A question on managing humidity with air-conditioning

[wrenchtech]

I was wondering about people‘s experiences with running their air conditioners during the summer months in humid climates while stationary (as opposed to when in motion). Were you able to control humidity effectively without over-cooling your bus? And what kind of air conditioning you were using, rooftop units, mini splits, etc. A lot of controlling humidity has to do with the air-tightness of the space.. My initial thought is that without a lot of work done on the windows and doors, that is not often very good. I would like to hear what your experience has been.

[CHEESE_WAGON]

Most rooftop units and mini-splits are thermostatically controlled to prevent over-cooling. However, the fans generally run non-stop when the unit is on whether the compressor is running or not.

[wrenchtech]

Originally Posted by CHEESE_WAGON

Most rooftop units and mini-splits are thermostatically controlled to prevent over-cooling. However, the fans generally run non-stop when the unit is on whether the compressor is running or not.

I'm thinking this might be an issue that has not gotten that much attention. I'm thinking about HVAC issues from the perspective of someone who would be living in their bus. As I gain more and more experience with my current bus more and more questions come up. I really don't like humid spaces and I am mold sensitive so I am interested in keeping any space that I will be living in below 60% humidity. But just as I learned that the winter weather moves easily through my windows and so does the humidity in the warmer weather. I am slowly tightening up my bus. I removed five of my large 54 x 28" windows a few weeks ago and replaced them with some nice 18 gauge sheet-metal. I will be working to get the remaining windows to be less leaky. Sealing the doors, especially the main door is high on my agenda as well. I also have to think about making the right choices with regard to the HVAC equipment. I am retaining the factory air-conditioning and I'm planning to supplement it with what will probably be a pair of mini split units. It seems like the biggest question with any HVAC system is the question of sizing it correctly for to the climate and the structure. It seems that this is crucial because if you oversize your system and it cools your structure too quickly then it won't be able to do a good job of dehumidifying. Instead, if I understand this correctly, you want your AC to run almost continuously and to do so without over-cooling your space. When you have achieved this happy state you will also have achieved the conditions necessary for optimum dehumidifying. Good HVAC contractors do a lot more than just sell and install equipment, they really have to have some pretty good knowledge about the science and technology of what they're doing. I'm thinking it couldn't hurt to try to apply some of that knowledge to what we are doing with buses. I'm hoping others have also thought about this and can enlighten me.

Or, as I keep saying, maybe I just need to move to a place where the weather is more moderate than it is here in Southeastern Wisconsin.

[cadillackid]

minisplits are the best option for humidity control , they will lower their fan speeds before lowering the compressor speed.. the result is ice cold air whiuch removes humidity well.. you can also run your minisplit forced into low fan speed which helps.. it will run the fan all the time which can cause humidity isues on cool but damp days.. where you dont need much cooling at all but its raining outside..

[wrenchtech]

[QUOTE=cadillackid;406000]minisplits are the best option for humidity control , they will lower their fan speeds before lowering the compressor speed.. the result is ice cold air whiuch removes humidity well.. you can also run your minisplit forced into low fan speed which helps.. it will run the fan all the time which can cause humidity isues on cool but damp days.. where you dont need much cooling at all but its raining outside..[/QUOTE

Thanks for that info. They occasionally have some good HVAC discussion over at greenbuildingadvisor.com They are followers of the Building Science Corp. and are evangelists for building houses in the best way possible using the latest tools and techniques. It is pretty interesting.

With regard to sizing my HVAC system. I think it will be fairly unlikely that I over size it, in part because of space and electrical limitations. Yeah because I’m having to be very frugal with my budget.

[Wnchstr156]

Being an HVACR tech for commercial buildings I can tell you that any AC needs to be charged correctly to dehumidify. Also 99% of people do not charge these to the correct super heat and sub cooling. The manufacturer usually lists what it should be on the unit. I can't tell you the amount of times I have seen refrigerant charge that seems fine until the super heat is calculated. Once it is adjusted customers tell me it works better than it ever has.

[cadillackid]

Originally Posted by Wnchstr156

Being an HVACR tech for commercial buildings I can tell you that any AC needs to be charged correctly to dehumidify. Also 99% of people do not charge these to the correct super heat and sub cooling. The manufacturer usually lists what it should be on the unit. I can't tell you the amount of times I have seen refrigerant charge that seems fine until the super heat is calculated. Once it is adjusted customers tell me it works better than it ever has.

Minisplits are a bit tricky because they vary both the compressor output and employ an EEV. Typically you can’t monitor head pressure . It’s rare to find a service manual that lists the superheat for them. They are a bit forgiving on overcharge due to having an accumulator but nevertheless I usually end up evac and weigh in a recommended charge. I’ve played with mine going over and under on charge and measuring performance. Thus far it seems like factory charge amount gives me best results.

The service books do usually tell you how much refrigerant to add for long line sets

An undercharged mini will cool decent but have very high sun cool in heat mode. A severely overcharged mini will trigger the computer to ramp down or periodically shut off the compressor ( it will be pulling high current).. takes quite a bit over to reach this point.

Superheat can be tested and charged for easier on the bus road AC. Superheat is usually set lower than on a commercial unit. These evaporators flow quite a bit of liquid at capacity more than you would ever flow in a fixed-system scenerio. Those are small compact coils with multi pass and very high volume blowers. Coil temperatures are often just above freezing.
This allows for the fluctuating capacity of the compressor to still maintain decent capacity for short idle periods..

[Native]

Wrenchtech, have you considered a dehumidifier?