duct mounted thermostat location?
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  1. #1
    Join Date
    Oct 2002
    Posts
    306
    I have an Indeeco electric duct heater that is going to be controlled by a duct mounted electronic thermostat. My question is how far should the stat be from the outlet of the heater.

  2. #2
    Join Date
    Jul 2004
    Posts
    68
    Need more info:
    type of space you are heating?
    why not sense r/a?
    wall mount stat and use remote sensor?

  3. #3
    Join Date
    Oct 2002
    Posts
    306
    The heater is being used to preheat air that is feeding a couple of vav boxes. The vav's heaters cant overcome the supply air temperature. It's feeding a couple of offices.

  4. #4
    Join Date
    Jun 2004
    Location
    Richmond Virginia
    Posts
    1,078
    We need some more information. Why not sense return air? Are you trying to maintain a certain discharge temperature on supply air or simply using the heater for reheat? Give more information as to application please.

  5. #5
    Join Date
    Oct 2002
    Posts
    306
    Basically what we have here is someone decided to put offices on the same unit feeding a computer room. They installed vav's with reheat. The main unit remains in cooling pretty much all year round, the vavs cannot reheat the air for the offices enough. The new duct heater will be used to preheat/raise the inlet temperature to the vav's. It will be controlled by a duct mounted stat set for 65 degrees and will be turned off in the summer. I just was wondering if anyone knew how far the duct stat should be from the outlet of the heater? Thanks.

  6. #6
    Join Date
    Mar 2001
    Posts
    740
    USMC

    like the others stated, more info on the setup would be desirable

    had a new install of a vav system where the suit/tie engineer figured a reheat indeco box on the med press air duct- only problem was by the time the sheet metal contrctr had to play houdini to get the elec reheat installed--- the factory mounted air switch would not make to allow reheaters to work $4000 + up in smoke

    customer went with a jap mini split heat pump to provide heat in the area

    fatboy


  7. #7
    Join Date
    Jun 2004
    Location
    Richmond Virginia
    Posts
    1,078
    OK. It looks like the posts got jumbled up as the emails were flying back and forth. The duct stat should be mounted far enough downstream from the heater so the radiant heat from the heaters do not affect the thermostat. I think I would put it close to the VAV's for those two offices and interlock it so it cannot come on unless one of the two VAV's is asking for heat. Had a similar situation to yours and used a Ranco electronic thermostat that is sold now for a very reasonable price. This set up is an energy hog as you are cooling air then heating it then heating it again!

  8. #8
    Join Date
    Jun 2004
    Posts
    160
    Originally posted by seaboard
    The duct stat should be mounted far enough downstream from the heater so the radiant heat from the heaters do not affect the thermostat.

    Like seaboard suggested, This is where I usually put them.., and why put another tstat in at all,
    just power a relay off the vav's heater input

    .

    Have you tried changing the minnimum cfm heating mode setting on the boxes?.. using enough cfm to make the dp switch so the heaters cut on but not so much that they overpower the heaters.

  9. #9
    Join Date
    Sep 2002
    Posts
    1,389

    usmc

    >>>> My question is how far should the stat be from the outlet of the heater.<<<<

    It should be far enough to reach a wall in the office.
    Putting it in the duct doesn't tell the heater how hot the space is. And setting it at 65 won't allow the offices to ever cool if they need it unless the control is tied into the vav positioner.
    Also is there adequate protection for airflow? As the VAV closes down, and not hopefully to zero minimum, there will be inadequate air flow to sustain the heater.
    Does this heater have SCR control as you mentioned "electronic" stat?
    Just another question for grins: what is the KW vs. air flow range?
    Thanks, and the above post is only a suggestion.

  10. #10
    Join Date
    Oct 2002
    Posts
    306
    Thanks for the replies. Seaboard thats what I was originally asking. I figured that if the thermostat was not in the line of sight it would not be affected by the radiant heat. This whole setup was an after thought be the engineer. I would have preferred to put in a seperate unit. I do like the idea of interlocking them with the vav's. Yes I know that this is an extreme waste of energy, but the customer wants what the engineer specified.
    Twcpipes, Yes the heater has SCR control. I was thinking of setting one of the vavs in a common area with a higher minimum setting to provide more airflow. The heater is a 9kw and the cfm of the vavs is about 2400 combined.
    Koolitman, yes I did try closing down the min settings on the existing vavs. The rooms could not get about 65 degrees last winter. They were actually not occupied then so it wasnt an issue. I suggested perimeter baseboard but they didnt want to loose wall space. Thanks again, I'll let you guys know how it works out.

  11. #11
    Join Date
    Dec 2003
    Posts
    1,739
    try to sell them on cove mounted radiant wall heaters, they take up no wall space, on the floor, and work really well and they are more energy efficient as they heat the person without heating the space. I have used them in offices before with great happiness by the occupants.
    there but for the grace of god, go all of us

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