Ugghhh...Need to Get a New A/C

It looks like our A/C finally crapped out. Actually it was the coil, but our equipment was so old that they don't make the parts anymore. We had a couple people out for estimates which has now raised more questions than I had before so I wanted to see if I could get any help here.

• Two estimates were for 2 1/2 ton units and replacing the lines (refrig. and elec.).
• One estimate that was a 2 ton and he was going to basically just replace the equipment (keep existing refrig. and elec.).

One question I came up with is the sizing of the unit. I started checking the internet and it sounds like a lot of contractors oversize which is more expensive up front and also costs more because an oversized unit dancingnaughty:...yeah, yeah, very funny...go ahead and get it out of your system) is actually less efficient. There's some manual HVAC guys are supposed to use to calculate capacity, but I guess it's all big secret because I can't find anything except references to Manual J of some HVAC book. I saw one thing that says a general rule of thumb is to use 1-ton of capacity per 500 sq. ft., but Manual J is supposed to take into account a lot more factors like how many windows you have, color of your roof, shade, etc.

We have ~1300 sq. ft. house but we have some pretty decent shade during the hottest part of the day, we don't have a ton of windows on the sunny side of the house, and we've got a pretty good layer of blown-in insulation in the attic. Oh, and the most important thing...we are going to be listing our house soon so we don't plan on being there more than a few months (I hope). So, is a 2-ton A/C actually what we should be getting?

I just got my new Carrier put in last week. Mine was leaking, but was also undersized for my house, so I went from a 3 ton unit to a 4 ton.

not cheap.

also, a ton per 500 sf would mean I should have a 6.5 ton unit. That doesn't sound right.

I looked into this for our house when we were still in Atlanta. There's no easy way to tell. You need someone to do an actual design if you want it to be done correctly. If you go with the bigger unit (yeah, go ahead, get it out of your system) then the compressor is going to be constantly starting and stopping, which is also inefficient. If you go too small then it will never turn off. Since you aren't going to be there that long I would go with the 2 ton if I were you. It'll save you some money for sure.

Sometimes, I like living in the land where all you need is a window unit for part of the day a few days a week.

Well, I finally ran across a site that had an equipment selector based on square footage and zip code, i.e. inside temperaturre -vs- outside temperature. Just based on those two factors, it says we need 2.1 tons so go with a 2 ton. Now I just need the guy to call me back to see if we can hopefully get him out before the temperature gets up into the 80's (we've been extremely lucky up to this point, highs in the 60's to low 70's so it hasn't been a big issue).

Apparently A/C gets more efficient at removing moisture the longer it runs. An oversized A/C actually cools too quickly which means it's turning on and off all the time and "short cycling". Properly sized it should turn on a run for a while which means it working more efficiently.

BTW, I found their website and it looks like they use Tempstar equipment. 10 year transferrable warranty so it can't be too bad.

jeb6294 said:

Well, I finally ran across a site that had an equipment selector based on square footage and zip code, i.e. inside temperaturre -vs- outside temperature. Just based on those two factors, it says we need 2.1 tons so go with a 2 ton. Now I just need the guy to call me back to see if we can hopefully get him out before the temperature gets up into the 80's (we've been extremely lucky up to this point, highs in the 60's to low 70's so it hasn't been a big issue).
Apparently A/C gets more efficient at removing moisture the longer it runs. An oversized A/C actually cools too quickly which means it's turning on and off all the time and "short cycling". Properly sized it should turn on a run for a while which means it working more efficiently.

BTW, I found their website and it looks like they use Tempstar equipment. 10 year transferrable warranty so it can't be too bad.

Click to expand...

That's still not enough to determine what size equipment you have.

I will tell you this though, if you're going from R-22 equipment to R-410A (which is likely), the manufacturer reps for Trane told me they will not honor the warranty unless the refrigerant lines are also replaced. The oil used in the R-22 compressor is incompatible with R-410A, so much so that even the trace amounts left in an evacuated system will gum up and cause a failure in the R-410A system. They make flushing agents that are supposed to remedy this, but with the cost of copper being low, replacing is typically the cost-effective method (providing you don't have extraordinarily long refrigerant pulls).

Yes, oversizing is a bad thing because you aren't able to control humidity. While you would prefer to be undersized as opposed to oversized (especially in humid climates), it is important to be properly sized in order to effectively, and efficiently, control your temperature and humidity.

Residential cooling loads are calculated using a "Manual J", or a program that replicates the calculations on the manual J. I think I have a copy of the old form laying around here somewhere (my father was and my brother is a mechanical contractor).

when I replaced mine last year, one thing we had to factor was the ac was in the attic, no trees to help outso our hvac was working against itself due to the heat, I cant recall the size, but we went .5 ton larger than the one we had.

in 2009 you can get a tax credit for putting in a new unit I beleive (do some internet searching)

jmbeck said:

That's still not enough to determine what size equipment you have.
I will tell you this though, if you're going from R-22 equipment to R-410A (which is likely), the manufacturer reps for Trane told me they will not honor the warranty unless the refrigerant lines are also replaced. The oil used in the R-22 compressor is incompatible with R-410A, so much so that even the trace amounts left in an evacuated system will gum up and cause a failure in the R-410A system. They make flushing agents that are supposed to remedy this, but with the cost of copper being low, replacing is typically the cost-effective method (providing you don't have extraordinarily long refrigerant pulls).

Click to expand...

The site I found was actually one of the ones preaching about not putting in a system that was oversized and it specified that the sizing was just an estimate, but at least their estimate took into account that we live in Cincinnati as opposed to Phoenix. They actually had a link to purchase Manual J if you wanted to calculate your own capacity, but I didn't feel like shelling out $50...maybe if we were buying a replacement for a house we were going to be staying in for a while.

All of the quotes we got were for R-22 units. Cincinnati generally runs several years behind the rest of the world so it shouldn't really be a surprise. A couple of the contractors said something about the new systems but kind of brushed them off because "R-22 will be around for years before they stop making it".

Road Guy said:

when I replaced mine last year, one thing we had to factor was the ac was in the attic, no trees to help outso our hvac was working against itself due to the heat, I cant recall the size, but we went .5 ton larger than the one we had.
in 2009 you can get a tax credit for putting in a new unit I beleive (do some internet searching)

Click to expand...

There's a link on Tempstar's website about tax rebates for HVAC's, but I think those are only for the really high efficiency (16 SEER or above, read $$$) units. Since we're going the cheapo route ours is only 13 SEER, but considering our existing A/C is about 30 years old, I'd imagine it'll still be a big step up.

just scan the invoice into photoshop and change the description to what it is so you can get the tax credit!

Road Guy said:

just scan the invoice into photoshop and change the description to what it is so you can get the tax credit!

Click to expand...

Brilliant!!! h34r:

JR

I was going to change the date on mine since for some reason in 2008 you couldnt take any tax credit for it, but you can in 2009....

I am nursing my A/C units (two on my house) until I have to replace them. I had to give them each a sip of R-22 a few days ago to keep them from icing up. As long as my source for R-22 holds out, I think I can keep'em running for a few more years.

Freon, P.E. with a gauge set

R-22 shouldn't be a problem until 2015, IIRC, but the new ACs HAVE to use 410 after next year (again, IIRC).