COOLING COIL BYPASS?
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  1. #1

    COOLING COIL BYPASS?

    I am getting a new forced air/natural gas system installed and I have a question about the cooling coil configuration.

    I note that the aperture area at the bottom of the coil assembly is considerably smaller than the supply plenum in which it sits - maybe half the area - but 100% of the air driven by the furnace fan passes through the coil. This is a furnace with a fan that can run at up to 2,000 RPM.

    Although this makes some sense to me for cooling operation, my question is whether it's also acceptable when the system is used for heating. Intuition tells me there ought to be a movable baffle to allow for bypass of the coil or even some full-time bypass going on. Right now there is maybe 25% of the area blocked off with sheet-metal.

    Your advice/comments on this point would be much appreciated.

  2. #2
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    Does the existing system have 25% blocked off, or was the new furnace/coil install that way.

    They should have installed a coil that was the correct dimensions for that model furnace. Most oil sized are made in several different widths to match to several furnace sizes. Sometime you have to upsize a coil a 1/2 ton if needed to get into a matching size range.

  3. #3
    Join Date
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    air passes through the evap coil 100% of the time regardless of what the stat is calling for.
    FILL OUT YOUR PROFILE!!

  4. #4
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    Jun 2006
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    I was thinking about doing this same thing for my personal system. The pressure drops for a coil range from .23 to .45. That is a huge restriction that wouldn't be tolorated in any heating only system. In my area heating is usefuly for about 30% of the year and cooling for about 15%. With that split I see a the chance to have some savings. Maybe only $15 a year though. More if they system is used in ventilation mode. Less wear an tear is another area for savings. This is something I can do myself. For you the cost to have it done might not be recouped. More siginfant for you would be to make sure you have enough filter area and proper ducting. Those you will definatly see the return on investment.

    Dan
    If you don't notice, I'm doing my job.

  5. #5
    Motorguy: If the width of the aperture at the bottom of the cooling coil had been bigger I am saying that I could have gained an extra 25% of throughput - as it is, this area is blocked off with sheet metal. The coil is exceptionally tall to my eye but of course that doesn't help the restriction much.

    Derb: when you say there's a pressure drop of .23 -.50 across the coil, what units would those be, or do you mean 23-50%? I am more concerned with adequate heat distribution and taking full advantage of the 2,000 RPM fan than saving $15 a year.

    As you know, we householders must take what our licensed specialist tells us we need - we are not supposed to ask detailed questions like this as they present themselves as all-knowing ...

  6. #6
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    Quote Originally Posted by spottymaldoon View Post
    Motorguy: If the width of the aperture at the bottom of the cooling coil had been bigger I am saying that I could have gained an extra 25% of throughput - as it is, this area is blocked off with sheet metal. The coil is exceptionally tall to my eye but of course that doesn't help the restriction much.

    Derb: when you say there's a "pressure drop" of .23 -.50 across the coil, what units would those be, or do you mean 23-50%? I am more concerned with adequate heat distribution and taking full advantage of the 2,000 RPM(?) fan than saving $15 a year.

    As you know, we householders must take what our licensed specialist tells us we need - we are not supposed to ask detailed questions like this as they present themselves as all-knowing ... (Tell'em what You want...)
    Do you mean a 2000-CFM (Cubic Feet per Minute) airflow volume rather than RPM Revolutions Per Minute Rotation?

    Is it a 5-ton air handler?

    The more CFM you force through a specific evaporator coil the higher the static resistance.

    In the northern states heating is used close to 9 months each year, cooling is only used about spottily 4 months &, many days of those 4 months it is not used at all.

    In the north country, it would make sense to engineer ways to mechanically by-pass the coil during the heating season.

    It is also wise to use an oversized tonnage coil that is a match &, that will flow more air at a lower static pressure drop during both heating & cooling.

    For example: you only need a 1.5-Ton A/C (600-CFM) you should always use at least a 2-Ton coil, because your furnace may require 2.5 to 3-Ton a 1000-cfm or, 3-Ton 1200 to 1300-CFM which is way above the .5" static limit rating of a 1.5-Ton coil.

    A 1.5-Ton E-Coil is only rated at around 600 to 700-cfm, the static will skyrocket trying to flow say, 1200-cfm for heating to keep the temp-rise at 60-F or below. These are very important component selection factors toward decent efficiency & longevity of performance that calls for intelligent engineering... even the 2-Ton coil on a 1.5-ton condenser is rated to a 900-cfm limit, therefore is not actually large enough if you need to flow 1000 or 1200-cfm for heating
    Last edited by udarrell; 06-03-2012 at 12:50 PM. Reason: 2-Ton not big enough for 1000 or 1200-cfm airflow...

  7. #7
    I'm located in Toronto with similar requirements to, say, Buffalo NY. My actual furnace is an Amana AMV91155DXBB. The unit was sold to me as having a fan speed (and therefore circulating capability) at the top of the line: the motor being a variable speed unit with a 2,000 RPM maximum. That impressed me since the old one was about half the speed.

    The manufacturer's specifications for this unit are at

    http://www.amana-hac.com/Portals/1/pdf/SS/SS-AMV9.pdf

    My home has a floor area of about 2,300 sq. ft. and the air conditioner compressor is rated 2.5 Ton. I don't know the rating of my coil.

    I don't believe I am anywhere near 2,000 CFM

    Although I have certainly paid for 'intelligent engineering' I am "here" because I want to be sure that's what I got!

  8. #8
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    Some people have different was of dealing with transitions.
    Some do this:

    Better to do this:

    And some guy has a Patent on this:


    All except for the first are usually not done by the lowest bidder.

    Dan
    If you don't notice, I'm doing my job.

  9. #9
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    Quote Originally Posted by spottymaldoon View Post
    I'm located in Toronto with similar requirements to, say, Buffalo NY. My actual furnace is an Amana AMV91155DXBB. The unit was sold to me as having a fan speed (and therefore circulating capability) at the top of the line: the motor being a variable speed unit with a 2,000 RPM maximum. That impressed me since the old one was about half the speed.

    The manufacturer's specifications for this unit are at

    http://www.amana-hac.com/Portals/1/pdf/SS/SS-AMV9.pdf

    My home has a floor area of about 2,300 sq. ft. and the air conditioner compressor is rated 2.5 Ton. I don't know the rating of my coil.

    I don't believe I am anywhere near 2,000 CFM

    Although I have certainly paid for 'intelligent engineering' I am "here" because I want to be sure that's what I got!
    It is Rated 109,000-Btuh Output.

    The blower motor HP is Rated at one HP @ 1750-RPM
    It is shipped on hi-speed which can be @ (+) 1948-CFM with a 52-F temp-rise in high stage heating mode.

    I'd use A (-) providing 1109-CFM low stage & 1594-CFM on high stage & 63-F temp-rise.
    It would usually need a 4-Ton evaporator coil to maintain 0.5" static at 1600-CFM; Says not to exceed 0.5" ESP in heating mode..

    Now; it depends on what tonnage A/C evaporator coil they installed; a 2.5-Ton coil will flow up to 1000-CFM @ 0.5" stat. Says not to exceed 0.5" ESP in heating mode. Yep, derb, having proper transitions is very, very important.

    Well, what size evap-coil do you have?

    For cooling I'd use B normal 1100-CFM for the 2.5-Ton A/C. It can also go all the way up to 0.8" ESP in cooling mode.
    Last edited by udarrell; 06-03-2012 at 02:54 PM. Reason: Clarity...

  10. #10
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    Quote Originally Posted by spottymaldoon View Post
    Motorguy: If the width of the aperture at the bottom of the cooling coil had been bigger I am saying that I could have gained an extra 25% of throughput - as it is, this area is blocked off with sheet metal. The coil is exceptionally tall to my eye but of course that doesn't help the restriction much.

    Derb: when you say there's a pressure drop of .23 -.50 across the coil, what units would those be, or do you mean 23-50%? I am more concerned with adequate heat distribution and taking full advantage of the 2,000 RPM fan than saving $15 a year.

    As you know, we householders must take what our licensed specialist tells us we need - we are not supposed to ask detailed questions like this as they present themselves as all-knowing ...
    Bear in mind the fan laws and static pressure.

    For example:

    Imagine in cooling mode the blower can move 1,200 CFM against 0.70 total external static pressure (TESP) and meet the cooling needs of your home on the hottest days of the year.

    Now, imagine in heating mode the blower moves just 800 CFM, since with gas furnaces the blower speed is typically dropped in heating mode (if electric heat and/or heat pump, this does not apply). At 800 CFM, the TESP is around 0.31. That's less than half the former static pressure at 66% of the former CFM. This is due to the fan laws...static pressure varies as a square of the CFM variance.

    Also, in heating mode, the cooling coil is dry. In cooling mode the coil is wet because it is dehumidifying the air. The OEM of your equipment calculates the TESP the blower will be expected to move against once the equipment is installed into the ductwork, and the coil is wet from dehumidifying the air.

    Therefore a coil bypass should be the least of anyone's worries. Duct design with low pressure drop fittings and layout should be the primary concern.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
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    Shophound: Now, imagine in heating mode the blower moves just 800 CFM, since with gas furnaces the blower speed is typically dropped in heating mode (if electric heat and/or heat pump, this does not apply).
    It is true that in the hot southern states heating has a lower CFM requirement than cooling however, in the real cold northern climate the opposite is true & many heating furnaces are way oversized to boot.

    We don't need as much cooling up here; my late brother Don had a 112,000-Btuh output Oil furnace matched with a 1.5-Ton A/C. The Oil furnace requires at least around 1200-CFM the A/C 675-CFM - tops.

    That setup in heating mode requires about twice the CFM that the A/C requires, which is a nominal 600-CFM.

    Yes, the furnace is oversized; it's only a one story with an unconditioned basement at only around 1100-sf.

  12. #12
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    Heated air is less dense than cold air, hence it does not need as much CFM to move.

    All depends on the temperature of the air delivered to each room via the registers, however (discharge air temp). The cooler the air, the more of it you must move to meet the calculated heat loss per room. The warmer the air, the slower it can be moved to each space.

    Cooling air delivery more or less revolves around 400 CFM per ton across the evaporator, to prevent the coil from freezing. I've never done HVAC work in northern climates...how common is it to have a CFM for heating higher than a CFM for cooling?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
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    SW Wisconsin
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    Shophound: I've never done HVAC work in northern climates...how common is it to have a CFM for heating higher than a CFM for cooling?
    Well, it all depends on whether they properly size the A/C or oversize it, as they have been doing for ages concerning the heating & also the cooling here in the north country.

    If the heating is way oversized & the cooling is sized right, then the heating could require near double the CFM of the cooling in order to keep the temp-rise within its acceptable range.

    If they are both sized right.

    I just had a properly sized 60,000 input, 57,500-Btuh Output propane furnace installed in my home last year. I don't have a central A/C, however, a 1.5-Ton will cool my home, or 600-CFM.

    I don't have any tools anymore, but the installers did a Home Energy Efficiency Audit using a blower door & infrared camera that you could see the insulation in the walls with, put an additional R-30 in the attic, ending R Value at R-50. Some new windows..., & caulking. Handled coldest heat-loss conditions with ease & plenty of reserve!

    They had a True-Flow they used in-place of the filter which showed 1248-CFM with a 51.5-F temp-rise, under the 60-F max. That does not figure right using my math...

    I don't see the heating mode needing more than 900-CFM which would allow a 2-Ton evaporator on the 1.5-Ton system - to flow that amount of air.

    So, in my situation in a 1937 farm home with a lot of windows & a 2-story home with a basement, the airflow for the new furnace in heating mode would be double the airflow of the A/C. Well, somehow their tests showed the furnace as achieving 97.5% efficient.>?

    The first floor 3 duct runs are all 7" rd metal the other 3 go up ducts in the walls. The upstairs heated perfect; I set the SWING TH for about a 3-F swing for 16 to 22 minute runtimes, varying due to the weather conditions.

    All the ducts, before they go up the walls have dampers just off the take-offs, which I adjust to get a balanced heating situation.

    Many still oversize the A/C here in the north. However, we have a few 2-Ton A/Cs on 2400-sf homes here in SW WI. I have an (I.) page showing my home & other data... all those little cedar trees have been cut down.

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