In order to feel comfortable in the cooling season conditioned air has to be at the right air temperature AND relative humidity. Conventional AC dehumidifies when it is running but it is thermostatically controlled by air temperature only. Which means that when air temperature is reached the AC turns off and stops dehumidifying. Not a big problem if you live in an arid climate such as Pheonix but if you live in New England where the summers are very humid and not THAT hot then you most likely will never get that perfect comfort combination of cool and dry air.

To exacerbate the problem further a recent North East utility study determined that 100% of AC systems are over designed and that more than 50% are over designed by more than 100%. Which is why summer AC in Connecticut (my home state) is typically cold and dank, (I hate it). Why does this happen? For example; let's take a typical July day, 86 degrees and 95% relative humidity, put the AC on and the system blasts on at 100% capacity and quickly lowers the air temperature and doesn't cycle long enough to dehumidify. Now the air temperature is OK but the occupant is still not comfortable because the air is too humid, so what do they do? Right, they lower the thermostat some more to make the air even colder but it is still too humid. It is actually impossible for the conventional AC system to achieve comfortable conditions.

Bottom line on conventional AC is that is works poorly most of the time. Primarily because it cannot load match and therefor can't cycle properly to control air temperature AND humidity at the same time. My recommendation is to switch to an inverter driven air source heat pump that can modulate to always match the load and actively dehumidify (many actually have a dry mode). This type of operation not only optimizes comfort but greatly contributes to overall system efficiency that is not reflected in the outdated SEER rating system that only measures systems at full capacity working only to lower air temperature. An added bonus is that inverter heat pumps also work amazingly well in heating mode too. Let's stop trying to even make conventional AC work and move on to the next generation of HVAC equipment that does the job right.

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Tags: AC, air, comfort, dehumidification, heat, indoor, pump


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Comment by Bob Blanchette on May 28, 2012 at 12:20pm

HVAC contractors have an issue with LUX thermostats because of the high failure rate, we change out a LOT of bad ones. Contractors could put MORE in their pocket by installing LUX thermostats, well until they have to run a free service call to replace it.. Perhaps the newer stuff from LUX is better, at least they have common terminals on their pro stuff. Looks like they are headed in the right direction with their recalls on the front page.

Then there is Hunter, who requires batteries for their MECHANICAL thermostat. Pure junk IMHO. I like the Honeywell products but I must admit their website is a bit of a mess. WR is my current favorite with their "big blue" lineup.

Comment by Bob Blanchette on May 28, 2012 at 6:56am

Where did you read the review at? Do you suspect Pexsupply is deleting bad reviews?

Here's what HVAC contractors think about LUX: HVAC TALK

Comment by Bob Blanchette on May 28, 2012 at 5:29am

IMHO the WR 1F95-1291 is a MUCH better stat than the LUXpro. It's easily worth the extra $30.

If somebody was REALLY sharp they could make a thermostat with built in enthalpy compensation. It would reduce/increase setpoint based on the actual humidity in the space, and could even control the blower speed of the HVAC system.

Comment by Bob Blanchette on May 28, 2012 at 5:15am

I'd like to see more thermostats implement "minimum run time" as a spec instead of a temperature spread or cycles per hour. Helps a lot with short cycling during low load conditions while not overshooting under moderate/high load conditions. Energate is the only stat I've seen doing this and it works well IMHO. Over time more thermostats will go to communicating technology such as Zigbee to talk to the power meter and adjust temperature based on utility rates. If you are going to use a programmable stat, you might as well get one that actually saves money by talking to your smart meter. We're already seeing it in our area, the utility is installing them for free in exchange for switching ot VPP pricing.

I have respect for the thermostat companies that have been around awhile and still have operating thermostats that are 20+ years old. Most WR and Honeywell stuff works even after 2 decades in the field, the same can't be said for most other brands. Lux has improved over the last 20yrs but they still have a ways to go in the reliability department. IMHO any quality thermostat should at least have a "C" terminal, and it should be HOOKED UP !! So many installs I see are simply running off battery only power, even though there is a common where already in the wall. Yes, most of the cheap "big box" models DO NOT have a common terminal, although HW is starting to put them on ALL their stats, not just their pro line. WR has a C terminal on ALL their stats that I've seen, but they don't sell @ the big box stores. I've seen a few LUX stats (I think it was a pro model) with a C terminal, but nobody ever hooks it up. Hunter = good fans, bad thermostats. Never have seen a C terminal on one.

Comment by Bob Blanchette on May 27, 2012 at 6:51pm

As for the Luxpro thermostats, I'm not impressed with the ones I've seen in the field. White Rodgers and Honeywell make thermidistats also, and they are better quality IMHO.

Comment by Bob Blanchette on May 27, 2012 at 6:48pm

Steven, in the real world it doesn't happen that way with the low airflow. 2 things happen to prevent this. One is the high latent load keeps evaporator pressures high. The other is the unit will rarely cycle longer than 10 minutes at a time during times of low load conditions. As a safety measure you could always add a 10 minute delay timer to "jump out" the humidistat and call for high speed blower after 10 minutes of A/C operation.

Comment by Steven Lewis on May 26, 2012 at 11:36am

GE and other heat pump water heaters will dehumidify the area they are located in and use the heat gathered to heat the water. 

Bob, is right Trane VS systems have the capability to vary fan speed to the humidity and work very well in my experience.  Running a "Jury Rigged Switch" to lower fan speed on a demand for cooling without paying attention to the run time will cause problems if the unit doesnt have the ability to step up the fan over time.  Coils will start to freeze in approx 10 minutes with low airflow while loading the condensor coil with the extra refrigerant, lowering the efficiency.  With the newer low volume condesor coils this will cause the system to load the cond coil and damage the compressor. 


Get the load right and look at staged a/c units is still the best solution today but tommorrow will bring other options.

Comment by Bob Blanchette on May 26, 2012 at 10:09am

Top of the line variable speed systems do the same thing, reduce blower speed in order to help with latent capacity. Not a "jerry rig", they even have a specific terminal on the board for the $20 humidistat to be hooked up to :)

Of course if there is no sensible load to trigger the system even the $10,000 inverter system won't help. In that case the $200 dehumidifier is the way to go :)

Comment by Bob Blanchette on May 26, 2012 at 5:07am

Tom, it does in fact work well. In times of high outdoor humidity it stays on low speed mode, medium it switches, and when dry it stays on high. Interestingly outdoor humidity seems to have more of an influence than AC run time.. Well worth the $30 in parts and 30 minutes to hook it up, it did take some experimenting to determine the best humidity "balance point", where the blower only ran on low speed when needed.

Comment by Bob Blanchette on May 25, 2012 at 8:01pm

The problem is COST. Few are willing to shell out $10,000 for an inverter system, especially in primarily heating climates. Much of the desired effect can be achieved with a $20 humidistat and possibly a $10 SPDT relay. Have an interior humidistat switch the blower speed based on latent load. Newer system that have a board can use the humidistat break the connection to the "cool speed" Y connection and have the fan run on the slower "fan only" G speed during times of high humidity. Older systems can use a SPDT relay to switch from high to low based on humidity. IMHO great solutions that won't break the bank and consumers will actually be willing to pay for.

Battling high humidity levels in comfort cooling systems

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