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Multi-speed equipment and attic ducts

Hello everyone. This is my first post. Thank you in advance for reading and responding.
Many of our customers have systems with duct work in the attic, where temperatures can drop to 20F or less in the winter, and 130F or higher in the summer. Energy codes require insulation over the ceiling to be r-38 (here), but only r-8 on the duct work. Obviously, the attic is not a desirable place for your HVAC system or ducts. But… add to that a multi-speed system! Now, when a system is operating at a low speed, you have air moving more slowly through over-sized ducts. I am sure that in some circumstances the lowest operating speed produces no net heating or cooling due to the high duct heat losses or gains.
A premium model digital thermostat's algorithm should eventually figure out that it is useless to operate in the lower speeds when it is hot/cold out. So now we have a customer that paid a premium for a multi-speed system that at best only runs on high speed, or at worst is of a much lower efficiency than they thought.
The conclusion I come to is that variable speed equipment, and possibly even 2-speed equipment, is not appropriate for systems with ducts that are in extreme environments, though I can find no literature that backs this up. Anyone care to weigh in on this?

Comments

  • HVACNUTHVACNUT Member Posts: 2,619
    Your right the the insulation on trunk ducts and flex ducts is not the greatest. There is a choice in R value with the flex duct. I've seen something call Safety Duct with a high R value and a reflective green jacket. Supposed to be hypoallergenic. Trunk duct R values I'm not sure, but I can't stand the bubble wrap.
    Are you talking about an ECM blower motor?
    A properly sized system and ductwork shouldn't have a problem moving the air with minimal temperature drop between leaving the evaporator or hydro coil to the conditioned space.
    Typically you dont need the velocity for heating like you do for A/C, but in a PSC motor, for hydro air, the fan speed should be set to the speed that mostly maintains your set DAT or with a furnace, within the manufacturers specs for heat rise.
    With A/C, depending on some factors, you should have a 17 to 22 degree TD between the supply and return at the system.
    With an ECM motor, once it adjusts to the static pressure of the system, there shouldn't be any issues getting the correct tempered air into the conditioned space.
  • unclejohnunclejohn Member Posts: 1,420
    I went from a 14SEER single speed heat pump to a two speed 18 SEER heat pump and my bill went up. It never shuts off, it lives on low speed all night long.
  • MikeMike Member Posts: 94
    There is always air moving in the ducts.hot is always going to cold. Natural circulation. Hot air rising through the ceiling diffuser in one room, and pushing the air out another. So, not as extreme as you are thinking. The more insulation, the better. I recommend to my clients is using a t'Stat with fan recirc. Or constant fan. Not the best, but less drastic Temps when the blower turns on. BTW, most of the manufacturersame are pushing more cfm in heat.
  • GBartGBart Member Posts: 753
    re: The conclusion I come to is that variable speed equipment, and possibly even 2-speed equipment, is not appropriate for systems with ducts that are in extreme environments, though I can find no literature that backs this up. Anyone care to weigh in on this?


    Depends on the ductwork, it must be sealed and well insulated, for decades we've ignored the ducts, they are a major part of the system.
  • GBartGBart Member Posts: 753
  • delpheedelphee Member Posts: 6
    GBart said:
    True, but it doesn’t address how multi-speed equipment magnifies the problem. My point is that single speed equipment would be better, with no constant fan.
  • delpheedelphee Member Posts: 6
    GBart said:

    re: Depends on the ductwork, it must be sealed and well insulated, for decades we've ignored the ducts, they are a major part of the system.

    Thank you, GBart. In many homes this is true. But even if the ducts are properly sealed and insulated, they are only an r-8 at best. With a delta T from inside the ducts to ambient of over 60 degrees Fahrenheit, by my calculations, a home with r-6 and an average duct system will lose over 6,000 btu’s per hour Most of the losses are in the flex run outs, since there is more total surface area. So the less it runs, the better. Won’t help humidity much though.
  • GBartGBart Member Posts: 753
    delphee said:

    GBart said:
    True, but it doesn’t address how multi-speed equipment magnifies the problem. My point is that single speed equipment would be better, with no constant fan.
    It doesn't, multi or variable speed is always better and more efficient, they won't work well and neither will single speed if the ductwork isn't to snuff, and when ducts are in a non conditioned space you need to go beyond R-8, beyond code, that's what the article is talking about.
  • GBartGBart Member Posts: 753
    edited July 2018
    Flex duct is garbage, it should only be used for the last few feet if
    at all and stretched taut, you control humidity by properly sizing the equipment not over sizing. In a desert climate it's different, you really need to add humidity, not here, not from the Texas side up to Maine.
  • delpheedelphee Member Posts: 6
    GBart said:

    delphee said:

    GBart said:
    True, but it doesn’t address how multi-speed equipment magnifies the problem. My point is that single speed equipment would be better, with no constant fan.
    It doesn't, multi or variable speed is always better and more efficient, they won't work well and neither will single speed if the ductwork isn't to snuff, and when ducts are in a non conditioned space you need to go beyond R-8, beyond code, that's what the article is talking about.
    I didn’t mean to imply that I didn’t appreciate the article. I did, and thank you. But I’m not convinced that multi-speed equipment is always better for every application.
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