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  1. #1
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    Default Ducting Insulation

    Well they said off topic
    Anyone know their way around heat transfer rates?
    I have a fan that moves air from a fire to two bedrooms. After the fan its a 10m run in 8" duct then splts into two 3m runs in 6", with only a 10W* fan it moves air pretty slowly. It is insulated ducting but likely some where around R1 at a guess.
    Between the fan and the registers there is a 20C to 25C** drop in temp. Getting some of that back would be nice. The first plan was to get some 14" ducting and run the 8" ducting through that but that sounds painful in the roof cavity. I'm currently thinking that a couple of 10m x 1.2m rolls of 1.8R foil faced fiberglass insulation wrapped around the 8" would be a better and easier way to go.

    Will this significantly effect the heat lose?

    *from memory

    **fan inlet is about 55C, roof cavity something less than 16C, the thermocouple is at the high spot near the chimney, the ducting just above ceiling , outlet is about 33C

  2. #2
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    Default

    I'm interested to see what you get in response, as there might be something I can use to improve the efficiency of my gas ducted heating.

    Dim recollections of thermodynamics (which I was never any good at anyway) makes me think though that it probably won't help much. If I'm thinking about this correctly, you might be losing a lot of the heat energy simply from the warm air expanding as it travels down the duct, and you might be better served by upgrading the fan. Which then may leave you with the issue of the fire being able to keep up the volume of heat required? I'm probably wrong though....

  3. #3
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    Hi Guys,

    Thermodynamics was never a strong point with me either ! But yes expansion loss would be a function of airflow rate and distance assuming perfect insulation.

    About the only thing to do would be to actually measure the temperature differential at set points along the duct. Not forgetting that the metal duct would also loose heat by conduction along its length irrespective of loss through radiation. Minimised of course by the insulation around it.
    Best Regards:
    Baron J.

  4. #4
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    Air movement fans raise the pressure very little, so expansion of air will not drop the temperature significantly. The most likely solution is, as Stuart suggests, more insulation. Either wrap some around or visit an A/C supplies place and see what they have that will fit. To check your heat loss, measure the temperature differential across the ducting (temp inside the ducting vs temp on the outside surface of the ducting).

    Changing the fan out to move more air won't do much, and if the air being moved speeds up too much you will feel a draught.

    Michael

  5. #5
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    Default

    How long has the flexible duct been in the roof?

    The internal liner is usually perforated so that noise from air movement and motor is absorbed but probably not an issue in your case, however and if the outer liner is perished which they usually do after some time in the hot roof, then some of the airflow escapes into ceiling reducing what should be going to registers.

    Guessing the fan is axial?..what diameter?

    If i was doing the the job I would just replace the lot with minimum R2 250/300 dia and then branching off with minimum 200 dia keeping the 200 as short as possible.

    Have attached retail duct prices for you to consider...you should expect to see pricing less about 30% if your good at bargaining from right supplier

    duct pricing.jpg

    pm sent

  6. #6
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    Can you measure the temperature of the ducting (outside)
    If it is warmer than the ambient roof space then you have heat loss and insulation will help, in proportion to the difference in temperature.

  7. #7
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    Ok that's a lot to cover but I will try.

    Firstly, Thank you all.

    Something else to add, the 150mm ducting lays on the ceiling insulation, it's taped directly to the register so makes a pretty crappy 90deg bend. I am going to try adding a 90 pvc duct to increase airflow, mostly because it irks me, but as I don't really have anything to measure airflow I thought I would do the insulation first.

    Quote Originally Posted by Jekyll and Hyde View Post
    I'm interested to see what you get in response, as there might be something I can use to improve the efficiency of my gas ducted heating.
    Not sure it would help as much on ducted heating as it doesn't run all the time(well most of them don't, some do), though it might make the cold draft when it turns on less chilly?

    I'm fairly sure the fire can supply more heat if/when required.

    Quote Originally Posted by Michael G View Post
    Changing the fan out to move more air won't do much, and if the air being moved speeds up too much you will feel a draught.
    The way I see it, increasing airflow would increase heat lose, sure you might not notice it as much as you would be moving more heat so losing less as a %.... but I could be full of it.

    Quote Originally Posted by eskimo View Post
    How long has the flexible duct been in the roof?
    Not exactly sure but believe its something less than 15 years.

    Quote Originally Posted by eskimo View Post
    the outer liner is perished
    The outer looks fine. At least being in the roof its much easier to get at.
    Quote Originally Posted by eskimo View Post
    Guessing the fan is axial?..what diameter?
    Yes. I'm pretty sure it is 200mm but will check tomorrow.
    Quote Originally Posted by eskimo View Post
    If i was doing the the job I would just replace the lot with minimum R2 250/300 dia and then branching off with minimum 200 dia keeping the 200 as short as possible.
    I like the bigger ducting idea, but I think I will go ahead with my plan as it will cost about $160 and I can do it now and then see what happens. Mainly because I can see myself getting that done, replacing the system will likely see me scratching my ass for 3 years lol. What I didn't say in may first post was there are plans to add to the system in the future but I cant see it happening this year. Wont have to save much wood to cover $160 and I can always use the insulation as "extra" on top of the batts if its not needed in the future.

    The thought is to add a separate system to supply master bedroom + some living areas, rather replace everything to make one larger system as the bedrooms we heat with the current system are about the only place we don't need to heat all the time, damn "open plan". crazy idea?

    Quote Originally Posted by russ57 View Post
    Can you measure the temperature of the ducting (outside)
    If it is warmer than the ambient roof space then you have heat loss and insulation will help, in proportion to the difference in temperature.
    It is, I just haven't actually measure that


    ok I think thats it

  8. #8
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    How long is the ductwork lengths...ie how much 200 and and each branch of 150..i can tell you what you should have
    how big are the rooms? I assume the wood heater has plenty of capacity

    200 dia fan would only push around 215litres/sec @ 20pa and 100l/s @ 70 pa...as you can see any restriction eg small duct would increase pressure and hence very little airflow...

    first thing I would do ASSUMING duct is good is to ensure no airleaks ie all duct joins and connections are properly taped.
    Duct has not collapsed internally....rae but can happen in straight runs
    Make doubly sure that duct has not collapsed at the 90degree bends or at least are not too too restrictive ..if not too tight these do not generally pose too much of a problem, or and if ceiling space is available try hanging duct from roof thus getting it off the ceiling to allow for make a larger radius for bend where it connects to ceiling register. We use that blue nylon package strap available in small lengths from bunnings....you should still have plenty left over for some packing later.

    If low air flow still seems to be a problem you have two options.
    1. Start again with properly sized duct and new fan
    2. Put in additional 200 fan so that each branch if supplied independently with its own but this would mean that you need to increase size of ducting from ceiling of heated area to inlet of fan/s and will mean that you need to fit a Y or branch take off into this duct.

    Can do a drawing a drawing for latter (2) if you not sure what I mean.

    The only problem I see with doing temp stuff is that costs may well exceed starting again from scratch...beware

    One other thing, you do leave the door open to the other rooms dont you?. if not the fan will also be trying to push air into a closed space ( less natural leakage)..its akin to puting a stopper in the duct..ie no air.

  9. #9
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    Ok another thing I should add as I'm not 100% happy with my last post.

    The system as is actually does its job just fine. Its just me looking at the numbers that sees a 15ishC rise out the register, but knows there is an extra 25ishC being lost to the roof cavity.

    Helpfully both the ducting and the fan have dates on them. The fan is 200mm, it is in fact a whopping 16W. What looks like 22 R/S is that revs per second?
    The ducting has the R value on it R0.6 and is dated 7/2003

    So now I'm wondering if the insulation halves the heat lose and 90deg ducts increase airflow I could end up with 3 times as much heat so my next post will be "how to throat this back" lol

  10. #10
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    Quote Originally Posted by eskimo View Post
    How long is the ductwork lengths...snip
    Was typing, will get back to you

  11. #11
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    okay so how can we help if it aint broken? lol

    yeah more insulation will help temp losses/gains depending on which way its flowing...inside to out or outside to in.
    Melbourne should be minimum R2. so .6 is low

    and what do you mean 90 degree ducts increase airflow

  12. #12
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    Quote Originally Posted by eskimo View Post
    How long is the ductwork lengths...ie how much 200 and and each branch of 150..i can tell you what you should have
    how big are the rooms? I assume the wood heater has plenty of capacity
    10m of 200mm, Y into 3m 150mm on each branch
    17sqm and 14sqm(but we leave the doors open because for the most part we aren't really trying to heat those rooms), just getting heat to the other end of the house).

    Yes fire is plenty big enough, built into brick so has good heat store as well.
    Another reason for some insulation improvement and some automatic control. As we currently run the fan over night there must be some points when 90% of the heat it is moving is going into the roof.

    Quote Originally Posted by eskimo View Post
    200 dia fan would only push around 215litres/sec @ 20pa and 100l/s @ 70 pa...as you can see any restriction eg small duct would increase pressure and hence very little airflow...
    I was thinking about it sort of thing but didn't mention it. While I cant measure airflow I can measure pressure drop to that sort of level I believe*. Do I just need to measure the pressure difference across the fan(should be easy)or do I need to measure just inside the inlet and the outlet?(I don't have enough hose.... there cant be an real difference between the two aside from any leaks...right?)

    *just checked, can measure kPa to 0.01 which I guess will give me a number to work with but only just.
    Quote Originally Posted by eskimo View Post
    first thing I would do ASSUMING duct is good is to ensure no airleaks ie all duct joins and connections are properly taped.
    Duct has not collapsed internally....rae but can happen in straight runs
    It all looks ok. Will a pressure test pick up an internally collapse?

    Quote Originally Posted by eskimo View Post
    Make doubly sure that duct has not collapsed at the 90degree bends or at least are not too too restrictive ..if not too tight these do not generally pose too much of a problem, or and if ceiling space is available try hanging duct from roof thus getting it off the ceiling to allow for make a larger radius for bend where it connects to ceiling register. We use that blue nylon package strap available in small lengths from bunnings....you should still have plenty left over for some packing later.
    You just want me to "walk" 15m through the trusted roof again don't you
    I think they are pretty "squished" over, and I don't think there is the length to do much about that. I'll go check when I finish this post. I have blue tape left from installing ducting on another place(I may even know where it is lol)

    Quote Originally Posted by eskimo View Post
    If low air flow still seems to be a problem you have two options.
    1. Start again with properly sized duct and new fan
    2. Put in additional 200 fan so that each branch if supplied independently with its own but this would mean that you need to increase size of ducting from ceiling of heated area to inlet of fan/s and will mean that you need to fit a Y or branch take off into this duct.

    Can do a drawing a drawing for latter (2) if you not sure what I mean.
    Covered this in my last post.... don't you just love trying to deal with half the story, sorry about that.

    Quote Originally Posted by eskimo View Post
    The only problem I see with doing temp stuff is that costs may well exceed starting again from scratch...beware
    For sure. Perhaps I will start with just the one roll this, wrap it around the 200mm ducting and just see what I get from that.
    https://www.bunnings.com.au/earthwoo...ation_p0810985


    Quote Originally Posted by eskimo View Post
    okay so how can we help if it aint broken? lol
    Well I dont want to make it to easy for you! hehe

    Quote Originally Posted by eskimo View Post
    yeah more insulation will help temp losses/gains depending on which way its flowing...inside to out or outside to in.
    Melbourne should be minimum R2. so .6 is low
    Quote Originally Posted by eskimo View Post
    and what do you mean 90 degree ducts increase airflow
    Placing a 90 bend(elbow) above the ceiling register.(perhaps not necessary given your last post)

    Back up into the roof I go.

  13. #13
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    When you pull the insulation back it would seem the bends aren't as bad as I thought.
    Attached Images Attached Images
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  14. #14
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    Quote Originally Posted by Stustoys View Post
    When you pull the insulation back it would seem the bends aren't as bad as I thought.
    When you put that back together make sure the inner liner is taped to fitting. ...Then tape the outer liner also.

  15. #15
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    Quote Originally Posted by eskimo View Post
    When you put that back together make sure the inner liner is taped to fitting. ...Then tape the outer liner also.
    Will do.
    Thanks

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