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  1. #14
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    Apr 2008
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    Mark,
    Are your outdoor condenser and indoor coil both 5ton?
    Key1
    In the land of the blind.....the man with one eye is king....

  2. #15
    Quote Originally Posted by key1cc View Post
    my installation manual indicates to not use comfort r on the 350/ton setting. it states to only use it on the 400cfm/ton or higher setting......which is 1600cfm......that is how i determined it was adequate...
    Ah, ok, that was the missing tidbit. So in effect they are telling you that you will be fine if you keep the flow above 320 cfm/ton (400 * 0.80 = 320). 80% of 350 would be too low according to the manual, which makes sense. So that confirms that you are fine , since you are within their guidelines.

    First of all the outside unit which is the 4-ton condenser is what is generating the cooling btu's. The cooling btu's are not being generated by the inside coil.
    Understood , and it makes sense. So it would imply that as long as the outdoor coil is "smaller" than indoor, then the outdoor tonnage would be the limiting factor. And so you would not worry about the coil freezing if you are not meeting the 320 cfm per ton of indoor coil, as long as you are meeting it for the outdoor unit size.

    If a pro wants to jump in and confim or deny this ....please feel free.
    Yup, would be nice to have it confirmed that this is indeed the case .

    Quote Originally Posted by key1cc View Post
    Are your outdoor condenser and indoor coil both 5ton?
    In my case, both are 5 ton, so that makes it "simpler" . So if 320 cfm/ton is considered OK, I can have the blower lowered from current 1800 cfm to 1600 cfm, which should reduce the static to around 0.70-0.80". Sounds like a good approach!

    Thanks Key1!

    Mark

  3. #16
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    Jan 2004
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    320 a ton won't hurt any A/C under normal ambient conditions.

    Now if your going to run your A/C when its 65° outside and set your stat to 70, it wll probably freeze the coil.

    Very few furnace A/C systems with PSC blowers manage to even move 350 a ton, on 5 ton system.

    Mostly due to improper return set up at the furnace.
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  4. #17
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    Apr 2008
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    From my perspective with the VS motor it came down to airflow.
    If the unit was set for 100% airflow the high static forced the air flow down to ~320 CFM/ton anyway according to my airflow versus static tables.....So I concluded that the coil was seeing the same airflow whether the unit was set at 400 CFM/ton with a high static > 0.9 or if it was set at 320CFM/ton with a more reasonable static of 0.75.

    Key1
    In the land of the blind.....the man with one eye is king....

  5. #18
    That all sounds reasonable to me. I'll report back on the static I measure after switching to 1600 cfm (320 cfm/ton). Hopefully it will drop to 0.70-0.80".

  6. #19
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    The Heatload Equation...

    Quote Originally Posted by key1cc View Post
    my installation manual indicates to not use comfort r on the 350/ton setting. it states to only use it on the 400cfm/ton or higher setting......which is 1600cfm......that is how i determined it was adequate...

    Now let me help you out with your concerns on the 5-ton evap coil because i don't think your current thinking is accurate. First of all the outside unit which is the 4-ton condenser is what is generating the cooling btu's. The cooling btu's are not being generated by the inside coil.
    the inside coil could have just as easily been 4-ton.....but 5-ton was used for greater efficiency...but still only 4 tons worth of cooling is going through it because the cooling btu's are generated by the outside unit......soooooo it is that 4 tons of cooling that we need to protect from too low air flow not 5 tons).
    If a pro wants to jump in and confim or deny this ....please feel free.
    Key1
    Actually, the heatload absorbed by the indoor coil determines the BTUH output of the condenser. You can only go so far above the Nominal Rating of the condenser.

    With reasonable outdoor temps, the combination of a very high humidity, plus a high temp sensible load will usually put the condenser load WILL ABOVE its Nominal BTUH Rating. (Everything is variable in psychrometric equations.)

    A 4-ton condenser under some lower outdoor temps, & high indoor temp & humidity loads, would put the NOMINAL Tonnage Rating of the outdoor condenser well above its NOMINAL BTUH Rating Output.

    That is why I would always want, as near as possible, a Nominal Rated indoor heatload through the E-Coil. Most A/C's operate well under their nominal load because they can't adjust to keep the evaporator near its nominal load Rating.

    That is why you should want a duct system sized so it can raise the airflow to meet the load conditions.

    You could actually have a blower relay setup that would change the blower speed according to the Ratio (IQ brain) between the Outdoor Ambient Temp (OAT) & the Indoor Wet Bulb temps, ha.

    A computer controlled variable speed blower motor could possibly do an acceptable job controlling the E-Coil heatload in relationship to the OAT conditions. - Darrell
    Last edited by udarrell; 09-03-2008 at 05:26 PM. Reason: raise not rise the airflow

  7. #20
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    Quote Originally Posted by udarrell View Post
    Actually, the heatload absorbed by the indoor coil determines the BTUH output of the condenser. You can only go so far above the Nominal Rating of the condenser.

    With reasonable outdoor temps, the combination of a very high humidity, plus a high temp sensible load will usually put the condenser load WILL ABOVE its Nominal BTUH Rating. (Everything is variable in psychrometric equations.)

    A 4-ton condenser under some lower outdoor temps, & high indoor temp & humidity loads, would put the NOMINAL Tonnage Rating of the outdoor condenser well above its NOMINAL BTUH Rating Output.

    That is why I would always want, as near as possible, a Nominal Rated indoor heatload through the E-Coil. Most A/C's operate well under their nominal load because they can't adjust to keep the evaporator near its nominal load Rating.

    That is why you should want a duct system sized so it can raise the airflow to meet the load conditions.

    You could actually have a blower relay setup that would change the blower speed according to the Ratio (IQ brain) between the Outdoor Ambient Temp (OAT) & the Indoor Wet Bulb temps, ha.

    A computer controlled variable speed blower motor could possibly do an acceptable job controlling the E-Coil heatload in relationship to the OAT conditions. - Darrell
    Thanks for the explanation but I must admit it went right over my head
    Are you saying that when the installation manual states that a givin CFM/ton should be used....it is referencing the indoor coil? and not the outdoor coil?
    Also when the pro's here talk about the rule of thumb of 400 CFM/ton, they are referring to the indoor coil?

    Key1
    In the land of the blind.....the man with one eye is king....

  8. #21
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    Jan 2004
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    Its a combination of many different things.

    For all practical purposes.

    I can hook a 5 ton condenser to a 3 ton coil, move 2000 CFM through it, and only have a 4 ton system.

    But.
    Your description was close enough and practicl, for the vast majority(99.95&#37 of resi systems installed.
    As you said, air flow is set more to the outdoor unit, and the metering device is selected for the outdoor unit.
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  9. #22
    Quote Originally Posted by key1cc View Post
    Thanks for the explanation but I must admit it went right over my head
    Are you saying that when the installation manual states that a givin CFM/ton should be used....it is referencing the indoor coil? and not the outdoor coil?
    Also when the pro's here talk about the rule of thumb of 400 CFM/ton, they are referring to the indoor coil?

    Key1
    I'd like to see these answers too . In my case they are one and the same, since both units are 5 ton, but I would be curious to know the answer for cases like Key1's.

  10. #23
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    Apr 2008
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    Quote Originally Posted by beenthere View Post
    Its a combination of many different things.

    For all practical purposes.

    I can hook a 5 ton condenser to a 3 ton coil, move 2000 CFM through it, and only have a 4 ton system.

    But.
    Your description was close enough and practicl, for the vast majority(99.95%) of resi systems installed.
    As you said, air flow is set more to the outdoor unit, and the metering device is selected for the outdoor unit.
    Thanks Beenthere for clearing that up.

    Key1
    In the land of the blind.....the man with one eye is king....

  11. #24
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    Jun 2005
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    SW Wisconsin
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    Here is the mfg'ers performance data

    Quote Originally Posted by key1cc View Post
    Thanks for the explanation but I must admit it went right over my head
    Are you saying that when the installation manual states that a givin CFM/ton should be used....it is referencing the indoor coil? and not the outdoor coil?
    Also when the pro's here talk about the rule of thumb of 400 CFM/ton, they are referring to the indoor coil? Key1
    Go to "Expanded Cooling Data" & you will see my points plainly illustrated as to which coil, the indoor or the outdoor coil determines the BTUH heat transfer rate.

    Here is the mfg'ers performance data on my brother's 1-ton 12-SEER Heil unit with a 2-ton A-Coil.

    At 400-CFM, 75-IDB, 71-IWB; 75-Outdoor temp. Rated at a Nominal 18,000-BTUH it delivers 20,300-BTUH. At 80-IDB 71-IWB 20,200-BTUH; At 85-IDB 71-IWB 20,100-BTUH. That is the mfg'ers data, not mine.

    An oversized indoor coil has just a little effect on the point I am making. Other tonnage units vary some as to whether they go much above the listed nominal Ratings with those conditions. - Darrell

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