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  1. #1
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    Jul 2011
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    Electric Baseboard Heating CPH Cycle Rate with Relay

    Hi All,

    Looking for some advice. I've replaced a bunch of baseboard heating thermostats with the Aube T209 (http://www.aubetech.com/products/pro...161&noLangue=2) which really works great. The T209 cycles the heat every 15 seconds. It has resulted in the room having very comfortable heat with a tight temperature swing (not the old wild swings of almost 5C with the old bi-metal thermostats I have replaced).

    My question is this:

    One room has the baseboards connected to a regular 24vac thermostat using an older honeywell relay (and older R841C1227 - http://www.pexuniverse.com/honeywell...c-heater-relay). In my research, it says that this relay will allow "4 - 6 cycles per hour".

    I am installing a honeywell TH5110D thermostat to connect to the room with baseboard heaters linked by this relay. Honeywell recommends 9 cycles per hour (9 CPH) for any electric device. Judging by my experience with the Aube T209 with 15 second cycles on baseboard heat, I want the thermostat to be set for either 9 CPH or 12 CPH so there is tight temperature control and even comfortable electric heat in the room (and a higher CPH if the thermostat will allow it, but I believe off the top of my head the TH5110D has 12 as the highest CPH setting).

    Will setting the CPH on the TH5110D to either 9 or 12 end up blowing the relay since it is rated at "4 to 6 cycles per hour"?

    ... on a further note, will there be any efficiency loss if I set the CPH this high? I can't imagine there being such with electric baseboard heat being 100% efficient, but would love to hear any ideas.

    Many thanks

  2. #2
    Join Date
    Sep 2002
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    I will run the 15-second cycling affect efficiency question past one of my sparky engineer buddies next time I see one. I would have to think cycling that tightly would affect efficiency in a negative manner. With regard to the Honeywell relay, exceeding the duty cycle rating by that much would most definitely reduce it's life expectancy.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  3. #3
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    Jan 2004
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    15 seconds on 15 seconds off, will use more electric.

    Set the 510 to 3 CPH. It will use less electric, an still keep the room temp within 1 degree.


    Mythbusters did a thing on light bulbs a year or so ago. I think a regular incandescent bulb used 1.5 minutes of electric when you fist turned it on. Your electric baseboard is basically the same as a incandescent light bulb. Just a minute and half for your baseboard is a lot more electric use.
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  4. #4
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    Quote Originally Posted by beenthere View Post
    15 seconds on 15 seconds off, will use more electric.

    Set the 510 to 3 CPH. It will use less electric, an still keep the room temp within 1 degree.


    Mythbusters did a thing on light bulbs a year or so ago. I think a regular incandescent bulb used 1.5 minutes of electric when you fist turned it on. Your electric baseboard is basically the same as a incandescent light bulb. Just a minute and half for your baseboard is a lot more electric use.
    Any idea why Honeywell recommends 9 for electric?

  5. #5
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    Quote Originally Posted by nk416 View Post
    Any idea why Honeywell recommends 9 for electric?
    Just a WAG.
    In a tight house(and I mean tight), with it cycling like that, the people would be able to set the temp lower and feel just as comfortable. And perhaps save money by keeping the stat set 3 to 5 degrees colder then normal.


    And I won't set any aux, emergency, or straight electric furnace to use 9 CPH. Its a waste, and the blower motor of air handlers don't do well running like that.
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  6. #6
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    Quote Originally Posted by beenthere View Post
    15 seconds on 15 seconds off, will use more electric.

    Set the 510 to 3 CPH. It will use less electric, an still keep the room temp within 1 degree.


    Mythbusters did a thing on light bulbs a year or so ago. I think a regular incandescent bulb used 1.5 minutes of electric when you fist turned it on. Your electric baseboard is basically the same as a incandescent light bulb. Just a minute and half for your baseboard is a lot more electric use.
    I saw that Mythbusters show. I remember they came to the conclusion that the time was so short it didn't make any difference. Electricity used has to be converted to something, where does the "wasted energy" go? Energy cannot be created/destroyed, only changed in form.

  7. #7
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    Quote Originally Posted by 54regcab View Post
    I saw that Mythbusters show. I remember they came to the conclusion that the time was so short it didn't make any difference. Electricity used has to be converted to something, where does the "wasted energy" go? Energy cannot be created/destroyed, only changed in form.
    Sounds to me like you're saying keeping the CPH at 9 or higher will have no effect on efficiency... ?

  8. #8
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    Quote Originally Posted by nk416 View Post
    Sounds to me like you're saying keeping the CPH at 9 or higher will have no effect on efficiency... ?
    I'm saying CPH makes no difference on efficiency. Any "wasted electricity" will be converted to heat. Electricity cannot simply disappear. As far as relay life goes just reduce the expected lifetime by the amount of extra cycles. Some relays are rated for 100,000 cycles which is a very long time... The rated CPH wil only occur at about 50% load, there will be less CPH at lower or higher loads.

  9. #9
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    Good man. Thank you for your help.

  10. #10
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    Quote Originally Posted by 54regcab View Post
    I saw that Mythbusters show. I remember they came to the conclusion that the time was so short it didn't make any difference. Electricity used has to be converted to something, where does the "wasted energy" go? Energy cannot be created/destroyed, only changed in form.
    I think one of us is remembering incorrectly, on how much it mattered.

    The waste is lost in the wires, as heat, in walls where you don't want it. Areas where they are doing or going to switch to demand electric rates will pay a higher bill based on that inrush current.
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  11. #11
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    Quote Originally Posted by beenthere View Post
    I think one of us is remembering incorrectly, on how much it mattered.

    The waste is lost in the wires, as heat, in walls where you don't want it. Areas where they are doing or going to switch to demand electric rates will pay a higher bill based on that inrush current.
    With an inductive load I agree with you about peak inrush current. With a pure resistive load there is no peak inrush. A light bulb has high inrush because it acts somewhat as a thermistor, it's not pure resistance. Take an ohmmeter to a standard light bulb, it's resistance is much lower than ohms law would indicate compared to the bulb wattage. Heaters ohm out close to their rated wattage.

  12. #12
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    Quote Originally Posted by 54regcab View Post
    With an inductive load I agree with you about peak inrush current. With a pure resistive load there is no peak inrush. A light bulb has high inrush because it acts somewhat as a thermistor, it's not pure resistance.
    Electric heating elements work the same way. Their resistance changes with their temp.

    I've watched more then one meter when the baseboard was turned on. You can see the meter speed up then slow down. of course that was on meters that you can see the dial move.
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  13. #13
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    I did find that episode's results. And they did claim that it made no real difference with a light bulb. To bad they didn't try it on an electric heating element. They would find a different result.
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