manual J and stack effect/buoyancy
I’m trying to estimate how much seasonal adjustment I’m likely to need for heating vs cooling upstairs vs downstairs. I used loadcalc.net to work out room by room loads, and it’s telling me:
Heating: 57% Downstairs, 43% upstairs
Cooling: 45% downstairs, 55% upstairs
However, I don’t think that accounts for the stack effect or tendency for hot air to rise up the open stairway. I would think the amount of effect that will have probably depends heavily on how air-sealed the house is (since leaky increases stack effect) and how well the registers are able to mix the air.
But I have no frame of reference for estimating just how much of a difference that could make. Is there a “cushion” of extra capacity for adjustment I could plan for that would be pretty safe?
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Airtightness makes a huge difference, and insulation does as well when it comes to temperature stratification. The stack effect is pretty weak; air currents move most of the heat around. Ideally you would have separate systems for each floor. If you're going with a ducted system, I would include either mechanical or manually adjusted dampers so you can fine-tune the levels.
If you want to try to dial it in from the start, it would help to know your climate zone and building details, including glazing specs.
Thanks Michael. And yes, I'm planning on manually adjusted dampers and upsizing the ducts enough that the system can run at full capacity while dampening one portion or another while staying within static pressure limits. Just trying to estimate how much adjustment I should plan for.
Climate zone is 4a. Building is a duplex w/ shared wall on one side. Slab on grade. 2 stories. Vented attic. I just did a large addition so 2/3's of exterior perimeter will have taped zip and caulk for airsealing. Other 1/3 will still be quite leaky, at least for now, and is only R-10 walls. I plan on replacing attic insulation and air-sealing ceiling. Thinking might as well go R60 or so there since blown cellulose is cheap. Addition walls will be R15. Glazing is pretty standard low E double pane. I think Uvalue of 0.26 or so.
Let me know if any other info would be helpful. And thanks again,
Ben
In most houses solar gain is more of an effect, when the sun is out the sunny side of the house gets a lot warmer than the shady side. The Manual J should account for that for cooling but usually doesn't for heating.
Let me pose a question back at you: what is your strategy for dealing with imbalances? Separate zones? Duct dampers that you move seasonally? Minisplits?
It also helps to know the size of the heating and cooling loads. Let me throw out a hypothetical as an example. Let's say your heating and cooling are about the same, each requires 1000 CFM at design temperature. You have zoned ductwork. In the summer, the upper level ductwork has to deliver 550 CFM. In the winter the lower level ductwork has to deliver 570 CFM. Even though you never do both at the same time, you have to size the ducts for 1120 CFM total even though you never need more than 1000 CFM.
Alternately, let's say you're heating dominant, you need 1000 CFM in the winter and 500 CFM in the summer. Your winter sizing will be fine for the summer, it just needs to be dampered differently.
Thanks DC. My strategy is duct dampers that I move seasonally. And following your example, I agree that I would need to size the ductwork for the 1120CFM. My question is basically whether that 550cfm upstairs and 450cfm downstairs in the summertime is going to be sufficient. Manual J suggests it is, but things like stack effect and hot air rising make me concerned that I might really need more like 600cfm upstairs and 400 downstairs, or perhaps even more than that. So I'm curious how much "safety margin" I should build in. For example, if I sized upstairs ducts and downstairs ducts to each flow 600 cfm, then I've got extra margin for seasonal adjustment, but potentially unnecessarily large ducts. On the other hand, judging by my house before the renovations and how leaky it was, I doubt that a 60/40 split would have been enough. So just looking for any guidance on how to quantify how much seasonal adjustment I should expect to need over and beyond the manual J calculations (which already account for solar gain through windows and ceiling vs floor loads).
Thanks again!
ben