Very low humidity levels in new construction - over ventilation?

Discussion in 'HVAC Heating & Cooling' started by bradforj287, Jan 1, 2018.

  1. bradforj287

    bradforj287 New Member

    Jan 1, 2018

    I just purchased a new home in the suburbs of Boston with move in date of November 2017. The home has felt unusually drafty and cold and i've had to increase the heat about 4 degrees above what I usually use. Ordinarily i'm comfortable at 68 but in this house the heat needs to be at 72 for equivalent comfort. In addition to feeling a bit uncomfortable, it has been very dry inside the home for the past 2 weeks. I purchased 2 ThermPro Hygrometers and they have consistently read 10-13 RH for the past 2 weeks. Right now it is 5 degrees outside and the RH is measured at 11% both upstairs and downstairs.

    We have 2 exhaust fans that appear to run 24/7. They are both located in bathrooms on each floor. I removed the vent cover and there is a switch that reads they are both operating at 110CFM. This means total venting is 220CFM? Curiously, in our home energy audit packet we received it says the mechanical ventilation rate is 145CFM. I wonder why the difference?

    I tried turning both exhaust fans to 50CFM for a total of 100CFM and the home seems a little more comfortable but the very low humidity hasn't changed. These fans are running ALL the time. The wall switch will not turn them off.

    I'm worried that even at 100CFM the house is being over ventilated causing cold air to be pulled into the house through the building envelope and exhausting warm moist air too quickly causing the house to dry out. I'm also worried about the opposite problem in the summer - too much humid air being pulled into the house.

    Here are the specs on the home:

    Basic stats of home:
    2700sq feet
    unfinished attic + basement
    spray foam in attic + basement
    fiberglass batt in exterior walls
    forced hot air heat. Dual zone w/ furnaces in Basement and Attic
    8 foot ceilings.

    Info from energy audit
    Infiltration rate: Htg 688 Clg: 688 CFM50
    Method: blower door test

    From 2016 IECC r-406 Confirmed Energy Index Report
    R402.4 - Envelope air leakage maximum leakage rate: (3 ACH50 for CZ3-8) PASS

    Are indoor RH levels of 10-15% in winter months unusual for a cold climate like Boston?

    Could my home be over ventilated?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Depending on how many people live there and the types of indoor air pollution (does anybody smoke?) you can turn one or both off whenever you're out, and run one at it's lowest speed while you're there without lowering indoor air quality much.

    Current code-maximum air leakage for new construction in MA is the IRC 2015's 3ACH/50 (three air exchanges per hour at 50 pascals pressure, measured with a calibrated blower door.) A measurement of 688/50 is a pretty tight house, the leakage equivalent of about a 3" diameter hole in an otherwise hermetically sealed enclosure. Houses that tight need active ventilation, and are required to be able to ventilate at ASHRAE 62.2 standard rates (there is a formula based on house volume and number of occupants/bedrooms,etc.), but that doesn't meet you have to ventilate at that rate.

    There is no real science behind ASHRAE 62.2- it's just the collective opinion/guesses of a much of HVAC people, not public health scientists, and the levels are actively disputed by people in the building-efficiency world as being excessive, with only vague health theories, but very real and measurable energy use consequences.

    Using exhaust-only ventilation (like a couple of continuously operating bath fans) is cheap to implement but a pretty lousy approach, and in some cases can reduce indoor air quality if the air leak points into the house happen to be below grade (say, the foundation walls and/or the seam between the foundation slab and foundation), bringing in radon and other soil gases. With a house as tight as yours exhaust only approaches can't be counted on to even meet the cfm rating of the fans. A better approach is balanced heat recovery ventilation, but for the time being just backing off on the existing fans will probably work. To make it work well you must use kichen exhaust ventilation whenever you're cooking (and for 10-30 minutes thereafter), with a similar approach on the use of bath fans. Timer switches and occupancy/vacancy sensors can make that almost automatic, if you like.

    Many hot air heating systems are also designed take in ventilation air whenever the air handler is operating. The amount of ventilation air is usually adjustable, and there is nothing wrong with adjusting it to zero.
    Last edited: Jan 2, 2018
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    For the record, the ASHRAE 62.2 spec is 7.5 cfm per occupant + 3 cfm for every 100 square feet of floor area (not including unfinished basements or conditioned but unfinished attics.) So for your house, assuming 4 people that ends up at:

    3 cfm x 2700/100=81 cfm

    7.5cfm x 4 occupants= 30 cfm

    ...for a whopping 111 cfm, and that's at the full (overblown) ASHRAE 62.2 ventilation levels. Cutting it back to 100 cfm (2 fans x 50 cfm) is still overventilating even in ASHRAE terms when there are only two people (or fewer) in the house.

    If you cut it to one fan at 50 cfm while you're home and ZERO cfm while you're away during colder weather you'll most likely still be OK, as long as you're religious about using kitchen & bath fans at higher cfm while actively in use.
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