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Exposed duct doubles as radiant heat source?

My energy audit client's condo building was converted in 1995 from a factory, keeping brick walls uninsulated (aesthetics), and with only standard double pane windows (budget). Not the current standard for a Chicago climate & clients feel chilly regardless what the thermostat says! There's an 80afue furnace and ND DHW tank. There's exposed spiral duct in this "lofty" condo, aligned (you guessed it!) with exterior walls/windows. Nothing prevents installing 95% heating equipment.
Question: how about securing PEX (or other) tubing Zig-Zag fashion to the outside of spiral duct, oriented toward people, circulating water from a tankless boiler? Paint the spiral duct a nice matt radiating color and make it function as a radiator. How to bond any tubing to transfer heat into the duct? Any history of this among our august and imaginative Wally's?


  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225
    Convection on the inside of the duct would probably rob any benefit and just deposit more warm air at the ceiling.

  • Gordy
    Gordy Member Posts: 9,537
    edited November 2014
    I'm assuming by "lofty" you mean high ceilings, and large windows. The ductwork runs parallel a long the exterior windowed wall.

    This is a case of probably poor window quality creating the cold 70 effect. The room is probably at set point, but the window temps give a radiant cooling effect.

    I think rads under the windows would give far better results.

    The duct work idea is a waste of time, and money. It's as Harvey put it.
  • RobG
    RobG Member Posts: 1,850
    Baseboard or panel rads would be the way to go. I would never try an experiment for a customer (unless I had a signed waiver).
  • Gordy
    Gordy Member Posts: 9,537
    Thinking about this in a way as not to over heat the space......is there window treatments utilized at all?

  • Harvey Ramer
    Harvey Ramer Member Posts: 2,225
    There is something you can do here. Easy and cheap. You need a thermostat that has a fan circulate feature on it. The fan motor should be wired such that it turns on the lowest speed when there is a signal present at only the g terminal. When there is a call for heating or cooling it reverts back to normal speed.

    This is easily accomplished with a couple relays and some wire. Or your furnaces may already have that logic in place. Better yet, upgrade to an ECM motor. Then it won't use much power at all in low speed and you can run constant circulation for increased comfort.

    I have done this fan trick a lot. The results are good.

  • SWEI
    SWEI Member Posts: 7,356
    Take a look at the registers/nozzles as well. Might be worth paying for a bit of CFD on the project.
  • Jack
    Jack Member Posts: 1,046
    Or, you could add a Rinnai EX22 or 38 and work them together. How big is this place?
  • Therm_lag
    Therm_lag Member Posts: 30
    Harvey, Gordy, RobG, thanks very much for your responses!
    Yes, building was a factory, has concrete floors w/ nice flooring installed, and ceilings 10 ft and greater in ht. Floor radiant not an option here, though ceiling radiant panels are.
    I have included suggestions for window insulation and upgrade.
    Resident is President of Condo Association and wishes to have a solution that is less expensive, so trying something that has less certain results is OK with them. Panel rads will probably solve the issue but are not the less expensive option.
    I like the idea of keeping the air moving, which would keep the ducts "radiating" without getting involved with attaching a hot water pipe to the ducts.
    I'll get back to client and see how far they've progressed with initial suggestions and let you know.
  • andreea
    andreea Member Posts: 4
    edited January 2017
    Hello everyone, I know the topic is quite old, but I saw that SWEI mentioned that you could pay for CFD to test this. If there are other of you who want to test their HVAC system, you just need to put some time into it, there is this software that can be used with a free account. This duct airflow analysis can actually be copied and used after you create the account: Duct Design Airflow Analysis