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Old 05-16-2009, 09:26 AM   #1
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


I have a 4 ton Trane 15i condenser paired to a 5 ton ADP coil with a 100k btu XV95 2-stage variable speed furnace.

When it was installed the blower speed dip switches were set at 400CFM per ton and the resulting external static pressure was over 1.0 even though the nameplate called for 0.5 max heating (and no mention for cooling.)

After I discovered the high esp the unit was adjusted to run at 80% of blower speed or 320/ton. I pretty much ran all last summer this way with no issues except that my esp is still high at ~0.75.

I know the dips can be set to 350/ton which would mean my airflow would be 80% of 350 which is 280/ton.

Does anyone see a problem with the air flow being that low (280CFM/ton)?
What are the risks?
The indoor coil has a TXV.

Thanks in advance for your insight

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Old 05-16-2009, 09:37 AM   #2
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Potential freeze up problems at low ambient conditions.

I have mine down to 280CFM per ton.
During mild ambients, it doesn't run long enough to freeze up.
But its not something I would recomend for most units.

Would be best to have a freeze stat installed if you want to run it that low.

I keep saying I'm going to install one on mine. But I just never get around to it.

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Old 05-16-2009, 09:57 AM   #3
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Quote:
Originally Posted by beenthere View Post
Potential freeze up problems at low ambient conditions.

I have mine down to 280CFM per ton.
During mild ambients, it doesn't run long enough to freeze up.
But its not something I would recomend for most units.

Would be best to have a freeze stat installed if you want to run it that low.

I keep saying I'm going to install one on mine. But I just never get around to it.
Thanks for the quick response. Two questions.

1. Please define "low" ambient. Less than 70F outside?

2. You indicated that you run your system at 280/ton. May I ask why?

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Old 05-16-2009, 10:39 AM   #4
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Low ambient can be in the low 70s. If the humidity is low, then there is very little load.

OD 72 X 65%RH has roughly 3 more BTU's per pound of air, then 72 X 50%RH.

So with that loss of latent load, a coil can freeze.

My A/C is oversized for my place(It was here before me).
2.5 tons for 1650 sq ft. I could get away with a 1.5 ton.
I have good shading in the afternoon.

Give you an idea of how over sized. When we had 98 weather last year, I could maintain 72F indoor(ran 6 plus hours straight). If I speeded the blower up again, I could probably do 68(or lower) when it was 100 outside.

So to get good humidity control, I slowed it down.
I can maintain less then 50%RH most of the time(usually 48%RH).

Keep in mind. That I did a lot of tweaking to get it to work at 280 under the varing OD temps.
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Old 05-16-2009, 12:39 PM   #5
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Quote:
Originally Posted by beenthere View Post
Low ambient can be in the low 70s. If the humidity is low, then there is very little load.

OD 72 X 65%RH has roughly 3 more BTU's per pound of air, then 72 X 50%RH.

So with that loss of latent load, a coil can freeze.

My A/C is oversized for my place(It was here before me).
2.5 tons for 1650 sq ft. I could get away with a 1.5 ton.
I have good shading in the afternoon.

Give you an idea of how over sized. When we had 98 weather last year, I could maintain 72F indoor(ran 6 plus hours straight). If I speeded the blower up again, I could probably do 68(or lower) when it was 100 outside.

So to get good humidity control, I slowed it down.
I can maintain less then 50%RH most of the time(usually 48%RH).

Keep in mind. That I did a lot of tweaking to get it to work at 280 under the varing OD temps.
Sorry, I'm a little dense in these matters so please bear with me. I am unclear when you talk ambient if you mean indoors or outside (I suspect outside but I want to be sure).

For example, if my indoor temp and humidity is 85F and 65% (which is a high indoor load) yet the outdoor conditions are 65F and 35% humidity....

Can the AC be run without issue?

Said another way is the load on the AC coming from the outside conditions or the inside conditions?


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Old 05-16-2009, 12:59 PM   #6
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


I did say it a bit confusingly.

Although the outdoor RH doesn't have a great effect on the condenser.
It does have a big effect on the infiltrations latent load.

The outdoor humidity being high, would cause the indoor humidity to be high from infitration. Same as low outdoor humidity would cause the indoor humidity to be low.

Your outdoor example of 65X35%RH. Just about makes it impossible to have an indoor temp of 85 and 65%RH.

You would have to have a humidifier, or 30 active people in your house.

A low outdoor temp, causes a low liquid saturation temp, and a lower liquid pressure fed to the evap coil.
With a lower latent load. The evaporators saturation temp drops closer to freezing.
As both the indoor DB and WD temps drop, less heat is contained in the refrigerant. Causing the liquid pressure to drop more.
As the liquid temp and pressure drop more, so does the saturation temp of the evap coil drop, lower and lower until it drops below 32F.
Its an accumlative effect.



PS: if you take 65F 35%RH air. and heat it to 85F, its RH is now .3%
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Old 05-16-2009, 01:32 PM   #7
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Quote:
Originally Posted by beenthere View Post
I did say it a bit confusingly.

Although the outdoor RH doesn't have a great effect on the condenser.
It does have a big effect on the infiltrations latent load.

The outdoor humidity being high, would cause the indoor humidity to be high from infitration. Same as low outdoor humidity would cause the indoor humidity to be low.

Your outdoor example of 65X35%RH. Just about makes it impossible to have an indoor temp of 85 and 65%RH.

You would have to have a humidifier, or 30 active people in your house.

A low outdoor temp, causes a low liquid saturation temp, and a lower liquid pressure fed to the evap coil.
With a lower latent load. The evaporators saturation temp drops closer to freezing.
As both the indoor DB and WD temps drop, less heat is contained in the refrigerant. Causing the liquid pressure to drop more.
As the liquid temp and pressure drop more, so does the saturation temp of the evap coil drop, lower and lower until it drops below 32F.
Its an accumlative effect.



PS: if you take 65F 35%RH air. and heat it to 85F, its RH is now .3%
Sooooo essentially what you are saying is that if the outdoor temp is high and the indoor humidity is high those are the conditions that put the most load on the condenser..... and those are the conditions my coils are least likely to freeze if I decide to operate at 280 CFM/ton? Additionally, from what I can surmise it sounds like there are two loads on the the AC system; the latent load which you refered to I presume is the load required to bring down the humidity which I presume leaves the "sensible" load as the one refered to in bringing down the temperature. As I think about it I guess it makes sense that faster airflow impacts the temperature more and slower airflow allows the e-coil to get cooler and condense more moisture out the air resulting in lower humidity. I think

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Old 05-16-2009, 01:38 PM   #8
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Yep.

Sensible, is the heat you can measure with a regular thermometer.
Latent, is the moisture/humidty load.(heat added or removed to vaporize or condense moisture).
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Old 05-16-2009, 01:48 PM   #9
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Quote:
Originally Posted by beenthere View Post
Yep.

Sensible, is the heat you can measure with a regular thermometer.
Latent, is the moisture/humidty load.(heat added or removed to vaporize or condense moisture).
Thanks.
I spent the heating season learning about my furnace and how it works and ways to maximize efficieny, comfort, and equipment life, as well as improve air quality....(as you know being one of the key instructors .)

Now it is time for me to focus on the cooling system

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Old 05-22-2009, 09:17 PM   #10
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Quote:
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PS: if you take 65F 35%RH air. and heat it to 85F, its RH is now .3%
WOW, thats unbelievable
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Old 05-22-2009, 11:13 PM   #11
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


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WOW, thats unbelievable
But, thats the way it works.

Thats why, when its 20F and 45%RH outside, and your house has a lot of air infiltration. Your house is dry, if you don't have a humdifier.
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Old 05-23-2009, 08:28 AM   #12
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


ok, to get back on topic. I have my 2 ton ac unit runningat med hi on my blower setting. That roughly equals about 1000cfm. I havnt had any really hot/humid days, but yesterday it was about 78 degrees and about 55% humidity outside. Inside was 75, and 40% humidity. I turned on the ac and in an hour the humidity dropped 10%, but temp only went down about a degree. It felt much more comfortable though, i guess due to the lower humidity. Should i lower the fan speed? my next lowest option would be med low on the fan, which should blow about about 750cfm. What do you think? Thanks
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Old 05-23-2009, 08:35 AM   #13
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


Unless you have your manual, and know what static pressure your system is operating at.
You don't know what CFM a speed change will give.

Your still probably over 1100CFM at the med hi setting.
Setting it lower, wihtout knowing what teh CFM will be. Can cause harm.
When you get a mild day, it could damage teh compressor.
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Old 05-23-2009, 08:50 AM   #14
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


i do have my manual, and have the graph. How do i figure out esp? i need to take readings before the filter, and after the coil, correct? What do i need to measure that? Thanks
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Old 05-23-2009, 08:57 AM   #15
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Can I reduce my AC CFM/ton to under 300 to lower static pressure?


You need a manometer.
Weather a digital or water/u tube. Doesn't matter. As long as you can read in tenth and or 100th of an inch.
Between filter and blower.(at the furnace)
Between coil and furnace.

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