What size unit? - Contractor Talk - Professional Construction and Remodeling Forum

MJM · 03-10-2009, 04:23 PM

I know it will be difficult/impossible to give me an accurate size based on the info I provided, but I have had 4 different quotes from HVAC contractors and 2 of them recommended a 3.5 ton packaged unit and 2 recommended a 5 ton packaged unit. To me this seems to be a big discrepancy and I thought I would ask for some other pro's opinions on here.

1600 sq ft. house, 3 bed 2 bath, 8ft ceilings, living room is 14' x 16' and has 10 ft. vaulted ceiling.
average summer temp in the area 93-98 with about 10-20 days in the 100's
The current unit is a packaged 3 ton, I have no idea how well it worked as I just bought the house.

Thanks in advance.

MALCO.New.York · 03-10-2009, 04:26 PM

Quote:

Originally Posted by MJM

I know it will be difficult/impossible to give me an accurate size based on the info I provided, but I have had 4 different quotes from HVAC contractors and 2 of them recommended a 3.5 ton packaged unit and 2 recommended a 5 ton packaged unit. To me this seems to be a big discrepancy and I thought I would ask for some other pro's opinions on here.

1600 sq ft. house, 3 bed 2 bath, 8ft ceilings, living room is 14' x 16' and has 10 ft. vaulted ceiling.
average summer temp in the area 93-98 with about 10-20 days in the 100's
The current unit is a packaged 3 ton, I have no idea how well it worked as I just bought the house.

Thanks in advance.

To begin................

One can NEVER have TOO big an A/C unit!!!! It will work less-hard than a unit that is "Fringefully Adequate".

If the 5 ton is NOT Economically Prohibitive..............Buy it!!!

Be sure you are buying "good stuffs" with a sufficient EER.

GET THE WARRANTY. I HATE Warranties............But with an A/C, something that NEITHER of us know how to fix, it does make sense!!!

macatawacab · 03-10-2009, 05:28 PM

Not a mechanical engineer but I use 750sqft per ton as a starting place. What is the insulation in the house? how are the windows? Attic ventilation, pitch of roof (volume of attic vented), out in the open or shaded, how is house to southern exposure? 3 ton seems minimal. Malco usually has some good advice.

knothole · 03-10-2009, 05:56 PM

Have the contractors do a "Manual J" load on your house. One that is to big will not run long enough to keep the humidity down and your house will feel damp.

flashheatingand · 03-10-2009, 06:36 PM

Sizing a unit is a lot like Goldilocks and her porrage. If it's too big, the unit will "short cycle" on & off, on & off. This is rough on the equipment. Also, it will not effectively bring down the humidity level as Knothole said. If the system is undersized, it will not be able to cool/heat the house during the extreme climates which isn't desirable. It is better for the equipment to run steadily as opposed to on & off.

Another thing in regards to oversized equipment, if you have 5-ton gear you need to have ductwork that is sized accordingly. The system may function, but, one is wasting a lot of energy dollars in the process. I am not there, and obviously don't know a lot of the variables. Based on what was stated, I am prejudiced in favor of the 3.5 ton estimates. But an actual heat load calc. should be performed.

I hear frequently people say that the equipment works "harder" if there is a big temp difference between actual temperature and desired temp. That isn't true. The appliance works steadily no matter what the conditions. It's an appliance, not a burro that carries bags of coffee beans up a hillside...). Actually, it works "harder" if it cycles on & off throughout the day.

Bottom line, require the contractors to show you a copy of their manual j heat-load calculations. That way, you know you are getting the right size of equipment.

MALCO.New.York · 03-10-2009, 06:44 PM

I used a 5200 BTUh to cool a 10 x 11 room. MORE than Sufficient.

Put a 15,800 (bought it a the right price) in and it did MUCH better than the 5k unit...........

Short Cycled?????

Nope! Set the differential by 4 Degrees.

Reduced my "Summer" Power Bill by $40!!!!

flashheatingand · 03-10-2009, 06:59 PM

It works, great, especially during the extreme days good deal for you. But, when you oversize the equipment, you risk setting yourself up for system that short cycles which is less than desirable. If you want the house to be meat locker cold, oversize it, but it isn't a good practice to oversize the equipment on a regular basis.

Nitwit · 03-10-2009, 07:14 PM

If you need to, run the blower at a slower speed. It will cycle longer and bring down the humidity at the same time. Thats what I had to with my new unit.

bwalley · 03-10-2009, 07:16 PM

Quote:

Originally Posted by MALCO.New.York

To begin................

One can NEVER have TOO big an A/C unit!!!! It will work less-hard than a unit that is "Fringefully Adequate".

If the 5 ton is NOT Economically Prohibitive..............Buy it!!!

Be sure you are buying "good stuffs" with a sufficient EER.

GET THE WARRANTY. I HATE Warranties............But with an A/C, something that NEITHER of us know how to fix, it does make sense!!!

You obviously have no idea how an HVAC system works.

MALCO.New.York · 03-10-2009, 07:20 PM

Quote:

Originally Posted by bwalley

You obviously have no idea how an HVAC system works.

And you are a Plumbing, Electrical, Ad Infinitum (http://dictionary.reference.com/browse/ad%20infinitum?qsrc=2888..... I know you need to look it up.......) Pro!!!!!!!

bwalley · 03-10-2009, 07:20 PM

Quote:

Originally Posted by MJM

I know it will be difficult/impossible to give me an accurate size based on the info I provided, but I have had 4 different quotes from HVAC contractors and 2 of them recommended a 3.5 ton packaged unit and 2 recommended a 5 ton packaged unit. To me this seems to be a big discrepancy and I thought I would ask for some other pro's opinions on here.

1600 sq ft. house, 3 bed 2 bath, 8ft ceilings, living room is 14' x 16' and has 10 ft. vaulted ceiling.
average summer temp in the area 93-98 with about 10-20 days in the 100's
The current unit is a packaged 3 ton, I have no idea how well it worked as I just bought the house.

Thanks in advance.

You need to have heat load calculations done.

Where do you live?

Based on square footage alone, I have a hard time believing you need a 5 ton system, 3.5 tons even seems a bit big, but without having heat load calculations done, there is now way to tell.

flashheatingand · 03-10-2009, 08:53 PM

Some people may decide they want to oversize a system. They might want colder air for whatever reason. The post does say 10-20 100 degree days That is a lot of hot days. How Many 95+ days? Who knows the exact details, but I would think its better to size it for about 25-30 degrees td as opposed to the 20 td standard. That is something to be worked in the heat load calc.

BigMikeB · 03-10-2009, 09:23 PM

Quote:

Originally Posted by bwalley

You obviously have no idea how an HVAC system works.

I agree with you 100%, judging by his answers here not only does he have no clue as to what comfort cooling is, he has no knowledge of moisture removal either.

MALCO.New.York · 03-10-2009, 10:21 PM

Quote:

Originally Posted by BigMikeB

I agree with you 100%, judging by his answers here not only does he have no clue as to what comfort cooling is, he has no knowledge of moisture removal either.

Oy Vey!!!

MALCO.New.York · 03-10-2009, 10:26 PM

Quote:

Originally Posted by BigMikeB

I agree with you 100%, judging by his answers here not only does he have no clue as to what comfort cooling is, he has no knowledge of moisture removal either.

Too many Codes and Seminars!!!

Cycle or Constant?????????????????????????????????????????? ???????????????????????

beenthere · 03-11-2009, 06:10 AM

Quote:

Originally Posted by MALCO.New.York

To begin................

One can NEVER have TOO big an A/C unit!!!! It will work less-hard than a unit that is "Fringefully Adequate".

You really don't know much about air conditioning.

Over size like that. And you have all sorts of problems.

MALCO.New.York · 03-11-2009, 07:32 AM

Quote:

Originally Posted by beenthere

You really don't know much about air conditioning.

Over size like that. And you have all sorts of problems.

Precisely............But I do know what has worked, from EXPERIENCE, for me.

A W Smith · 03-11-2009, 08:11 AM

[beavis and butthead] I just like the thread title [/beavis and butthead]

Chris Johnson · 03-11-2009, 08:52 AM

I know with ICF homes that the Heat/Loss calc is harder since there is no thermal transfer. Most systems are oversized causing short cycling and higher energy bills.

I'm betting 1 in 10 may know how to size an ICF properly. Not that I know how to do them, but my experience always was smaller unit and a larger fan motor.

MALCO.New.York · 03-11-2009, 09:06 AM

Quote:

Originally Posted by Chris Johnson

I know with ICF homes that the Heat/Loss calc is harder since there is no thermal transfer. Most systems are oversized causing short cycling and higher energy bills.

I'm betting 1 in 10 may know how to size an ICF properly. Not that I know how to do them, but my experience always was smaller unit and a larger fan motor.

I have found the Opposite to be true.........Just me $.02.

.Hey!!! Nice AVATAR!!!.