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  1. #1
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    May 2007
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    Are Taking Room Pressures Std Practice?

    Had a split system heatpump replaced in my Chandler, AZ home and later found all bedroom doors blow closed due to excessive Pa readings.

    My contractor does did not test for this or anything else inside the home besides checking the split temps.

    Is checking bedroom pressures covered in any HVAC best practices doc or HVAC building code somewhere?

    My bedrooms range from 5-8Pa with the master at 17Pa and suspect the ideal # is near 0?

  2. #2
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    The original install was where the problem statrted.

    It would have ben nice if the replacement contractor had pointed this out and included a solution in his price.however if he did ,not you ,but some owners would go with a lesser cost from another contractor that might have said you don't need to correct this.

    My point is ,you should pay to correct the problem.

  3. #3
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    Aug 2003
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    Fort Worth, TX
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    A colleague of mine in the trade carries around with him one of these:




    It's a digital manometer made by Testo. He uses it to measure exactly what you're talking about by standing outside the closed door with the system running, with one of the tubes going into the room to obtain a reading. He can then know if the room is overly pressurized or not.

    I only mention that because he's likely one of the few technicians in the area that actually does that kind of a check. It should be more commonplace, as the more a tech understands airflow problems, the better he can work to resolve them.

    In your case I recall you being the initiator of the "jumper duct" thread that somewhat strayed into a chest beating contest. Being that you are experiencing room pressurization, and have data to confirm it, adequate return air paths is the answer and you should seriously consider getting that done. Dedicated or jumper, whichever works best for your particular house...either should be sized properly so room pressure is essentially neutral.
    Building Physics Rule #1: Hot flows to cold.


    Building Physics Rule #2:
    Higher air pressure moves toward lower air pressure


    Building Physics Rule #3:
    Higher moisture concentration moves toward lower moisture concentration.

  4. #4
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    >>Is checking bedroom pressures covered in any HVAC best practices doc or HVAC building code somewhere?

    I believe it is addressed by Florida codes, have not heard of anywhere else:
    http://www.tamtech.com/PDF/R.A.P.%20...L.%2008.02.pdf
    'Section 601.4 of the New Florida Building Code:
    ...Provisions shall be made in both residential and commercial
    buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to .01 inch WC (2.5 Pascals) or less...'


    But Florida is a hot-humid climate, I can imagine moisture problems there but not in Arizona. S.Texas is a hot-humid climate too, and many houses have stood for years with AC ducts designed by people who have *no* concept of this. I have seen Texas houses which apparently have 40% of air flow behind potentially closed doors. You do not hear many stories of mold or other horrors due to pressure problems, so one could make the argument that apparently houses are fairly robust despite pressure differences.

    My own house had approximately 400 cfm behind closable doors in the master bedroom suite, you could really feel it blow when doors near closed. I measured room pressure with a more primitive (for this app) instrument, a Dwyer micro-manometer bought on Ebay. My room pressures were about half the size of your problem. Added a thru-the-wall vent, functionally equivalent to a jump duct, to allow a return path. Now my pressures are in the 3-4 Pa range, I think acceptable.

    I should confess my added vent violated a couple broad rules, my personal opinion is rules should be intelligently questioned and analyzed -- though many times that questioning will reveal the truth and wisdom behind a rule. A very firm rule is no returns in bathrooms, so I chose a transfer vent which exited air to another bathroom. This followed the letter of the rule but not the intent. I was prepared to wall the thing up if bad effects were observed (or strong enough spousal disapproval), but it seems to have good effect in my case. The additional return path seems to be responsible for a 5-10% reduction in relative humidity in the shower area. But I am not advocating that any other person do that without consulting an HVAC pro first.

    Hope this helps -- Pstu

  5. #5
    Join Date
    May 2007
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    Thread Starter
    nice.... sent you an email Shophound too...

    A colleague of mine in the trade carries around with him one of these:

  6. #6
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    May 2007
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    Thread Starter
    Thanks.... Wonder if this is suggested as part of some HVAC best practices guidelines?
    Quote Originally Posted by pstu View Post
    >>Is checking bedroom pressures covered in any HVAC best practices doc or HVAC building code somewhere?

    I believe it is addressed by Florida codes, have not heard of anywhere else:
    http://www.tamtech.com/PDF/R.A.P.%20...L.%2008.02.pdf
    'Section 601.4 of the New Florida Building Code:
    ...Provisions shall be made in both residential and commercial
    buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to .01 inch WC (2.5 Pascals) or less...'


    But Florida is a hot-humid climate, I can imagine moisture problems there but not in Arizona. S.Texas is a hot-humid climate too, and many houses have stood for years with AC ducts designed by people who have *no* concept of this. I have seen Texas houses which apparently have 40% of air flow behind potentially closed doors. You do not hear many stories of mold or other horrors due to pressure problems, so one could make the argument that apparently houses are fairly robust despite pressure differences.

    My own house had approximately 400 cfm behind closable doors in the master bedroom suite, you could really feel it blow when doors near closed. I measured room pressure with a more primitive (for this app) instrument, a Dwyer micro-manometer bought on Ebay. My room pressures were about half the size of your problem. Added a thru-the-wall vent, functionally equivalent to a jump duct, to allow a return path. Now my pressures are in the 3-4 Pa range, I think acceptable.

    I should confess my added vent violated a couple broad rules, my personal opinion is rules should be questioned and analyzed -- though many times that questioning will reveal the truth and wisdom behind a rule. A very firm rule is no returns in bathrooms, so I chose a transfer vent which exited air to another bathroom. This followed the letter of the rule but not the intent. I was prepared to wall the thing up if bad effects were observed (or strong enough spousal disapproval), but it seems to have good effect in my case. The additional return path seems to be responsible for a 5-10% reduction in relative humidity in the shower area. But I am not advocating that any other person do that without consulting an HVAC pro first.

    Hope this helps -- Pstu

  7. #7
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    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by shophound View Post
    A colleague of mine in the trade carries around with him one of these:



    It's a digital manometer made by Testo. He uses it to measure exactly what you're talking about by standing outside the closed door with the system running, with one of the tubes going into the room to obtain a reading. He can then know if the room is overly pressurized or not.

    I only mention that because he's likely one of the few technicians in the area that actually does that kind of a check. It should be more commonplace, as the more a tech understands airflow problems, the better he can work to resolve them.
    That poor little manometer gets a real workout every day.

    The closest thing to a manometer most residential techs have in our area is a gas pressure tester.
    6 years ago the same could be said about me though.

    One thing that is fun when a customer doesn't seem to grasp the concept of what is happening when their bedroom pressurizes relative to the rest of the house is to break out the flow hood, when I happen to have it on my truck.
    I have the customer stand there holding it up to the register and have them read the numbers off as I close and open the door.
    I had one that the CFM dropped almost in half within 30 seconds of closing the door, lol.

  8. #8
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    Dec 2006
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    Dallas
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    Instruments are nice,but...

    I just observe all doors as I close them...When a door is 1/8" open, you can tell if a room has positive or negative pressure. If the door jams are covered in dust, and you are a thinking man, you can tell which way the air flows.

    In my home, one of the bedrooms has a 3x return to supply surface ratio. Even though I own a sawzall, I'm not going to hack up the walls. This can be fixed with ductwork in the attic.

  9. #9
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    Aug 2002
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    Southern NJ
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    You need to have the house air balanced...simple as that.

  10. #10
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    Quote Originally Posted by gonekuku View Post
    Instruments are nice,but...

    I just observe all doors as I close them...When a door is 1/8" open, you can tell if a room has positive or negative pressure. If the door jams are covered in dust, and you are a thinking man, you can tell which way the air flows.
    Dark lines in the carpet under doors, and even along the basebords of walls, is a good clue too, the instruments mainly serve to show the customer what is going on, and quantify the problem.

    Its one thing to tell someone what the problem is and give them a price to fix it, but if you can show them the problem, and quantify it, selling the solution is easy.

  11. #11
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    May 2007
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    Thread Starter
    I'm going to have my contractor install dedicated returns in each bedroom & master to resolve my positive pressure problems with doors closed.

    Seem to recall seeing a formula for sizing dedicated returns (some say twice the supply) but I'd like to be armed as I know the current CFM's for the rooms from the flowhood tests?

    Read where to big a return and it will go negative and too little and I'll still have a problem?

    Thank you.

  12. #12
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Just make sure they understand that adding return ducts will increase the static pressure,and may reduce airflow below the required minimum.

    This can be avoided by redesign of existing ducts,if needed,or jumper ducts .

  13. #13
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    Apr 2005
    Location
    illinois
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    Dash
    Could you please explain how adding returns increases the static pressure
    and decreases the airflow. Thanks

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