Oversized equipment and dehumidification

ChrisJ

So something is bothering me.

I often see that oversized equipment will not dehumidify sufficiently.

Why?

If you have a 2 ton system in a space and it runs for 100% of the time it will move the same amount of air as a 4 ton running 50% of the time.  So in theory the grossly oversized 4 ton should remove the same amount of water.

I guess on really cool days it could get where the 4 ton system doesn't run long enough to pull the evap down low enough but it seems like most of the time it would do fine especially with a txv.

I have limited exposure to systems so I'm asking for help.

The few I've seen with issues were not running the coil cold enough.

Matt_67

The equipment doesn't run long enough to get the coil cold enough to dehumidify well and actually drain off the coil - it might only run 4 or 5 minutes and the stat is satisfied. My neighbors condensing unit is not far from my bedroom window and his unit does this constantly. I suppose in theory by adjusting the cycles per hour you could get long enough run times but then that leads to uncomfortable temperature swings.

motoguy128

It’s coil size and coil temp

To remove water a 4 ton coil had 2x the surface area that needs to get wet. IF you had a plexiglass door on the coil you’d see that it takes 3-5 minutes to get fully wet and water to get the pan wet and start draining. To the percentage of time that a 4 ton coil is wet than a 2 ton with the same sensible load, will be much lower.

To level the paying field, you need ramp profiles on the blower to allow the coil to get cold an wet sooner. Although I’ve down this any it still performs poorly.

Also keep in mind that a modern TXV system set SH to around 8-14. So a percentage of the coil will always be dry. That portion is more significant on a oversized system because there are less air changes due to shorter run time. SO some air is effectively “bypassed” without any moisture removed. Add to that any leaks around the coil also are magnified.

Then there is the lineset. Every cycle it takes a little time to get the lineset coil and the lineset outdoors Inside the condenser is also sweating. This also becomes greater.

For all of those same reasons, a larger system will effectively be less efficient. Compounded by high static pressure on the same duct system.


Case in point. I’m cooling a 3600sqft all brick Victorian with just 4.5 tons cooling capacity. Conventional sizing would probably install 6 tons, Normal oversizing would be 7 tons. MY upstairs system is 2 stage and will often run for 8-16 hours straight holding 74F with a setback down to 72F for comfort overnight. indoor RH is 42-48%. I live in the SE Iowa, design is 91F, wet bulb 78F. For comparison Atlanta, GA is 92/74F and For Lauderdale, FL is the same at 91/78F.

Another home I owned in the same town had a 3 tone 2 stage system on a 1200sqft home. Grossly oversized. IT struggled even with slowing the blower and Blower delay profiles To keep Indoor RH under 55%. IT had a 4 tone coil “chasing SEER” Compounding the issue. A 2 ton or 1.5ton system would have been more efficient, cheaper to install as well. In that case a 13 SEER 2 ton would have been more efficient than a 16 SEER 3 ton And more comfortable.


Downsizing takes a leap of faith and the customer has to understand they can’t just turn down the temp and have it quickly change. It takes 2-4 hours for my upstairs to cool off by 1F. Even if it’s 60F outdoors and there’s no sensible gain. I have hundreds of tons of brick and plaster to cool off.

MY whole house if modern would have maybe 10 tons of drywall. With plaster than number is probably around 50 tons when dry. The poor plasterers probably hauled around 70 tons wet. Then compared OSB sheathing and siding at maybe another 10 tons, compared to I’d guess 100 tons of brick and mortar or more.

100 tons of brick has the same specific heat as 20 tons Of nearly 5000 gallons of water. IT no wonder brick homes takes days to fully cool off or heat up with an outside weather change.

ChrisJ

@motoguy128 you lost me on the lineset.  I have no idea what was meant there?  I'm not sure how much of an effect a properly sized lineset has on the coils behavior.  Though it's best to keep the suction line insulated to prevent sweating and I insulated my entire liquid line due to sun exposure and attic concerns. Suction line I used 3/4" thick insulation.


Duct system is moot because it should always be sized properly to the system.  I can understand the point but I feel it's a separate subject.

Btw, I have a 3 ton 2 stage 16 seer system in 1600sqft and have no problem keeping my humidity 46-48% most of the time.  It does get into the low 50s during extreme conditions like a dew point of 74+.  The house is too drafty.

But temperature matters too.  I do 72 during the day and 69 at night.  That means my system runs more than someone that wants 75 or 78.

SH I'm actually running on the low side, 6-8 degrees or so.   I like it that way.

motoguy128

Id freeze at 69 at night.  It won’t get there anyway in hotter weather.  Barely holds 73 if it’s been sunny all day.  


3 Ton on 1600 isn’t really oversized in lower mass typical 1950s-present construction.  2.5tom was probably spot on but with 2 stage you go bigger.   Easy to tweak airflow closet to 350/ton for 23-25f temp drop and you’ll have great latent capacity.  If your closer to the “nominal” 20f temp temp drop, the coil probably isn’t getting fully wet.

motoguy128

The point about the lineset is that’s it’s 5-10lbs of copper that you have to cool off every cycle. The shorter the cycle, the more energy is wasted cooling it off and that’s all time NOT spent with a wet coil.  It might only be 10-20 seconds.  But it all adds up just a little.  

ChrisJ

motoguy128 said:

Id freeze at 69 at night.  It won’t get there anyway in hotter weather.  Barely holds 73 if it’s been sunny all day.  


3 Ton on 1600 isn’t really oversized in lower mass typical 1950s-present construction.  2.5tom was probably spot on but with 2 stage you go bigger.   Easy to tweak airflow closet to 350/ton for 23-25f temp drop and you’ll have great latent capacity.  If your closer to the “nominal” 20f temp temp drop, the coil probably isn’t getting fully wet.

Obviously it depends on the humidity in here but typically what I've seen is 18-19 drop in high stage and 20-23 in low stage.  I'm running closer to 400 per ton.  I tried lower last year but I like the extra sensible on hot days and the thermostat can slow the blower down a lot when it needs to.  

I think in dehu mode it runs 330 per ton.

I have a freeze stat on it as well as CCH and a fan controller for low ambient.   I often run it when it's in the low 50s out and I didn't really like it without the extras. 

jumper

Even more complicated is chilled water A/C. Then stuff like coil circuiting and chilled water temperature comes into play. Modern energy saving variable flow•varying effective temperature•variable volume run into humidity issues.

EBEBRATT-Ed

Yeah, Some of the new technology, chilled beams, variable flow bla bla has not been perfected yet. Just ask the people in the building where its not working