Air Conditioning - sizing

i know it's complex. Three diff guys want to put in a 4ton system to replace exisiting one. I measured all my rooms and this house totals about 1400-1500 sf and is one story stucco/cement block on a cement slab (Miami). (4 bedrooms NOT big ones) i read at acdirect.com that (as a guideline) a 1500-1800 sf house would need only a 3 ton unit. I'm thinking a 3.5 ton would be more than sufficient.

At least two of them took measurement but i think they're trying to sell me what the previous owner had in.

any opinions? advice ?

thanks in advance

Mike

Not an ac pro, but that sounds like overkill to me. We have just under 2000sf, block on slab, and I think ours is a 3.5. You have a lot of windows maybe? Or poorly insulated?

If you are only replacing the outside unit you have to stick with the same as the inside or are you replacing the complete unit?

Correct me if I am wrong(as I have been retired for a bit) But does not the EPA limit the sale of equiptment with refigerent in it to be limited to techs with a licence? acdirect.com Seems to sell to anyone.

They are trying to sell you a 4 ton unit because it costs more, and they will make more off of it...

If you had the existing system awhile past operating experience is a good sizing tool.
Providing it was installed and maintained properly the run time and humidity level of the house would say a lot.
If you found that the unit did not run much on the hottest days then there could be room to downsize the unit.
A unit that is oversized will cool quickly but not dehumidify in a humid climate.

You really need someone to do a proper load calculation or you could buy a program and try it yourself.

http://www.hvaccomputer.com/

As GregH mentioned, how well did the existing unit perform. Does it have short run times? Do you have anything that would increase the heat load (as already mentioned), such as lots of windows, poor insulation, high air infiltration, etc.?
It is entirely possible you need a unit that large, but this is just speculation at my end. A manual J calculation should be considered. However, many a/c contractors don't like to do them because of the time and hassle involved.

on the hottest days the unit would be running for say 12-15 minutes, and rest for 12-15 minutes - but my feet (in sandals) would get COLD!!.

Everyone was ready to slap in a 5ton unit, AND 13 SEER. I'm gonna get 2 or 3 more guys to come in (and check angieslist.com again too)

That pretty much says that your existing unit is too big.
Maybe 3 1/2 ton would do it but a load calc might even show a 3 ton to be right for you.
If you plan any energy saving measures in the future now would be the time to factor this in.
A window or insulation upgrade could put a 3 1/2 ton unit in the oversized category.

I believe your system is oversized. But some people like it that way. A "properly" sized system will actually be a little bit too small on the hottest days. In other words the system will be on continuously and may not achieve a 20 degree difference between outdoor and indoor temperatures. Some people will find this completely unacceptable and will insist on a larger system. Many a/c techs are afraid they will irritate their customers with a properly sized system and therefore will go up 1/2 or 1 ton just to be sure.

I had posted that my unit was on about 12-15 minutes and off 12-15 minutes continuously on the hottest parts of the hottest days.

One response said that was a good indicator that the unit was oversized (or was it that my sandaled feet got COLD??)

Just curious, with the newer 13-15 seer products out there now, for example, what kind of cycles should one expect on the hottest hours of the hottest days?

continuous or LESS time running? like 10 on 20 off? anybody have an idea? k

again i have an 10-12+ yr old 4 ton unit and about 1500SF one story stucco/cement block on a slab - but it sounds like that cycle time i mentioned up top rang a bell with the moderator here (i believe).

Mike

Moderator edit:
Please keep questions on the same equipment in one thread.
It is unfair to ask people to bounce around to follow your progress.

However, a larger system will cost a bit more money, sometimes drastically, because of the increase in cooling capacity. Sometimes, it doesn't pay off getting a bigger unit (and a higher initial price as well) than have a proper one running for long periods of time.

I know what you are saying when you say techs are afraid of customer reaction to a properly sized a/c unit.
I feel though this is just a cop out for a tech who can not properly explain how comfort control works or can not properly size the unit.
There may be an odd day when temperatures are above the expected highs but it is poor practice to size for these few days.
I have had complaints from customers who obsess over inside temperatures when we get the occasional record high.
A digital hygrometer/thermometer will often set them straight.
When they realize that their slightly undersized system will not drop the temp to the 20 degC/68 degF they want, they understand that their unit is pulling buckets of humidity out of the air making the environment less muggy.
When properly sized in damp parts of the country, the lack of temperature drop should be made up by moisture removal.

In response to your question about seer rating it will make no difference to the running time.
You still select the unit based on btu/h.
It is the power consumed that would be different.

on the HOTTEST days how much might a "reasonable" size AC run (assuming a reasonable setting, say 74)? it sounds from a previous post that 12-15 minutes on and 12-15 minutes off continuously is OVERsized . . . (oh and my feet get COLD lol) - just curious what other people have seen on a reasonable size with the HOTTEST days -

almost continuous ? 15 on 5 off?

sorry i dont remember how much my current system ran on moderate days - probably 10 minutes on and 20-30 minutes off?

Of course the bottom line will be found in the complete writeup (sizing) but also how that standard is applied, and whether it's TOO conservative (likely to result in OVERsized systems).

We dont EVER set our AC lower than 74!!

Monday morning i'm calling acouple of AC names back and finding a couple more and insisting (nicely) on the previously mentioned write up - AND getting figures for Sound level db and 3.5 ton prices .

thanks to all for their time advice

Mike

What controls the on time is the capacity of the unit and the off time is the insulating factor of your house.
IMO the run time in the hottest weather should be measured in hours.
The off time you can only control by adding (or removing) insulation.

miami_mike I have had 3 homes down there in West Palm Beach. All 1800 to 1900sq ft all had just 3 ton AC with electric heat no heatpump. Worked just fine. Just turn it on and let it run . It will get the R/H down so you feel good at a higher temp. Down there for sure go for a unit with a seer of 15 and a V/S blower that has a humidistat control on it. This will pull out more humidity for you and you need it .
Now also spend the $$$ and get a heat recovery unit on the new AC. This will give you FREE hot water for about 8 months down there. I know it did for me with 4 people in the home. I turned the hot water tank off. It also will kick up the seer of the AC unit also.
1. REDUCED HOT WATER COSTS Heat Recovery typically has a two-year payback... or less. By heating domestic water whenever the air conditioning system is in operation, the Heat Recovery unit supplements the hot water heater or boiler... significantly reducing the cost of heating hot water.

2. REDUCED A/C COSTS In addition to Hot Water heating cost savings, the AC Series Heat Recovery unit also reduces the operating costs of the Air Conditioning system by 10% to 15%. This means additional energy efficiency for the overall system when the Heat Recovery is in use.

http://acforsale.com/online/product_info.php?products_id=1158

Go here and read about them


Now I know we here always say get 3 bids for the same job. BUT down there I will say get 5 bids for the same job.
" TRUST ME" ;)

DOES sound like i'm in for a smaller unit - i'll press these guys a bit, and get the numbers (dB, calculationns, etc). and . . . that heat recovery sounds awesome AND a "green" energy conserving idea!

thanks again

Mike

Contractor B said, oh yeah those tranes are quiet, but
1) they come on and go off with a loud snap
2) the install manual specifically says "do not install under a bedroom window"

Anybody verify this or do i have to go Trane-Hunting? This would apply to the XR and maybe the XB series also.

Of course i'm thinking of relocating the outside unit because it IS right under one of/very near to the bedroom windows.

TNX

Down there you want the XL15i with a V/S blower coil. That way you will have the humidistat control on th unit . This can pull out much more humidity for you

i'm not gonna get ANY TRANE if it makes a loud noise at startup and shutdown!!! cuz if it does, i'll have to MOVE IT for at least $1000 to other side of the house, or build a wall around 3/4 of it!!!

like i said it's directly between 2nd and 3rd bedroom windows (1 story house) - so if it aint QUIET i wont care about the electric bill OR the humidity or much else!!!

Then go find one and listen to it. What does this guy sell that is saying they make all this noise? I have not been called back on any for noise.

may be the best thing to do - i was given that option. - The competition is Carrier - 10yr comp/5 yr parts/ 1 yr labor - new coolant (they call it Puron) seers at 14 and 15.5.

the Trane guy points out the 10/10/5/1 warranty - i doubt there's much difference - the extra 10 is for what? i'll have to ask him again. Plenty of time - its cool today in miami!

I'd LIKE to get the trane maybe but only because of the vs air handler and the FREE air cleaner they're pushing with the XR system (which he says listed at like $1500, he sells them for less, and probably u can find em for $500 IF they're worth it).

I am not sure what you or they mean by the noise on the Trane???

I have it, and so do a few of my family members, and we never had an issue with this snap noise.

Id still stay with the Trane XL15i and the V/S blower. Also it comes with the R22 yet that will be around till 2030. Ask them how much R410a cost per lb. Also if you do have a leak with the R410a and its over half you have to pull all the old freon out and put all new freon in cost$ cost$ cost$
You will like the new trane filter.

i'm not gonna get ANY TRANE if it makes a loud noise at startup and shutdown!!! cuz if it does, i'll have to MOVE IT for at least $1000 to other side of the house, or build a wall around 3/4 of it!!!

like i said it's directly between 2nd and 3rd bedroom windows (1 story house) - so if it aint QUIET i wont care about the electric bill OR the humidity or much else!!!

I really hope you are not running your A/C with the windows open,, if they are shut you should not even hear anything..

As for the original question, the unit for the house should be a 3 ton 3.5 at the largest it is better to go smaller as oversizing a unit is a huge no-no and a good rule of thumb I believe is 1 ton per 600 sf

A good rule of thumb is to have a pro do a manual J on the house so you do not throw money down the drain.

A good rule of thumb is to have a pro do a manual J on the house so you do not throw money down the drain.

this is true, or find a student to do one for you as a class project