Space Pak AC - is this an acceptable level of performance?

Motz

Greetings to all the AC experts here on the Wall.

I recently had a second Space Pak AC system installed in my home. The first was installed in 2012 to cover the 2nd floor of a big house, the new system cools the first floor and replaces a number of window AC units. Both systems were installed by the same vendor.

The performance between the two systems is very different. The 2nd floor system has no problem coping with very hot days and will easily drop the air temp by 1* per hour or more. It cycles on and off about every 20 minutes or so.

The first floor system runs for 12 hours or more without cycling off and is unable to drop the air temp by even 0.5* per hour on a day of moderate summer temperatures.

Example:

System is turned on at 8:30 am, the interior air temp is 78*, the target temp on the thermostat is 75*, the outdoor air temp is 75*. After running non-stop for over 10 hours, the interior air temp is 74.4*. The outdoor high for the day was 84*.

Therefore: On a moderate summer day with a high in the mid 80’s, the continuously running 1st floor system lowered the air temp from 78* to 74.4*, a decrease of 3.6*. That averages out to a drop in air temp of 0.36* per hour. i.e. a temp lowered by about 1/3rd of a degree per hour.

Meanwhile, the 2nd floor system is capable of dropping the air temp on that floor by more that 1* per hour and routinely cycles on and off.

Conclusion:

My perception is that the 1st floor system lacks sufficient cooling capacity or contains some other flaw as the performance is vastly inferior compared to the 2nd floor system. The vendor states that the new system is ‘functioning as designed’ and will not address the inferior performance of the new system.

Are there any comments on this disagreement? Thank you for your attention to my problem.

Jamie Hall

Obvious question: are the system's cooling units (compressor etc.) identical?

Ironman

Proper Cooling capacity is NOT determined by how many degrees per hour a system can lower the temp of a structure since that number will continually vary.

The real question is: does the system maintain a proper indoor temp when it's operating at design temp outside (90 -95*)? If it does, then it's sufficiently sized.

Since we're not on site, it's impossible to tell if the unit's operating correctly, but if it's maintaing indoor temp, it may be that the other system is oversized.

EBEBRATT-Ed

you stated that your outdoor air is 75, indoor air at 78 and takes 10 hours to drop the indoor air to 74. yeah, something is wrong. Were you running the 2d floor unit also at this time??

I would suspect the first floor unit should be smaller than the second floor with a cool basement below most of your load comes from the roof. Maybe they undersized the FF unit.

Did they do a load calculation??

Motz

Ed,

Yes, the 2nd floor unit was running intermittently during the same period.

Yes, they did a load calc, which is part of why the vendor thinks everything ought to be ok despite all the data to the contrary.

I to suspect the 1st floor system is undersized and the vendor does not wish to address this possibility.

Thanks for your input.

Motz

Yes, both the compressors are 2 ton.

Also, perhaps I ought to have mentioned that the first floor is between 1.5 and 2 times the square footage of the 2nd floor and probably 3 times the cubic air volume. i.e. There is a much larger volume of air to be cooled on the first floor.

Thanks again for the feedback.

GBart

Motz said:

Yes, both the compressors are 2 ton.

Also, perhaps I ought to have mentioned that the first floor is between 1.5 and 2 times the square footage of the 2nd floor and probably 3 times the cubic air volume. i.e. There is a much larger volume of air to be cooled on the first floor.

Thanks again for the feedback.

if that is true they ought not to be the same size, also bear in mind that all of our equipment is made to maintain temperature, leave it set at 74 and leave it alone, don't allow the temp to go 80 or whatever then turn it on, if it won't maintain 74 something is wrong

Jamie Hall

1..5 to 2 times the area and 3 times the volume and both units are the same size? And you seriously expect them to perform similarly?? Ah... no.

Ironman

There's really not enough info to determine if the size is correct.

First, as I mentioned previously, none of us are on site to know if the system is designed, installed and commissioned properly. How are we, or you, to know if it's properly charged? Etc, etc, etc.

Second, you can't measure it by the performance of the other system. Again, that system may very well be oversized. Given the fact that 2 tons is the smallest size for most single zone high velocity, that might have been their only choice. The 2 ton system for the lower floor MAY actually be properly sized, but if it's compared to the other system which may be oversized, it's not gonna cool as quickly.

Third, and as mentioned, a properly sized a/c system is designed to be left at one temp, not turned off then on when the structure has reached 80* inside.

Do have the load calc from the contractor? If not, please get it and post it so we can see it.

Also, ask him for the following data from the commissioning:
1. Indoor dry bulb and wet bulb temps
2. Outdoor ambient temp
3. Suction pressure, head pressure
4. Sub cooling and superheat
5. Indoor air flow cfm
6. How many duct outlets and their sizes (2 1/2" or 2")?
7. Return air duct size?

unclejohn

Unit is sized by load not room size. Are there the same amount of air outlets on both floors?

Motz

Almost, 13 upstairs, 14 downstairs. I will see if can get the load calc and other data from the vendor.

Thanks again for the input.

GBart

Some other things to look for, does the lower level have a lot of glass, windows facing the Sun perhaps?? Are both units in the attic, hi velocity systems lose on long runs, from the Space Pak info-- The most important rule of thumb when
installing a SpacePak system is having the proper
number of outlets. Six (6) to Seven (7) outlets per ton are
recommended for optimal 35-40 CFM airflow from each
outlet under normal conditions to maximize aspiration. Supply Duct – Ideally, all runs should be as equal in length
as possible. Keep the 2" duct length between 9 ft. and 30
ft. for best performance. The longer the run, the lower the
CFM capacity.

Motz

re: "does the lower level have a lot of glass".

In a word, YES.

2nd floor has 10 windows of about 41 sq ft in area.
1st floor has 29 windows of about 216 sq ft in area.

Is this a big deal from a load calc perspective?

Thanks again for your expertise.

Motz.

Ironman

It's a huge difference! Glass is the largest percentage of heat gain of the building's envelope. If it's South, SW, or SE facing, figure figure 80 btus per sq. foot when the sun's striking it. East or West facing, 120 btus per sq. foot when the sun is full on it.

Those numbers can vary a lot depending upon shading, angle of the sun, e value, time of day, etc. But for any of those directions and variables, I'd allow no less than 60 btus per sq. foot of glass.

GBart

Wow and BINGO, you just probably found your problem.

you can try doing your own load calc with this

http://www.loadcalc.net/

GBart

There's a company near me they always run ads in the local paper that say " get AC now, 1500 sq ft home 1.5 ton, 2000 sq ft home 2 ton, etc etc, it doesn't work that way.

GBart

Like Ironman said most use 60btu/hr per sq ft, this is off an engineering site, if it the windows are calculated in the load calc program then it's already factored in providing the correct info was put in. Usually the programs ask for type of window, single pane, single pane with storm, double pane, etc.

The theoretical model and calculation of this number is so complex that you might just want some rules of thumb. Here are six:
Use 60 btu/hr / square foot of window, and skip the rest.

Always sunny, curtains usually open, majority of windows get sun during hot parts of the day, use 120 btu/hr / square foot of window.

Always sunny, curtains usually shut, majority of windows don't get sun during hot parts of day, use 60 btu/hr / square foot of window.

Not always sunny, curtains usually open, majority of windows get sun during hot parts of day, use 60 btu/hr / square foot of window.

Not always sunny, curtains usually shut, majority of windows don't get sun during hot parts of day, use 40 btu/hr / square foot of window.

Clear skylight, use 150 btu/hr / square foot of skylight.

41 x 60 = 2,460
216 x 60 = 12,960

you're short 1 ton of cooling

mikeg2015

WHat’s your location? In much of the country “design” tempo is is 90F or lower. Sounds like its sized perfectly.

the real question is whether it’s removing humidity. The system is intended ot make it comfortable at 74-76F where a conventional system would need to be 72-74F, That saves energy.

My upstairs Unico runs for 10-12 hours continuously and just maintains 75F and catches up after sunset. It’s actually oversized and has some air leaks from poor installation. But holds humidity under 45%. So its comfortable.