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HVAC 2 or 3 zones when adding the basement? What’s the most efficient?

eppnet757 | Posted in Mechanicals on


I am remodeling a 2 story home with a partial finished basement in Philadelphia, PA which I think is climate zone 4A. The basement is 700 sq ft, 1st floor is 1200 sq ft with an open above foyer and the 2nd floor is 1450 sq ft including the open foyer. The house has R19 fiberglass in the walls, R30 fiberglass on the attic floor, I am adding 2” of closed cell spray foam in the bathroom walls and closed cell on the band joists in the basement since I am gutting those areas. I am also installing all new energy star vinyl double hung windows with added foam insulation within the frames. The current plan is to install a 3 ton 16 seer AC unit with 80,000 btu – 95% eff 2 stage for the basement and 1st floor. For the 2nd floor we will install another 3 ton AC unit and 80,000 btu furnace but 80% efficiency since we are putting it into the vented attic. My goal is energy efficiency and I am getting conflicting advice on what to do with the basement. The basement will only be used as storage and a game room. I have a few questions for the community below:

How much harder will my 1st floor unit work to supply conditioned air to a rarely used basement?

Would it benefit me to go with a smaller 1st floor unit and dedicate that unit to just the 1st floor and install mini splits in the basement? This option obviously costs more upfront but will I make up for it later with energy savings?

Do I need to heat and cool the basement when not in use?

Are my units sized correctly for this house?


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  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    Get ready for a flood of stock answers. Your furnaces and air conditioners appear grossly oversized.

    We hear about cases like yours all the time. Here's the stock answer:

    1. The first step to designing a heating system is to perform an accurate Manual J heating and cooling load calculation. For more information, see these articles:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Calculating Cooling Loads

    Who Can Perform My Load Calculations?

    2. It may make more sense to install a few ductless minisplits (or ducted minisplits) rather than two conventional ducted forced-air systems.

  2. Expert Member
    Dana Dorsett | | #2

    A 3 ton air conditioner even for the ENTIRE 2650' of above grade space could be oversized by 2x, and two of them is on the insanely ludicrously oversized end.

    Similarly, a single 80,000 BTU/hr furnace would be somewhat more than 2x oversized for my inefficiently shaped 2400' 1.5 story 2x4 framed house + 1600' of conditioned basement with ~R20 in the attic at a design temp 10F colder than Philadephia's. And you're planning on TWO of them? That would be fine if you expected it to get down to -150F or so during the next Polar Vortex, but never got anywhere near that cold in Philly even during the last ice age.

    Basements have essentially no sensible cooling load, and very low heat loads that don't track as closely with outdoor temperature as fully above grade floors. A hydronic heating loop operating off the water heater or a half-ton PTHP would handle the heating load of a 700' basement just fine, but you'll have to calculate the load to not over-spec the basement's heating system too.

    Frequent contributor Allison Bailes compiled this plot of the Manual-J calculated air conditioning square feet per ton ratios against house sizes:

    Most of those houses were in the gulf coast states, where the 1% outside design temps and latent loads are higher than in Philadelpia. But for a 2650' house the middle of the cluster would be a ton per 1400' or so, meaning it's as likely as not that the total cooling load for your house is less than 2 tons (not six.)

  3. eppnet757 | | #3

    Thanks for the quick response. My name is Justin. I am meeting with another HVAC contractor this weekend and he is willing to provide a manual J. As for the basement what is optimal from an energy standpoint? 1 stystem for the 1st floor and a separate for the basement? Or 1 system for both? I realize the loads are low for the basement but I find it hard to believe 1 system for both won’t be working harder and using more energy.

  4. walta100 | | #4

    My gut says the basement will take care of itself and skip the upstairs furnace, more than enough heat will find its way up there from down stairs. You will want a mini split head up stairs mostly for cooling. Assuming a blower door test and air seal to less than 2 ACH 50.

    In my opinion putting duct work and or equipment into a vented attic is inefficient, dumb and lazy. The goes for 80% furnaces north of Florida.

    The most efficient system is the works the hardest and has the longest run time. The least efficient system is the one run for a short and turns off in just a few minutes.


  5. eppnet757 | | #5

    What is everyone’s thoughts on putting an HVAC system in a vented attic and building an insulated closet around the equipment and insulating the ductwork? I have a huge attic with steep rooflines and it will be very expensive to insulate the underside of the roof with close cell and bring the attic into the building envelope. I searched this site for HVAC closets in attics and could not find much. Thanks!

  6. GBA Editor
    Martin Holladay | | #6

    You can build an insulated mechanical room in your attic if you want. Several issues to consider:

    1. This "solution" doesn't address the ductwork, so it is an inferior solution compared to creating an unvented conditioned attic.

    2. Most homeowners who build an insulated mechanical room in their attic make the room too small. Remember to leave room for maintenance workers to enter, put down their tool box, and gain access to all the equipment and filter slots. Small rooms can overheat.

    3. If you have an atmospherically vented furnace in this mechanical room, you will need an outdoor air duct to bring fresh combustion air to the furnace.

    4. Many homeowners who are shocked at the cost of converting a vented unconditioned attic into an unvented conditioned attic conclude that the most logical solution is to abandon the attic ductwork and just install one or two ductless minisplits.

  7. Expert Member
    Dana Dorsett | | #7

    "I am meeting with another HVAC contractor this weekend and he is willing to provide a manual J."

    Unfortunately Manual-J's performed by HVAC-contractors are worth less than the paper they're printed on 19 out of 20 times.

    That's not necessarily out of malicious intent, but human nature, and less than total focus on the correctness of the numbers. An HVAC contractor's natural inclination is to err to the conservative side on all assumptions to guarantee that they never get the 5AM call on the coldest night of the year from an irate customer. But that is in direct contravention of the actual instructions in the Manual. They make their money by installing and maintaining mechanical systems, not by doing the math.

    It's far better to pay a qualified engineer or RESNET rater whose reputation & living is made or lost on the accuracy of their numbers. In my neighborhood that service typically runs between $500-1000, depending the complexity of the house (and sometimes the perceived wealth of the neighborhood- they're human too.) But it almost invariably saves more than the fees charge in upfront cost of the smaller, more right-sized equipment, with dividends in comfort & efficiency over the lifecycle of the equipment.

  8. eppnet757 | | #8


    Converting my attic to unvented will be around $14K. Mini splits don’t make financial sense since I have 4 bedrooms, 2 baths and a large foyer all on the 2nd floor. I talked to my HVAC contractor who is also a Leed and energy star builder and he states that if we seal the ductwork properly with mastic and insulate the ductwork there is minimal energy loss with having the 2nd system in an unconditioned attic. After my research and reading the article you posted on ductwork I tend to agree with my HVAC contractor. If the main issue is energy loss through ducts then problem solved with insulation and mastic right?

  9. Jon_R | | #9

    Never trust air sealing - always measure it.

  10. walta100 | | #10

    OK if the contractor owns a blower door and a duct blaster and will be testing his work.

    If not it is just a snow job. They will slather some mastic around for you to see and take your money and run.

    You will not need a mini head in every room.

    All the contractors now play bait and switch game. They say they can install mini splits and then price them very high and talk you into the same old systems they have been installing for the last 50 years.

    Find the guy that has sold minis not just bidding them as straw dogs.


  11. Expert Member
    Dana Dorsett | | #11

    Do you have room by room Manual-J load numbers (even an HVAC contractor's Manual-J) for those "...4 bedrooms, 2 baths and a large foyer all on the 2nd floor..." yet ??

  12. GBA Editor
    Martin Holladay | | #12

    Q. "If the main issue is energy loss through ducts then problem solved with insulation and mastic, right?"

    A. You still need to figure out what to do with with the furnace. Have you decided to build an insulated mechanical room in your attic to house the furnace? Did you figure out where the furnace's combustion air is coming from?

    Assuming that the furnace is located in an insulated mechanical room, you have to consider the performance of the ducts that will extend through your vented unconditioned attic. This type of attic usually has R-49 or R-38 insulation on the attic floor, whereas your ducts (if you're lucky) will have skimpy R-8 insulation. So to answer your question: No, the problem is not solved with mastic and duct insulation.

    For more information on this topic, see Is R-8 Duct Insulation Enough?

  13. Expert Member
    Dana Dorsett | | #13

    In Philadephia it's probably going to be better to install a right-sized modulating 2 stage or modulating heat pump than going with gas furnace. It's better still to go with a high efficiency modulating mini-ducted mini-split solution. A 1.5 ton Fujitsu 18RLFCD is almost certainly has sufficient capacity for 1450 square feet of top-floor, and it has more than a 6:1 turn down ratio in both heating & cooling modes. (A 1-ton 12RLFCD might do it too- get the load numbers.)

    It would require some amount of duct design, and would probably need to use very short duct runs from a central plenum to ceiling registers well away from the exterior walls, with grilles designed for horizontal throw, as seen in John Semmehack's pictures in response #14 on this thread:

    Mini-duct cassettes are sufficiently low profile that they can usually be installed on a platform/attic floor above joist level and still have adequate space to install code-min attic insulation over the top. Build out small service chamber for the cassette and short duct runs, with air tight sides and top with door/hatch access from either the top or the ceiling below for service/maintenance. They can also be installed under the ceiling in the top of a closet or large cabinet, if works from floor plan point of view.

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