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Thread: HRV location...

  1. #1
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    HRV location...

    Two techs, one says the HRV duct cannot be tied into the return after a split. That the HRV duct must be before any splits in the return. The other tech has installed both ways.

    On our install the return duct make a split very shortly before the AH plenum. This makes accomadating a HRV connection in that area nearly impossible. If the HRV duct can be after a split in the return system, there is lots of room in the attic to facilitate this and vent it out the side of a 30" closed soffit.

    Any insight?

  2. #2
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    Will the HRV only run when the system runs.
    Is the duct big enough to handle the added air.
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  3. #3
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    If I understand the HRV's function correctly it is motorized damper that will be opened according to set time intervals. When the air handler comes on the damper will have opened a 6" fresh air vent to the outside adding to the normal conditioned return air.

    So I believe "yes", the HRV will only run when the system runs.

    The return leg that the 6" HRV duct would go on is a 12" round. That return duct is for 600 cfm of supply feeding the second story. It is already sized for a small positive pressure of the second story. I certainly expect the HRV would be drawn on. It's location would still be closer to the air-handler than a couple of the other return locations.

    The home also has a 2nd story bath fan the electrician wired on a timer. I was sort-of under the impression that "it" was the fan that met the fresh air requirement. Needing an HRV along with the timed bath fan was a surprise to me.

    I was going to go to the munincipality on Monday for clarification.

  4. #4
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    Bath fans are usualy exhaust, not intake fans.

    Most people refer to a floor being under a slight positive pressure, when the return for that area is undesized. Which could mean that return duct is undersized.

    The HRV blower should be able to force its air into the duct even if that is the situation.
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  5. #5
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    You are right on the bath fan, I was not thinking.

    Your example of positive pressure is what we have. The return system as a whole is not undersized, but the second floor itself is by about 100cfm using normal design perameters compared to the 2nd floor supplies.

    HRV blower? am I under the wrong assumption that the HRV system is just a motorized damper that is still relying on the air-handler to actually circulate?

    Is the wired/timed bath fan suppose to work in concert with the HRV system. Like, the bath fan timer turns the bath fan on and the HRV damper is signaled to open the damper simultaneously so the system can re-circulate fresh air through the conditioned space?

    Seems to me these two should be connected as one working circuit. I'm trying to understand thier individual function if not. It does not compute.

    Can anyone help me make sense of this from the perspective of what the code is trying to do, if my thinking is wrong?

  6. #6
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    Red face WA state code requirements

    Quote Originally Posted by mlo1 View Post
    Can anyone help me make sense of this from the perspective of what the code is trying to do, if my thinking is wrong?
    Residential ventilation air ( outside or makeup air) requirements are stated in ASHRAE Standard 62.2. Std 62.2 is generally only applicable if it it adopted in the state or local building or mechanical code.

    Washington state is rather aggressive in writing specific up-tp-date ventilation air requirements in Chapter 3 for residences.

    http://www.sbcc.wa.gov/docs/codes/VIAQ07.pdf

    The current Table 3-4 requirements for buildings other than residential are outdated as they are based on 1989 edition of ASHRAE 62.1.

    I am not aware of a mechanical code at this time that strictly requires a HRV for residences. Each state has their code for addressing overall building energy efficiency.
    Last edited by dan sw fl; 03-02-2008 at 07:11 PM. Reason: add link / WA state code
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    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  7. #7
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    All the HRV's I've done, have their own fan.

    It could be ducted to be part of the bathroom exhaust.
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  8. #8
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    Thank you gentlemen.

    The electrician wired the second story bath fan onto a 24hr. auto timer. He explained this fan was sized for the home and occupant load and was wired to meet the whole house ventilation code.

    Then the HVAC techs included the HRV motorized damper and duct into thier bid automatically. Claiming that it will need to be installed to meet the whole house ventialtion code.

    Neither of these guy's talked as if these two items would be integrated. Seems to me they should be. How can you have the bath fan exhausting with-out a make up air source?

    Seems a trip to the munincipality is in order for a code interpretation.

  9. #9
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    Yes.
    Because it sounds like what the hvac company wants to install, is a standard motorized damper.
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