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Old 02-19-2014, 11:40 AM   #1
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3 ton evap 2 ton compressor


what happens when you have a 2 ton evap coil in a air handler and a 3 ton condensing unit? Just found out that is what I have. I would think it would be common to flood the evap, but I check supper heat every summer, and it is always 10 - 15 degrees, so I cant be flooding the evap.

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Old 02-19-2014, 11:46 AM   #2
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3 ton evap 2 ton compressor


flooding may not occur as the orifice controls the amount of flow. a tx valve would have a hard time but an orifice could not care less. must get some weird readings on the compressor though.

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Old 02-19-2014, 12:46 PM   #3
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3 ton evap 2 ton compressor


. . and the bottom line is I'm only getting 2 tons of heating and cooling, right? Wouldn't that wear out the compressor faster? suction pressure would be low I would think, I've never had a problem freezing in the summer. Now I'm wondering what to change it out with? I was going to go the other way a 3 ton inside and a 2.5 ton outside. Just to take care of humidity. good idea ?
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Old 02-19-2014, 12:48 PM   #4
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3 ton evap 2 ton compressor


the outside unit needs replacing, so maybe I could put a 2 ton compressor in place of the 3 ton compressor? that way I would have to spend money for an inside unit or the condenser, just the compessor
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Old 02-19-2014, 01:03 PM   #5
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3 ton evap 2 ton compressor


There was a contractor in Texas, that was using 5 ton condensers on 3 ton evaps. Even had 1-5ton condenser matched to a 2 ton evap coil. All of these units were Lennox units. And lennox approved the matches.

In reality, an evap coil has no BTU rating. So if your moving 1200 CFM through that coil, your getting close to the 3 tons your condenser is rated at.

The match you have, was probably/may have been done for humidity reasons.
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Old 02-19-2014, 05:34 PM   #6
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Uh huh. Lennox uses the same evap coil for 1.5 and 2 ton units and the same 1 coil for other combos. uses a different orifice or tx valve but yeah the coil does not have to exactly match the condenser. should match for proper SEER though.
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Old 02-19-2014, 05:35 PM   #7
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3 ton evap 2 ton compressor


geez 5 down to 2. The suction had to be low. how do you keep it from freezing? You have to assume a 2 ton indoor coil is only going to have a 800 CFM fan.
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Old 02-19-2014, 06:58 PM   #8
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3 ton evap 2 ton compressor


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geez 5 down to 2. The suction had to be low. how do you keep it from freezing? You have to assume a 2 ton indoor coil is only going to have a 800 CFM fan.
No, it was on a furnace with a 5 ton blower. The thing is, you have to move the air that the condenser needs. The smaller surface area evap coil runs much colder then a regular 5 ton would. So the latent capacity is greatly increased.
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Old 02-20-2014, 10:25 AM   #9
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3 ton evap 2 ton compressor


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No, it was on a furnace with a 5 ton blower. The thing is, you have to move the air that the condenser needs. The smaller surface area evap coil runs much colder then a regular 5 ton would. So the latent capacity is greatly increased.

what would happen if say. one were to use a 1200cfm blower on a 4 ton condensor and coil system?
ive been thinking about this issue myself, and it seems to me one would have problems with condensation on grills perhaps walls etc, as the air would get significantly colder? much colder than the dew point in my case.
would a clean coil possibly freeze even?
since the air presumably gets colder, don't you achieve better dehumidification? on the other hand less air is passed across the coil so perhaps overall dehumidification suffers?

is there a rule of thumb or even an area of study i can look to for real hard answers to this? ive been through the ASHRAE manuals looking for psychrometrics dehumidification etc to no avail, ive seen mention of percieved comfort with respect to humidity etc, but no maths or tables etc for equipment sizing etc.

what about the opposite situation where one has a 3 ton coil and condensor... but wants better air quality ( by this i mean more aggressive filtration) and installs a 1600cfm air handler with the appropriate sized duct work.

in this situation what are the adverse effects? minus of course the additional electrical cost?
does dehumidification suffer? does sensible cooling suffer?

i ask this because id like to update the duct work in my home, but after attic renovations etc the house will only need like 1/3 less sensible cooling, and id like to ride the current system till the wheels come off so to speak.

ive thought about creating an indivitual thread to ask these questions, but after seeing this post, i thought id just bring it up here?

anyway sorry for the mild hijack.

Last edited by Queequeg152; 02-20-2014 at 10:33 AM.
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Old 02-20-2014, 10:50 AM   #10
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Originally Posted by subcool View Post
the outside unit needs replacing, so maybe I could put a 2 ton compressor in place of the 3 ton compressor? that way I would have to spend money for an inside unit or the condenser, just the compessor

You have to size the condenser for the heat load. After that you can change the evap coil size but I would not as you will mess up the SEER rating. These days it is very critical to have them matched for SEER and energy use. If the owner does not care and only wants better humidity removal then go ahead. The newer Carriers can run 350 cfm/ton and with a ECM motor and good sized ducts that is safe and keeps the SEER. You are playing with fire and could damage the compressor by mismatching coils and even though the readings may be good at one point what happens when the customer forgets to change the filter or it gets partly plugged. You won't be there to see the readings or know if there is a problem. Not something I would fool around with. If you boost the fan speed thru a small coil it can whistle or actually lift the water off the coil due to the extra high velocity. Seems to be a huge problem with horz units in my experience or they won't drain properly for that reason. LOTS to go wrong when mismatching coils.
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Old 02-20-2014, 10:55 AM   #11
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QUE

Read my post below yours. whenever you fool around with a AC system you never know how it will react under ALL conditions from 70F to 110 F outside. may work safely at one temp and cause a lot of trouble at other temps. this is all experimental what people do with mismatching so there is no proven way to do it. basically an experiment that can go horribly wrong. ASHRAE cannot help as this is not normal industry methods.
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Old 02-20-2014, 11:54 AM   #12
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3 ton evap 2 ton compressor


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QUE

Read my post below yours. whenever you fool around with a AC system you never know how it will react under ALL conditions from 70F to 110 F outside. may work safely at one temp and cause a lot of trouble at other temps. this is all experimental what people do with mismatching so there is no proven way to do it. basically an experiment that can go horribly wrong. ASHRAE cannot help as this is not normal industry methods.
thanks for the reply. with respect to the coil condenser mis matching. i wholly understand what you are saying.
But id like to know more about the effects of mismatching an air handlers to a matched system IE a 4 & 4 ton condenser evaporator setup or 3 &3 etc.

ironically i can find all sorts of documents about miss matched coils from york at least; total and sensible cooling with a 4 ton coil 3 ton condenser and visa versa, but they make no note of say a 3 ton coil with 1600cfm whistling through it. or the opposite, a 4 ton coil with only 1200 across it.

you mention that carrier goes with 350cfm per ton. is this for purely efficiency reasons, or does this offer better control of humidity? but presumably their evaporator is designed to operate at lower flows, so they are probably less densely finned?
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Old 02-20-2014, 12:11 PM   #13
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3 ton evap 2 ton compressor


I think Carrier has one of the deepest pockets $$ for R@D and spent the time to fine tune their systems for better humidity control. ECM motors can maintain a constant static pressure to .8"WC so there is less chance of poor airflow freezing up the coil and liquid refrig slugging to the compressor and destroying it which is the major concern when mismatching coils. Most people go with the rule of thumb 400 cfm/ton but if the ducts are large enough and the airflow proper then 350 cfm/ton works. Rule of thumb is old school so in the future getting more precise with airflow is a good thing.
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Old 02-20-2014, 12:33 PM   #14
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3 ton evap 2 ton compressor


In Carriers system of decreasing to 350 CFM/ton it is done with ECM motors and high end control systems which communicate with the furnace blower, this is driven by a priority for dehumidification, it is also done with a coil using a TX valve as a metering device, so flood back is not an issue. Not all manufacturers are using TXV's on all coils, so some caution should be used if trying to make this work in all systems.
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Old 02-20-2014, 01:33 PM   #15
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In Carriers system of decreasing to 350 CFM/ton it is done with ECM motors and high end control systems which communicate with the furnace blower, this is driven by a priority for dehumidification, it is also done with a coil using a TX valve as a metering device, so flood back is not an issue. Not all manufacturers are using TXV's on all coils, so some caution should be used if trying to make this work in all systems.
is there anyway to calculate refrigeration temperature/pressure with respect to CFM over a given heat exchanger? or is this all empiracle data provided by the manufacturer?

im not an hvac tech, or tradesman, but i think i know what you mean by flood back... the compressor seeing a liquid rather than a gas im assuming? this leads to a cylinder trying to compress the uncompressible as such with water flooded engines damaging themselves etc.

couldn't you simply take the temp of the line returning to to the compressor and verify gas or liquid phase accordingly?
if one were to try to run a 4 ton with 1200cfm he could presumably check to make sure no flooding was occurring in this or a similar manner? moreover one might wire a surface mount thermocouple to the line, and cut power to the contactor if/ when the temperature dropped?

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