Cold Basement - Potentially air flow issue?

Discussion in 'HVAC Heating & Cooling' started by MK_Chicago, Jul 10, 2019.

  1. MK_Chicago

    MK_Chicago Member

    Joined:
    Nov 23, 2013
    Location:
    Illinois
    Hi Everyone,

    Hoping to get some good advice or recommendations on this forum. I have expertise in HVAC so any real advice I can get would be helpful.

    Background
    Our house was built 3 years ago and we moved in 1 year ago this week. The home has a finished basement. In January we had a contractor put up a wall in our basement to create a true bedroom area. The area of the basement we made into the bedroom was where the one air return is (~15 x 15). When the contractor added the wall, they added a second air return (~12 x 5) on the wall outside of the bedroom (see pictures). There are a total of three ~4x10 registers in the finished area of the basement plus one in the basement bathroom. We did change the registers with the wall addition.

    Issue
    With the vents open in the basement, it will stay ~62 to 64 degrees while the main floor is 69 and the top floor is ~71-73. With the vents closed, the basement at most will get up to 66. It definitely feels cold down there and my kids are starting to complain. I do not remember it being that cold last year (could just be bad memory) so I am wondering if it is possible that adding the wall or modifying the return duct has caused issues with air flow. I have noticed that when in the bedroom with the door shut and the air conditioning is running, you can feel the cold air coming in under the door and into the bedroom pretty significantly.

    With these symptoms, is it possible I need a larger / smaller return or any other possible causes or solutions?

    Thank you

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  2. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    There isn't enough information to fully diagnose it, but there are many possible issues, which may point to different solutions.

    Before adding the bedroom was the basement temperature tracking well with the first floor, or was it a "don't know, didn't care" situation then?

    From a duct design point of view supplies and returns need to be balanced, and from an implementation perspective the ducts need to be sealed (preferably with mastic) at EVERY seam and joint, and the duct boots need to be sealed to the wall/ceiling gypsum. Flex duct needs to be fully stretched/taut to deliver the designed flow.

    To meet Energy Star spec on the duct system room to room pressure differences cannot exceed 3 pascals (0.012" water column) under all air handler speeds and all door conditions (open/closed, or combinations thereof with different rooms.) To get a handle on duct balance issues, this could/should be measured everywhere in the house. The ~$150 dual-port manometers of the type used for measuring static pressures on duct systems will usually only have resolution to 0.01" increments. That's not good enough to commission an Energy Star system, but it can be good enough to find and fix the most egregiously imbalanced rooms.

    Are the foundation walls insulated to the IRC code minimum? (R15 continuous insulation for US climate zone 5, northern IL, or R10 c.i. for zone 4, southern IL.)?

    If the foundation walls are insulated on the exterior, does it extend all the way to the foundation sill, or is it (as is all to often the case) only to grade level, with some amount of un-insulated above-grade exposure?

    The changes in heat loss characteristics of basements (insulated or otherwise) with changes in outdoor air temperatures differ quite a bit from those of fully above grade floors. If the entire house is being operated as a single zone, balancing the basement's temperature with the other floors so that it stays within a few degrees year-round is nearly impossible, ESPECIALLY if the HVAC is oversized for the loads and cycling often. With modulating or multi-speed systems right-sized for the loads it's possible to tweak in the flows with balancing vanes so that it does a lot better. The typical furnace is 3x oversized for the 99% design load, which can make it impossible to fix if operated as a single zone. What are the BTU/hr ratings on the furnace or heat pump? Is it multi speed? Roughly how many square feet of conditioned space is there on the first & second floors, and for the basement?

    After verifying and fixing any duct imbalance or insulation deficiencies, the solution might be as simple as (paradoxically) strapping your multi-speed furnace to ONLY run at it's lowest firing rate. Since you have a heating history on the place you can use wintertime-only fuel use to measure the design heat load reasonably accurately, and from there figure out the oversizing factor on the furnace. If the furnace is 5x oversized for the design load you may need to get rid of it and install something more appropriate to really fix it.

    ASHRAE recommends no more than 1.4x, which is enough to cover the Polar Vortex disturbance cold snaps. AFUE is tested at 1.7x, and while that is less optimal for dealing with temperature imbalance issues it's not terrible, especially if it's a 2-stager that can be tweaked a bit.

    Before spending any more money on this, spend some time reading up and watching the videos on Nate Adams' blog site, which are a pretty good primer on how to deal with comfort and temperature difference issues:

    http://www.natethehousewhisperer.com/home-comfort-101.html

    http://www.natethehousewhisperer.com/hvac-101.html

    http://www.natethehousewhisperer.com/hvac-102.html

    Nate Adams works primarily in the Cleveland Ohio area, which is zone 5A, comparable in most respects to northern Illinois/Chicagoland.

    BTW: Is there any insulation under the slab? If not, those carpets & underlayment are at risk for mold issues in summer unless you actively dehumidify the basement air to a dew point lower than the slab temperature.
     
    Last edited: Jul 10, 2019
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  4. MK_Chicago

    MK_Chicago Member

    Joined:
    Nov 23, 2013
    Location:
    Illinois
    Hi Dana,

    Thanks for taking the time to provide so much information. I will definitely review all the resources you sent over. I will also try to answer as many of the questions that were asked in the reply if I can.

    My assumption is that since the house is newer that it does comply with any code minimums. From what I can tell in the unfinished storage area of the basement, it appears to be insulated halfway down the wall.

    I can't say for 100% certainty that this was not an issue last summer before the new bedroom wall was put up but I feel like my family would have noticed it. I know the basement has always been cooler than the upper floors but I don't remember it ever being this cold and feel like I would have noticed it last year. We are ~62-66 degrees in the basement both in the summer (90+ degrees outside) and winter. The basement humidity is pretty steady around 60%.

    The finished basement is 1000 square feet.

    I need to see if I can dig up the info on the furnace and also try to estimate the upper floors.

    Thanks again,
    Mike


     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    That would not be a good assumption.

    Three years ago Illinois codes were generally at the IRC 2012 level, and have since moved up to IRC 2015, but the basement wall R values did not change between those versions. See TABLE N1102.1.1 (northern IL is zone 5.)

    Code minimum is R15 continuous insulation (or 2x6/R19, which is a problematic build from a moisture/mold point of view) all the way down to the slab, not merely "...half way down the wall." But if there is R30 between the floor joists between the basement and first floor the walls are allowed to be uninsulated, except where the basement was being heated, as in your newly finished section.

    But it sounds like the whole basement is/was always heated, which means this is more likely a case of "whatever the inspector let us get away with".

    What sort of insulation (and at what R value) is that half-wall insulation?

    How much of the foundation is above grade?

    Was insulation added to the lower half when the space was built out into fully conditioned space?

    If the lower half of the wall is un-insulated and completely below grade it represents a constant heat loss to the ~50-55F deep subsoil, changing only a small amount seasonally rather than in response to daily outdoor temperature swings. The thermal conductivity of concrete is much higher than the surrounding soil, so there is some response in load to the outdoor temperatures due to the thermal bridging between the above grade exposure and the lower half of the wall, but not nearly as much as it would be if the above-grade portion were completely uninsulated. It's still a fairly substantial heat loss though.

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    Last edited: Jul 11, 2019
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