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  1. #1
    Join Date
    Mar 2013
    Posts
    16

    Max number of supplies off one trunk and max distance between register and furnace?

    I am planning on adding heat/AC to a sun room that's connected to our family room. The trunk line for the family room side of the house stops at about 3' from the sun room. This trunk line also provides heat/AC to the kitchen, dining room, living room, and the foyer (8 registers total). This trunk line is 16" x 8". Will I be able to take 2 or 3 more supplies from it to heat/cool the sun room? The furnace is a Carrier ICS 58MVC, which should be sufficient to handle the extra 13 x 13 sqft of the sun room. But I am concerned if the heated/cooled air can reach the sun room, since the end of the trunk line is already 38' away from the furnace, and it's another 16' to the sun room window where one of the registers will be.

    I got 2 HVAC guys who will do it. One will extend the trunk line to the sun room, and take 3 supplies off. One will just take 2 supplies off near where the current trunk ends. Another guy won't do it, saying there will be no air reach the far end. I am really confused.

    I think the questions are: How many supply lines can come off one trunk, and is there a max distance between the register and the furnace?

    Thanks in advance!

  2. #2
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    2,068
    it's a lot more complicated that what you are asking. How much air with the equipment move total design? How much air is it moving now? How much air is needed in each room fed by this duct? How much is being deloivered? How much air is needed in the sunroom? When the sunroom is added how does that affect the load and air flow on the other rooms?

    These aren't all the questions I would be considering but it's a good start. Until these and other questions are answered it's all a guess. I haven't even got into the type of duct fittings that are involved.

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,660
    To begin with, the existing 16"x8" is "normally good for 700 cfm (7-6" supplies). You have 8 registers off of it now. Are they 6'' supplies or maybe some 7'' supplies?

    Sunrooms are really hard to heat/cool. Will it have a crawl space under it or a full basement? If at all possible, replace some of the 16"x8" with 24"x8", plus which ever rooms you mentioned are closest to the furnace, reroute those supplies off the furnace plenum.

    Zoning could be added if you must have ideal sunroom temperatures.

  4. #4
    Join Date
    Mar 2013
    Posts
    16
    Thanks for the replies!

    My house is a rambler with full basement. The basement is almost all finished so it's hard tell exactly how the ducts are laid. As far as I know there are 3 trunks off the furnace plenum. One for 3 bedrooms and 2 bathrooms, one for about 1150 sqft of living space, and the third is for the basement. All three trunks are 16"x8", and the one for the basement branches off to two 14"x8" going opposite directions. Based on what I see, I believe all supplies are 6" round. The room under the sun room is finished but not heated/cooled, and as for now I have no plan to add HVAC to that room. The rest of the basement is all heated/cooled. The whole house is 4400 sqft (4400 cfm?)

    The guy who did not want to do it mentioned making the trunk bigger, but he also said that since the basement is all finished, enlarging that trunk means knocking all the ceiling from the far end of the house all the way to the furnace, and the repair will be too costly. Even worse, the entire trunk that needs to be bigger lays in between the cold air return and the trunk for the basement, so the other trunk has to be moved to make room. He wanted to do mini-split.

    I am quite against mini-split because it takes up space (it's a small sun room), and most importantly mini-splits are not good at heating below 0. In Minnesota we have cold winters, so for the cold days I need another heat source (ie baseboards, which take more space).

    From the numbers it seems that my existing trunk for the living room is not adequate to begin with. However we don't have any issues with the comfort level. The supply for the foyer may be from the bedroom trunk, if so 100 sqft (100 cfm?) could be deducted. I highly doubt it, though. Because if the foyer supply comes from the bedroom trunk, it has to make two 90 degree turns; if it's from the living room trunk, it just goes straight off the trunk. The foyer register has one of the best air flow, so maybe it's on the living room trunk?

    I understand that sun room are hard to heat/cool. I am just hoping that with good insulation and good windows, such a small room will not put too much stress onto the existing system. In one overnight test, when outdoors was 20F I opened the door between the living room and the sun room. The sun room was kept at 63F without a heat source, the corner in the living room by that door was 66F, and the other end of the living room was 69F. Am I too optimistic/naive to believe that adding 2 or 3 registers can bring the temperature up by 6-10 degrees? I should know better if I did the test when it was -20F outside!

  5. #5
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,660
    The 8 supplies that you mentioned probably do not need to be "wide" open. Example, the foyer could be shut off entirely (it'll still get some airflow). Hopefully you can get to the dampers for balancing.

    Adding on to the 16"x8" and installing 3-6" supplies to the sun room might just do the trick 95% of the time for the least amount of money. With the 3 main supply ducts, if I'm understanding your lay out correctly, it sounds like zoning may be a possibility if the temperature needs to be spot on. The basement under will help. Adding some heat there might just do the trick as well.

  6. #6
    Join Date
    Mar 2013
    Posts
    16
    George2, thanks for suggesting shutting the foyer register. That room doesn't need to be that warm anyway.

    I was told by the two guys who will do the duct work that zoning may not help the sun room much if the thermostat is not in the sun room. But it doesn't make sense to put the thermostat in the sun room - it should be in the living room where people spend time the most.

    From what I heard from the contractors who came looking, the problem seems to be that the sun room is too far away from the furnace (41'-54'). So is there such a rule saying there is a maximum supply length, providing the size of the trunk and the BTU of the furnace?

    From the information here I got the idea that it's the actual CFM needed vs. available for the particular space, that will determine whether I can get sufficient heat/AC. So is it true that the total number of registers off the trunk is not an important factor?

    George2, are you saying I should extend the basement trunk to the room underneath the sun room, or I should add a register off the living room trunk for that room? That room is the only one completely below grade, and its temperature stays above 43F all winter and below 72F all summer. I already closed 50% of the registers in the basement (with look out windows but no walk out doors). In the winter it's 7-10F lower than the main floor (which we don't mind, since we are there only for the laundry - yeah, 2200 sqft laundry room), in the summer it can be even cooler than the main level.

  7. #7
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,660
    Quote Originally Posted by rapida View Post
    George2, thanks for suggesting shutting the foyer register. That room doesn't need to be that warm anyway.

    I was told by the two guys who will do the duct work that zoning may not help the sun room much if the thermostat is not in the sun room. But it doesn't make sense to put the thermostat in the sun room - it should be in the living room where people spend time the most.

    From what I heard from the contractors who came looking, the problem seems to be that the sun room is too far away from the furnace (41'-54'). So is there such a rule saying there is a maximum supply length, providing the size of the trunk and the BTU of the furnace?

    From the information here I got the idea that it's the actual CFM needed vs. available for the particular space, that will determine whether I can get sufficient heat/AC. So is it true that the total number of registers off the trunk is not an important factor?

    George2, are you saying I should extend the basement trunk to the room underneath the sun room, or I should add a register off the living room trunk for that room? That room is the only one completely below grade, and its temperature stays above 43F all winter and below 72F all summer. I already closed 50% of the registers in the basement (with look out windows but no walk out doors). In the winter it's 7-10F lower than the main floor (which we don't mind, since we are there only for the laundry - yeah, 2200 sqft laundry room), in the summer it can be even cooler than the main level.
    Where do I begin? A furnace can move only so much air. Of course the number of registers will effect the airflow of each supply. Example: 1,200 cfm blower with 12 - 6" supplies equal (approx.) 100 cfm each. Add more supplies and the amount of airflow gets reduced. Most homes have more supplies than the blower can fully supply. That is where duct design and balancing come into play.

    You what to extend the supply duct and add the new supplies to that. Do not "tap" into the existing living room supplies. It's okay to omit one and re-routed it to the sunroom if that's what you meant.

    Zoning would help because what it does is shut off the ductwork to the areas not being used. This in turn will increase the airflow to the areas farthest away from the furnace. It's a matter of common sense. Most contractor are not experienced with zoning, are afraid of it and tend to not recommend it. F.Y.I.........the thermostat should be centrally located (to that zone) and not located in the sunroom.

  8. #8
    Join Date
    May 2009
    Location
    Savannah, Ga/H.H. Island, S.C.
    Posts
    1,460
    For situations such as these, I always suggest installing a mini split system.

    Sun rooms have a large heat load. If you do get enough air into the sun room to make it comfortable, the rest of the house will suffer because your unit and duct system were designed for a home without a sun room. Your robbing Peter to pay Paul.

    Any of the contractor do a load calc on your new room? I'm guessing they didn't.

    Go with a ductless mini split and you won't regret it.

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Sunroom is ideal for a minisplit. Especially since you may want to design it with garden doors so it can be closed off and widnows opened without having to open up you whole house.

  10. #10
    Join Date
    Mar 2013
    Posts
    16
    On either coast or anywhere USDA zone 6 or above I'll just go with a mini-split. The problem is, in MN (zone 4) a mini-split is only half the solution because the heat pump will stop working before outside temp drops below 0F. That's 1-2 months here. If I have to supplement with baseboard heat I might just do the baseboards without any AC. Here AC is far less important than heat.

    None of the contractors did a load calc. Can I do it myself?

    I can't even find the blower capacity for the Carrier ICS 58MVC on the Carrier web site. Where else can I find that info?

  11. #11
    Join Date
    May 2009
    Location
    Savannah, Ga/H.H. Island, S.C.
    Posts
    1,460

    Max number of supplies off one trunk and max distance between register and fu...

    Mitsubishi's hyper heat series will go below 0 degrees and still have a decent heat output.

    Google it. It states it has up to 75 percent heating capacity @ -13 degrees.

  12. #12
    Join Date
    Mar 2013
    Posts
    16
    Sorry for my ignorance, George2. Is each 6" supply 100 cfm max? Or with a bigger furnace and fewer supplies, each 6" supply can provide more than 100 cfm?

    I didn't understand what you meant by "tapping" into the existing living room supplies. Does it mean drawing 6" supplies directly from the 16"x8" without extending the length of the 16"x8" duct? If so, that was one of the contractors suggested to do. He didn't want to extend the 16"x8" duct because it will "increase too much air space for the furnace to handle". Was it a valid point?

  13. #13
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    For a 13x13 room a load calc can be just done manually. Its' not like you need to size to within 500BTU's here.

    There are minisplits with "hyper heat" modes that pruduce plenty of heat down to 0F. Below that they will be on heat strips, but given your other options and requirement for AC, I don't see any other economical alternatives. Minisplit has ceiling casettes that take up no wall or floor space or wall mounts that take up only wall space up high. You can also install ducted units that could be placed in the basement and you run ducts to and from the room.

    Otherwise another option is electric radiant floor heat with a minisplit or just a portable unit for cooling only

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