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Old 02-20-2014, 02:20 PM   #16
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3 ton evap 2 ton compressor


You are dealing with variable conditions within the air itself going through the evaporator coil ( I believe Yuri was referring to this earlier), so determining what is going to happen under all circumstances will be difficult.

Some examples would be varying humidity (latent heat content) and temperature (sensible heat content) in the air as it passes through the coil, coil depth (number of rows of construction) speed of air flow through the coil ( based on surface area), obstructions to consistent air volumes (cleanliness of filter) even the amount of condensate moving down the coil itself can be a concern.

Systems could have an accumulator added to ensure that liquid coming back would have a place to boil off before returning to the compressor as a vapor.

What is your intended purpose for the mismatching again? I am loosing track of the original intent of your question.

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Old 02-20-2014, 02:39 PM   #17
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Quote:
Originally Posted by Queequeg152 View Post
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?
You can do all these things and some have been done and worked poorly. We had freezestats on suction lines B4 but then you get nuisance dropouts, short cycling of the compressor etc etc. The cost would be prohibitive for accumulators etc and there is NO demand for any of this. If there is no huge demand they won't develop or produce these systems. The rest of this conversation then becomes theory but is interesting.

One major concern I see is that the manufacturers have to try develop systems for extremely hot humid climates like Florida and Texas and then have them work in moderate climates like mine and very dry climates like other states. There will never be a one size fits all unit and once again it is all a matter of economics and demand as to what they will produce.

The other BIG problem especially in some of the states is ductwork sizing being too small due to the builders being too cheap to do it properly or not caring. Always having issues here with undersized returns etc. where I am we have our issues but it is not as bad as it usually is large and sized for heating and old school furnaces with slow moving fans. You can invent all the technology in the world but if the ducts are too few or small it ain"t worth squat and won't work.
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Last edited by yuri; 02-20-2014 at 02:45 PM.
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Old 02-20-2014, 03:47 PM   #18
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Quote:
Originally Posted by HVACDave View Post
You are dealing with variable conditions within the air itself going through the evaporator coil ( I believe Yuri was referring to this earlier), so determining what is going to happen under all circumstances will be difficult.
yes i understand incoming temp etc all effect the refrigerant temperature pressure, but these variables can all be approximated towards the worst case scenario very easily. if there is still a significant margin of safety, then you can assume the system will work in all conditions.

what i cant approximate, is the effect of flow, because simply i am too ignorant in this area.

i was hoping that there was a known rule of thumb or calculation based on empirical data or otherwise i could interrogate, so i can better understand the relationship. if there is not no big deal, i understand its an unusual question.

Quote:
What is your intended purpose for the mismatching again? I am loosing track of the original intent of your question.
long story short. i have a 4 ton oversized unit that badly needs duct work replaced. along with said ductwork attic will be renovated. air sealing, new soffits, r30+ insulation etc.

im assuming current ac will live another 5 years(10 years old right now, last checkup was positive), so duct work will be servicing both the current(larger), and future (smaller) system.
due to the attic improvements, any future AC will need to be smaller, 3 tons at the highest, 2 tons at the very lowest ( with no additional margin +r40 ceiling/roof assmbly ) according to manual J.
im trying to size the system to operate well with both systems.

my assumption in the past was that I could simply size the duct for the current 1600cfm( for current 4 ton coil/comp), then simply running a mismatched 1600cfm air handler with the future 2-3ton system.

now however im wondering if i could move the system to a lower ( albeit potentially less efficient), cfm design like 1250cfm. this saves a significant amount of material cost (r8 ductboard) in the trunks alone. but the obvious boon would be in that the new system would operate in conditions it was designed for.

again note that i am not proposing to mis match coil and condenser. im sorry if i didnt make that clear earlier on. i just want to mis match air the air handler.

i do have a real nice inclined manometer accurate down to like .005" w.g. maby ill just cram the pitot into the plenum and get a real rough idea of the flow at the moment. it could very well be 1200ish cfm at the moment due to the insanely poor duct work.

Last edited by Queequeg152; 02-20-2014 at 03:49 PM.
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Old 02-20-2014, 04:27 PM   #19
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Quote:
Originally Posted by Queequeg152 View Post
what would happen if say. one were to use a 1200cfm blower on a 4 ton condensor and coil system?

At only 300 CFM per ton. The indoor oil would frost over on milder outdoor temp days, with low humidity. Might take 15 minutes to start occuring, but it would happen. Shorty after that, liquid can start to return to the compressor.

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.

Only if room air was drawn back toward the register, or the register wasn't sealed to the ceiling/wall.

would a clean coil possibly freeze even?

Any coil can freeze over.

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?

Dehumidification is greatly increased/improved.

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.

You need to first determine how much dehumidification you need. and then look at the coil performance charts at different saturation temps/pressures.

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.

If it had appropriate sized duct work, then there is no need to have a 1600 CFM blower on a 3 ton system for better air filtration.

in this situation what are the adverse effects? minus of course the additional electrical cost? Read above answer.

does dehumidification suffer? does sensible cooling suffer?

Read above answer.

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.

Measure your current TESP(Total External Static Pressure)

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.
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Old 02-20-2014, 07:10 PM   #20
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thanks beenthere, few followups tho.

would you say, that anything short of 1600 cfm is could cause issues? or is there a middle ground? say 350cfm or 375? Im going to look at the condenser specs( if i can find them), but im just wondering what you think.

with respect to the hypothetical 3 ton coil and 1600cfm blower air filtration comment. i should have said better rate of air exchange. a 1600 cfm air handler would increse the rate at which air in the house is cycled through the filter by 12.5 percent.

why should i measure my TESP if im designing new ductwork? you lost me on this one im afraid. what does this show me beyond how badly the existing duct is?
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Old 02-20-2014, 08:46 PM   #21
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Quote:
Originally Posted by Queequeg152 View Post
thanks beenthere, few followups tho.

would you say, that anything short of 1600 cfm is could cause issues? or is there a middle ground? say 350cfm or 375? Im going to look at the condenser specs( if i can find them), but im just wondering what you think.

350 CFm is fine for most areas. Except those that have irtually no humidity, or are at an elevation of 5,000 ft or more.

with respect to the hypothetical 3 ton coil and 1600cfm blower air filtration comment. i should have said better rate of air exchange. a 1600 cfm air handler would increse the rate at which air in the house is cycled through the filter by 12.5 percent.

Also removes little to no moisture from the air and causes high humidity.

why should i measure my TESP if im designing new ductwork? you lost me on this one im afraid. what does this show me beyond how badly the existing duct is?

Your duct work may be fine for 3 tons. A TESP check would tell you if it is or not.

A 4 ton duct system moving 160 CFM at a TESP of .9" would only have a TESP of .5" moving 1200 CFM.
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Old 02-21-2014, 02:08 PM   #22
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Your duct work may be fine for 3 tons. A TESP check would tell you if it is or not.

A 4 ton duct system moving 160 CFM at a TESP of .9" would only have a TESP of .5" moving 1200 CFM.
thanks for taking the time beenthere, its appreciated.

based on your recommendations im going to redesign the system to 1400 cfm at normal trunk velocities...700fpm down to 400 in the branches..
the air handler im thinking of buying is rated at 1380(medium tap) and 1580(high speed tap) at .3 tesp... does this sound decent?
should i perhaps, look to an ecm blower?


with respect to my current ductwork. you said it yourself!, " its garbage, replace it"(or something similar) some time back in another thread of mine, lol.

its not even "duct" by my definition. the inside sleeve is actually a fiber mesh glued to the spiral wire... only the outside sleeve is keeping the air inside the duct work.
this is why i never considered testing it, just sealing the leaks i could find etc.
this duct is from the 1970's! the homeowner made the grievous error of not having it replaced when a new unit was installed in the 1990's.

however it would be a good learning experience, and it would be enlightening to see how bad it really is. so i probably will do this.

to the OP, again i apologize for running this thing off the rails so to speak. should a just made this a thread of its own.
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Old 02-21-2014, 05:05 PM   #23
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.3" TESP, sounds like a Nordyne air handler. They have less then desirable blower performance.

Supply register, .03" PD, return grille, .03" PD, see through fiberglass air filter new, .03" PD, supply balancing dampers, .03" PD. Total device PD equals .12

.3-.12=.18" ASP

Use a good media air filer, and the filter PD is more like .17". So the ASP would be .08" to have rated air flow. Means real big duct work. An air handle with a .5" ESP rating is better.
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Old 02-21-2014, 06:23 PM   #24
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yea idk why its rated at such an absurdly low static pressure. my old york furnace manual had a huge table of cfm's @ dozens of diff, static pressures. Stuff for the heat exchangers etc.

this blower ( a goodman MBR 1600), only has the one cfm rating. from what i can tell... its possible the legit manual is unavailable online?

i was planning on running it on high assuming the cfms will drop to 1500ish at .5-.6 TESP. this would yield slightly higher velocities than i designed for, but it wouldn't be terrible...

Honestly, i was just told that the goodman stuff (they have giant warehouses here in houston), was ubiquitous, and could be bought locally whereas other stuff would need to be ordered through a supply house, and would cost more. i know little about their quality, perceived or otherwise.

do you think i should look to an alternative air handler? im more than willing to fudge the budget a bit to get things squared proper.

the current design for 1600cfm yields a total static pressure of .52 at the critical path, the lowest register will be .508.
This does not include the electric heating coil, as i only recently found the smacna manual detailing pressure drops across them. im guessing the heat coil ads at least .05-.1 or so.
however the lower cfm will drop the losses across the media air filter and coil, so it might be a wash. my current coil is by far the biggest loss at .25 in w.g.

i hear you on the media air cleaner, and im ahead of you on this. im going to use the trion air bear at 25x20 owing to its cheap replacement filters.
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Old 02-22-2014, 06:41 AM   #25
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I would use a different blower/air handler. The MBR's static ratings don't include the coil. So you have to add in the PD across the coil to find your ASP.

Goodman has other air handlers that are better.
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Old 03-08-2014, 11:41 AM   #26
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I love this thread lots of good info and theory! But one last question if my blower is one speed(I think it is) how can I verify a new blower will be 1200 cfm? I didnt think they sold them like that, I thought they sold by horse power and RMP. Is there a conversion for RMP to CFM? Back to my original problem - I did buy a 1.5 ton case coil, what I am hearing is I don't need to install that just upgrade the blower to 3 tons, right? Is there any advantage to installing 2 coils in the same air handler. I did only pay 180.00 for the 13 seer coil, it has a piston. In the future can I just change the piston for R-410?

Thanks again
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Old 03-08-2014, 02:12 PM   #27
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The ducts have to be sized to move 1200 CFM using that blower. And then either a static pressure check, or a volumeter used to make sure it is. Its very doubtful your old et up was moving 1200 CFM.
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Old 03-08-2014, 05:43 PM   #28
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so I thought I would go ahead and install a new compressor. but now are the charging charts still good? I assume charging charts assume matched equipment. My numbers are nowhere near the charts.

ODDB 68 degrees
INDB 70 degrees
low side 39 psig 51 degrees
LL 173.2 psig 59.5 degrees
That gives me a super-heat of 33.9 under charged right?
look at subcool 32.8 overcharged!
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Old 03-08-2014, 06:03 PM   #29
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Sounds like a restriction in the refrigerant circuit.
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Old 03-08-2014, 06:06 PM   #30
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could it just be low ambient? I did replace the LLFD - because of the size dif I think I am going to run 30 degree supper heat. there is not a lot of load now, but I would think a 30 degree now would mean at least 10 in the sumer


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