I live in a hot, humid climate. My Mitsubishi mini splits do a great job of dehumidification compared to single-speed systems. A lot of people are left with the perception that they work as well as a dehumidifier. That is not the case. I still have cool, humid days when a dehumidifier is desirable.

In order to provide the best performance for multi-mini split systems, some care should be taken to match the indoor units as best as possible with the outdoor units in rooms that will be at partial load. The ability to throttle down the compressor to minimal output is important and the multis will not go as low as a 1:1 compressor:indoor unit will.

Something sounds a little strange about your system. Your assumption about poor dehumidification is correct but 2.5 tons doesn't sound way oversized for 2200sq ft. Where are you located? Is your house unusually well insulated/sealed? Are your bathrooms/kitchen/laundry room vented to the outside? How many people are living in the house? A multistage system might help but you should try to figure out if something else is amiss before throwing away your old system

PS: Leaving the fan on all the time will worsen your summer humidity problem.

Thanks for the responses. We're not throwing out the old system, I'm just planning ahead for when it fails. We installed the unit in Jan. 2001 and nothing lasts forever. The cheapest option would be to just replace the compressor when it fails with a 2 ton R22 Copeland scroll model, assuming the coil is still in good shape. Our fan coil is compatible with this and I see that they are still for sale online for around $700-$800.

Our house is a split level in S. Indiana. It's a common style from the 60s-80s and is very energy efficient by its nature, that being, the finished lower level is partially below ground. There are shared rigid metal ducts between the floors; registers downstairs are on the ceiling and upstairs are on the floor. R42 insulation in attic and newish two pane windows with low E film. Air infiltration is not terrible. We are maintaining app. 50% RH at 68F with the temperature outside 5F right now. 3 people and some cats. Bathrooms/stove have exhaust fans and are used when needed.

There is no door between the two levels (and one won't be installed), so the cold air falls and there is 5-7 degrees F temp. stratification in the summer without a low speed coil fan turned on. Yes, we leaned long ago to turn off the fan in the summer due to higher RH in the house. Unfortunately, there is no way to bypass the indoor coil for low speed airflow in the summer. That would be a nice option. I'm a big fan of the low speed "always on" function for a comfortable home. Aprilaire filtration is used. Of course, all of this comes from a single speed compressor mentality, circa 2001.

I guess the question is this: will a new variable speed compressor operate "on" at a low capacity constantly to provide proper humidity levels and less air stratification than our current system in the summertime?

I see that Emmerson has released the 2nd generation high efficiency Copeland scroll compressor: 25 Seer, 13 HSPF. Awesome! Now if Carrier can address NVH with their expanding/contracting big outdoor metal boxes during the wintertime, then I'm interested.

This post was edited by newtalk on Thu, Jan 8, 15 at 10:09

"I guess the question is this: will a new variable speed compressor operate "on" at a low capacity constantly to provide proper humidity levels and less air stratification than our current system in the summertime?"

Well, that's a bit of a loaded question. Yes, a smaller A/C (by virtue of a smaller size or first stage operation) dehumidifies better. Alternatively, if the system has "dehumidify priority" it will dehumidify the house to the humidity setpoint no matter how cold the house gets (and you probably don't want that). If you leave the fan on constantly at a low speed, with no compressor operation, it may produce a more uniform temperature throughout the house, but at the expense of increased humidity (the fan is blowing and evaporating condensate off the coil and drain pan instead of letting it drain out). Sounds like you may need a whole house dehumidification system.

Just to clarify, the RH levels in the summer are acceptable when the low speed fan is turned off. It's the air stratification and floor to floor temp. differential that we would like to improve upon. Why not circulate that colder air upstairs instead of letting it sit on the lower level floor? We will pass on a whole house dehumidifier, as it's not really necessary.

After calculating electric use for heating, cooling, and baseload (all electric house) for the last year, and converting kWhs to BTUs, it appears that our total heating and cooling loads are app. 10K-16K BTUs/hr for the various months. I will refine this spreadsheet in the next year. I may even purchase a monitor to log the on times for the compressor.

They missed by a mile when they sized us at 2.5 tons in 2001. 1.0- 1.5 tons is correct. A 1.5 ton single speed R22 compressor is still available and is compatible with the air handler. Can this be done and is it advisable? Staged 15 kWh strip heat already installed would make up for the occasional winter BTU shortfall.

I guess many people are not familiar with the concept of a split-level home where the whole lower level is 4.5 feet below grade? This is a large, long established HVAC company that did the work. It's amazing that this kind of stuff happens.

Live and learn. Next time will be better. I'm waiting for Copeland to release a 1-1.5 ton variable speed, high HSPF rated, scroll compressor. If not available, then we will go with a rotary compressor HP of the proper size and with 5 stage or variable speed capability, when it comes time to upgrade.

I don't have personal experience with variable speed ducted cooling, compressors or blowers. I doubt that a mini split ducted system is substantially different in its performance characteristics, save capacity, from a "maxi split" system. By that, I mean the humidity control and their ability to throttle to partial capacity. My perception is that the cooling world is changing very rapidly. Daikin, for example, is now producing compressors with DC motors and Swing compressors. They may allow output to be cut to even lower levels than today's inverter-driven AC compressors.

It is a no-brainer that a large, climate-dependent imbalance between heating capacity and cooling capacity can be fixed with variable-speed equipment. The imbalance between upstairs and down is likely best attacked with additional ducts or manual dampers on the ducts, new or old.

I don't think that I would like 50-60%. I fell kind of clammy when the RH goes above 50%. My house is leaky and in a humid environment. I generally have no problem staying below, way below, 50% with my mini splits.

Yes, hopefully more improvements and convergence between mini-splits and maxi-splits in the next few years. Less energy use, greater efficiency, and better comfort dialed-in to any location in the home.

I'm intrigued by the Panasonic system using standard mini-split wall unit(s) combined with advanced, modular "bathroom" fan(s) to move air through ducts to other rooms in the home. The return air flows back to the wall unit, so only one condensate drain line and one evaporator coil per floor. This might work with our setup, although I would have to have separate the combined duct runs for each floor. Two small systems instead of one larger system. No large air handler in the laundry room is a big plus!

As for now, the reliable Copeland Scroll compressor on the Carrier system keeps going strong. Impressive longevity.

"modular "bathroom" fan(s"

Are you proposing to cool and heat the whole house with one or two units and jump ducts between rooms? I don't think that will work except maybe in a very mild climate and then not well.

It's not really a "bathroom" fan, ventilation fan is a better word for it. Panasonic seems to lump all ventilation fans together on their website.

The Whisper Ceiling fan with 4" or 6" duct connection is more of what I mean. Anyway, with up to 390 CFM capacity, these fans can move some air. For reference, the comfort settings of the latest high efficiency Carrier split products have 500 CFM airflow for the 2 ton model.

Panasonic has 9K or 12K BTU mini-split units, and combined with such ventilation fans to distribute the air to various rooms, it might be a solution. There is no electric backup heat with this type of setup. I will be upping attic insulation to R60, new doors, and possibly solar thermal heat supplement in the near future, so strip heat is not really needed. It only comes on below 5F right now, and even then not all the time.

Watch the "Total Home Comfort Solution" video and tell me what you think.

I've been using a box fan to move air between rooms and it keeps them within a few degrees until is in single digits. Are like the little ones put into the wall to circulate air?