HVAC-Calc help and opinions

[catmanacman]

why are you going from 2 units to 1

[Doral]

Without getting into all the particulars..I have posted them before....we have a physical layout issue with regard to the compressor and running a new lineset. It is indeed a BIG problem with no solution other than one that is prohibtively expensive. This has been done successfully in the past in other houses similar layout to ours. We know that everybody votes for 2 units. That solution is a mess as we MUST relocate the compressor (code violations in current location) as it is. There is NO room for two compressors...trust me.

[jimj]

We are using and indoor temp of 75F and outdoor temp of 98

-Sensible: 32586
-Latent: 6444
Sq. feet 2324

Ask your dealer for the performance data on the selected equipment match up, you will find the info your looking for there. My guess ( and it is ONLY a guess) is you will need the quoted 5 ton. A Trane XL19i 5 ton with matching air handler would give you 30,070 sensible @ 98 DB outdoor, with 68 WB and 75 indoor DB. If your contractor can't give you this information find a new contractor.

[Doral]

Jimj-
That was a good tip. We were able to access the specs. and that did indeed alter a lot of the numbers after matching up the manual j latent and sensible to the performance data.

[dcmcm5]

I am not familiar with HVAC-Calc. I use Wrightsoft's Right Suite based on Manual J Ver. 8.

How many people did you use in the calc? Are you able to input overhands and shading for windows? What did you assume for air infiltration and duct leakage?

As an added note, you can assign up to on half of your excess latent capacity to remove sensible heat. So even though you may be a little short on a particular unit, you may reallyl be ok with the excess latent. I looked at some Rheem specs and that was the case for their 3.5 ton unit, which would work with your specifics.

[Doral]

Yes you can put in information for overhangs, house orientation, 3 levels of air infiltration and duct leakage (although most of our ducting is going to be replaced) and we followed the manual which said to use 5 people.

My husband was very precise in entering measurements and specifications.

[BigJon3475]

If you were able to get the specs for the Infinity system then you probably saw the 4 Ton system in low speed would take care of most of your load. The 5 Ton is to big IMO.....It would only come on days that are above 95º. I would have to say that since your 1% cooling load is set @ 92º then you would probably hardly ever see the hi speed kick in. Maybe someone else can chime in but the 4 ton system to me seems to fit your situation best.


@ the 95º outside ambient and 80º inside with a 67º wetbulb the system efficiencies will go down if the outside temp rises or your inside temp drops. You want the system to run the majority of the time in low speed to remove the high latent load I'm sure you guys have in New Orleans.

[dcmcm5]

Would you consider the house to be well insulated and built tight with good energy efficient windows? What level of air infiltration and duct leakage did you use? I ask that because your load seems a little high for the size of the house if well built.

[Doral]

I am trying to attach the file with all the information.

This was my husband's best estimate based on his evaluation of where we actually are. He also did two other evaluations downgrading our insulation (maybe it is a bit worn out..) and a few other things. I have those tw versions available also. He took many hours, blue prints and measuring tape to try to be as accurate as possible. The house is 27 years old.

[ampulman]

I am not familiar with HVAC-Calc. I use Wrightsoft's Right Suite based on Manual J Ver. 8.

How many people did you use in the calc? Are you able to input overhands and shading for windows? What did you assume for air infiltration and duct leakage?

As an added note, you can assign up to on half of your excess latent capacity to remove sensible heat. So even though you may be a little short on a particular unit, you may reallyl be ok with the excess latent. I looked at some Rheem specs and that was the case for their 3.5 ton unit, which would work with your specifics.

When you say 'assign', are you really saying that up to 1/2 of unused latent capacity will boost your sensible by a like amount, or is this accomplished by adjustment (dip switches, or some other aspect)?

AM

[jimj]

[QUOTE]

As an added note, you can assign up to on half of your excess latent capacity to remove sensible heat. So even though you may be a little short on a particular unit, you may reallyl be ok with the excess latent. I looked at some Rheem specs and that was the case for their 3.5 ton unit, which would work with your specifics.

Opps, you are exactly rite. I keep forgetting everyone doesn’t live in Arizona like I do.

[jimj]

When you say 'assign', are you really saying that up to 1/2 of unused latent capacity will boost your sensible by a like amount, or is this accomplished by adjustment (dip switches, or some other aspect)?

AM

As per Manual S guidance if more sensible capacity and less latent capacity
is needed you add one half of excess latent to the sensible capacity.
No dip switch or other adjustment.
Using HO data she has target CFM of 1559 CFM at a 19 degree delta T.

[dcmcm5]

Jimj is correct - the assignment of excess latent comes from Manual S. This can be significant and I have seen units that were just a little below on sensible that they went to the next larger size, which unfortunately was a 1 ton jump, and resulted in a unit that was way too large.

A couple of comments/questions:

- You show walls with R-2.5 1/2 in. Extruded poly board. This is not common. Do you really have a continuous insulation board on the outside of each wall? That's great if you do.

- If the walls are insulated with fiberglass, unless you know for sure, they are probably not giving you a full R-11. May want to do a couple of runs with something a little less to see how it comes out if you have little margin on capacity.

- You list several windows as Double pane; Metal frame; Low emittance. Did you have a film put on the windows? Otherwise, I don't think you would have a low-e metal frame window.

- If the attic insulation is R-19, you should look at adding additional insulation (you can blow cellulose on top of the batts) to get you to at least R-38. This is usually not that expensive and will really help where you are located.

- You are not following Manual J, and I believe you are making a mistake, by using design temperature values beyond the design. I show for New Orleans the 1% values are 92 deg F for cooling and 34 deg F for heating. The ASHRAE mean extremes are 96 and 23. You are outside of the extremes. You load calculation is too high assuming all the other parameters are correct.