Which is more important? Register placement or velocity/duct length?
Context: 1991 home in the high sierra (zone 6B). House has double pane windows but being built in 1991 is not particularly airtight in my opinion. Furnace and ductwork all in the attic in the center of the house. I’m enclosing the furnace in a mechanical room. Goal of the project is to remove heat from the attic to lessen my severe ice dam problem. I can design the mechanical room to encompass most of the supply ducts as they are all in the center/interior of the house… supply registers on center walls at ceiling level.
However, one duct runs ~13 feet across the attic floor to supply the master bedroom, dumping warm air from the ceiling in front of a wall of windows which are always cool to stand next to.
I could eliminate that run and supply the master bedroom from the interior wall over the bed, which would take supply from the main supply trunk about 3ft from the furnace. This will result in a super short run and probably high velocity, and may be louder since the supply is almost direct from the furnace.
So which is more important? Getting heat out to the exterior wall of windows? Or very short runs with higher velocity?
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Alright, let's tackle this like we're solving one of the classic building science puzzles in the high Sierra (Zone 6B), where the stakes include battling severe ice dams and making a 1991 home act like it’s learned a few things since the 90s.
You’ve got a situation that’s all too common: a home that’s as leaky as a sieve when it comes to air sealing, with a furnace and ductwork stashed in the attic like some sort of afterthought. Enclosing that furnace in a mechanical room? Smart move. It’s like putting your quarterback in protective gear rather than letting him run around in pajamas. This isn’t just about keeping the furnace cozy; it’s strategic, tackling those ice dams by cutting down on attic heat.
Now, onto the crux of your dilemma: the long duct run versus the short and shouty alternative.
Long Duct to the Master Bedroom:
Sending warm air on a 13-foot hike across the attic to reach the master bedroom has its merits, especially with that wall of windows acting like an icebox. It's intuitive, right? Warm air meets cold surface, balancing out the temperature differences. But here’s the kicker: that long duct run is like a marathon runner going through a freezer; you’re losing heat before it even gets to the finish line. Plus, you’re feeding those ice dams with all that heat escaping into the attic.
Short Duct, High Velocity:
Shifting gears to a short run from the furnace, you’re essentially turbocharging the air delivery to the master bedroom. It's efficient, like taking a shortcut through the park on your way to work. But hold your horses – this isn’t a quiet stroll; it’s more like a sprint. High velocity means more noise, and that direct shot from the furnace might make you feel like you’re sleeping in a wind tunnel.
So, What’s the Play?
Here’s where we blend science with a bit of common sense. The goal is twofold: keep the living spaces comfortable without turning the attic into a sauna that’s cooking up an ice dam feast.
Mitigate the Noise: If you opt for the short run, look into dampers or other noise-reducing measures that can slow down that air just enough to keep the peace. Think of it as putting a muffler on that furnace sprint.
Insulate and Seal: Whether you go long or short, the real MVP here is insulation and air sealing. Wrap those ducts like they’re precious cargo. Every bit of warm air that stays in the duct is one less bit melting snow on your roof and feeding those dams.
Consider the Distribution: Getting heat to the exterior wall is crucial, especially in front of a chilly window parade. But it’s not just about getting the air there; it’s about getting it there efficiently. If the short run makes sense structurally and you can manage the velocity and noise, it’s worth considering. Otherwise, enhancing the long run’s efficiency with better insulation and airtight ductwork could be your golden ticket.
In essence, you’re not just picking between two duct runs; you’re engineering a solution that respects the home’s need for warmth without inviting the attic to the party. Ice dams are a symptom, and you’re on the hunt for a cure that goes beyond just rerouting ducts – it’s about sealing, insulating, and thinking like the air you’re trying to corral. Choose the path that keeps the heat where it’s needed, respects the peace of your bedroom, and doesn’t arm wrestle with physics more than necessary.
Thanks BSBot.
You summarized well. I just don't know if the master bedroom will feel a lot cooler heated from the center instead of the window?
I have the quotes for the ducting rework and the mechanical room. Just trying to decide if I'm creating a much colder bedroom, or if it won't really be enough of a change to be noticeable.