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5Kw w/line voltage therm heater controlled from Nest

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Comments

  • JUGHNE
    JUGHNE Member Posts: 11,268
    edited August 2015
    The Nest site didn't state the amp ratings of the of its internal switches. But if it is intended to run the typical HVAC system that has a 40 VA supply transformer then we might assume the R-W contacts can handle the coil load which would resemble the R-Y load for AC.

    This posting caught my eye because I've never seen or heard of a 115KW furnace..................such a let down ;) (they do exist somewhere though).
  • andrewco
    andrewco Member Posts: 104
    Already have one (fluke). I've used it extensively in this project to determine if the contactor is doing the right thing, where 24v is coming from etc. I've just never used it on 240v...but I guess it's just twice as nice.
  • hot_rod
    hot_rod Member Posts: 23,231
    Then all you need is a transformer. Most are multiple input voltage 120- 208/ 240. Then remove the line stat and break that with the relay. Should be simple enough. Run a 4 conductor line to the thermostat, the Nest needs a common, hot, and a switched leg to the relay.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • andrewco
    andrewco Member Posts: 104
    Okay - not to leave well enough alone - let's say I wanted the Nest to control the fan alone on the garage heater without running the heating coil (as well as the full unit). From what I can tell, the fan circuit is run off of a single leg of the 240 feed. It would still require switching 120v but I don't have a clue on how (or if) the Nest could control a fan like this. Posting the heater control circuit again for reference.

    The Nest has the ability to schedule the fan to run periodically for a normal HVAC system so I was thinking it could be cool (no pun intended) to be able to do the same for this heater.
  • Paul48
    Paul48 Member Posts: 4,469
    edited August 2015
    No.......The fan runs off both legs and is controlled by the fan control. When you drop one leg of 240 you have nothing. There are circuits, such as drier circuits that run 120/240, but you have to provide a neutral. This doesn't appear to be the case for your heater. The fan control is probably temperature dependant, like a thermostat.
  • andrewco
    andrewco Member Posts: 104
    Paul48 said:

    No.......The fan runs off both legs and is controlled by the fan control. When you drop one leg of 240 you have nothing. There are circuits, such as drier circuits that run 120/240, but you have to provide a neutral. This doesn't appear to be the case for your heater. The fan control is probably temperature dependant, like a thermostat.

    I was afraid of that. I've noticed that the fan would sometimes run without the heater element. It was worth a try. Thanks for the analysis.
  • SWEI
    SWEI Member Posts: 7,356
    Once you install the 240V-24V transformer, one more (pilot) relay should do it.
  • andrewco
    andrewco Member Posts: 104
    edited August 2015
    SWEI said:

    Once you install the 240V-24V transformer, one more (pilot) relay should do it.

    SWEI - can you expand on this diagram to explain how to wire an additional pilot relay to control the fan? Would having it controlled via a pilot relay affect the native functionality of the fan?

  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Check out Taco's HAFC201 control. This may do what you want.
  • andrewco
    andrewco Member Posts: 104
    Bob Bona said:

    Check out Taco's HAFC201 control. This may do what you want.

    A bit too rich for my blood - just to turn a fan on and off. I was originally hoping I could just do it with a relay triggered off of the Nest...but there seems to be an issue with that Paul48 mentioned above.
  • Paul48
    Paul48 Member Posts: 4,469
    It kinda reminds me of a skit, Eddie Murphy did years ago on Saturday Night Live, about George Washington Carver. He said," the man spent his entire life, trying to make a peanut into a phonograph needle". All that aside....I'd buy a cheap ceiling fan and throw it up.
  • andrewco
    andrewco Member Posts: 104
    Paul48 said:

    It kinda reminds me of a skit, Eddie Murphy did years ago on Saturday Night Live, about George Washington Carver. He said," the man spent his entire life, trying to make a peanut into a phonograph needle". All that aside....I'd buy a cheap ceiling fan and throw it up.

    I hear ya - but I figure I have a control device (thermostat)and a fan (integrated into the heater) so why not use whats I gots.
  • unclejohn
    unclejohn Member Posts: 1,833
    I would get a 40 amp single pole contactor and remove the tstat from the circuit. Wire the contactor contacts where the t'stat was and power the contactor coil with the "W" circuit off the nest. You need a 240 x 24 volt transformer to power your nest and that's all. easy peesy japanessy
  • unclejohn
    unclejohn Member Posts: 1,833
    If you want to run the fan only you can most likely use a 90-340 relay for that, they are good for 12 amps I think. And just parallel the temp switch or fan control through the open contacts of the relay and connect the coil to "G" off the nest.
  • Abracadabra
    Abracadabra Member Posts: 1,948
    Get rid of the contactor and r8845

    All you need is this:

    http://www.supplyhouse.com/Honeywell-Aube-RC840T-240-240v-Relay-w-Built-In-24V-Transformer

    handles 5280 W @ 240 V
  • andrewco
    andrewco Member Posts: 104
    Abracadabra said:

    Get rid of the contactor and r8845

    All you need is this:

    http://www.supplyhouse.com/Honeywell-Aube-RC840T-240-240v-Relay-w-Built-In-24V-Transformer

    handles 5280 W @ 240 V

    Okay - now that is cool and reasonably priced. I just want to be sure based on the wiring from the heater - is the thermostat switching 240v or just one phase (120v) of the circuit? Looking at the diagram it looks like it's just breaking L1. Does that change the application of this?
  • Paul48
    Paul48 Member Posts: 4,469
    http://www.nojolt.com/Understanding_240_volt_circuits.shtml
  • andrewco
    andrewco Member Posts: 104
    I just received the rc840T - looks pretty cool. I'm a bit confused on the wiring so some clarification would help. The RC840T (diagram below) has 3 wires on the load side blue, black and and red. According to the diagram they hook up to L2, L1 and one side of my line voltage thermostat coming from L1. But the diagram shows it coming from L2. Does this matter?

    I'm including the diagram from my heater as well. I'm replacing the thermostat show in the diagram with the rc840t.
  • andrewco
    andrewco Member Posts: 104
    Ok - wired it up but would probably like to get confirmation prior to plugging it in. Diagram below is what I did.
  • JUGHNE
    JUGHNE Member Posts: 11,268
    I would reverse the positions of the black and red.

    Your transformer must have 240 vac (assuming you have the 240 VAC version, not the 120 VAC) going to it at all times in order to produce the 24 volt control voltage.

    The red is the switched wire from the black leg which will take place of your existing T-stat.

    As it is now nothing will happen unless you leave the old T-stat in place and closed, which you don't want.
  • andrewco
    andrewco Member Posts: 104
    Excellent! I'll switch it and give it a try. I'm completely removing the t-stat - that's what the yellow was supposed to signify.
  • JUGHNE
    JUGHNE Member Posts: 11,268
    Personally, I would leave it physically in place and label it as non functioning. You may want to restore it as was, upgrade and sell it, etc.
    There are many things I have saved on the shelf and can not locate them if wanted later. But left in place you would always know where it was.
  • andrewco
    andrewco Member Posts: 104
    Sorry - that's what I meant. I'm completely removing the t-stat from the circuit - it will remain in the unit.

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