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Thread: An observation

  1. #1
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    An observation

    and fair warning to those that might want to install a large capacity range hood.

    Late yesterday evening, I happened to notice that the thermostat claimed the temperature in the house was 76, even though the setting had not changed from the 74 that we keep it at in the summer.

    This is something very out of the ordinary in my house, so I immediately checked the air conditioner. The output air temperature was nice and cold, the humidity was only 41% in the house, and the condensate drain had almost a constant flow to it, so I knew it had to be running correctly. So, that left the question of where did the extra heat load come from?

    I mentioned this to the wife, and she admitted that she had forgot and left the range hood exhaust on much longer than necessary - one of the disadvantages of having a very quiet exhaust hood. My exhaust hood is only rated for 600 cfm. I'm quite glad I did not put one of the 1,200 cfm monsters in!

    An hour or so after turning the exhaust hood off, the house was back to its normal temperature again.

  2. #2
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    Quote Originally Posted by paul42 View Post
    and fair warning to those that might want to install a large capacity range hood.

    Late yesterday evening, I happened to notice that the thermostat claimed the temperature in the house was 76, even though the setting had not changed from the 74 that we keep it at in the summer.

    This is something very out of the ordinary in my house, so I immediately checked the air conditioner. The output air temperature was nice and cold, the humidity was only 41% in the house, and the condensate drain had almost a constant flow to it, so I knew it had to be running correctly. So, that left the question of where did the extra heat load come from?

    I mentioned this to the wife, and she admitted that she had forgot and left the range hood exhaust on much longer than necessary - one of the disadvantages of having a very quiet exhaust hood. My exhaust hood is only rated for 600 cfm. I'm quite glad I did not put one of the 1,200 cfm monsters in!

    An hour or so after turning the exhaust hood off, the house was back to its normal temperature again.
    You are required by design to have make up air to compensate for exhaust from the hood. A huge lack of design error in today's market with large exhaust being installed by contractors and individuals.

  3. #3
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    Quote Originally Posted by second opinion View Post
    You are required by design to have make up air to compensate for exhaust from the hood. A huge lack of design error in today's market with large exhaust being installed by contractors and individuals.
    I am not convinced that a dedicated make up air duct is the right answer for residences. The make up air is bringing in additional heat and humidity no matter where it is located and it is foolish to think that your exhaust hood will only draw air from that dedicated input. Proximity has practically no effect on the air flow. Only the relative air pressures at inlet and outlet have an effect.

    With a dedicated make up air duct, you also have to add a powered damper that is controlled by the exhaust unit, or you are just opening another big hole into your conditioned space.

    My preference is that as much of the make up air as possible comes in through the fresh air duct into the air handler. By pressurising the house with that fresh air duct, I have more control over where the fresh air comes in at and that at least gives me a little more control over the humidity of the incoming air.

  4. #4
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    Quote Originally Posted by paul42 View Post
    I am not convinced that a dedicated make up air duct is the right answer for residences. The make up air is bringing in additional heat and humidity no matter where it is located and it is foolish to think that your exhaust hood will only draw air from that dedicated input. Proximity has practically no effect on the air flow. Only the relative air pressures at inlet and outlet have an effect.

    With a dedicated make up air duct, you also have to add a powered damper that is controlled by the exhaust unit, or you are just opening another big hole into your conditioned space.

    My preference is that as much of the make up air as possible comes in through the fresh air duct into the air handler. By pressurising the house with that fresh air duct, I have more control over where the fresh air comes in at and that at least gives me a little more control over the humidity of the incoming air.
    The fresh air intake being your primary make-up air source, given how tight your house is (if I recall correctly), explains the temperature drift. With the exhaust hood operating, considerably more air entered the fresh air intake, loaded with our nice North Texas humidity (this summer has been so much more humid than last summer. An amazing and stark contrast). This loaded up your cooling coil with an elevated latent load, reducing its sensible heat removal capacity. Space temperature inside the house then drifted upward as the system no longer could stay ahead of structural sensible heat gain.

    If there was a dedicated make-up air source adjacent to the exhaust hood, and it was sized properly and its outlet was at the perimeter of the hood, it should (key word should) not escalate infiltration or fresh air intake through the a/c volume elsewhere in the house. This is the proper way it is done in commerical kitchens; make up air enters at perimeter of exhaust hood and mixes with air drawn up from stove, oven, etc.
    • 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.

  5. #5
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    Quote Originally Posted by Shophound View Post
    The fresh air intake being your primary make-up air source, given how tight your house is (if I recall correctly), explains the temperature drift. With the exhaust hood operating, considerably more air entered the fresh air intake, loaded with our nice North Texas humidity (this summer has been so much more humid than last summer. An amazing and stark contrast). This loaded up your cooling coil with an elevated latent load, reducing its sensible heat removal capacity. Space temperature inside the house then drifted upward as the system no longer could stay ahead of structural sensible heat gain.

    If there was a dedicated make-up air source adjacent to the exhaust hood, and it was sized properly and its outlet was at the perimeter of the hood, it should (key word should) not escalate infiltration or fresh air intake through the a/c volume elsewhere in the house. This is the proper way it is done in commerical kitchens; make up air enters at perimeter of exhaust hood and mixes with air drawn up from stove, oven, etc.
    And is also the proper way to do it in residential.

  6. #6
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    Quote Originally Posted by second opinion View Post
    And is also the proper way to do it in residential.
    Agreed.
    • 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.

  7. #7
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    Yup... at 100F outdoor temperature, that almost 1.5 Tons of sensible cooling alone for 600CFM with a 74F indoor temp... and that doesn't take into account another 25% for the humidity removal.

    +1 on MUA unit when you have a large exhaust source. I beleive it's now a code requirement. IF you have a point source of exhaust and you repalce it near the same point source with MUA, the only air you have to condition is a zone of air beween the two points plus whatever convection occurs along with some migration of humidity. You might end up conditioning maybe 50% of hte air. If your MUA is at a central location, you have to condition 100% of the MUA.

  8. #8
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    I have a customer that remodeled their kitchen and put in a huge Wolf range, along with an equally huge exhaust hood, with a matching huge fan.
    Nobody thought about where the 1000 to 1600 CFM of air was supposed to come from when they fire the exhaust hood up.

    When they fire the thing up, the house gets depressurized enough to pull air past the seals between the panels of their double hung windows, all 18 of them, causing the seals to vibrate.
    The combined noise of all of them sounds like a giant wasp nest when all the wasps are fanning the hive.

    It sounds kinda cool to me, not so much to the homeowner.
    If more government is the answer, then it's a really stupid question.

  9. #9
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    Agrred,Just like a mini version of a commercial kitchen

    If there was a dedicated make-up air source adjacent to the exhaust hood,
    how about the 75% of exhaust rule for make-up?
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  10. #10
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    In my house, a dedicated make up air duct would not be cost effective. We typically run the exhaust hood less than an hour per week.

    The cost of the motorized damper alone would overwhelm any possible monetary savings.

    When the air handler is running, the fresh air intake into the return provides positive pressure to the house. If I wanted most of the make up air to come from a dedicated duct, I would also need an inline blower in that duct to make it the lowest pressure source of air for the exhaust.

    For one of the really large exhaust hoods, the inline blower would not be needed, but a motorized damper would still be required.

  11. #11
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    Quote Originally Posted by paul42 View Post
    In my house, a dedicated make up air duct would not be cost effective. We typically run the exhaust hood less than an hour per week.

    The cost of the motorized damper alone would overwhelm any possible monetary savings.

    When the air handler is running, the fresh air intake into the return provides positive pressure to the house. If I wanted most of the make up air to come from a dedicated duct, I would also need an inline blower in that duct to make it the lowest pressure source of air for the exhaust.

    For one of the really large exhaust hoods, the inline blower would not be needed, but a motorized damper would still be required.
    It may not be cost effective and give you a great return on investment, but in your scenario of running it less than one hour you can create enough co to kill each member of your familiarly and any one visiting along with any pets, and god forbid if you should happen to leave it on by mistake and the HVAC is not running.

  12. #12
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    Quote Originally Posted by second opinion View Post
    It may not be cost effective and give you a great return on investment, but in your scenario of running it less than one hour you can create enough co to kill each member of your familiarly and any one visiting along with any pets, and god forbid if you should happen to leave it on by mistake and the HVAC is not running.
    According to the latest statistics I can find, less than 1% of all CO deaths in the US are attributed to gas cooking appliances. And that includes ovens wich are typically much worse than cooktops when it comes to generating CO.

    However, since I have a total electric house, none of that applies to me anyway.

    I am somewhat paranoid about any form of open flames in my house. My wife has learned to love LED candles.

    I doubt that god has any prohibitions against me turning my AC off. The thermostat is set to circulate which turns on the blower in the air handler about 20 minuts out of every hour enen when it is not calling for heat or cooling.

    In case you have not seen some of my previous posts, and it has been awhile since I last posted, I am an engineer by trade. I designed my house for energy efficiency. I heat and cool 4,000 sq. ft. in north central Texas with a 2 ton heat pump. At 112 degrees outside, the system keeps my house at a constant 74 degrees.

  13. #13
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    Quote Originally Posted by paul42 View Post
    According to the latest statistics I can find, less than 1% of all CO deaths in the US are attributed to gas cooking appliances. And that includes ovens wich are typically much worse than cooktops when it comes to generating CO.

    However, since I have a total electric house, none of that applies to me anyway.

    I am somewhat paranoid about any form of open flames in my house. My wife has learned to love LED candles.

    I doubt that god has any prohibitions against me turning my AC off. The thermostat is set to circulate which turns on the blower in the air handler about 20 minuts out of every hour enen when it is not calling for heat or cooling.

    In case you have not seen some of my previous posts, and it has been awhile since I last posted, I am an engineer by trade. I designed my house for energy efficiency. I heat and cool 4,000 sq. ft. in north central Texas with a 2 ton heat pump. At 112 degrees outside, the system keeps my house at a constant 74 degrees.
    If you have no fossil fuel equipment in your house the co would be a moot point unfortunately your post did not state that and hundreds of people with fossil fuel appliances read these post and interpret that it is ok to install high volume kitchen exhaust without following makeup air criteria leading to the production of co when the appliances are running.

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