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Community and Q&A

Basement as cold/heat source

srivenkat | Posted in Energy Efficiency and Durability on

Hi All,

My home has about 1313 sq. ft. living/dining/kitchen area (open floor plan) with about 723 sq. ft. of bedrooms on the side. Similar layout in the finished/conditioned full basement. The house (2012 ICF/EIFS construction) has been cooled and heated with a 3.5 ton AC and 15 kW heatstrips respectively.

The air handler is in a mechanical room in the basement, supplying to registers in the floor of first floor and the ceiling of the basement with returns high up on the walls on first floor and near the floor on the walls in the basement.

What I am planning to do is to replace the AC/Heatstrip system with a couple of mini-splits in the basement open area and then use the current Air handler for circulation of air between the basement and the first floor. Essentially the idea is to turn the basement into an extended air handler with the heat/cold added by the mini-splits instead of the AC coil and heat strips.

Has anyone done this before and any pitfalls with this approach?


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  1. GBA Editor
    Martin Holladay | | #1

    Of course, any time that you are designing a heating and cooling system, the first step is to perform a Manual J calculation. You need to know your design heating and cooling loads.

    My first reaction: Instead of installing "a couple of minisplits" in the basement with the hope that some of the conditioned air can be convinced to go upstairs, why on earth wouldn't you install one minisplit on each floor? It's always better to put a minisplit close to the space you are trying to heat and cool, rather than arbitrarily putting it far away.

    My second reaction: Most furnaces and air handlers have energy-hog fans; some of these fans draw 600 watts or 800 watts. Every hour that you run an air handler fan is an hour you don't want. One of the main advantages of a minisplit is that it comes with an optimized fan. So put one of your minisplits upstairs, and turn off your air handler fan.

  2. srivenkat | | #2

    A couple of things I missed mentioning. The stair case from the first floor to basement is thru an open landing area. I plan to have the airhandler draw the air from the returns near the floor in the basement and supply to the first floor in summer and vice versa in winter (draw air from return high up on walls on First floor and supply to the basement), effectively circulating air between the basement and first floor.

  3. srivenkat | | #3


    Thanks for your response. A few considerations are driving me in the direction of running the Air handler:

    1. In summer, the basement is already cold and if I have a mini-split on the first floor cooling, without any circulation of air between the basement and first floor, eventually the cold air will settle into the basement, making it even more cold, right? I would be forced to then run a heater in the basement even in summer. Right now, with the AC running periodically, it's turning the air over so it's somewhat bearable in the basement.

    2. A whole house air circulator will allow me to filter and treat air with UV bulbs in the main ducts. Regarding the efficiency of the air handler, are there any efficient replacements for it available?

    3. You mentioned elsewhere that a mini-split running on high fan speed would be most efficient. I am for deriving the highest efficiency and am thinking a noisy mini-split head would be tolerable in the basement rather than on the main floor, especially if I have an air handler/circulator circulating the air.

    4. Running all the mini-splits in the basement, besides giving me the ability to run them on high fan setting (most -efficient), would avoid the aesthetics issues on the first floor. And in a heating dominated climate zone (5a), we would want the heat to start at the lowest level of the house anyway, right?

    I would be grateful for your thoughts on these.




    5. Bed rooms will benefit from the air moved/circulated by the air handler/circulator, thus not needing expensive or less efficient ducted solutions to cater to them solely.

  4. Jon_R | | #4

    Air that is comfortable for people is not efficient (in CFM and fan operational cost) for moving heat/cooling around.

    You have central AC and apparently like many aspects of it. Have you looked at converting to a central heat pump?

  5. GBA Editor
    Martin Holladay | | #5

    I've provided my advice and my reasoning. You've explained why you want to do it your way. It's your house.

  6. user-2310254 | | #6


    As Martin said, it is your house. But it seems to be that you are proposing an experiment. It may work out--or not. Will you have the budget to install a more conventional solution if your design proves ineffective?

  7. srivenkat | | #7


    I have considered the central heat pump, and the reason I don't like it is that when the set temp is reached in summer, I have to have the blower fan off as well, otherwise I risk blowing the moisture that has accumulated on the coil and drain pan back into the house, right? At least that's what I have seen with my current AC. I feel an uncomfortable mugginess if I leave the fan running after the set temp is reached and the AC condenser is off.




    Not sure if the symptom I have experienced above is the result of a poor drain pan design, holding too much undrained water at the bottom of the pan or if it's a universal issue.

  8. srivenkat | | #8


    Thanks for your response. I would imagine some of the considerations I have mentioned would be pretty standard, especially in cold-climates, are they not?

    1. Cold air from main floor settling into the basement making it uncomfortable there.
    2. UV conditioning of air.
    3. Re-evaporation of moisture from drain pan.

    What I am looking for is any potential pitfall/disadvantages with the approach. Martin already mentioned the energy penalty with the Air handler. I would be interested in hearing any alternatives that take care of 1 and 3 above. I would also appreciate any advice on efficient whole house circulators available out there.


  9. Jon_R | | #9

    A not-oversized, variable speed central heat pump will have fewer issues with a continuous fan. When it doesn't dehumidify enough (which occurs even with mini-splits), a standalone dehumidifier will.

    I suppose you could make some system with dampers that bypasses central AC coils when cooling isn't needed.

    Additional pitfalls with mini-splits in non-load areas - you may not have enough CFM to make both areas comfortable. And there will always be some delta-T.

  10. Expert Member
    Dana Dorsett | | #10

    A 3.5 ton AC could cover the cooling load of a 1300' tent, and ridiculously oversized for a 1300' house, which pretty much guarantees the air handler is ridiculously oversized too. (The 15kw heat strip is also likely to be ridiculously oversized too, unless you sleep with the windows open during the coldest hours of the year.)

    Unless the ducts and house are both fairly air tight, running the oversized air handler will increase the actual loads too, since it will create room to room pressure differences, with some of the return path being "the great outdoors".

    In heating mode moving great volumes of tepid room air around with a big air handler lowers comfort- there's a wind-chill effect. With the heat strips on the air coming out of the registers is north of 110F, (as it usually is right at the mini-split head) so it's a warm draft that feels OK to bare skin. But 70-75F room air is well below human body temp, and blown at the same velocity has a very real chilling effect. It may be sufficient for heating a high-R house, but it's a serious hit in comfort.

    BTW: Running mini-splits at the highest fan setting is NOT the highest efficiency, and more likely to be the LOWEST efficiency. The fastest blower speed is always the noisiest. Most cold climate mini-splits are quite a bit quieter than your refrigerator when running at their lowest speed (which is usually correlated to their highest efficiency), which is where they will be running most of the time. Even at high speed a mini-split is still usually quieter than the oversized air handler & duct system.

    For some bench tested efficiency vs. speed data, see: Figure 5, page 10 (p18 in PDF) , and figure 14, p19 (p27 in PDF) of this document:

    In Figure 5 note that when it's 35F out the efficiency maxes out when both the fan & compressor are running at their minimum speed, and literally twice the efficiency at that temp when it's all running at maximum speed. At 47F efficiency is maxed out at lowest fan speed combined with medium compressor speed. Under no outside condition is maximum fan correlated to maximum efficiency.

    Similarly in Figure 14, when it's 83F outside the maximum efficiency is at minimum compressor & blower speed, more than twice as efficient than when both are at maximum speed, and running the fan at high speed never correlates to a higher efficiency, independently of the outdoor temperature or compressor speed.

  11. srivenkat | | #11

    Thanks for the info, Dana. The house is actually 2036 square feet (1313 + 723) main floor + full basement for a TOTAL of 4072 SFT. Would what you mentioned in the earlier part hold true for this square footage?

  12. Expert Member
    Dana Dorsett | | #12

    If the full basement is mostly below-grade (not a walk-out with big glass sliders) it's heating & cooling loads are "in the noise" of measurement error. For the ~2000' of above-grade conditioned space anything over 2 tons of cooling would be on the high side even for a code-minimum house. With favorable shading factors and better windows it could be lower than 1.5 tons. With a lot of unshaded west facing glass you might see 3.5 tons, but that would be a very unusual house.

    For comparison purposes, my ~1700' basement in zone 5A that has insulated walls, uninsulated slab but is not actively conditioned. It never strays more than a couple of degrees from 65F, winter or summer. There is 2400' of 2x4 framed 1.5 story (~2000' downstairs, ~400' upstairs) bungalow type house with clear glass windows above that basement, but with 2' overhangs on the south facing windows and good afternoon shading factors, the design cooling load is under 1 ton @ 83F (the local 1% outside design temp). It might be as high as 2 tons if the neighbors got together and clear-cut the trees, and the hill to the west got a bit shorter. The heat load at +5F (the local 99% outside design temperature is between 10-11kw (under 38,000 BTU/hr.)

    Your outside design temperatures are likely to be within 5F of mine, and certainly within 10F, so even my house in your location woudn't have a heating load more than 10% over where it's actually located. But you have both higher R (twice the whole-wall R!) and a thermal mass benefit of the ICF working for you to lower both the heating & cooling peaks, and you should have better windows too. Your house probably has a more efficient shape (I have 14 corners to the footprint, and three dormers), and with ICF construction it's likely to be more air tight than mine (if not it could be with retrofit air sealing.)

    So, where would the extra tonnage of cooling or kw of heating be coming from? There is no way your house would have a heat load as high as my slightly bigger, leakier, substantially lower-R house. Though it may have a cooling load higher than my pipsqueak cooling load, it's probably not 4x higher.

    To find out where it really lives, run an aggressive Manual-J, using a tool that is ICF-aware, or a site specific BeOpt simulation.

  13. Jon_R | | #13

    Air handler sized to move 3.5 tons of normal AC = ~1400 CFM
    Air handler sized to move 15KW of heat at a not so comfortable 5F delta-T = 9500 CFM

    Even if 15KW is too high and some heat moves without the ducts, I doubt you have enough CFM.

  14. srivenkat | | #14

    Thanks for the info, Dana and Jon.

    The basement is below (or at grade) at the front of the home and the outside slopes to about 4 feet being below grade on the back. The back of the home is about 80% glass and is almost North with the Sun from the west hitting the glass on the back at about a 15 degree angle or so. The basement back (the portion above grade) is 80% glass, about 4 steps up leads to a landing area which leads to the patio on the back. I guess this is called a partial-walkout basement?

    Also, I have briefly turned on heat and measured the Amps being drawn with a clamp meter. The meter shows 40 Amps with 3 Amps of it being drawn by the blower. So, I figure the Heat Strips Watts to be 37 Amps x 240v = 8800 Watts = 8.8 Kw and the Btus/hr to be 8.8 x 3412 = 30,299 Btu/hr. I am surprised that the Amps being drawn are only 40 Amps while the installation docs show 14/15 kW heat strips. Perhaps not all strips are connected. The thermostat is a plain non-staging thermostat, so I was expecting all connected strips to come on in one go.

    Dana, you mention your basement keeps at a steady 65F. Does it mean you are not conditioning the air in the basement, i.e., no supply/returns in the basement? I am having to close the dampers that supply to the basement in summer so it doesn't get colder. I open them up only for winter. This is obviously messing with the ESP for the blower. What's the ideal solution for those that want to have their basement at the same temperature as the rest of the home, especially in summer? Does one have to run a heater in the basement in summer? Would a good alternative to running a heater be turning the air over between the cold basement and hot upstairs?



  15. Jon_R | | #15

    3A continuous = ~$650/year.

    In cases where moving large volumes of air can't be avoided for temperature equalization, I'd keep ducts big and short (perhaps just the thickness of a floor or wall) and, where possible, use hallways/doorways/stairwells as part of the air flow path. This allows lots of CFM with little power draw.

    Keep an eye on basement humidity if you push warm humid air down there.

  16. Expert Member
    Dana Dorsett | | #16

    VENKAT Y: Yes, there is no active conditioning of the basement, but the hydronic heating distribution plumbing is down there, as well as the (rarely used in winter) uninsulated ducts. There is also the hot water heating, washer/dryer, etc all in the basement.

    My air conditioning loads are quite low, and the duty cycle on the central air is also extremely low, so it's not making a significant difference in the summertime temperature.

    If the slab were insulated the temperature would likely rise a handful of degrees in summer, since the average temp of the slab is within a couple degrees of 60F everywhere. In summer the subfloor of the rooms above are running 70F or higher.

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