I just spoke to one of the companies that gave me a quote. He indicated that he would run the Manual J numbers if/when I commit. I pointed out that I have most of the room, door, window, and ceiling measurements already and he said he could run a quick calculation but he is very confident that it will match what he has estimated.
I also found a spreadsheet on the ACCA site (https://www.acca.org/Files/?id=128) and i might try plugging the various numbers i have into the spreadsheet to get a sneak preview (assuming i'll be able to make sense of this). From what I can tell so far, it appears to be based upon the abridged edition of the 8th edition of Manual J so i can't judge whether it will be a reasonable approximation of what a skilled professional will generate using the full calculations but worst case maybe I'll learn something?
I have one more company that I've just emailed that appears both on the NATE and ACCA lists and has good BBB and angieslist ratings--I'm not sure how I didn't see them before. Hopefully I can get them to come out in the next week or so. I've taken dash's advice and have asked if they will do Manual J, S, and D.
Speaking of which.. are all three 'must-do' for my situation? I hadn't really heard of either Manual D and Manual S prior to dash's reply.
Man. S is always a good idea ,and a must if you want an indoor temperature of 77 or less.
Man D is to be sure the ducts aresized to deliver the airflow called for by J and S.
Sounds like you found one decent contractor.
If you look up at the top of the page you will find a link for load calc, it's a manual J clac of which alot of people have used and purchased the trial version for a couple of months and did there own load calc, you may wish to use it as well and if you have any questions the guys here are willing to help you out
Personally, I don't use Manual S, I have extended performance data, for my brand.
You could get a contractor that has extended data also, and they may not use manual S.
OK I wanted to give folks an update and ask a few more questions:
I got the ACCA/NATE certified folks to come out, do the Manual J calcs, and give me an estimate.
After doing the Manual J calcs (I believe the computed value he ended up with was 35712), he is recommending a 3-ton AC unit (vs. the 4-ton unit recommended by the other two companies).
Specifically, he is recommending an American Standard 4A7A3036A1000 and a Heatcraft TE32636 with R-410A refrigerant. My existing furnace is a Carrier 4-ton 58stx110-16 (80% AFUE).
A few questions:
1. Does the choice of coil seem reasonable for the suggested condenser? I seem to sense a preference for matched (as in brand) coil and condenser based upon the posts I've read here.
2. Given my desire to limit the overall footprint of the condenser unit (to preserve the boat parking on the side of the house), I've been trying to find out the recommended clearance for the condenser but haven't been able to find it. Can anyone tell me how close the condenser can be to the wall?
3. They guarantee the cooling system will provide a 75* indoor temperature on a 95* summer day. However, we have had a number of very warm (100*-105*) days already this summer, and I tend to like the temperature a bit on the cooler side. Is 75*/95* a standard guarantee? Is it even feasible to talk about changing the recommended system to accommodate a larger spread between indoor and outdoor temps? IOW, would a larger condenser unit or a different coil allow me to get more like 70* indoor temperature on a 100* day? AIUI, using an oversized condenser unit would mean shorter cycles and thus higher RH (and higher perceived indoor temps)..
4. Are there any other condensers in the Trane/American Standard lineup that have smaller dimensions than the suggested unit (akin to the Carrier/Bryant Streamline units)?
Thanks again for all your help. I feel like I'm closing in on a decision here, and I have more confidence in this last company than the other two based upon experiences with them so far.
Oh, and added to the other questions above:
5. I assume having a 4-ton furnace (which the last guy says is oversized) will mean I'm using more gas than I need to.. any guesses how much more? Is there any other consequence to having an oversized furnace? Given that the existing unit is only ~4 yrs old, unless there is a significant difference in cost between this and a more efficient and properly sized unit, I'm planning to just keep it until it needs to be replaced.
4 ton, is the blower drive.
How many BTU is it rated at, and how many BTU's does the heat loss come to.
Bear with me, as I'm not quite sure what you're asking (I'm new to all this).
A quick search turned up a doc (http://www.xpedio.carrier.com/idc/gr...t/58st-5pd.pdf) that seems to suggest that the furnace is a:
58STX Low NOx version
110 110,000 Btuh Input capacity
16 1600 CFM Nominal cooling size (Airflow at .5 ESP) (400 CFM per 12,000 Btuh)
Looking further in the doc I see it is 105,000 INPUT BTUH for Downflow/Horizontal (which IIUC is what I have).
Output capacity BTUH is said to be 85,000 for Downflow/Horizontal.
Your sp may be high enough that you are only moving 1300CFM.
Your contractor can check this for you.
Your furnace may be slightly over sized. Have them do a load calc to find out. You might be able to save some on your heating bill.
The Manual J calc he did to estimate the size of my AC system suggests it is (see my earlier post). is there a different load calc to be done, or is this it?
I only see a cooling load. Not a heat loss load.
ah i see. I didn't realize these were different.
Most houses have a higher heating load, then cooling load.