2 STAGE OVERSIZE?

[CUMULUS]

Please forgive my ignorance, but I had a load calc performed on my 2300 sf 50 year old home in Houston and was on the cusp of a 4 ton and 5 ton. I currently have a 6 year old 4 ton 10 SEER shabby unit that runs like crazy and gives me outrageous bills in the summer. I inspected my old metal ducts and they appear to be in reasonably good shape. I do need to have more insulation blown in after AC installation. I have had 5 quotes from installers and 1 quotes 4 ton single stage, one quotes 4 ton two stage and 3 quote 5 ton two stage. All 14 to 16 SEER and all recommend variable speed blower. I guess my ductwork has the capacity for a 5 ton since 3 have quoted it. My question is is it worth coughing up the extra dough for a 5 ton two stage? I know the first stage will run at 70% to 80% in non-extreme conditions, but will a 4 ton two stage actually end up running at full power more to not justify having dual stages. This stuff is confusing and prices are all over the place. Any info would be greatly appreciated.

[amd]

Make sure that the ductwork is insulated and sealed properly.

Definitely do everything you can to reduce heat gain prior to investing in new equipment. Also get a load calculation done after the improvements are made.

If the load calculation is accurate and a 4 ton unit is having trouble maintaining setpoint, something's wrong. (Running continuously at design temps is okay) I wouldn't be so quick to replace a 6 year old unit because it's 10 SEER - something as simple as insufficient air flow or imporper charge can significantly reduce efficiency.

It is my understanding that the first stage capacity on many two stage systems isn't much lower than that of second stage. (wait for confirmation on that)

I guess my ductwork has the capacity for a 5 ton since 3 have quoted it.

Please don't find out the hard way.

A variable speed blower might be able to overcome restrictive ductwork within limits, but you could end up with a really noisy system and high fan energy consumption.

[beenthere]

What did the load calc say you need.

[CUMULUS]

Approx 53000 BTU of heat gain. This equated to a 5 ton.

[beenthere]

What was the sensible, and latent load.

Some new 5 ton units, won't meet that total gain.

Also, are any of your current supplies or return/returns slightly loud?

[CUMULUS]

They didn't get that specific with me. Supplies and returns are not loud. If some new 5 tons can't meet that gain, I guess you answered my question.

[beenthere]

Ask those contractors, what the actual capacity of the unit they are proposing is.

Did the load calc take into account the extra insulation you are planning.
Can you decrease your homes infiltration rate. The lower you get that, the smaller A/C you need. Which will save you far more then just increasing the efficiency of the A/C you get will.

Caulking around all windows, repairing/replace all worn door seals, sealing all receps and switches on outside walls.

Could lower your requirements by ½ ton, or more. And save you money year round, weather heating or cooling.

Does/did your current 4 ton, maintain temp, but just ran 24/7, or did it lose temp during the hot days?

Few duct systems were installed sized for the next ton larger system.

[classical]

They didn't get that specific with me. Supplies and returns are not loud. If some new 5 tons can't meet that gain, I guess you answered my question.

You have not found the right contractor because they have been unable to assuage your concerns or your needs. If they could you would not have the need to come here for guidance. We here cannot properly answer your questions to do so properly we would need to see and evaluate your home in person.

Who are the companied you are working with? It sound like they are offering what they want to sell you not what is best for your needs.

If your metal duct is in good condition sealing it and re-insulating could reduce total system load by as much as a full ton.

If the load for your home is 53,000 BTU's there are 4-tons units available that will reach that threshold, there are also 5-ton units that cannot achieve that capacity. What degree day are you predicating your load on 93* or 95* it makes a difference. There are many variables that can be put into a program that will affect your final results.

If you have a leaky house (which I suspect) you can make numerous inexpensive upgrades that will reduce your load to below the 48K threshold so a 4-ton will be a viable unit.

[skippedover]

I'd recommend the following to provide you with the best bang for the buck and most economical to operate:

1. Have a blower door test done on the home to determine total leak rate (air exfiltration/infiltration rate/24hrs) and the locations of the leaks. The test should include testing your ducted system for leaks.
2. Tighten up: Take all the steps you can to eliminate the leaks as indicated by the blower door test, including sealing the duct system as necessary to reduce duct leaks to no more than 10% of the system airflow in cubic feet/min (CFM)
3. Increase you insulation in the cap and/or other areas as desired.
4. Have a new load calculation done on the home with the new leak rate (should be a tight house by then) and increased insulation level.
5. Determine what if any changes you need to make in your AC system.

As an aside, you might consider having a HP installed to act as a dual fuel system during the heating season. This changes your AC costs from an expense to an investment, since the HP will lower your gas consumption. It's also a 'greener' approach as you'll be lowering your local greenhouse footprint, which is impossible to otherwise address. The electricity producction facility is much easier to regulate about greenhouse gasses. If your 10-SEER 4-Ton system is just keeping up with the hot days but costing more than you want to spend on monthly electric bills, then I'd recommend the highest efficiency unit you can purchase. Please know that at 4 and 5 tons, the ACTUAL SEER rating is NOT what the manufacturer labels the LINE of equipment. In other words, if you want a 16-SEER unit, have your installing company provide you with documentation showing that your equipment is capable of operating at 16-SEER. In most cases 4 or 5 ton systems will need to be at 18, 19 or 20-
SEER ratings just to provide you with 15 or 16 ACTUAL SEER operation. This is a little like buying a car in that respect. You never really get the efficiency that the manufacturer claims. I lowered my electrical consumption when I went from a 3.5-ton, 10-SEER AC system to a 3.0-Ton, 3.0-Ton HP rated nominally at 18-SEER/2-stage. The overall electrical decrease was about 27.7%.

[CUMULUS]

Ask those contractors, what the actual capacity of the unit they are proposing is.

Did the load calc take into account the extra insulation you are planning.
Can you decrease your homes infiltration rate. The lower you get that, the smaller A/C you need. Which will save you far more then just increasing the efficiency of the A/C you get will.

Caulking around all windows, repairing/replace all worn door seals, sealing all receps and switches on outside walls.

Could lower your requirements by ½ ton, or more. And save you money year round, weather heating or cooling.

Does/did your current 4 ton, maintain temp, but just ran 24/7, or did it lose temp during the hot days?

Few duct systems were installed sized for the next ton larger system.

Yeah, added insulation, radiant barrier and energy efficient windows would drop it below the 48000 threshold for a 4 ton. I am still on the cusp. My current 4 ton seems to run continuously in the summer with the temp held at 78 during the day and 76 at night. My bills get up to $550 even with a decent rate. I want to try and bring this way down. As for the ducts, I currently have eight 8" metal ducts coming out of the plenum. A couple of those have smaller splits to feed small bathrooms. From what I've read it seems my duct sizing is also on the cusp. It does suck to have to come here and ask these questions. Out of the five quotes I received, only one really explained stuff to me.

Added: My home is 2300 square feet built in 1958 and is one story with 8' ceilings. Nothing out of the ordinary. Just old and a little airy. The houses were built well then, just not very energy efficient.

[dash]

Sounds like your old 4 ton did okay,improve the attic insulation,and a new 4 ton is likely all you need.Load calc will tell for sure.

Ask about having the duct static(resistance to airflow) tested,to determine if they are correctly sizd for a 4 ton,many aren't.

[classical]

Yeah, added insulation, radiant barrier and energy efficient windows would drop it below the 48000 threshold for a 4 ton. I am still on the cusp. My current 4 ton seems to run continuously in the summer with the temp held at 78 during the day and 76 at night. My bills get up to $550 even with a decent rate. I want to try and bring this way down. As for the ducts, I currently have eight 8" metal ducts coming out of the plenum. A couple of those have smaller splits to feed small bathrooms. From what I've read it seems my duct sizing is also on the cusp. It does suck to have to come here and ask these questions. Out of the five quotes I received, only one really explained stuff to me.

Added: My home is 2300 square feet built in 1958 and is one story with 8' ceilings. Nothing out of the ordinary. Just old and a little airy. The houses were built well then, just not very energy efficient.

CUMULUS send me an E-mail or call me the info is in my profile, you need help ypu are not getting.