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Old 04-21-2020, 06:47 PM   #16
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Re: Differential temps


low dehumidification, more noise.

lower than normal airflow can cover up being oversized which in itself reduces dehumidification.
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Old 04-21-2020, 07:42 PM   #17
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Re: Differential temps


Right. I would rather not move it to high. I do have a bonus room that I can run a duct to if the numbers allow for it. I'm guessing that would help with the oversizing.
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Old 04-22-2020, 05:04 AM   #18
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Re: Differential temps


Is your A/C a high velocity system?
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Old 04-22-2020, 07:03 AM   #19
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Re: Differential temps


I dont think so. Standard single stage 3.5 unit. 1700 sq feet but we have unconditioned bonus room that I was thinking of running a duct or two to. On the TD I had read to take the average return temp. I have 83-84 degrees coming from the attic from a smaller return duct but I have 79 degrees from the low return. If I put the meter in the air filter slot I get 79 degrees. If I use 79 as the return temp my delta T is 21-22 which could be in line depending on other factors.
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Old 04-22-2020, 11:37 AM   #20
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Re: Differential temps


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Originally Posted by dorlando View Post
Right. I would rather not move it to high. I do have a bonus room that I can run a duct to if the numbers allow for it. I'm guessing that would help with the oversizing.
Why not if you can lower your utility bills in doing so? Easy, cheap.

There's a way to slow it down and improve dehumification on demand and have it run at high speed the rest of the time.

Of course cfm has to be verified before adjusting the speed, it may be set correctly now.
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Old 04-22-2020, 12:55 PM   #21
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Re: Differential temps


I have a call into my tech. I have a separate question. How much should the suction line outside be sweating? It is about 82 degrees and the unit was running for maybe 8-10 minutes when I checked. The line was cold and there was hardly no sweat under the insulation. The uninsulated piece, short piece, only had a little sweat on it. Humidity outside is 30%. Should i be seeing more sweat? I also did a delta T on the condenser and ambient temp and was only getting around 10 degrees.
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Old 04-22-2020, 01:02 PM   #22
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Re: Differential temps


Depends on amount of humidity outside.

Depending on operating conditions, the suction line should be between 45 and 60F.

Without measuring pressures too, it's a guess if it's properly charged or not.

I don't think your unit is low on refrigerant with a 26F drop across the coil.

Quote:
was only getting around 10 degrees.
Depends on the design of the condenser - cfm vs heat rejection.

Also depends on where you measured, some parts of the coil may be cooler than others.

Outdoor unit airflow always exceeds indoor airflow, hence the lower delta-t.

It's easier to get accurate measurements on the indoor side than outdoor.
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Old 04-22-2020, 08:29 PM   #23
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Re: Differential temps


Quote:
Originally Posted by dorlando View Post
How much should the suction line outside be sweating? It is about 82 degrees and the unit was running for maybe 8-10 minutes when I checked. The line was cold and there was hardly no sweat under the insulation. The uninsulated piece, short piece, only had a little sweat on it. Humidity outside is 30%. Should i be seeing more sweat?
With 30% humidity you won't see much sweating.

Have your guy look it over but if the coil isn't freezing over, and you don't want to increase the airflow (because of noise and drafts), I'd just leave it alone.
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Old 04-22-2020, 09:05 PM   #24
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Re: Differential temps


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With 30% humidity you won't see much sweating.

Have your guy look it over but if the coil isn't freezing over, and you don't want to increase the airflow (because of noise and drafts), I'd just leave it alone.
Increasing airflow reduces capacity loss to attic and overall improves efficiency.

The normal coil temp for a older 10 seer with 80F return air is well above 40F so it can still have low airflow and yet not freeze up.

If the flow rate is under 350 cfm per ton it absolutely should be increased.
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Old 04-22-2020, 10:27 PM   #25
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Re: Differential temps


Not knowing the specifics of your home I have made some generalizations.
Keeping the thermostat as high as you do will limit the dehumidification you get.
Do you keep the fan on all of the time? That will help equalize temperature when the unit is not calling for AC. You mentioned 1700 sq. ft.
How well is your home insulated? How old is it?
The temp. differential is fine, very unlikely that you are under charged.
The sweating is relative to outdoor humidity. Years ago, before all of the sophisticated testing equipment, we measured the effectiveness of the HVAC by touching the suction line....it was supposed to be "beer can cold".
All of that has changed.
Your humidity will be lower if you lower the temperature. If you don't want to do that then you can get a de-humidifier. There are a lot of factors to consider.
I wouldn't worry too much about the supply air temp, the sweating of the refrigerant lines. With HVAC the bottom line is comfort.
That is really all that matters.
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Old 04-22-2020, 10:33 PM   #26
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Re: Differential temps


Energy consumption and utility bills matter to a lot of people.

Quote:
That will help equalize temperature when the unit is not calling for AC.
great way to re-evaporate condensed moistured and boost the humidity level 5 to 10+ % points up.
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Old 04-23-2020, 12:21 AM   #27
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Re: Differential temps


Quote:
Originally Posted by dorlando View Post
The line was cold and there was hardly no sweat under the insulation. The uninsulated piece, short piece, only had a little sweat on it. Humidity outside is 30%. Should i be seeing more sweat?
The Dew Point temperature of the ambient air is what determines sweating. Siri has a problem telling us that but if you don't have a Sling Psychrometer the DP temperature will be the same as your local weather station or nearest air port.
http://www.usairnet.com/weather/maps...a/temperature/
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Old 04-23-2020, 04:31 AM   #28
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Re: Differential temps


82F 30%RH=47.6F dew point. So should be very little to no condensation/sweating on the suction line. Unless your keeping your indoor temp below 70F.
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Old 04-23-2020, 05:13 AM   #29
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Re: Differential temps


Thanks for all the replies. Sounds like unit might be working properly but I may still have it quickly checked out to get the airflow rate. I think I recall the manual stating a medium speed puts out 1250-1400 cfm depending on pressure. How about return air? Will more return air increase the cfm per ton output? My return air vents calculate out to 130 sq inch per ton. I have read that is kind of low. I have a 20x13 opening grille going directly into bottom of air handler and two 13x7 openings going into a 3 1/2" 2x4 wall, so probably not a whole lot of air there. I know other calcs would need to be made but does it look like I might need more return air?


House is 1700 sq feet, built in 1970's, concrete block and we have had insulation blown in but we get all day sun.
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Old 04-23-2020, 12:02 PM   #30
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Re: Differential temps


What's the full model number of the air handler?

What you need is a static pressure test and measurement of wetbulb return temp in addition to temp drop, otherwise you can only guess at what's going on, if airflow is low or not.

I'm unclear on what problem you were originally out to solve -> bad distribution? High energy bills?

If you buy the cheapest digital manometer out there, tubes and test tips you'll probably spend less on tools than what a tech would charge.

Not to mention it may be hard to find someone who actually knows to check static.

There are tons of videos on youtube - checking static and what the readings mean.

Wetbulb you can get by wrapping the thermometer end in moist cloth.
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