Basement HVAC Questions
I am finishing my basement and am wondering how many supplies and returns I should put in the finished space. The finished area of the basement is about 14í x 35í ( about 490 sq ft, just under 4000 cubic feet). Itís a pretty well insulated space and Iím not even done insulating yet. I have about 1 inch foam insulation on the exterior, 1.5 inch thick EPS foam on the interior, and will eventually have R-13 fiberglass in the stud cavities for a total of R-24 on the exterior walls. The floor has 1 inch XPS insulation covered by 5/8Ē OSB. The rim joist has all been insulated and sealed with 2Ē XPS and spray foamed the edges. The room temperature as it sits right now is 65-68 degrees without any real supply of conditioned air. Itís been that temperature consistently over the past few weeks, even in the morning when it is below freezing right now in central Ohio. We normally run our upstairs heat in the 70-72 range, so we don't have far to go to stay comfortable in the basement... So I donít feel like Iíll need a bunch of supply ducts, but maybe Iím wrongÖ
I should note that it will be REALLY easy to put three supplies in. I can branch off each end of my bulkhead and create two supplies at each end of the finished space and one more near the middle of the room. Any more than that and it gets more difficult because I have to cut holes in the top or sides of the bulkheads elsewhere (which are difficult to get to). Is three supplies going to be enough for this space? Or should I go more?
As for return air I have direct access to an existing return duct that is practically right in the middle of the finished area. What is the rule of thumb for how big the return should be in relation to the supply? From what I've read does the square area of the return grill have to equal the area of the supply duct work? So if I put in three supply ducts at 6 inches in diameter each I would need close to 90 square inches of return grill or is it 90 square inches of return duct also? (I calculated the 90 square inches from radius squared * pi. Or about 28-29 square inches per 6 inch supply duct)
Any help would be greatly appreciated. Thanks!
I am not an HVAC expert, but I think one additional consideration is how many rooms you have in the basement. Our house in Texas had a large return in the main part of the house, and then individual returns in each bedroom. This ensured we had air good flow in the bedrooms regardless of how many doors were shut, which was key to having balanced cooling during the hot months. However, if your basement is mostly one large room (although it sounds bigger than that), it may be sufficient to have the supply ducts near the edge of the room and one large return toward the middle.
All good heating system designs start with a heat load calculation. But as you've empirically discovered, the space heating loads of insulated basements below fully-conditioned first floors are pretty miniscule. The sensible-cooling loads are even smaller- it's all latent-load.
Since the heat loss/gain characteristics of basements are so radically different from above-grade floors it's usually impossible to control the whole house as a single zone and have it comfortable. With too much furnace aire directed into the basement you'll cook most of the time in winter, and it'll be clammy-cold with the AC running in summer. Heating it with a hot air furnace without zoning & zone valves would be a recipe for discomfort. A hydronic loop off the hot water heater with a low-temp panel radiator sized for the calculated heat load at the 99% outside design temp, with the pump controlled via thermostat with a zone relay would be pretty cushy heat, and wouldn't mess up the duct balancing issues of the upper floors with hacks.
If that turns out to be too complicated or expensive, any 3/4 ton mini-split air source heat pump would more than cover the load, and would be able to both heat & cool the basement as it's own zone.
BTW: After factoring the thermal bridging of the 2x4 framing, with 1.5" XPS of continous insulation interior 1" EPS on the exterior, assuming 8-10" concrete and half-inch sheet rock on the interior you're looking at about R21 for whole-wall R value, not R24 (not that it much matters, eh? :-) ), or a U-factor of ~0.048. From a load calculation point of view it's easiest to only count the above grade wall area (including band-joists), down to about a foot below grade- the rest will be pretty much "in the noise" of error on real vs. calculated U-factors. Assuming only 2 sides of the 14 x 35 section are exterior walls, and from the top of the band joist to a foot below grade is about 4', and assuming the 99% design temp in your area is maybe +5F, with a 70F interior the heat loss from walls is:
(14 + 35) x 4 x 0.048 x (70F - 5F) = 12,740 BTU/hr
To that you'd have to calculate the window losses based on U-factor and area, but unless you have a walk-out basement with sliders or something you're typically talking 8-10 square feet with a U-factor of 0.5, which is about 260-325 BTU/hr, which is less than the heat output of a conscious human sitting in the dark. With any power loads like lights or a TV, etc it starts peeling off from the heat load, so in an occupied basement room, at 5AM on the coldest day of the year the heat load might be as high as 12,000BTU/hr, which is about the output of 3/4 ton mini-split at +5F.
But run your own crude calc on the real wall & window areas. With the radiated losses off the supply ducts for the upstairs zones into the basement you can probably peel another 2-3KBTU/hr off any heat load that would have to be supported by a heating zone or system just for the basement. Be aggressive on your plug-loads (at 3.412 BTU for very watt of power being used) and any other heat sources (standby losses from a hot water heater, or something?) It all adds up- whatever you come up with based on U-factors and indoor/outdoor temperatures alone, it'll be less, possibly MUCH less.
With R20+ whole-wall R on the exterior walls, even if the 490' is divided into 2 or three spaces it can be heated and cooled with a point source (like a mini-split), if you can tolerate ~5F temperature deltas on the coldest days, less the rest of the time. Just pick the room that will be the warmer room, or calculate the room-by-room loads and put the heating source in the lossiest room- the rest will pretty much track if they share much common wall area.
If you did hydronic loop you can size radiation for each room based on your room-by-room load calc and it'll usually be somewhat better balanced. I say "usually", since one big-screen TV and X-Box will more than heat the room it's in, with a coupla sweaty gaming teenagers at the controls- any radiator output would be only adding to the localized overheating issue.
But as I said, ducted air heating into the basement controlled by a first-floor thermostat never really works out well, due to the dramatically differing loss/gain characteristics.
The answer is zero.
Since the heating loads are low, the cooling load zero and warm air is not ideal for space heating in any structure, you should be taking heat from a gas domestic water heater and driving it through panel radiator, radiant walls, ceilings or floors for perfect year round comfort.
Dana has it again!