[tiger08]

Originally Posted by ND_Irish

Can anyone provide the names of any good AC contractors who will do a manual j load calculation? Based on the various threads I have read, I believe our units are oversized by a significant amount for our home. Also, what should one expect to pay for a load calculation?

Got to HVAC-talk.com
They have a load calculation program you can download yourself. Your units are probably oversized as most units in vegas are.
Dont ever let a contractor come in and tell you that the rule of thumb is 400 square feet per ton. There is no such thing as rule of thumb for that.
I ran a load calc on my home and sized my unit properly and it was the best move I ever made.

[ND_Irish]

Originally Posted by tiger08

Got to HVAC-talk.com
They have a load calculation program you can download yourself. Your units are probably oversized as most units in vegas are.
Dont ever let a contractor come in and tell you that the rule of thumb is 400 square feet per ton. There is no such thing as rule of thumb for that.
I ran a load calc on my home and sized my unit properly and it was the best move I ever made.

I appreciate the post. I have been on HVAC-TALK.com and have received some really good advice. One contractor even offered to assist me with doing the load calculation at no charge. I should have the results by the middle of this week. It will be very interesting to see what his results are. I have had six (6) highly rated AC companies come to my home to give me quotes, and every one of them simply proposed replacing the units with the same type of units. They made no mention of doing load calculations. When I went back to the three most responsive vendors and mentioned I wanted to do a load calculation, one would not do it insisting the original calculations from the late 1990s must be right, and the other two want quite a bit of money to do the load calculations. One vendor insisted all of my duct work would need to be changed if the size of the equipment was reduced. The folks on HVAC-TALK.com said that was not correct; that would only be an issue if the current equipment was undersized. The consensus seems to be our equipment is significantly oversized for our house.

[WestieJeff]

IMO, since the home is already built and has had AC for years, you already have all the information you need and a load calculation is not necessary. Does your home cool too quickly? Then you might have an oversized unit. Does it not cool enough? Undersized.

The impact of having an oversized unit in Vegas is minimal since we have little humidity. In humid climates it becomes a problem when the unit is oversized and cools too quickly but hasn't run long enough to remove humidity. That's why it's safer here for the contractor to use the rule of thumb and potentially oversize a unit. If they oversize, minimal risk of problems. If they undersize, the owner is more likely to notice it not cooling adequately and complain.

It is possible if you had an extremely oversized unit that there would be some energy savings since units may work slightly more efficiently once they are up in running, but I suspect that cost savings would be minimal.

[beachhead]

Originally Posted by WestieJeff

IMO, since the home is already built and has had AC for years, you already have all the information you need and a load calculation is not necessary. Does your home cool too quickly? Then you might have an oversized unit. Does it not cool enough? Undersized.

The impact of having an oversized unit in Vegas is minimal since we have little humidity. In humid climates it becomes a problem when the unit is oversized and cools too quickly but hasn't run long enough to remove humidity. That's why it's safer here for the contractor to use the rule of thumb and potentially oversize a unit. If they oversize, minimal risk of problems. If they undersize, the owner is more likely to notice it not cooling adequately and complain.

It is possible if you had an extremely oversized unit that there would be some energy savings since units may work slightly more efficiently once they are up in running, but I suspect that cost savings would be minimal.

Just because the home is established, doesn't mean the "system" was designed properly. We moved into our last house, brand new in 1983, with a similar climate to Las Vegas. I had them out several times under warranty to "fix" the a/c, as it took running an hour to get the house to cool down one degree. (1700 sq ft, 4 ton unit) They swore it was working "as designed". Over the years when I'd have it serviced, I'd have them check it, they all said it was working fine (based on temp at the vents). Finally, in 2014, the unit died. We ended up having to replace the unit, and due to the age and condition of the ducting, we had to replace the entire ducting system so it would seal to pass final inspection.

While getting bids, every contractor said our unit was perfect for the size of the house. The one we chose (Costco dealer) said he guaranteed he could keep the unit at 4 tons, and fix the issue with 1" bigger ducts feeding the rooms. (we also had one bedroom that never warmed/cooled the same) I reluctantly agreed, and they were completely correct. Once finished, it only took about 15 minutes of run time to drop the temp one degree inside and the starved bedroom had plenty of airflow.

So, there is more to the "system" than just the unit. If it's not moving the right amount of air, even if it's been there for 20+ years, it may be worth re-calcing the install. But you are right, if everything is acceptable, no need to do more than replace the malfunctioning unit.

And, I feel for anyone in this situation. It hits the wallet a lot harder than you'd like...