Nervous about 2-ton AC for a 3400 sq ft zone in a new house

[mxarey]

Having trouble getting subs to pay attention to what my new house needs. Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit. I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows. I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain. I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

Anyone have any experience with this sort of situation? All advice gratefully appreciated.

Mark

[jtrammel]

Having trouble getting subs to pay attention to what my new house needs. Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit. I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows. I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain. I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

Anyone have any experience with this sort of situation? All advice gratefully appreciated.

Mark

I would look into an IQ drive system from Nordyne (maytag, broan, fridgidaire) they are completely variable from 30%-118% of size equipment and are very good at keeping a constant temp and rh with any outdoor temp. Also the trane xl20i would be another option. I think the 3 ton would short cycle too much and not dehumidify at all because it would satisfy the temp before the coil even got loaded and began removing humidity.

[beenthere]

I'm surprised that a 3400 sq ft foamed house comes out to 2.5 tons. A member here "paul42" has a 4000 sq ft house in Texas that came out to 2 tons. He report that when it hit 104 outside it still maintained 74 degrees inside.

http://hvac-talk.com/vbb/showthread....C-is-oversized

[catmanacman]

Its my opinion that all foamed houses need a dehumidifier and fresh air ventilation. I woud think the guy saying 4 ton just does not believe a foamed house is that efficient and is worried it wont do the job .go with a2 ton 2 stage unit

[dan sw fl]

Having trouble getting subs to pay attention to what my new house needs.
Two different subs say they ran a Manual J on the same plans, but one recommends a 2.5 ton 2-stage unit and one a 4 ton 2-stage unit.

I decided I better check, so I bought software and did Manual J calcs on the zone covering the lower two floors of this 3-1/2 floor, tight house with spray foam insulation and good windows.

I get 18,500 Btus of sensible heat gain and 3,500 Btus of latent gain.
I like Ruud/Rheem equipment, and when I look at the specs for their hardware a 2-ton unit does look like enough even for the very hot, humid days we get here in Virginia.

Anyone have any experience with this sort of situation? All advice gratefully appreciated. Mark

BOTH recommendations are Really Suspect.
Where does one get a 2.5-Ton 2 stage?
(Or is 2.5 typo, meant 2.0)

18,500 BTU/HR Sensible might be a bit low in my opinion.
Your calc - How many total square feet of windows?
Square feet facing each direction?
SHGC for 'good' windows means 0._ _?
U-value 0._ _

EXCELLENT WINDOWS
SHGC < 0.52
U-VALUE < 0.35
If sensible load is < 20,000 BTU/hr with typical 350 square footage window area (3,400 sq foot residence),
the windows must be excellent.

[tedkidd]

Its my opinion that all foamed houses need a dehumidifier and fresh air ventilation. I woud think the guy saying 4 ton just does not believe a foamed house is that efficient and is worried it wont do the job .go with a2 ton 2 stage unit

X2

Or iaq per JT, or carrier GREENSPEED.

Or ducted mini splits. How are you managing fresh air? How are you insuring qc on the foam work?

Btw, nice job uncovering bs work.

[teddy bear]

I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.

But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.

Anyone have any experience with this sort of situation? All advice gratefully appreciated.

Mark

You have figured out the great mystry of comfort in modern homes. Most a/c designers do not understand this problem.
The senisble cooling load of homes in green grass climates varies from 80% on a design day to 0% during wet, cool day. On the otherhand, the latent load is 20% of the total cooling load on a desigh day to 100% during wet cool weather. Yet our a/c systems operate with 75% sensible/25% latent cooling ratio. The tools used are slowing capacity and over-cooling. The new varible speed systems, while they work better are not the solution. You will end up with a cold damp home during cool wet weather. Also a/c designers are avoiding the necessary fresh air ventilation that all homes need to purge indoor pollutants, renew oxygen, and remove odors. In addition to the latent load in infiltrating/ventilation fresh air, the latent cooling from the occupants occurs throughout the cooling load range. 100 cfm of 70^F dew point fresh air plus the moisture from 4 occupants is 7,000 btus per hour of latent cooling with very little sensible cooling.
I will attach data from the a home with a variable speed a/c slow the inability to maintain <50%RH. The owner spared no expense to get have the best, yet had high humidity in the home and cold temps during certain weather.
I will follow later with data.
The solution is simple, add a whole house dehumidifier that compliments the home's load profile and a/c preformance to maintain the customer's desires. Typically, occupants want 70-78^F, <50%RH. A properly setup single speed a/c with a sized WH Dehu will maintain <50%RH during all of the ranges including no a/c during unoocupied times.
The total installed cost is comparable to the VS a/cs. In addition all quality homes should have mechanics to provide the amount of fresh filtered make-up air to purge indoor polutants and renew oxygen at a minimum when occupied. Whole house dehus like the Ultra-Aire unit provide fresh air ventilation as an option.
Again congrats on figuring out the variableness of the sensible cooling loads, while having a steady latient load.
Regards TB

[vstech]

have you considered a Heatpump water heater in the home to provide humidity control when design conditions are low? or if you have one, are you factoring it's cooling/dehumidification in the calc?

[54regcab]

When figuring the latent capacity on the 2 ton unit did you use low speed?

[alex_in_fl]

What Teddybear said.

[54regcab]

http://optimalbuilding.com/files/ele...n_the_room.pdf

[teddy bear]

VS AC RH Control .pdf

The varible speed a/c do better at removing moisture than a single a/cbut are unable to maintain <50%RH during low/no sensible cooling loads. The attached data illistrates the problem of not being able to maintaining <50%RH during should seasons and wet cool weather.
While adding a small whole house dehu even with fresh air ventilation easily maintains <50%RH throughout the home.
Regards TB


Sept WI Basement Ultra Aire 70 .pdf

[jtrammel]

VS AC RH Control .pdf

The varible speed a/c do better at removing moisture than a single a/cbut are unable to maintain <50%RH during low/no sensible cooling loads. The attached data illistrates the problem of not being able to maintaining <50%RH during should seasons and wet cool weather.
While adding a small whole house dehu even with fresh air ventilation easily maintains <50%RH throughout the home.
Regards TB


Sept WI Basement Ultra Aire 70 .pdf

I agree that fresh air is needed in any home and controlled ventilation is a must in a tight home. I also agree that dehus are great. Although when it is cooler out outside rh is lower and internal loads alone in milder weather will produce enough gain in a typical house for a true variable capacity hvac system such as the iq drive or mini split to run on low almost constantly thus removing enough humidity to keep comfortable at a reasonable temp setting. These higher end systems also have dehumidify functions that will run the unit long enough to maintain rh.