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Matching new heat pump to 40 year old ducts – GreenBuildingAdvisor
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scottperezfox | | #2
How invasive will it be to update your ductwork? I’m more worried about practices of design and installation 40 years ago than the size of the ducts per se. Back then, folks just threw in some metal, clipped it together — if you’re lucky — and called it a day. It’s probably bulky, loud, inefficient, and leaky. Updating the ducts along with the equipment will be a major improvement to the home’s comfort all together.
That is, if you can access them. Destroying the walls and ceilings does change the calculus, I admit.
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I have little basis to judge, but the ducts look like they were well done. Replacement wouldn’t have to be invasive–it would just be a huge pain working up in the trusses and among the loose-blown cellulose.
I did ask one contractor about moving ducts inside. The trunk runs up a chase on a firewall, so I imagined connecting to it on both sides and running duct across the ceiling on that wall, blowing out. He didn’t think that would work because the room is too deep, but even that he said would cost around $6000, mostly labor. Are ducts really that difficult to work with? Or maybe we miscommunicated and he was thinking more like having ducts completely around the perimeter. I can’t run them anywhere but the perimeter because there is too little headroom as is.
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Expert Member DCcontrarian | | #3
Forty years ago computers weren’t used at all, it could be anything.
Ductwork presents resistance to air flow. To overcome that resistance the fan has to pressurize the air. Ductwork will be described by how much pressure is required to create the needed airflow. This is done by hooking up a fan and measuring airflow and pressure.
At the same time, a fan will move less air when it faces higher pressure. The manufacturer will provide what’s called a “fan curve,” which is a graph of airflow vs pressure.
If the system has given satisfactory performance, and you know the fan curve of the existing fan, you can assume that a fan with a similar curve will also give satisfactory performance. You can also assume that a system with a much lower fan curve won’t. That’s probably what he’s going on.
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Thanks, I can see that. This has led me to look at the manuals of the proposed air handlers, and I think I have gleaned that even the truly variable ones are meant to be used at (more-or-less) constant airflow. And at least the one I’m looking at claims a flat curve which is entirely above the curve of the one proposed by the contractor in question (they’re roughly equal at zero pressure, but the one drops with pressure).
I suppose another issue could be if the register layout relies on a long throw. If a VRF unit (which was what I meant by “variable”) calls for less airflow then there will be less air speed at the register. That could result in poor mixing, but the area in question has well-distributed registers, so I’d think the constant running of a VRF would more than compensate for any lack of throw. Never mind that I would *like* to more the ducts inside an rely on throw…