needed, as well as dumped to trhe air. i would not advise it as I doubt the local talent is available to do the work.

Seems like I just need to:
1. run PVC pipe in and out of the Pool
2. Modify the Heat Pump, remove the fan, Stick the condenser in a little box for water to flow around it.
3. Figure out a way to turn both the water pump and heat pump at the same time.