Pennsylvania Finishing Basement w/ Geo System

Discussion in 'General Discussions' started by csementuh, Oct 17, 2018.

  1. csementuh

    csementuh Member

    Hello! Long-time lurker, have only posted a little in the past! Hoping to get some basic HVAC advice here since it relates to my geo system. Been satisfied with the geo for the past couple years and now it's time to finish the basement and add some heat!

    Two story 2600sq/ft house in PA, so we have cold winters and hot summers, pretty good mix. House is 2.5 years old with a Carrier GTX 3 ton system in it with 2 zones (1st floor and basement one Zone 1 and 2nd story on Zone 2). House is moderately well insulated and tight but nothing crazy. Basement is entirely underground with only inside access and a 36" exterior door with steps up to grade. Ceiling height around 9' and basically one big open room with a couple smaller rooms (cold cellar, bathroom w/ no heat, utility room w/ no heat and a small office). Total basement footprint is around 1,125sq/ft and the finished portion is around 800sq/ft.

    The builder's HVAC vendor knew I was finishing the basement when we built. They told me the Manual J was effectively a 'net zero' on the basement since it's totally underground and well insulated in foam down there. They said capacity on the system was fine. The HVAC vendor put in two heat runs in the basement with a 6" round duct off the main trunk that finish with a 10"x4" register in the ceiling. So that's around 100CFM each run. These two positions are now at the two edges of the basement, and I need to consider adding another couple source runs to the other two corners, plus a 3rd to a small office room (9'x13').

    Here's a picture... The existing two runs are shown, as well as the proposed three new runs. The cold air returns I'd put as green rectangles, those spots will work OK with the floorplan. The big room is bordered in blue and the smaller office room is red for reference.


    First assumption: I think I'm more than OK with capacity to tie into the 1st floor/basement zone as-is and run some more source ducts. This system is a little oversized I think for the house, since it has pretty short runtimes, so I doubt it will hurt. I'll likely kick it on manually for a little heat if needed and/or stick a 3rd thermostat onto the Zone 3 spot on the Honeywell board and jumper it to Zone 1 so they run together. (I know this will work, no worries there).

    Here's my big questions... I've seen this debated both ways and couldn't find a concise answer.

    1. Run the supplies in the ceiling and returns down the wall close to the floor?
    2. Run the supplies down the wall close to the floor and returns up in the ceiling?

    I've received both answers from local HVAC places and I don't know who to trust here since they're conflicted..

    Ceiling is much easier (as I'm half done as-is), so I'm hoping it's the way to go! Theory is that registers will blow the heat down onto to floor warming it, and the returns will pick up cold air off the floor and cycle it through the house. This seems to be a good compromise with a heating climate since the basement will be naturally cool in the summer so little to no AC needed. Thoughts?

    If the ceiling is OK I just need to know how what size I can run for the other three runs.

    The main two in the large basement room has a dilemma.. This is the big open room of the basement comprised of two open rooms at a right angle (L shaped). 22x15 and 14x17 approx 550 sq/feet open space.

    If 12,000 BTUs is 1 ton and 1 ton is roughly 400CFM needed, then my room LxWxH times climate zone 5 = 24,288 and then divided by 12,000 for tons, makes it need roughly 2 tons of HVAC or 800+ CFM which just seems crazy high for a basment! Especially one that is already a very decent temp year round as it is.. If I use the formula that uses cubic feet times 6 runs per hour divided by 60 mins in an hour I get roughly 400 CFM which sounds more reasonable. Though I still question if a basement even needs that much..

    The main trunk of Zone 1 ends before the joist bay where I need to run the supply.. My only option would be to cut into an existing 8" round duct run there and tee it off both directions to add the basement runs. That 8" round trunk feeds a laundry room above (9'x12' room, 2 supplies) and also a storage room below it in basement (9'x12' with a small 2" ceiling diffuser). So the million dollar question is whether or not I can steal some CFM from that run without messing things up too badly. 8" duct is around 200+ CFM. My thoughts are probably.. The laundry is open to the kitchen and has an exterior door to the garage, like a mudroom. The 1st floor of the house is very open and the air would likely stratify to there and the small registers would temper it enough. I could take smaller 4, 5 or 6" runs to cut down on how many CFM I'm stealing from the 1st floor there. Sadly due to plumbing there's literally no other place to tap into that 1st floor zone on the main square trunk. Thoughts?

    For the small office room I was planning to just put a 6" supply off the main trunk in the ceiling (or wall bottom) and call it a day. This room is only about 125 sq/feet, so maybe even a smaller run like a 4 or 5" duct for less CFM. Then put a small return in a stud bay on the opposite side of the room. That room has the HVAC inside it in a closet, so the runs are easy.

    Last final question/thought.. Is insulated flex tubing really the devil? I know it will cut down on airflow, but it's soo easy. ;-) In my case maybe the cut down on CFM isn't a bad thing?

    I could also 'do nothing' and keep only the runs I have and add a return. Only putting out maybe 200CFM and the runs are off to one side, so not sure how that will affect things?

    Any and all help is very greatly appreciated! Thanks!
    Last edited: Oct 17, 2018
  2. Josh Hicks

    Josh Hicks New Member

    That’s a lot of post up there, lol.

    Basically your basement is not going to put a measureable load on the system. You are putting in heat runs and returns to temper and or just exchange the air, more for keeping the basemyfrom getting stale more then an exact temperature.

    High or low position won’t effectively change how the air moves.

    I would (were mine) add one 4” run in the new room ceiling and put a few small returns in and call it good. No need to rob any more air from the system when the basement will stay at a relatively constant temperature to begin with.
    Last edited: Oct 17, 2018
  3. csementuh

    csementuh Member

    Thanks for the input, most appreciated!

    For the office I will definitely add a 6" source with damper in the ceiling and then a 6" round to 6" oval to 12"x6" grille for the return air. That will be super easy to fit in the ceiling and stud bay.

    The two HVAC contactors I got quotes from were all planning on putting 2x more source runs in the main big room, one on each side, then a 3rd new run in the office room. For returns they were going to put one big one to service the big room and the office. I'm torn on whether or not that is 'needed' for the HVAC in this room or not.

    You really think just the existing 2x sources are enough? I could put a couple/few returns opposite them in the room down to the floor and that would fit fine. Just not sure if it would move air around enough to be worthwhile with only 200CFM coming out into that room.

    I'm a "do it right the first time" kinda guy. :) Any further input is appreciated!

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