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Fix or replace duct work?

5K views 27 replies 5 participants last post by  user_12345a 
#1 ·
Hello All,

I have a house that was built in 1958. It is a ranch on a crawl space. The furnace sits in the garage. This is a Carrier Downflow 100k btu furnace w/3 ton AC. The house is a total of 1700 sqft (Including the garage) Maybe 1400 ish for living only. Supply's are from the crawl and returns are in the attic.

I am in need of a new system. The current system was installed in 2003. I have been told it has a cracked heat exchanger, and I know the exhaust motor need replaced. It leaks enough water that it filled my burner box. I currently have this band aided with a Mountain Dew Bottle diverting the water to a condensate pump.

SO far I have had two companies out. On guy said i needed all new duct work. One says I need only the return duct work fixed. The supply is 8X16 from the garage to the other end of the house (30 feet ish). The return is where I know I have an issue. Its 14 inch spiral round duct. I have one central return in my hall. There are 3 bedrooms that are tied into the same panning as the 14in return.

First guy told me that each bedroom needed to have its own return line to the main plenum box on top of the furnace. This is 20-1/2 X 20-1/2. He said he would change the 14 metal pipe to 14 flex r6 duct, and then run the 3 bedrooms with 6in R6 duct to the main box. He would add a return to the Living room and install Boots at each location because they are only panned right now.

2nd guy just said it all needs to be replaced. He would put a plenum box 15 feet into the attic with a 14 in duct back to the furnace and run each bedroom to that new plenum box.

My question is this. Because each bedroom is cold when i close the door. Who is right in fixing this issue? Im fairly handy and think i can tackle the attic issue. I just don't want to mess something up more than it is. THe current piping all not attached to anything and is drawing outside air. This caused issues last summer with 80% and higher humidity in the house.

With the supply duct in the basement im leaning toward leaving it unless there is a real reason to change it out. I have 10 runs off that main trunk. I want to delete or move one run. 1st off the main trunk is a 10inch pipe to the kitchen then a 6in into the kitchen maybe 4 feet away. I have 2 6in supplies into the LR and one each into the bedrooms that are 6in. Then I have 2 6in runs into the Family room (flex duct going through a brick foundation wall) and one 6in into the Bathroom. I would like to delete the 10in duct unless it is needed for air flow. If that is the case then I want to move it to the family room because it stays warmer in the summer or cooler in the winter.

I want to thank you in advance for your help. I can make up a drawing if it helps. Sorry for the long post. Each HVAC Company has quoted ~1200 for the Attic duct work and the one company quoted me 3500 for replacing all the crawl duct. I don't know if that is out of line or not.
 
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#18 · (Edited)
That's of very questionable accuracy and isn't room by room.

The air infiltration is set to poor which may not reflect your house. If it's that bad, fix the house. i suspect poor would be for a super-drafty 100 year old house with no improvements.

Doubt you have summer design of 98f in indiana.

The duct heat loss is 21 000 btu per hour; that's absolutely crazy. If your ducts are that bad, fix them -> insulate them or better yet convert the crawlspace to a semi-conditioned space.

It shows floor over crawl-space with no insulation. If that reflects reality, deal with that.

Fix the house first. Insulating accessible things like crawlspace walls or ducts and air sealing is affordable and saves money on heating system upgrades.

If you don't have the funds to deal with the heating system and fix the house at the same time, deal with the house now and wait on the heating system.

It's march already, you probably have only 2 months of the heating season left.

Heck - the others will frown upon this - it's not proper, but if the crack(s) in the heat exchanger is (are) hairline, not open and not producing flame disturbances, I would be tempted to even keep running it for this season, leaving enough time to do things properly.

The heat exchanger runs under negative pressure and the real risk is having flames or exhaust being blown out of the front of the unit.

You can get a monoxide alarm with digital display (which reads low levels) and ride it out.
 
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#19 ·
i suggest playing with this -> http://efficientcomfort.net/asp/ResDuct_Web/ResDuct_Web.asp

Set the friction rate between 0.08 and 0.1" and play with the numbers to get an idea of what airflow different ducts can carry. the actual rate you need may be different than what you input.

I was wrong about the 8x16 being for 600 cfm or less, closer to 750. Still too small for the 1200+ cfm 100k/3 ton need.
 
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#2 ·
First guy told me that each bedroom needed to have its own return line to the main plenum box on top of the furnace.

Because each bedroom is cold when i close the door. .

(i can't speak for your other problems).

there could be scenarios where that is true = large houses perhaps.

but, i can say, that at least in my case = BS !!!
i had just 1 return at the furnace wall in my old place. and here, i replaced all of the ducting. with just one return close to the furnace. then just 1 supply in each room. this works just fine.

if the room is ok with the door open. then closing the door seals the room to much. 1" spacing under the door will work well.

YMMV
 
#3 ·
Do the common door test to determine how much return is needed and if the common 1 inch door clearance at the bottom will suffice.

Hollow core doors work well for the test. When the door ceases to close when the unit is on cool and or the highest blower speed setting with new filter(s), calculate the square inch of the opening top to bottom and compare to a 1 inch gap at the bottom.

A pressurized room is a loss of conditioned air you've already paid for. Where does that air go? Some out the window seals, some from non sealed areas into the attic from baseboards, receptacles, switches and ceiling light fixtures. All this escaped air into the attic area promotes ice dams.

If our ears pop when we enter or exit the room , Houston, we know we have a problem.:surprise:
 
#4 ·
I can say that the doors don't close. If i leave them half open or 1/4 open they stay that way.

Ears don't pop when we walk in or out of any room. I have done this with no air filter installed because i have a 4inch thick filter. i wanted to take this off the table as a possible issue.
 
#5 ·
I believe the first guy's solution is better. As I understood the second guy would get a return from the ceiling. As the hot air gets out of the register it will float to the ceiling and short-circuit to the return. Your furnace would reheat the same air and on the next run the segregation of cold and hot air in the room would be even greater. The short-circuit would be even more pronounced.

Adding new return grills in the floor and connecting those to the main plenum seems to make much more sense to me.

If your main problem was having A/C being distributed properly, I'd pick the returns in the ceiling, because the segregation would work the other way.

Of course the short circuiting is not that direct. A good amount of heated air would get mixed with cold air and increase your comfort compared to the situation right now. However, if you avoid it at all you will be better off.
 
#8 ·
Is the existing unit 80 or 90%+?
What are the branch duct sizes?

Just based on your description, it really sounds like a grossly oversized furnace on undersized ducts. Same for the a/c.

It is not normal to lose a heat exchanger after 13 to 14 years. A furnace from 2003 should have lots of life left.

Your heat exchanger is probably covered by a 20 year warranty, but this is not due to a manufacturer's defect. you could pay the labour and it will crack again.

No, you do not need return in each bedroom although it would be nice - a properly sized central return can do fine.
 
#9 · (Edited)
The supply is 8X16 from the garage to the other end of the house (30 feet ish). The return is where I know I have an issue. Its 14 inch spiral round duct. I have one central return in my hall. There are 3 bedrooms that are tied into the same panning as the 14in return.
8x16 is good for 600 cfm, tops. (with larger sizes being needed for longer length of the longest run.)

Your old 100k furnace will need at least 1200 cfm if it's a 17" wide cabinet and 1400+ CFM if it's 21" wide with a 4 ton blower.

3 ton a/c needs at least 1050 cfm but preferably 1200+.

With a 3 ton blower if you're lucky it's was probably moving 800 to 900 on high, on that duct system. Furnace was probably cycling on limit hence the short heat exchanger life.

Return flex i'm not familiar with, but doubt 14" can flow more than 700 cfm on a return, possibly less if flex.

Now, what needs to be done is a accurate heat loss and gain calculation (including duct losses) so you can see how much capacity you need.

The smaller equipment you can get away with, the less money you have to spend on ducts.

{ on a related note - Do not rule out improvements to the house to get smaller equipment. It's cheaper in the long run to cut heat loss than pay to re-do ducts, and be stuck with high utility bills.

For example, your crawlspace? I'm betting it's vented, the same temperature as outside and you have floor insulation. and the ducts are losing heat to the crawlspace. (or gaining heat in the summer) A heck of a lot of energy can be saved closing the vents and air sealing, insulating the walls, putting a barrier over the floor if it's dirt. It's so much better to do that than rely on insulation between floor joists - having thermal bridging and relying on R6 to R8 insulation on air ducts, trying to stop heat moving from 120F air to less than 50F air.}

If you end up only needing a furnace and ac which need 800 cfm to work right, you may be able to get away without re-doing any ducts.

The trick is to get a furnace with a 1200 cfm/3 ton blower, 17" cabinet even if you need less. With an oversized blower, you will be able to get 800 cfm through those ducts; it will be noisier, it will not be as efficient when it comes to electricity use, but it save you lots of money having to re-do ducts.

The fan speeds are adjustable; the key is getting a good contractor who will check airflow in heating and cooling mode and adjust speeds accordingly.
 
#10 ·
Few things. As promised I am attaching 2 diagrams. One is of the supplies and where they are located. As mentioned earlier I would like to get rid of the 10 inch supply in the kitchen. Tonight I will see about going under the house to measure the duct work. I want to make sure I am telling you right that it is indeed 8x16.

The 2nd picture is the return duct in the attic. On this one, you will notice in the room LR i drew a line through the duct and labeled it C1. This is where one company is suggesting they place the plenum and then duct the bed rooms into it. There is no return in the Living room. where it looks like the main trunk of the return takes a 45, it is open to the attic on the ends. I have tried using foil tape to seal it. In the past they tried to use the insulation blanket to seal the open joints. This is my Major concern/issue with the system.

In a perfect world I would like to get rid of the central return and put individual return in each room. I do have 14 inch insulated duct that I can use to replace the 45 bend. I can rework the panning (I think) and take it back to the top of the furnace in the attic.

I tried the open the bedroom door and see if it closes. It does not. I opened it a crack and it stays there. no movement of the door.
 

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#11 ·
With 6 supplies before the bedrooms I can see why they would be cold in winter and the door test I suggested produced a no positive pressure result. The blower is loosing a large percent of its energy in those first 6 supplies. With those air flow conditions returns in each bedroom won't be of any value that I can see. Don't feel alone because it is a typical duct design but has never worked well. We see this complaint frequently.
 
#12 ·
With 6 supplies before the bedrooms I can see why they would be cold in winter and the door test I suggested produced a no positive pressure result. The blower is loosing a large percent of its energy in those first 6 supplies. With those air flow conditions returns in each bedroom won't be of any value that I can see. Don't feel alone because it is a typical duct design but has never worked well. We see this complaint frequently.
There was 7. I capped it off. It went into a panned floor joist that went to the original 4 foot long baseboard return that was converted into a supply. Would capping off the 10 inch supply (First one) help?
 
#16 ·
That 10" kitchen vent off trunk where it starts is probably the only thing that enabled your furnace to last as long as it did.

It was probably added specifically to stop limit trips or something.

You can't just remove it and expect the system to get sufficient airflow.

What I recommend you do is keep the 10" but have it run to the furnace -> have it as a separate trunk and use it to supply areas that may not be getting proper airflow now.

It's obvious that this system was pretty much done without real plan, they just put 6" ducts everywhere -> whether the supply is feeding a bathroom or a larger bedroom.

Diagram shows all return is only connected to a 14", there's no other return trunk. Is that correct?

I wouldn't worry much about getting a return in each room; I would worry more about getting enough return for the equipment, period.

At this point, if you want to fix this system up, you really need to get a room by room load calculation. The calculation can actually save you money -> like I said before, if you can heat/cool this place with smaller equipment the need to fix the ducts will be greatly reduced. A 2 ton a/c and a 60000 btu 80% furnace for example may serve you very well, needing far less airflow.

That, along with the airflow required by the equipment provides the blue-print for designing a system.

Otherwise, you're pretty much just guessing at how much capacity you need and how much supply you need.

the only affordable room by room software I know of is hvac-calc, you can get a short term license for $50.
 
#20 · (Edited)
Diagram shows all return is only connected to a 14", there's no other return trunk. Is that correct?

I wouldn't worry much about getting a return in each room; I would worry more about getting enough return for the equipment, period.

-> like I said before, if you can heat/cool this place with smaller equipment the need to fix the ducts will be greatly reduced. A 2 ton a/c and a 60000 btu 80% furnace for example may serve you very well, needing far less airflow.
Lets hope the 2nd time is a charm. Lost my fist attempt at posting this.

1st, I missed a few questions you asked yesterday and in this post i hope to address them with some pictures.

2nd, I tried the door test again last night. BR1 i oiled the hinges as the door sticks. All the doors in the house are original to 1958. There is a 1 inch gap at the bottom roughly. at 1/4 and maybe a little more the door will move to sit on the latch. more than that I get nothing. Can feel air movement around the gap. This was done with the 10 inch pipe closed. With that open I can feel air movement around the door just not as much.

On the returns question. Yes ALL returns feed into the single 14 inch pipe and I do have pictures that I intend to add. The family room is tied in some how (I cant see the connection). Last night i went into the attic and learned that BR1 and the main return are panned together. THe panning is with insulation board and it needs replaced as it has started to degrade and is letting in outside attic air. MBR and BR2 are panned together as well with over 6 feet of insulated board. off this there are 2 return ducts that tie in next to the main return trunk. This is also pictured.

What I would like to determine is can I remove enough of the 14inch trunk (Where there are already issues due to pipe fitment) and attach 14 inch insulated flex duct, and then take each bedroom and put it on the return plenum on top of the furnace (Pictured as well)? I would plan to use either 6 or 8 inch flex for each return also adding a return to the Main living room where I do not have one. I would not use panning for this but I would plan to install a boot into the ceiling. Would this give enough air return for the system?

Keep the 14 inch main return and add 4 new 6/8 inch returns to the top of the furnace?

Each HVAC company i have had out has said my furnace is too big for the house. They say i do not need a 100,000 BTU furnace but would be happy with 70-80,000 BTU. They have said I am also on the edge between 2.5 and 3 tom AC. Again all this is their word with no Heat Load.

I ideally would like to keep the furnace for atleast one more winter season so I can pay the furnace and AC off at one time. I have been told the heat exchanger is cracked when they placed a meter on the supply vent and read 4 PPM/PPB (not sure which unit) of CO. I have a CO detector outside the bedrooms next to the return duct. Because the exhaust blower is bad I have a pop bottle diverting the water and that has worked out really well so far.
 

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#22 ·
QUOTE:
On guy said i needed all new duct work.


The supply is 8X16 from the garage to the other end of the house (30 feet ish).


THe current piping all not attached to anything and is drawing outside air. This caused issues last summer with 80% and higher humidity in the house.

I don't understand this not attached statement above.
*********************************************************************************************
With the unit in the garage the problems you are having is fairly common and the one guy may be correct with the 30 ish ft. distance to the troubling areas of the house. I'm certainly no expert but from observations it seems as if the trunk supply is small for that distance and that many supplies and if the 4 supplies near the unit are all 10" that's a lot of supply to lose near the unit and I can see the far end being cut short of supply air.

The return side of the equation I'll leave that to someone else, but I feel it is inadequate according to what a very qualified expert recommended installing in my ranch with 1,800 sq. ft. ground level floor + 400 sq. ft. basement. According to him one 14x24 return grill was borderline for the 2.5 ton unit so he went with the 2 attic returns with 14" flex double insulated.

I'm sure you won't want to relocate the unit as seen in my crude drawing ( I flunked drafting :surprise: ) but it works very well for 30+ years with PVC supply ducts under concrete slab and this duct design.



 

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#23 ·
QUOTE:
On guy said i needed all new duct work.


The supply is 8X16 from the garage to the other end of the house (30 feet ish).


THe current piping all not attached to anything and is drawing outside air. This caused issues last summer with 80% and higher humidity in the house.

I don't understand this not attached statement above.
*********************************************************************************************
With the unit in the garage the problems you are having is fairly common and the one guy may be correct with the 30 ish ft. distance to the troubling areas of the house. I'm certainly no expert but from observations it seems as if the trunk supply is small for that distance and that many supplies and if the 4 supplies near the unit are all 10" that's a lot of supply to lose near the unit and I can see the far end being cut short of supply air.

The return side of the equation I'll leave that to someone else, but I feel it is inadequate according to what a very qualified expert recommended installing in my ranch with 1,800 sq. ft. ground level floor + 400 sq. ft. basement. According to him one 14x24 return grill was borderline for the 2.5 ton unit so he went with the 2 attic returns with 14" flex double insulated.

I'm sure you won't want to relocate the unit as seen in my crude drawing ( I flunked drafting :surprise: ) but it works very well for 30+ years with PVC supply ducts under concrete slab and this duct design.



The supplies are all 6 inch duct Except one run that is 10 inch. the 10 inch is the very first run off the main trunk. I intend to go under the house and measure everything and report back here with what i find out.

The return is in the attic. There is a section of pipe that makes a 45 deg turn to the vent in the hallway. This section of pipe is stuffed in the pipe and then wrapped with a insulation blanket in hopes of covering the gaps. I think if you look at the drawing you can see this? in the pictures i posted earlier you can see a section with foil tape on it, this was my attempt to fix this short term. Before i found out that was the main problem, last summer it was drawing in Hot Humid attic air. This would cause my AC to freeze up and not work.
 
#24 · (Edited)
Some points - to sum up what I was saying:

1. If the furnace is actually putting monoxide into the supply air, you have a dangerous situation on your hands and should not run it one more season. If it's 4ppm in the exhaust, different story.

2. Panning off joists for returns results in leaks no matter how well they're sealed. Get rid of all the panned returns! They don't belong in attics or crawlspaces, only in basements and ceilings with no exterior exposure.

3. The pictures show this is a high efficiency furnace, not what I thought. The BTU output is what counts, not the input, and airfow requirements are dictated by output and temperature rise range.

4. The 100k 90%+ is huge and actually needs closer to 1600 cfm to work right. If what you have is 1-stage, it's definitely been riding the high limit switch.

80000 btu is huge too - the 17" cabinet unit may need 1200 cfm but won't get that on the duct system, if it's a 4 ton drive 21" you're looking at more than that, they're designed around a lower temperature rise. (the 3 ton drive may be designed for 40 to 70f difference between supply and return, 4 ton drive could very well be 25 to 55 or something)

Just based on guidelines of 0.8 to 0.1" per 100 ft which doesn't take lengths into consideration (u may need a lower rate than that), your ducts are sized large enough for no more than 750 cfm

If you get a new 80000 btu 3 ton drive furnace which needs lets say 1150 cfm minimum, it will not move that on your ducts.

The contractors you've had out so far evidently suck. If you don't do the calcs yourself and specifically request what you want, you won't have good results.

If you don't want to replace and re-size every duct, the maximum size furnace you should be looking at is 60 000 BTU/hr input with 95% of that going into the house.

Anything larger would require completely re-doing the air ducts to prevent the heat exchanger from cracking in a few years vs 20 to 25.

Heat loss too high for that size says the calc? Let me give you some perspective; a space heater puts out 5000 btu/hr, and if you even need the 57000 btu/hr a 60k 95%, that would be just over 11 space heaters running full blast.

If your house needs over 11 space heaters (70-80k furnace) to keep it warm, it has really major problems that need to be fixed.

This 60k size will need 900 to 1100 cfm, but you could probably get 900 to 1000 with a 1200 cfm/3 ton blower if the heating speed is raise above factory setting. They come from factory under assumption that the ducts are properly sized.

The max a/c? probably 2 tons or 800 cfm, but if the house is in really rough shape and needs more than that, may be able to get away with 2.5 ton.

Just as with heat, they have to measure the airflow and set the fan speeds correctly after installation. If you end up with 2 ton a/c and 60k, the heating fan speed will need to be set a little higher than cooling speed. most guys probably won't do this.

I don't think you'll be able to use a unit with an ecm motor (they ramp up and try and maintain proper airflow on restrictive systems) on those ducts without shortening lifespan but the others with experience may say differently. So everything i said applies to 1-stage/standard motor units.

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With the returns, i don't know what problems you're trying to solve.

Central returns are fine, not ideal, but fine. Your hard earned money would be better off spent elsewhere.

Panned off returns in an attic are not the slightest bit fine - you have to get rid of them and run pipes.

The 14" return trunk may be undersized for 70 to 80000 btu and 2.5-3 ton a/c, but there may not be a point of adding more return if the entire supply side is undersized.

More return may help with airflow issues without re-doing the supply, but better to put the smaller unit which needs less airflow to begin with.



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New questions:

1. Is the 8x16 trunk line insulated in a cold, vented crawlspace? What about the branch lines?

If you're trying to cut heat loss, start there.

2. Is the crawl space like a mini-basement or is it ventilated?

3. how cold does the crawl space get? insulated walls if like a mini-basement? if it's vented, is the floor insulated?

Before i found out that was the main problem, last summer it was drawing in Hot Humid attic air. This would cause my AC to freeze up and not work.
drawing hot humid air reduces the risk of coil freezing, so that's not your issue. it actually raises the temperature of the coil.

freezing means you have an airflow issue or refrigeration problem on your hands.
 
#26 ·
Some points - to sum up what I was saying:

1. If the furnace is actually putting monoxide into the supply air, you have a dangerous situation on your hands and should not run it one more season. If it's 4ppm in the exhaust, different story.

2. Panning off joists for returns results in leaks no matter how well they're sealed. Get rid of all the panned returns! They don't belong in attics or crawlspaces, only in basements and ceilings with no exterior exposure.

The contractors you've had out so far evidently suck. If you don't do the calcs yourself and specifically request what you want, you won't have good results.

If you don't want to replace and re-size every duct, the maximum size furnace you should be looking at is 60 000 BTU/hr input with 95% of that going into the house.

Anything larger would require completely re-doing the air ducts to prevent the heat exchanger from cracking in a few years vs 20 to 25.

This 60k size will need 900 to 1100 cfm, but you could probably get 900 to 1000 with a 1200 cfm/3 ton blower if the heating speed is raise above factory setting. They come from factory under assumption that the ducts are properly sized.

The max a/c? probably 2 tons or 800 cfm, but if the house is in really rough shape and needs more than that, may be able to get away with 2.5 ton.


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With the returns, i don't know what problems you're trying to solve.

Central returns are fine, not ideal, but fine. Your hard earned money would be better off spent elsewhere.

Panned off returns in an attic are not the slightest bit fine - you have to get rid of them and run pipes.

The 14" return trunk may be undersized for 70 to 80000 btu and 2.5-3 ton a/c, but there may not be a point of adding more return if the entire supply side is undersized.

More return may help with airflow issues without re-doing the supply, but better to put the smaller unit which needs less airflow to begin with.



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New questions:

1. Is the 8x16 trunk line insulated in a cold, vented crawlspace? What about the branch lines?

If you're trying to cut heat loss, start there.

2. Is the crawl space like a mini-basement or is it ventilated?

3. how cold does the crawl space get? insulated walls if like a mini-basement? if it's vented, is the floor insulated?



drawing hot humid air reduces the risk of coil freezing, so that's not your issue. it actually raises the temperature of the coil.

freezing means you have an airflow issue or refrigeration problem on your hands.
I am planning to go get a CO meter and verify this. I figure $20 is not a bad investment. I will put it on the first supply vent.

I am planning to do away with all the panned returns. That has been decided. Im going to get a boot for each room and one for the central return (at the moment).

Im here to lear how to do the calculations. I like knowing the answers to my questions when I ask them. I plan to stay in this house for many many years so I want this to be done right. If I knew how to replace the duct i would attempt it myself.

So far I'm not real thrilled with the HVAC people i have had out. Ive stopped getting quotes till i can become more educated.

Im okay to get a 60K BTU Furnace. What want to avoid is the system running all day when its 0 out or short cycling (like it does now) in the winter and Short cycling and running all day in the summer.

Now to get to your new questions-

I have verified the supply duct is indeed 8x16. it is NOT insulated. The runs are 6 inch round with dampers (all open), also not insulated. The last run going to BR2 is on the very end of the trunk.

The Crawl Space has thick plastic on the ground and insulation board on the walls and batt insulation on the rim joist. All the vents are covered with a piece of insulation board and a piece of plywood. Not the best and i haven't decided the best way yet to seal them for good. Was considering Glass Blocks.
This last December I had to replace some plumbing, and it was nice and toasty under there.

Are you thinking that i could be low on freon? Or even that the system is just oversized for the duct work?

I have attached a few photos from under the house.
 

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#28 · (Edited)
The plug in co alarms aren't very good, the ones with digital displays are better but not for testing low levels, only alerting you to moderate levels, like 30ppm+. they all won't sound an alarm until 70ppm and even at that level, not for a long time.

there are some low level co monitors but be prepared to pay at least $100.

Bottom line though is if the is somehow pumping co into the living space (not just pulling residual co from garage from return leaks - which btw are super important to not have), you must shut down it down.

my comments only apply to tiny hairline cracks and no co being put into the living space.

You can be chronically poised by low level co for months without a peep from one of these ul approved alarms. They only make noise at life-threatening levels.
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Based on your responses, your crawl-space is not bad; it's already been insulated.

I also see taped seams which is a good thing. Actually have metal down there is a good thing.

You don't necessarily have a bad duct system, just ducts too small for the size of equipment you have.



I'm sure some rooms are over-supplied and others under-supplied though.

So your first load calculation is a gross over-estimate because it shows duct loss to an unheated crawlspace and floor loss as well. You do have to add the crawlspace walls, as well as floor area so the software can calculate air leakage and foundation losses accurately - otherwise you'll end up with an under-estimate.

You'll have to verify insulation in the walls but taking samples from around electrical outlets and any other place that can't be see. remove the covers of outlets and stick a long tooth-pick in, see if it hits insulation.

i take it the house is 2x4 construction.

check attic insulation as well.

With calculations, they're simple but you have to get the inputs right. If you do each room separate, you can actually gauge how much supply air each room needs. for example, your bathroom with little exterior area has a 6", potentially supplying 100+ cfm to a room that may need 30.

On the other hand, the family room addition with two 6" flex ducts which are equivalent to 5" metal pipe is probably under-supplied. In fact the 10" kitchen duct may have been put to correct an under-supply problem in that open area.

What want to avoid is the system running all day when its 0 out or short cycling (like it does now) in the winter and Short cycling and running all day in the summer.
Load calcs will make sure you have enough capacity. though the equipment has to be comissioned right to get the capacity.

Do not be scarred of undersizing the furnace.

over 90% of the time, the furnace is over-sized. It's sized for the most extreme cold you can plus there's always a safety factor since you're choosing in 20 000 btu increments.

Running a lot of not a bad thing; continuous operation makes for more even, consistent heating. Smaller units are quieter on a given duct system. They break down less and have longer lifespans because it's cycling on and off that causes the most wear. Relays and ignitors are rated in cycles; the heat exchangers get metal fatigue and crack as a result of heating up and cooling down, not staying hot.

Long cycles are very desirable.

For a/c the smaller unit does a better job of pulling out the humidity, so you can be comfortable at a higher temperature.

Assuming it cycles a lot now, going from 3 to 2 ton could make the house a lot more comfortable.

Are you thinking that i could be low on freon? Or even that the system is just oversized for the duct work?
Maybe. When it gets warm enough, turn it on and make sure the large insulated line is cold.

low refrigerant makes this pipe warmer. too little airflow makes it colder than normal.

You can get a thermometer and measure the difference between return and supply right at the furnace.

A difference greater than 22 could indicate low airflow (can also indicate very low humidity).

A difference below 17f plus a freezing coil would point to a refrigeration problem. or high indoor humidity or too much airflow (not a chance in your case) but without the freezing up.

Both low airflow and low refrigerant can cause freezing, and if you have both problems, the delta-t can look normal, one problem covering the other. This highlights the need for proper diagnostics done by a competent tech.
 
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