Timer Control for Aux Heat

Discussion in 'HVAC Heating & Cooling' started by Chezelle, Nov 20, 2010.

  1. Chezelle

    Chezelle New Member

    Messages:
    2
    Hello,

    I live in NC and we have around 3 months of cold weather most years, if that, and it's not usually too bad. I'm on that TOU Time of Use plan the power company offers where you get a discount on/off peak hours but must pay the demand charge for the demand KW that are used at on peak times. I've found my Honeywell 3611 thermostat turning the aux heat on so often it always spikes my demand charge several KWs which are close to $4 each usually costing me $20+ unnecessarily each month on the months I use the heat at all. Since I don't usually have the extreme cold winters I don't need the aux heat on all the time and only want it to work when at the off peak hours times. I tried a toggle switch on the aux wire and that worked ok but its easy to forget about turning it off before the On Peak times start. Right now I have the aux heat wire disconnected but sometimes the heat pump runs a long time to try and keep up.

    Are there any small programmable timer switches available that do not apply any power to the switch but only use the timer to open or close it? I'm wanting something to control my heat pumps aux heat on/off times to keep the aux heat off during On Peak power hours but still run the heat pump alone at those times.

    I'm currently doing this with a timer for my electric tank water heater and saving a lot more $$$ versus what my gas tank water cost me and also do not have to hassle with the gas man.

    Any help appreciated,

    Brian
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,013
    Location:
    New England
    I don't deal with this sort of stuff much, but if the ability to use aux heat is controlled by a low-voltage circuit, you could break the aux heat enable from the thermostat probably easier than trying to control the high amperage of the heat coils themselves. So, do a search for a SPST NO timer switch. then, run the aux heat control line through it. It would only be closed when the timer allowed it to. Since it would be controlling low-voltage and amperage (i.e., the power to enable the contactor to supply the heat), it would be fairly easy and not use much energy in the process.
  3. Thatguy

    Thatguy Homeowner

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    1,459
    Location:
    MD
    Since these HVAC transformers are usually rated at 40 VA the OP's timer switch contacts should be rated for an inductive load [the relay coil] at 40/24 = ~2 A or less. 1 A contacts will probably work fine.


    I've never understood this.
    If the heater loses 50W continuously through its insulation and has to supply make up kwh to water that has cooled due to the power being shut off, how on average do you save?
    Last edited: Nov 20, 2010
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    With peak electrical dual pricing structures, only allowing the WH to run when you are on the cheap electrical rate verses the peak rate, you might end up saving a lot of money. By gas man, I'm assuming he's dealing with propane verses NG, and depending on where you live, delivery and seasonal/market price changes can be quite diverse. On flat electrical rates, it probably doesn't make any difference on a timer unless it is off for days at a time and is one more component to fail/troubleshoot.
  5. hj

    hj Moderator & Master Plumber Staff Member

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    Depending on the amperage of the heat strips, you might be able to use the same timer you have on the water heater, otherwise get any dpst timer with the necessary capacity. Isolate the clock from the "load" terminals and give it its own power feed so it operates all the time, not just when your system turns the heat strips on. Then wire the heat strip power feeds through the timer's load/switched terminals.
  6. ballvalve

    ballvalve General Engineering Contractor

    Messages:
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    Location:
    northfork, california
    http://www.1000bulbs.com/product/3380/ELEC-GM40.html

    After years of looking for a great switch, this is the only one I would ever use again. Ad says steel case but it comes plastic, no matter.

    Big bonus: made in Germany. Most all the Intermatics went to China. 15 minute intervals with NO buying of trippers.

    The Germans bought Intermatic, and brought in this one from Europe which makes the old Intermatic line look like junque.
    Last edited: Nov 22, 2010
  7. Thatguy

    Thatguy Homeowner

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    1,459
    Location:
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    24,000 w/240 v = 100 a. How many 5 kw heat strips does your unit use?

    Another way to do this is to get a cheapo garden variety 24 hr. wall outlet timer and have it power a $3 relay from Hosfelt. That way you get the contact closure output that you need rather than a 120v output that you would get with this timer.
    Last edited: Nov 22, 2010
  8. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Good point - the timer I like can only do 40 amps. Likely not enough for heat pump back up coils. Some draw 60 or 90 amps and more.

    Use it with a relay, but a 3$ version will not switch 60 amps. And the plug in wall timers are unreliable in the extreme.

    http://www.grainger.com/Grainger/SQUARE-D-Contactor-5B099?Pid=search

    There is a heavy duty contactor. However, you likely wont need it ...

    because Heat coils in electric furnaces almost always are controlled by sequencers that switch the banks of 5kw strips with delays between. Most likely the feed to the sequencers are already fed by a contactor, so the spot to wire in the timer is the feed wire to the contactors magnetic coil, probably 24 volts.

    That is not a DIY project to find the correct point to add the time switch into!

    Next issue, you want the heat pump to run when the air temperature switch says to turn on the resistance heat coil banks. Your furnace will likely not allow this sequence. When the furnace calls for coil heating, the heat pump will be shut off as it has no capacity to extract heat from the outdoor air.
    Last edited: Nov 23, 2010
  9. Thatguy

    Thatguy Homeowner

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    Location:
    MD
    No, it just has to switch the 24 vac furnace control relay which must be less than 1.7A. It probably takes less than 0.5 A for this furnace relay coil.
  10. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I've found my Honeywell 3611 thermostat turning the aux heat on so often it always spikes my demand charge several KWs which are close to $4 each usually costing me $20+ unnecessarily each month on the months I use the heat

    Okay, so he has an easy wire to work with. But most heat pumps I have worked with "tell" the heat coils to turn on based on outside air temp or duct air temperature, without a wall thermostat adding its own "opinion".

    I would guess disabling the thermostat aux wire completely would still allow the heaters to come on when its too cold outside. [?]
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,013
    Location:
    New England
    No, if you disable the aux heat thermostat wire, they won't come on. Keep in mind, the only time the aux heat generally comes on (the exact control sequence depends somewhat on the h/w you have) is if say the heat was down and the house cooled off quite a bit, and the requested temperature rise couldn't be satisfied with the heat pump alone in a timely fashion, OR, it is too cold for the heat pump to work at all. You could make your control fancier and use not only the timer but also an outside temperature sensor to control the enable for aux heat. This gets much beyond a simple fix.
  12. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    He or she already said this works.
    Replace the toggle with a timer-driven relay.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,013
    Location:
    New England
    I suggested a NO (normally open) switch because my guess was the off-peak power period was longer than the prime rate. But, it has the disadvantage that if it fails, the switch will probably fail open. If you go with a NC (normally closed) switch, if it fails, then it would default to 'normal' operation, which would be probably safer. This would mean that the timer would have to run (open) the switch during prime time, and close (relax) during the off-peak time.
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