Chilly Finished Basement Needs Cold Return?

Discussion in 'HVAC Heating & Cooling' started by whacka.thumb, Mar 26, 2014.

  1. whacka.thumb

    whacka.thumb New Member

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    Our basement (in a raised ranch) is quite chilly in winter, and in the heavy a/c season. There is no cold air return in the basement, just upstairs.

    1) is a cold return likely to help take the chill off in the basement?
    2) assuming the return vent should be near floor level, can I route the cold air through a partition wall (2x4 wood stud + d/w), maybe using those flexivents I see in new construction? Not sure how I would get the vent through the top plate, but 1 step at a time I guess.

    AAA!

    Chris
  2. Dana

    Dana In the trades

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    All ducted air heating & cooling needs balanced supply & return. Leaving it un-balanced means you can't guarantee the that the designed-for supply volume of air is getting there, and the pressure differences the imbalance creates between rooms drives outdoor air infiltration rates sky high, since every air leak to the exterior becomes part of the return path.

    In a basement it doesn't matter a whole lot if the return is at ceiling height or not, but it should be sufficiently distant laterally to promote air-flow across the room(s) rather than short-circuiting a very short high flow path with very little air exchange in the room.

    The heat loss/gain characteristics of basements are dramatically different from the fully-above grade zones. Unless controlled as it's own zone the comfort levels will not be consistently comfortable. In most raised ranches half or more of the exterior wall area of the lower level is below grade, and there's maybe half the amount of window area which lowers both heating & air conditioning loads relative to the first floor, but they also don't track too well.

    What are the foundation insulation levels?
  3. whacka.thumb

    whacka.thumb New Member

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    We're not the original owners, so I don't know for sure. House is ~25 years old, but was originally built as a cottage. Heat is FAG, so we don't really have zones (other than via dampers).

    I can't see any foundation insulation from outside and all the interior basement walls are d/w. Have to open a few holes when it warms up.

    The air in the basement is stratified. Much warmer near the ceiling. This is at least partially due to lack of subfloor/dricore/etc on the floors, I guess.

    For the heat problem, I'm guessing that I need subfloors and an aux heat source (gas FP). I don't have a plan for the a/c season.
    Suggestions always welcome!

    Thanks.
  4. Dana

    Dana In the trades

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    The basement slab isn't going to be cold enough to be the primary cause of the wintertime stratification "coolth" unless you're way up on Hudson Bay or something. Since you have an air conditioning season I'm assuming you're in southern Ontario, no further north than Sudbury or Ottawa, but don't be coy, what is your location?

    The odds favor air leakage at the foundation sill or uninsulated foundation walls (at least part of which is above grade) as the primary heat loss or source of cold air. A cantilevered raised- ranch will often have huge air leakage at the cantilever, which is often stuffed with a few batts or something, without any real air-barrier between the conditioned space and that bit of leaky overhang. That's usually fixable without ripping the interior finish wall or ceiling apart, but takes some sleuthing to figure out for sure that it's the main problem. And if your foundation walls aren't insulated it's worth considering a do-over on the basement wall finishing, even though it's a major project to get it done properly.

    But start with making a return path a the other end of the space from the supply. It doesn't take huge air flows to break up stratification, and putting in a return that's balanced in size with the supply may increase the total air flow by an order of magnitude.

    If that doesn't quite do it, if you have a gas-fired hot water heater you may be able to manage the basement comfort issues in winter with a hydronic heating loop (baseboards or radiators) on a heat exchanger with the hot water tank. But that won't fix summertime air conditioning issues.
  5. whacka.thumb

    whacka.thumb New Member

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    :) Grand Bend, near the south end of Lake Huron.

    For now, I'm going to make a crude return in the ceiling at the far end of the room as a test.

    Sounds like I'll have no trouble keeping busy this summer!
  6. Dana

    Dana In the trades

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    The deep subsoil temps in Grand Bend are a relatively benign +8C, and the soil below the slab has both thermal mass and R value, so if you kept the temp at the basement ceiling at 21C, the slab temp would likely stay above 17C just on radiated warming alone, unless there is air leakage of much colder/denser air cooling the slab.

    [​IMG]

    If you have an infra-red thermometer (even a $50 pistol-grip version from a box store is good enough) you could measure both ceiling and floor temps. My suspicion is that you will find a measurable variation in the ceiling to slab temp differences a half meter from the foundation, and it is likely to be a larger difference than in the middle of the basement. You may be able to identify spots that are colder at both ceiling & slab that may be pointing to a higher-leakage area. The cold spots may change with wind direction too. A blower-door test in combination with infra-red imaging would be the ideal way to ferret it out, but you can still do a lot with a cheap IR thermometer and some good observational sense. This kind of snooping is best done while it's still relatively cold outside, and its worth making a sketch of the floor plan and marking both ceiling & floor temps at a dozen or so locations to get a better sense of it.

    Your binned hourly mean mid-winter outdoor temps for the 6 coldest weeks of winter is about -6C, which is ~ 13C colder than your deep subsoil temps, and 26-27C colder than your desired room temp.

    http://weatherspark.com/#!dashboard;a=Canada/N0M/ON/Grand_Bend

    With the lower R value of the wall as compared to the subsoil, and at twice the average temperature difference, average heat loss per unit of area through above-grade uninsulated foundation + finish-wall would be several times that of the heat loss through the slab and may be more than 10x, depending on the absolute thermal conductivity of the subsoil (which is affected by moisture content, soil type, etc.). If the wall is insulated the difference in average heat load per area will shrink, but it has to be a pretty well insulated wall to where slab losses at subsoil temps that modest begin to dominate the heat load number for the basement. A subsoil temp of +8C is nothing like perma-frost, and unless your water table is less than a meter below the slab you have substantial soil-insulation to cut the rate of heat loss.

    But leakage of cold outdoor air (colder than the subsoil temps) the air leakage could easily exceed the conducted heat loss, and would be a strong contributor to room air stratification.
  7. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    A return in the basement will likely exacerbate the problem. All basements should be radiated and ventilated with an ERV. Zoning is a must.

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