Deliberately oversizing an airconditioner

[eljeffe]

I have a situation (long story) where the system that needs to be installed in my home now is sized at slightly under 5 tons (4.5+); Later, phase 2, after I've completed some remodling it's closer to 4 tons. The lower rating is because phase 2 insulates / tightens the house and adds zoning to the HVAC. My question is this ... given that we'll be installing a top-of-the-line Carrier Infinity system how much of a problem is the extra ton?

Unfortunately I can't defer installation of the system until phase 2 of the project, and my intuition says that with a 2-stage compressor, VS fan, and zone controls, the system can handle the extra ton with no problems. The extra ton could also come in handy when we have large gatherings at our home, or we hit those really hot summer peaks here in Austin.

I'm asking here because neither my architect or either of the HVAC contractors bidding the job seem to have a definate answer.

[jeffshvac]

you need to do a load calculation to determind what size equipment is needed.did anyone do this?

[eljeffe]

Originally posted by jeffshvac
you need to do a load calculation to determind what size equipment is needed.did anyone do this?

Yes, the load calculation shows 4.5+ tons required in phase I (prior to remodeling) and 4+ tons (not allowing for peaks such as large parties) after the remodel and allowing for the addition of zoning. So the question is can you install 5 tons now to accomodate the current state without having a serious impact on the system's performance once phase II is completed? (i.e. does the proposed configuration provide enough flexibility to manage the prospective excess capacity without causing problems?)

[t527ed]

if you are showing over 4 tons after your phase 2 projects i would have no fear of using 5 ton 2 stage equipment. as long as your zones are sized to handle air flow required it should not be a problem.

[dash]

The five ton is 2.5 ton on low speed ,so even if the home ends up at 4 tons it will be fine.

It will run at the 2.5 ton speed most of the time,unlike other brands that go to high speed after 10 minutes.

What type of heting?It should be two stage as well to work the best with Infinity Zoning.

[teddy bear]

I assume you are concerned about oversizing and providing humidity control during partial load conditions. The claim that perfect sizing will provide humidity control during low load conditions needs clarification. A/c systems are unable to control humidity with no cooling load. Green grass climates have many weeks/months with no cooling load and plenty of moisture. Occupants activities in a home generate 30-50 pounds of moisture per day. Outside infiltrating fresh air with a 70^F outside dew point adds about the same amount of moisture, a total of 60-100 pounds of latent cooling per day to maintain <50%RH inside. Have your a/c people explain how their system will remove the moisture with little or no sensible cooling load. True that the new, very complex a/c systems are capable of operating at half speed and over-cooling a few degrees. This marginally improves the latent removal, but requires a constant 50% cooling load +10 hours/day to remove 60 lbs of moisture. Instead of attempting humidity control with a complex a/c, suggest a good supplemental dehumidifier like Ultra-Aire and high SEER a/c. The cost is about the same with positive humidity control without over-cooling and at any inside/outside combination of temperatures. Get humidity control in writing. There always an excuse as to why your a/c will not deliver <50% without a significant cooling load or over-cooling. TB

[Steve Wiggins]

1. Zoning doesn't have anything to do with sizing of equipment. Tightness/insulation & radiant barriers - Yes, Zoning - No.

2. My opinion here goes against most but I say size it larger than the load calc if you have two stage cooling. There are many times when you need that extra capacity. Load Calc freaks seem to think loads are constant, sorry guys but they are not.

3. Most of the sizing problems come from duct systems not sized right for the equipment. All this focus is on equipment sized right for the house and that is NOT what most of the sizing problems stem from.

4. You say "top of the line" but you don't mention what kind of air filter will be protecting your investment, which is the most important part of the system.

[dan sw fl]

Originally posted by eljeffe
My question is this ... given that we'll be installing a top-of-the-line Carrier Infinity system how much of a problem is the extra ton?

, and my intuition says that with a 2-stage compressor, VS fan, and zone controls, the system can handle the extra ton with no problems.

Seems reasonable to me
if the subject load analysis addresses
all the building envelope parameters.

Set-up the system for close to 350 CFM/ ton.

Also, the higher capacity is needed
if you wish < 76'F set-point.

[eljeffe]

Great feedback and based on everyone's feedback I'll double check the distribution and return duct sizing.

WRT to filtering, we're actually going a little overboard with a Trane CleanEffects and low-MERV fiberglass prefilters on each of the return registers. My HVAC guy is a little hesitant about using register filters but we're rather anal about periodic maintentance so I'm not too worried about it and, with the return located in the floors, the thought of keeping large stuff like cat hair, etc. out of the return ducts is simply too tempting.

[dash]

Filters and duct sizing are critical.

Here's a quick example of a Manual D duct design ,with and without restrictive filters.Your job will not be the same ,just and example to show what can happen with restrictive filters.

2000 cfm @ .5 ESP ,equipment specs.

.5 ESP - .15 for the pressure drops of grille and standard filters = .35 ASP

.35 ASP X 100 ,divided by 400 Equvilalent feet of duct = .0875 FR

2000 cfms @ .0875 FR = 18.5" round duct for the main trunks

__________________________________________________ ______

Now add the Trane filter:

.5 ESP - .40 for the pressure drop of standard filters,grilles and the Clean Effects

.10 ASP X100 , divided by 400 Eqv. feet of duct = .025 FR

2000 cfms @ .025 FR = 24" round duct

__________________________________________________

ESP =External Static Pressure
ASP= Available Static Pressure
FR = Friction Rate
PD = Pressure Drop


Be sure they size the ducts correctly including all the filters you plan on using.

We are curently doing a job with Carriers new filter,.25 PD at 2000 cfms.We will be using two on this five ton system to reduce the PD to keep duct sizes down so they will fit in the space allowed.

They can do this as well and be sure the floor filter grilles are oversized as well.Larger size filter = less PD,at the same cfms.

[Edited by dash on 02-02-2006 at 05:20 PM]