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Old 04-24-2020, 05:33 PM   #31
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Re: Differential temps


So I have someone coming out on Monday morning to run the diagnostic tests, static pressure, delta t, airflow, etc. It will be cooler here on Monday morning, like 70 degrees or so. Will that skew any of the calcs like Delta T? He won't be able to see how I get 83 degrees from the attic return because it won't be that hot. He will probably get a much cooler temp and I imagine it would show a 21 TD instead of the 26 I see at times.
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Old 04-24-2020, 07:11 PM   #32
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Re: Differential temps


Less heat gets picked up from the attic in cooler weather - having a hot attic doesn't increase the split at the air handler, just increases the temp at the vents.

Humidity level impacts the temp drop much more so than anything.

May get different readings but if the tech is competent and knows how to interpret them, shouldn't be an issue.

You should download and print the fan performance table for your air handler so the tech can determine airflow based on measured static.
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Old 04-24-2020, 07:15 PM   #33
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Re: Differential temps


Quote:
Originally Posted by dorlando View Post
So I have someone coming out on Monday morning to run the diagnostic tests,
From everything you have said your AC is cooling properly.
I think that any excess humidity you have is because of the relatively high temperature you keep your thermostat at.
With that being said....why fix it if it "ain't" broke?
Personally I think you are worried about something that doesn't need to be taken care of.
Call a tech when it breaks, not because of some "temperature differential" that you have been researching.
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Old 04-24-2020, 07:19 PM   #34
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Re: Differential temps


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Originally Posted by Missouri Bound View Post
From everything you have said your AC is cooling properly.
I think that any excess humidity you have is because of the relatively high temperature you keep your thermostat at.
With that being said....why fix it if it "ain't" broke?
Personally I think you are worried about something that doesn't need to be taken care of.
Call a tech when it breaks, not because of some "temperature differential" that you have been researching.
Missouri Bound - If I want to add a duct run from the plenum to a new room which may also require a return to be added do I need to have measurements taken first? I understand the possibility of loss of airflow but when I tested the run i don't think I lost much airflow in the house.
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Old 04-24-2020, 07:39 PM   #35
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Re: Differential temps


Quote:
Originally Posted by Missouri Bound View Post
From everything you have said your AC is cooling properly.
I think that any excess humidity you have is because of the relatively high temperature you keep your thermostat at.
With that being said....why fix it if it "ain't" broke?
Personally I think you are worried about something that doesn't need to be taken care of.
Call a tech when it breaks, not because of some "temperature differential" that you have been researching.
You can't tell if an a/c is cooling properly and efficiently based on drybulb temp drop alone, 26F is high, the humidity better be like under 40% to get that kind of drop.

At 80F and 50% humidity, should have a 19F delta-t +/- a little.

With the same dewpoint but hotter return, should have a 20F drop +/-.

This is probably an oversized a/c on undersized ducts with below normal airflow - which actually covers up for being oversized but wastes energy.

I don't believe high humidity was mentioned as being a problem in this case. 50% reported is pretty normal.
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Old 04-24-2020, 07:42 PM   #36
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Re: Differential temps


Quote:
Missouri Bound - If I want to add a duct run from the plenum to a new room which may also require a return to be added do I need to have measurements taken first?

The static pressure measurement is actually useful for assessing the duct system - if it's high on supply side, adding duct may help cut it down, if it's already low on the supply side, adding another supply run may starve existing rooms of airflow.

Individual trunk lines need to be sized large enough for what you're adding - you don't want bottlenecks.

You were never clear on what you were trying to do.
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Old 04-24-2020, 07:53 PM   #37
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Re: Differential temps


Great comments back. The 26 drop was from the attic return that heats up 7 degrees from morning till evening. If I measure the return at the filter I get a 22 drop. Since the attic return is bringing back so little air from the wall cavity maybe that isn't the best place to take the temp.

I know this isn't how you test it but the airflow is twice as much in some rooms compared to the old system. I can't imagine the flow being stronger but maybe it is supposed to be.

I have a few things going on. 1. Is the unit oversized and what do I do? 2. Can I add a supply run to a bonus room? When I tested it I didn't seem to lose much airflow elsewhere. Maybe that will help with the oversizing 3. Are my returns big enough. I calculated 130 square inches per ton. 4. Can I add a return if I add a supply in the bonus room. 5. When I tested running the supply, about 45 feet, I got good airflow but I lost 8-9 degrees in the attic heat. The temp on a hot day was coming out at 67 degrees. Not sure if that will cool the room

The temp at the plenum is around 56 degrees and flows strong. The unit cycles around 15-20 minutes in mid 80's. I can cycle for 13-14 minutes at times in late evenings when it is 78 outside.

Last edited by dorlando; 04-24-2020 at 07:56 PM.
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Old 04-24-2020, 08:05 PM   #38
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Re: Differential temps


Quote:
Great comments back. The 26 drop was from the attic return that heats up 7 degrees from morning till evening. If I measure the return at the filter I get a 22 drop. Since the attic return is bringing back so little air from the wall cavity maybe that isn't the best place to take the temp.
What's important to measure is how the unit itself is cooling- after that you worry about duct loss.

Quote:
I know this isn't how you test it but the airflow is twice as much in some rooms compared to the old system. I can't imagine the flow being stronger but maybe it is supposed to be.
Points to undersized ducts/vents or oversized a/c.

If the house is cooling unevenly, you can put dampers on the branches to the rooms getting too much flow and adjust. It needs to be done carefully - you can freeze the coil dampering down too much.

Possible longer branch runs should have larger duct.

Quote:
1. Is the unit oversized and what do I do?
Nothing until it needs to be replaced.

Quote:
2. Can I add a supply run to a bonus room? When I tested it I didn't seem to lose much airflow elsewhere. Maybe that will help with the oversizing
Yes, but if you have a trunk/branch system, trunk may need to be enlarged.

Or can be taken right from plenum.

Sketch out entire duct system with sizes (and rough lengths) noted and I can give you a general idea.

Quote:
4. Can I add a return if I add a supply in the bonus room.
Yes.

Quote:
5. When I tested running the supply, about 45 feet, I got good airflow but I lost 8-9 degrees in the attic heat. The temp on a hot day was coming out at 67 degrees.
Using metal duct and wrapping with insulation instead of pre-insulated flex can help - smaller duct required to get same airflow, less exposed surface area.

Very hard to work with in an attic though.

Increasing airflow and supply temperature by lowering duct pressure (adding run) or increasing blower speed will reduce loss to attic a little.
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Old 04-24-2020, 08:16 PM   #39
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Re: Differential temps


I can and did take it from the plenum in my test. Airflow in rest of house was still strong. Maybe points to oversized system but I'm hoping with an extra run or two into bonus room that will help with the oversizing. I might take one run off a trunk that has very strong flow, I think partially because the vents are directly under the trunk - there is no branch off. I have to close one of the vents more than halfway.


I'm trying to not have to turn blower on high to reduce noise. Is it possible to take the vapor barrier off the flex, wrap additional insulation around it with new vapor barrier? If I can't get the supply temp down from 67 I don't know if it will cool the room. I can increase the flex size as that may help push the air faster through the flex so it doesn't sit in the attic too long. I might also be able to shave 15 feet off the run which I would think would help.
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Old 04-24-2020, 08:24 PM   #40
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Re: Differential temps


Increasing duct size alone won't necessarily reduce duct loss, but u can get more airflow to that room.

Quote:
Is it possible to take the vapor barrier off the flex, wrap additional insulation around it with new vapor barrier?
You can try.
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Old 04-24-2020, 08:28 PM   #41
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Re: Differential temps


ok. Thanks. On taking the barrier off, do I create any condensation issues? I am in Florida where humidity can be 80% or more. I would take the barrier off, wrap faced insulation with the face out, and tape the seams. Since there is only one vapor barrier left would that eliminate any risks on condensation forming on the interior liner? Is a faced barrier appropriate for that use?
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Old 04-24-2020, 08:35 PM   #42
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Re: Differential temps


there should only be one barrier and on the hot side.

you should have the plastic flex duct, the insulation over it and barrier.

You may want to see if the existing duct is only R-4 or something -> it may be easier for you to just use new R8 than try to upgrade old duct.
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Old 04-24-2020, 08:38 PM   #43
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Re: Differential temps


It is currently R6. I don't know for sure but I would have to think the change from R6 to R8 would only be marginal. If I take the barrier off I was thinking of wrapping R13 over the R6. I would just have the one faced barrier on the hot side.

Based on our discussions do you think I need to have a tech take readings?
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Old 04-24-2020, 08:48 PM   #44
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Re: Differential temps


you're right, it would be a marginal improvement going from r6 to r8.

You pretty much only need a tech to deal with refrigerant charge.

If you're comfortable measuring static pressure (buy manometer and tips) across different parts of the duct system, you can see how the duct system is doing without a tech.

The purpose of taking static is to check for bottlenecks (for example undersized return) by checking different parts of the system separately and also determine airflow using the fan performance table.

The indoor coil and blower need to be clean before bothering to test and you can check.

You can tell a lot about how a system is running just with static pressure and wetbulb temperature readings.

In all this - what is the main problem you're trying to solve?
What prompted doing testing?

Uneven cooling? Too much noise? Hot room?
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Old 04-24-2020, 08:55 PM   #45
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Re: Differential temps


To be honest, there really isn't a problem, kind of like Missouri Bound said. I thought I had a high TD but that may only be because of the return duct picking up heat. It may still be a little high but that may not be an issue. The air blows very cold and strong. Every room is comfortable. Maybe I was just paranoid? However, I do want to run the duct from the plenum and need to get the temp to stay down and not 67 degrees at the end of supply.

I don't think the tech coming out is planning on taking static at different parts. He just mentioned at the plenum and said he wouldn't need to get into the attic.

If system is running fine maybe I should cancel, I can always try and do it myself.

I was a little concerned with whether or not the blower is getting enough air based on undersized returns, but my airflow is very strong and old. I know I can't compare to old system but I was barely able to feel the air in some vents before and now I feel it blowing on my face standing underneath or a couple feet away.
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