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Community and Q&A

Collecting Attic Heat

Ddddgghj | Posted in General Questions on

I have a 2 story home roughly 3,200 sq ft (2,000 downstairs, 1,200 upstairs). There is no door dividing the two floors. I have 7” of open cell around the envelope. There’s a 3 ton central for upstairs and a 3 ton central for downstairs. Right now while it’s cold outside, my upstairs unit never kicks on and the upstairs temp stays consistent with downstairs. The downstairs unit runs almost constantly. They are both inverter systems so I feel like I’m losing some benefit here with the downstairs unit running full speed all the time. The attic is sealed and invented so that’s where the heat ends up. Could I put a return from the downstairs system in the attic to pull some of that hot air back down instead of letting it waste up there? This house is in the final stages of construction so I have limited experience with the system. I’m afraid I will run into the same but opposite problem during summer with the cold air falling.

Any suggestions?

Thanks in advance!

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  1. Jonathan Blaney | | #1

    An HVAC system is intended to maintain a given temperature/level of comfort. In winter the system will provide the necessary amount of BTUs to hold the set point. The system will work within the entire conditioned envelope. You can not move heat from the attic to warm the lower level because it will only be replaced. The attic will continue to warm at the expense of comfort.

    These systems work best at maintaining comfort when left on. This avoids the need to have excess capacity to recover from long periods of chill or heat. If one unit is heating the whole house, looks like you have a significant oversize issue. We know this equipment is not inexpensive.

    1. Ddddgghj | | #3

      I’m still in the finishing stages of construction. My windows aren’t caulked yet, I’ve got a couple doors and thresholds that need adjusted to seal better, I don’t have a door handle on the door going into the garage, and my attic roof isn’t fully insulated yet (the foam guys have to come back and finish). Could this be the whole problem? Too much cold air is getting in so the bottom unit struggles to keep up while all the heat is rising and satisfying the upstairs T-stat? And with the attic not being fully insulated yet I guess that conditioned air is leaking out and being replaced by unconditioned air downstairs?
      Im new to the energy efficiency stuff and I’m self building, but I’ve been reading like crazy to learn. I just couldn’t find anything specific about what I’m experiencing.
      Thank you for your reply!

  2. Bongo30 Bongo30 | | #2

    Did you have Manual J done? We are building 2 stories with 1,440 sq ft on each floor, so 2,880 sq ft total, with vented attic, and our load as per Manual J is about 1.5 ton per floor. 3 ton per floor sounds grossly oversized. Who did your Manual J?

    1. Ddddgghj | | #4

      I did have a manual J done by an energy auditor in Little Rock. I have a ton of windows (19) and they aren’t the most efficient, I’m building slab on grade and the slab perimeter is not insulated, and the trunk lines for the 1st floor HVAC are in the garage on the wall between it and the house (R8 insulation on the ductwork I think); so these were all variables that went into my sizing I think. I started going for efficiency a little late in the game. They did tell me that I need to get variable speed units for this reason though. Also, for budget purposes I bought Mr. Cool heat pumps (pretty new to the market but had great reviews so far). They have dip switches that allow me to convert to 2 ton instead of 3 if need be. You can also read my reply above for a little more background on my situation.
      Thank you so much for your response!

  3. DCContrarian | | #5

    Maybe not the answer you're looking for, but: temperature stratification -- ie, it's a lot warmer upstairs than downstairs -- is usually a symptom of excessive air leakage. If your second floor gets hot in the heat of the day, that's usually a sign that your roof isn't well insulated. Six tons for a 3200 square foot house sounds like an enormous amount of cooling.

    I think what you really need is some air-sealing and insulating.

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