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Old 05-28-2010, 12:00 AM   #1
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mixed cond and evap sizes


Nothing I am doing right now but I always see questions about size matching evap coils and cond units and curiosity got to me. I suppose there is a reason for it but was curious as to what happens if you mismatch the parts


such as: what happens if you undersize the evap coil and maybe why if you can explain it somewhat


and what happens if you oversize the evap coil, and why if possible


I had read before that you sometimes mismatch intentionally for better dehumidification. What do you do and why

and is that a thing of the past since you can have variable speed fans now?

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Old 05-28-2010, 05:34 AM   #2
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mixed cond and evap sizes


Its actually not mismatching. In the sense you and 99% of everybody else thinks it is.

A mismatch, is when they don't work together, or work together at rated capacity

A 3 ton outdoor unit. May have a match to a 3, a 3.5, and a 4 ton indoor coil. Some 2 ton condensers, have a match to a 5 ton indoor coil.

Seldom is a smaller indoor coil used. As in its BTU rating. But once in a great while it is. But only for increased moisture removal. And then the installer better know what he's doing to get it to work right.

Most of the time. The larger coil is for higher efficiency from the system. However, larger coils tend to have a warmer surface temp, and reduce moisture removal capacity.

Larger coils are still used today for higher efficiency even with VS blowers. And actually work better because of VS blowers.

One of the bad things the HVAC industry did early on. Is to put a BTU rating on an indoor coil. In reality, the indoor coil has no BTU rating. Carrier has been getting away from that for some time now. And Trane has followed on many of their coils. Instead of using a BTU rating on their air handlers, they just give it a number. Like an 04, or a 31.

The things that will vary a coils ability to effect a condensers efficiency and capacity. Is the coils internal size/volume. How much room for the expansion of the refrigerant as it absorbs heat. The number of fins it has per linear inch. The tube spacing/configuration

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Old 05-28-2010, 02:10 PM   #3
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mixed cond and evap sizes


thanks. That's a great wealth of info.

If you don't mind more questions:

Quote:
A 3 ton outdoor unit. May have a match to a 3, a 3.5, and a 4 ton indoor coil. Some 2 ton condensers, have a match to a 5 ton indoor coil.
How is the proper match determined? What would happen if the coil were too large, or is that possible.

yes, I understand there is a lot of information needed to make a proper ac design, just trying to learn as much as I can.

I am going to have to have a new AC unit in the not too far future. Knowledge is power and I didn't have enough power last time and the system had never really worked, what I thought to be, correctly.
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Old 05-28-2010, 03:00 PM   #4
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The manufacturers(OEM) use computer programs to determine the effects of the different coils. Then when they have one or more that come up with the results they are looking for. They have it made. And test it in their test chambers with the different condensers the program said it would work with. This includes capacity, and efficiency testing.

Third party coil manufacturers, only use a computer simulation, to see if a coil design works with the outdoor units specs as provided by the condenser manufacturer. Their is no real testing done by them.

Some coil match ups are for efficiency only. They have a very large surface area. Allow an extra 1000, 2000, possibly 3000 extra BTUs of capacity. But, they run warmer, and don't remove much moisture.
Often used on 2 stage equipment where first stage provides a long run time. And in arid climates where removal of humidity id not desired.

The smallest coils that match up. Are usually designed more for moisture removal then high efficiency. And are best used in areas like south Florida, or Hot humid Houston. And some areas of other states, where the home may be located near a swamp, or heavily wooded area. They provide a lower total BTU capacity, and a lower efficiency.

The coils that are in between the above sizes, are good general use designs. Providing both outdoor rated capacity, efficiency some times exceeding the condensers rating, and reasonable moisture removal.

It is possible to be too large. As in a very large coil in a very humid climate. Won't remove enough moisture. And the home owner ends up needing to use a dehumidifier. or, setting the thermostat 4 to 6 degrees cooler, just to feel like it 74 in the house when its actually 68 in the house. An actual over sized coil. Could end up with a coil temp of 60. And at an indoor condition of 74 with 60%RH, the coil would need to be at 59 to remove moisture(59.3 to be exact). So it is possible to have too large of a coil.
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Old 05-28-2010, 03:20 PM   #5
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great gobs of info!!!

thanks, a ton (or 12,000 btu if you prefer)

I appreciate the tutelage.
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Old 05-28-2010, 04:14 PM   #6
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Here's another wrinkle for you. Lennox and others have 3 different 2 ton coils. Physical size is small, medium and larger. Sometimes a larger coil is too high or too big for the bonnet/plenum and literally won't fit. Smaller coils restrict airflow more than large ones and sometimes that is a BIG problem. What to do?

Answer: you go with what fits and not try to overengineer or micromanage every BTU out of the system as it is not possible. The biggest problem is not having a proper load calculation done and getting the right sized unit, proper sized ductwork and fan speed setup, proper weather and door stripping, shade trees or awnings, good quality windows. They all play a factor in a good system.
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Old 05-28-2010, 04:28 PM   #7
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Interesting info, considering I was debating on building my own water based AC unit and I'd have to consider what is the best size to make my coils. I'm sure a lot of that would still somewhat apply even with a water based system. Probably wont go with that project since I don't think it would work well enough, but still toying with possibly doing it.
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Old 05-28-2010, 04:38 PM   #8
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Canuck Tire has 18,000 BTU/1.5 ton window units on sale for $550 and you get our 2nd official currency. LOL. For that kind of dough I doubt you can build anything half decent or get it to work. They even have 2 ton models for $750. 1.5 tons will do up to 900-1000 sq.ft so I would not fool around with any DIY stuff.
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Old 05-28-2010, 05:57 PM   #9
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I installed (2) 14.5k units in the wall, they were $125 each after rebates on sale
We don't need much AC here so it doesn't make sense to buy a unit
One less thing to worry about
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Old 05-28-2010, 06:16 PM   #10
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For $550 thats a cheap easy way to cool a house. May need a 20amp circuit though.
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Old 05-28-2010, 11:25 PM   #11
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Quote:
Originally Posted by yuri View Post
Canuck Tire has 18,000 BTU/1.5 ton window units on sale for $550 and you get our 2nd official currency. LOL. For that kind of dough I doubt you can build anything half decent or get it to work. They even have 2 ton models for $750. 1.5 tons will do up to 900-1000 sq.ft so I would not fool around with any DIY stuff.
Problem is, our CT has sold out on all units! Well they had the Whirlpool models, but they have very bad reviews so did not want to spend the cash. Need to wheel it to the bath tub, lift it in, so you can empty it! Bad design.

I did end up ordering one online from Home Depot though. Probably safer then the DIY central air route I had in mind. But it would still be fun.

Though think once I get myself more setup with power tools, if I decide I want another portable unit, I will just use a window unit inside a custom built wooden box with a vent tube. The Window units are much cheaper.


Last edited by Red Squirrel; 05-28-2010 at 11:28 PM.
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