Very Technical Duct Insulation Question

[Carnak]

Well you have a multiple part problem beachwalker.

First off, you said these were variable speed, so they will run colder than 55F at times, assume 50F and take that as the outside duct wall temperature.

You are power venting the attic which I think is a bad idea because it just sucks humidity into the place.

So your heat transfer will be driven by the difference in the attic air temperature and the ductwall temperature assuming the foil back vapour barrier more or less reflects away most of the radiated heat.

So you can simplify your problem into two resistances in series.

Heat has to transfer from the air to the surface of the insulation , so lets just assume you have one mother of a power vent fan and treat this resistance like it was a blowing wind and give it a low and conservative value of 0.17 Btu/(hr ft^2 degF). Remember this is per square foot.

R convection = 0.17 Btu/(hr ft^2 deg F)

If this was a recatangular duct the rest of the problem would be much simpler. If you believe me skip everything that follows except the very last parargraph, else continue on.

Get the R-value of an insulated round duct, it is radial heat flow.

R insulation = ln (ri/rd)/(2 x pi x k x L)

ri would be the radius of the insulation and rd would be the radius of the duct. For example your 16 inch duct could be insulated with 2 inch duct wrap, 1.5 pounds per cubic foot density, that compressess by a maximum of 25% when properly installed. This formula works in the units of feet but since you are dividing you could still use ri=9.5 and rd=8, for your 16 inch diameter duct with a 2" insulation wrapped around at 1.5" thick.

You should be able to get the k values from a manufacturer, this is the thermal conductivity of the insulation.

2 x pi x L is part of the values used for the surface areas, however when they do the calculus to derive the formula, the 'r' term for radius has been manipulated out of this. The radial flow comes in to play as the surface area changes as the heat flows. Perhaps you can set L to being equal to a 1 foot of duct length.

When you use the convective R value of 0.17 remember to adjust it for the surface area of one foot of insulated duct. You would use the one foot length and the outside radius/diamter of the insulation to get this. 2 x pi x ri x L.

You should eventually be able to come up with two resistances in series and be able to add them up to get a total resistance.

total resistance = R insulation + R convection

Once you have the combined resistances, the heat tansfer per foot of duct would be equal to the difference of the attic and duct wall temperatures divided by the total resistance.

Btu/hr = temperature difference / total resistance

You can then isolate the convective resitance and or the insulation resistance to come up with the surface temperature of the insualtion, and hopefully that temperature will exceed the dewpoint of the ambient air.

Btu/hr x individual resistance = temperature difference.

The Btu flow through the convective air resitance and the insualtion are the same. Working from the convective air side, perhaps the surface temperature of the insulation would have a temperature differential of 2 degrres less than the attic air temp, therefore the surface would be attic temp minus 2.

If the surface temp is too low, like below the dewpoint you are shooting for, then you need a thicker insulation.

You could search "Steady-State Radial Conduction in a Long Hollow Cylinder" or "Critical Radius of Insulation"

OR

You can make sure that your ducts DO NOT LEAK, before you reinsulate them, and then have a 2 inch layer of ductwrap with an "INSTALLED" R Value of about 6.5 properly applied. Make sure the foil vapour barrier is intact so that humid air does not short circuit through the insulation and contact the cold ducts.

[jacob perkins]

Originally posted by Carnak
If you believe me skip everything that follows except the very last parargraph, else continue on.

Thanks Carnak.
I believe...I believe...I believe!

[Carnak]

Originally posted by ralphtheplumber
naima.org has a calculator for pipe insulation that'll do the same thing.

yes ralph they sure do, good suggestion

http://www.pipeinsulation.org/download3eplus.html

[beachwalker]

Carnak:
That link was right on point. Thanks...

Factory engineer was here today and their solution was to increase the cfm's to 1500 and jumper y1-y2 in air handler, so effectively, the variable speed fan is bypassed... so it runs at 1500 cfm for stage 1 and stage 2 compressor. This lowered the differential to about 12 degrees across the coil in an attempt to raise the duct temp and stop the condensation.

According to their meter, relative humidity in the attic was 100 % this morning and temp was 101.

I anticipate this change will allow sensible heat (air temp)reduction, but latent (humidity control) will suffer: humidity control now shut off. After running all day, temp is at 74 deg setpoint, but tstat indicates rel humid at 71%, which is up from where it had been at 65%. When humid control was on, it was between 50 and 55 percent.

They also suggested foaming the roof and sealing the soffits, then bringing in controlled amount of outside air for ventilation. Even maybe dehumidifying the attic. This in hopes of lowering humidity and temperature in the attic.

Any thoughts on these ideas?

[Carnak]

Originally posted by beachwalker
Carnak:
That link was right on point. Thanks...

Factory engineer was here today and their solution was to increase the cfm's to 1500 and jumper y1-y2 in air handler, so effectively, the variable speed fan is bypassed... so it runs at 1500 cfm for stage 1 and stage 2 compressor. This lowered the differential to about 12 degrees across the coil in an attempt to raise the duct temp and stop the condensation.

According to their meter, relative humidity in the attic was 100 % this morning and temp was 101.

I anticipate this change will allow sensible heat (air temp)reduction, but latent (humidity control) will suffer: humidity control now shut off. After running all day, temp is at 74 deg setpoint, but tstat indicates rel humid at 71%, which is up from where it had been at 65%. When humid control was on, it was between 50 and 55 percent.

They also suggested foaming the roof and sealing the soffits, then bringing in controlled amount of outside air for ventilation. Even maybe dehumidifying the attic. This in hopes of lowering humidity and temperature in the attic.

Any thoughts on these ideas?

101F and 100% no way. They need a new meter, a new battery or more time for it to stablalize.

The high speed will mean more sensible cooling, do you have a 3.5 or 4 ton system?

Are you running the air handler fan all the time, that will raise RH.

Still suggest you seal your duct work, and then when you re-insulate, ensure the vapour barrier is intact all the joints are taped.

Is the air handler sweating or just the ducts. If the temp and humidity readings were close to being accurate, that air handler and the auxiliary pan would be demonstrating the evaporation cycle. It would be raining into the pan.

Not up on all the lennox details, but I would be surprised if there was much more than 1/2 insulation in that cabinet, maybe 3/4. If that air handler is not sweating and your ducts are the problem is the duct issues already raised by me and everyone else replying to this thread.

[beachwalker]

Carnak:

I tend to agree with you relative to their meter... my own dry & wet bulb readings with digital thermometer have yielded closer to 61% rel hum and 75 deg. dew point in the attic.

To answer your other questions:
air handler is not sweating... only ducts
fan on auto, not constant
unit in attic is 36,000 btus

Not counting the flex, I have about 80 feet of 16" and 14" supply ducts. My next move is to cut off a section and put a new fiberglass wrap and see what happens. If that works, then put the cfms back to where they were and check for condensation again.

the foaming and sealing the attic is a foreign idea to me, but the factory guys claimed it is gaining popularity and solves the problem. It will be expensive to do, but as long as there is no potential bad things, like decreased shingle life, I will consider it.

Thanks for the perserverance.

[Carnak]

Air handler is in attic, it is not sweating but the ducts are.

The coldest thing in that attic will be the air handler casing just down stream of the coil. If anything is going to sweat it is going to be that.

It is a duct insulation/vapour barrier problem and or duct sealing problem as far as I can tell without being there.

[Carnak]

Fix the duct problem and put the blower back on its proper speeds.

[johnl45]

Originally posted by beachwalker
I have received lots of good info from this forum regarding my sweating duct problems with my two new Lennox high efficiency var spd air handler & 2 spd compressor heat pumps.

I am considering replacing the existing duct insulation and want to know if anyone has a link to software that can tell me, based on ambient attic temp and relative humidity, how much fiberglass duct wrap would be needed to eliminate condensation? Seems to me there must be a calculator out there somewhere.

Thanks.

R=TD/15

[AllTemp]

it would appear to me mor so than poor insulation is leakage in the duct system. Such as the gores on elbows, connector, start collars etc. I would concentrate on sealing these 1st. If your area of humidy is high you may require in addition to insulation a vapor barrier. I suspect if you are in an ara of hig huidy there are insulation company that perform this service proffesioanlly. I have seen some pretty nice viynal wrapped duct systems residentially in the easern part of Texas when I visited . Really looked nice almost like a heat wrapp job or something, all seamed nicely and tight no breach in the vapor barrier . Most important is those air leaks... Doesn't seem liek a lot to worry about but it is most likely your biggest problem at this time.

[dash]

1500 cfms on high and low,that's not acceptable ,IMHO.You are losing the benifits of dehumidification.

Having you insulate or condition the attic,although it will likely solve the problem,I question if it's needed,to stop the sweatig?

Ducts ,as stated are likely the problem.

Did the check and record the supply and return static?If the air handler is horizontial,a high return static will often "pull" water off the coil,blowing it into the ducts.

Your old air handler ,likely didn't have the static capability to do this ,but the VS fan does.

So if raising the cfm doesn't solve the problem,I'd have the static checked,above .3 on the return ,can be a problem.

[Carnak]

Dash

As an observation, with some high static and in particular, a high return static, they will pull moisute off of a coil.

In fact, the first time I saw this happen was with horizontal Carrier VS AHUs. What made this worse was they converted the air flow handing of the Carriers, but did not make a baffle change at the 'point' of the A coil.

But, this carried over moisture, in the horizontal air handler, would also soak the insualtion in the bottom of the cabinet, and the AHU would be sweating like crazy.

No condensation on this AHU speaks volumes to me.

[dash]

We have had them sweat ,in the cabinet,duct,or both.

Just remembered ,there can be a problem ,when turning the A-coil,wher the water blows off the top of the A,Those maybe the ones that carry a mist thru the blower and into the duct.

Yes,if it was backing up water in the pan and over flowing,the cabinet would be soaked.


This problem,at least with Carrier ,is more prevalent in humid climates,with Ft Meyers,FL,being named the worst.

[Carnak]

These were the ones that when you converted them to flow the opposite handing, you had to remove the plate on the top of the A and flip a baffle around. They were never properly converted.

FK4N maybe,don't have a catalog any more.

I honestly hate AHUs in the attic, and the variable speed ones can run the coldest and be the most prone to sweating.

I got the Fort Laud/Meyers humidity and then some

[beachwalker]

Carnak & Dash:

You're both right on.... the first air handler did not have the baffles changed over... so water missed the pan and soaked the insulation... and it rained under the air handler. Contractor replaced the air handler, now it doesn't sweat, which as Carnak says, speaks volumes.

Condensation has lessened on 1500 cfm setting, but I agree, loss of humidity control is not acceptable. As a test I am going to replace a 4' section of duct insulation and put everything back to the original cfms and enable humidity control (low stage compressor & 70% cooling cfm's) to see if that will stop it.

The bigger bullet to swallow is the garage because I will have to pull off a lot of sheetrock to get to the ducts.

Thanks, again, to everyone. You've been a big help.

[condenseddave]

Originally posted by prodiyer
I dont use a calculator, I do the math myself. NORMCHRIS WHAT DO YOU WANT A COOKIE !

So, you can't do the math, yourself, there, scooter, and are jealous?

Just ask, we'll teach ya.

No need to make your ignorance shine so brightly in front of everyone.

[control_noob]

Originally posted by condenseddave

Originally posted by prodiyer
I dont use a calculator, I do the math myself. NORMCHRIS WHAT DO YOU WANT A COOKIE !

So, you can't do the math, yourself, there, scooter, and are jealous?

Just ask, we'll teach ya.

No need to make your ignorance shine so brightly in front of everyone.

I thought the same thing Dave... Typical of someone who is new here and doesn't care to learn about the site and it's members before acting like an idiot. Kudos to Norm for letting it slide, I don't think I would have been able to.

[Carnak]

prodiyer sure sounds like eddy, was awaiting to see norm's math tho

[dan sw fl]

Originally posted by beachwalker
Thanks... According to their meter, relative humidity in the attic was 100 % this morning and temp was 101.

Any thoughts on these ideas?

ALTERNATE R & D:

Could I get your Attic SPECS
so that I can develop and copyright a Cheap SAUNA?

[beachwalker]

Looks like this thread has headed toward the comedic side, so as long as you bring up saunas, when we built this house in 1998 we installed a steam generator in the master shower. The builder asked me why I wanted to do that, since, as he put it, "This whole place is a steam bath".