# Can I reduce my AC CFM/ton to under 300 to lower static pressure?



## key1cc (Nov 21, 2008)

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

Key1


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## beenthere (Oct 11, 2008)

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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## key1cc (Nov 21, 2008)

beenthere said:


> 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.
> ...


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?

Key1


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## beenthere (Oct 11, 2008)

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 72°F 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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## key1cc (Nov 21, 2008)

beenthere said:


> 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.
> 
> ...


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?


Key1


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## beenthere (Oct 11, 2008)

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 65°X35%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 32°F.
Its an accumlative effect.



PS: if you take 65°F 35%RH air. and heat it to 85°F, its RH is now .3%


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## key1cc (Nov 21, 2008)

beenthere said:


> 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.
> ...


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 :001_unsure:

Key1


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## beenthere (Oct 11, 2008)

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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## key1cc (Nov 21, 2008)

beenthere said:


> 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 :thumbsup:.)

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

Key1


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## Yamvmax (May 16, 2009)

beenthere said:


> PS: if you take 65°F 35%RH air. and heat it to 85°F, its RH is now .3%


WOW, thats unbelievable


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## beenthere (Oct 11, 2008)

Yamvmax said:


> WOW, thats unbelievable


But, thats the way it works.

Thats why, when its 20°F 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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## Yamvmax (May 16, 2009)

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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## beenthere (Oct 11, 2008)

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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## Yamvmax (May 16, 2009)

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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## beenthere (Oct 11, 2008)

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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## Yamvmax (May 16, 2009)

any links to a "cheap" water/utube manometer? i am guessing homedepot or lowes wouldnt have them. i dont want to buy the wrong one. thanks again


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## beenthere (Oct 11, 2008)

No, I got my u tube from a wholesale supplier.
And the rest of manometers are digital, starting at 150 bucks, on up.


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## Yamvmax (May 16, 2009)

i dont know if links can be posted, but would something like this be ok?
http://www.dwyer-inst.com.au/htdocs/pressure/Series1221-1222-1223Price.cfm#CRA
http://www.dwyer-inst.com/PDF_files/1221_cat.pdf


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## beenthere (Oct 11, 2008)

You can use one of them.
they are hard to read at times though.

When you use it. And the level changes by say .1" on one side. Double that, and that is the actual reading.
Because the level actually changed on both sides.


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## key1cc (Nov 21, 2008)

Yamvmax said:


> 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


I have attached a video link to a site that may provide you some insight on testing ESP.
Watch the "Shawn on ESP" Video.

http://www.bacharach-training.com/ESP100.wmvhttp://www.bacharach-training.com/

Also, I have attached a link to the dwyer magnehelic guage that I use to check ESP (in the amazon.com link) below

http://www.amazon.com/Dwyer-1-0-Wc-...ref=sr_1_2?ie=UTF8&s=hi&qid=1243100291&sr=1-2

Key1


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## Yamvmax (May 16, 2009)

key1cc said:


> I have attached a video link to a site that may provide you some insight on testing ESP.
> Watch the "Shawn on ESP" Video.
> 
> http://www.bacharach-training.com/
> ...


 
Thanks. for some reason the video wont open. when i click on the link on the website, media player opens, but nothing happens.


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## key1cc (Nov 21, 2008)

Yamvmax said:


> Thanks. for some reason the video wont open. when i click on the link on the website, media player opens, but nothing happens.


I edited the link
Now try it.

Key1


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## Yamvmax (May 16, 2009)

well, 2 different computers no luck. Thanks anyway


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## key1cc (Nov 21, 2008)

Yamvmax said:


> well, 2 different computers no luck. Thanks anyway


I am sure this will work.
Go to post number 44 in the attached link and try it again..

Key1

http://hvac-talk.com/vbb/showthread.php?t=278462&highlight=video


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## Yamvmax (May 16, 2009)

ok, got it. media player was screwed up. thats a great video. thanks. so i guess i will need probes also? Damn. this is getting expensive. Is there a fairly cheap way to do this?


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## key1cc (Nov 21, 2008)

Yamvmax said:


> ok, got it. media player was screwed up. thats a great video. thanks. so i guess i will need probes also? Damn. this is getting expensive. Is there a fairly cheap way to do this?


You really only need 1 probe. Link attached. You can measure the positive and negative seperately. You do not need to measure them at the same time. 

http://www.amazon.com/Dwyer-303-Por...ref=sr_1_2?ie=UTF8&s=hi&qid=1243114950&sr=8-2

Personally I feel that static pressure is one of the most important things you can know about your system. That way, when you make changes you will understand if the impact is potentially positive or negative for your system. You will also be able to measure the "back pressure" caused by your filter or stated as the pro's say "the pressure drop across your filter". It will give you insight if you want to move to a better filter. It will also provide insight if one area of your home is less comfortable and you need to close dampers or register grills to get a more even temp. Essentially being able to measure static pressure is kinda like having xray vision to the changes you make on your system.
Have fun.

Key1


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## Yamvmax (May 16, 2009)

thats great. i really appreciate all the advice. i didnt realize how complex a HA system is.


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## Yamvmax (May 16, 2009)

Update guys. I got my magnehlic gauge. My esp is .55. I am guessing this is very good. Now back to my orig question. Based on the furnace stats, i am pushing 775 CFM at the fan speed i am using for the AC. I have a 2 ton unit, so i fairly close to the 400 per ton. If i were to lower the speed I would be at 536 CFM, which would be too low. If i increased the speed, I would be at 1025 CFM, which is slightly too high. So i should leave it exactly where i am, correct? Thanks for any input. BTW my ducts are 50 yr old galvanized. i am reading .40 just after the filter, and .15 right before the acoil. Do these sound like good numbers? I disassembled and thoroughly cleaned the insides, and sealed them back up with mastic. What do you guys think?


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## beenthere (Oct 11, 2008)

Must have big supply duct to have that low of a supply static with a wet coil.

One thing you may have made a mistake on though.

When you look at your blower cahart to see what CFM you would be moving if you lower the blower speed.
You don't use the .55" ESP.
When you slow the blower down. Since its moving less CFM, your static pressure decreases.

So you may have to look at .3" or .4" on the fan data chart.

EG"
At 775 CFM and .55" TESP. When only moving 536 CFM, the TESP would drop to .26".

Since you don't have a VS blower that controls RPM to regulate CFM.

Switch it to the lower blower speed, and see what Static you have in your ducts. And then compared that to your fan data chart.


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## Yamvmax (May 16, 2009)

beenthere said:


> Must have big supply duct to have that low of a supply static with a wet coil.
> 
> One thing you may have made a mistake on though.
> 
> ...


I do have a large return. It is only a 2 bed 1000 sq ft house, but has a return in main living area, and one in each bedroom. SO you are saying take an ESP at each blower speed? Then look at the chart?


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## beenthere (Oct 11, 2008)

Yes.

Set it to a lower spped, check and record esp. Then if you still have another lower speed. Set it to that, take and record it.
Then look at chart for which one is acceptable.


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## Yamvmax (May 16, 2009)

ok new numbers. I am getting confused.
esp at 
HI .90+.37=1.27
MED HI .80+.30=1.10
MED LO .45+.15=.60
LO .20+.05=.25

HI and MED HI are off the chart. My Chart only goes up to 1.0. Low at that ESP would be pushing 600 cfm, so i am guessing too low. Med LO at the esp would be pushing 760, a little low but very close. My next option at MED HI at that ESP with my guesstimate is about 700 CFM. So do i stick with the MED LO? ANy suggestions? There is no MED option. What are your thoughts? Thanks


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## beenthere (Oct 11, 2008)

I would stay with the med low, if your not having any humidity problems.

If you want your humidity lower then what med low is providing.
there is no harm from setting it to low, since its moving 700 CFM.


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## Yamvmax (May 16, 2009)

beenthere said:


> I would stay with the med low, if your not having any humidity problems.
> 
> If you want your humidity lower then what med low is providing.
> there is no harm from setting it to low, since its moving 700 CFM.


I will be leaving it at MED LO, I havnt had any really hot weather yet. It has been a bit humid though, and the few times i have turned it on, it has removed about 10% humidity in an hour. As far as using the LO speed, it is only pushing 600 CFM, was that a typo on your part? I guess the true test will come wth some higher temps, and humidity. It has been quite cool here on Long Island.


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## beenthere (Oct 11, 2008)

I just misread your post, and thought it said 700 for low. 
On med high, it would be moving closer to 950CFM in order to have that high of a ESP. When your med low ESP is .60" at 760CFM.


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