HVAC catch-22

tvrgeek

Scott
Corporate Member
Seems I did a decent job of insulating my shop. Tried to keep the load low. Put in a super efficient split system. Nice and cool. BUT, humid! Seems the variable speed compressors that are so super efficient don't always bring the evaporator cold enough for efficient de-humidification. Cool nights, so not running most of the time. When I set my temp down to 68, coils dropped to 50-ish from 65-ish. Probably more efficient to over cool than to add a dehumidifier. Back is the bad old days, coils ran 40 to 45 so they could really pump it out.
 

Bas

Recovering tool addict
Bas
Corporate Member
Definitely get a dehumidifier, that's a lot more efficient than trying to cool extra. Helps with keeping your tools free of rust as well.
 

Oka

Casey
Corporate Member
Depending on the unit you can program a de-humidification mode and duration.

Also, be careful not to introduce too much outside air. Most units are designed to handle 100% outside air. Most home units can get to about 20% outside air beyond that they cannot keep up.

Random trivia - A lot of the large food stores like Winn-Dixie's and alike will bring the fresh air (outside air) through the walk in cooler, and use it to de-humidify the air before they send out into the store. That is how they prevent the windows from fogging.

One last thing, the high efficiency unit does not describe how well the unit performs, only how efficient it converts the power to cooling.
 

tvrgeek

Scott
Corporate Member
Definitely get a dehumidifier, that's a lot more efficient than trying to cool extra. Helps with keeping your tools free of rust as well.
Disagree. A de-humidifier is a low efficiency AC where it blows the hot air back into the room. Very inefficient. I pay the price in my house with one in the crawl space. One of the $1000 jobs and no more efficient, and from what I see, no more reliable than the $200 ones. It has failed twice, and my GE portable failed. I am not sure of the reasoning for isolating a sealed crawl space with it's own de-humidifier and the included airflow using house air. I am beginning to think going house air , no difference as if it was a basement, is actually better. Shop is a slab, different problem. Ordering a small unit for the shop today.

Yes, it has a de-humidify mode, but it is still tied to the temp, so was not pumping any water. This morning, set @ 65, got to 66%. Still too high. Older AC ran 45 or so degrees on the coil. The new ones can be as warm as 65 so instead of 15 to 24 degrees air drop, less than 8. Longer cycle but lower current.

Unfortunately, right now outside air is much higher humidity. I am looking at an air to air changer for my house for quality of air, but the humidity being so high makes some of the paper element units not a good choice. Most residential HVAC guys do not know anything about them, yet SOP in bigger buildings.
 

Oka

Casey
Corporate Member
Well I guess how we stated was poorly articulated, the issue is around the level of how much de-humidification controls are programmable.

But to the 66% humidity that is pretty high, depending on the dew point that is a real challenge.

Last year, I installed some units for the Hilton Lobby in Honolulu, had a lot of the similar issues, eventually, we addressed with additional fans and longer dehumid mode, that kept the air moving which helped. Understand, not the same as this but the conditions as a shop, but the unit had a similar problem. We finally got it below 60% humidity which solved the issue.
 

tvrgeek

Scott
Corporate Member
Isolating the crawl space and installing a dehumidifier is a anti termite measure.
Why is this any more anti-termite and including the air with the house like it is a basement, keeping in the 50 to 55% range? My anti-termite program is chemical warfare. Termites are just as happy with dry wood as wet. Carpenter ants want wet wood. Different problem.

Shop target is 50 to 55%, same as my house. More than 55 you can have mold. Small unit on order. I actually like it closer to 45.

The point of this thread was the discovery the new super efficient systems work by running lower pressures and higher temps with longer cycles, so they are not as good at dehumidifying as the old high pressure fixed systems.
 

junquecol

Bruce
Senior User
Is your unit oversized? If so ,it will cool down to desired temp before moisture is removed. Also have you checked the suction and head pressures, after letting unit stabilize the air in shop?
 

tvrgeek

Scott
Corporate Member
No. Correctly sized. 12K unit. Being a heat pump, it needed to be big enough to heat in the winter. Again, this is the difference with the new variable speed compressors, so if the load is low, they run at lower speed, so lower pressure, so higher coil temps.
Got the portable dehumidifier and just plugged it in. I wonder if it catches up, if the split system can keep up so the less efficient dehumidifier won't need to run as much. I am also going to contact Mitsubishi as the problem is beyond the licensed company who installed it. I wonder of there is a sensor issue in the main controller.

I don't have gauges, plus these new units are charged by weight, not by pressure delta or temp delta. Another "feature of the variable speed compressors. No pressure specs in the service manual ( I found it online)

FWIW, 35 years ago I was certified for refrigeration as I had to redesign the condensing units in Seimens laser printers. We converted the from CFC to HCFC's. A lot has changed since then. GHG was new then ( R-409 now I think) so other than basic physics, I am totally out of date.
 

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