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Help! I'm a Texan, and it's about to get cold!

Howdy, first time poster, as the need for heat is uncommon where I live. But we're supposed to have quite a cold snap, and we're all as terrified as you might expect, so I'm making sure we've shored up out heating system to weather the cold (assuming we keep our power on this time...)

I have a single stage heating system where the electric heater coils are in the air handling unit, so real basic, not a huge heating load required. We recently replaced the ductwork to the home which has improved the balance of heat delivery in the house, but I noticed that the heat would cycle on and off frequently (usually on for 3 minutes then off for 3 minutes). I thought this was short cycling, but then I read that the recommended CPH for an electric heat system is 9 CPH!

I'm a real dummy when it comes to heating. Can someone explain why such a high cycle rate is optimal for electric heat? Seems like it would just lead to a real high electric bill... why wouldn't I want the system to just run a few times an hour? Doesn't the extra amperage to start the AHU and heaters every 6 minutes or so make this less efficient?


  • ratio
    ratio Member Posts: 3,543
    One half-hour on cycle in an hour uses exactly the same power as three 10 minute on cycles in an hour, & therefor adds the same amount of heat to your house. (You won't even be able to tell the difference in the electric bill.) Cycles per hour on electric resistance heat are greater than fossil fuels & heat pumps because the 'cost' of turning them on is insignificant (e.g. before a burner ignites it's literally zero % efficiency), so cycling it often will even out the heat & avoid large temperature swings.

    On many stats, cycles per hour can be adjusted; there's no harm in setting it longer—but you're not really going to save any energy.

  • ethicalpaul
    ethicalpaul Member Posts: 5,053
    If you really want to save money, (of course it will cost money to save money but there may be incentives that can lessen this...), ditch resistive heating which is only 100% efficient and switch to a heat pump which is more like 400% efficient
    I'm a homeowner thankful for this site. This post is my attempt to give back to HH.
    If that angers you, I apologize.
  • hot_rod
    hot_rod Member Posts: 20,725
    3 minutes on, 3 minutes off would drive me crazy. If I could hear that cycling or see lights dimming durning the cycle periods 😏
    My electric cooktop does the same PWM, but not 24/7, and I only hear it when I stand over it
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    JUGHNE Member Posts: 10,897
    If you set the fan to "on" for constant running, things will not be noticed so much.
  • WMno57
    WMno57 Member Posts: 1,012
    ERCOT seems to be on top of this.
    Good luck and stay warm.
    I DIY. If anyone has a problem with that, then I'd like to apologize to ABSOLUTLY NO ONE!
    credit to The Notorious
    EBEBRATT-Ed Member Posts: 14,472
    We would need to know what it is cycling on. High limit or thermostat. I would check the condition of the air filter...is it clean?

    Maybe increasing the fan speed, checking the temperature rise and electric heater amperage are in order. If the ductwork is sized for the proper airflow the heater shouldn't cycle UNLESS the thermostat is telling it to.
  • Nuke11
    Nuke11 Member Posts: 2
    Thanks for the information, everyone. The system is doing the best it can, but it's really an AC system with a bolt on heater, so it definitely is struggling to keep up with this cold. Thankfully we don't see temperatures like this often, so I can afford the higher bill to ride out the cold. I'd have to do some serious considering as to whether a heat pump would be worth the cost (or how long it would pay back in heat savings).

    Stay warm out there!
  • JDHW
    JDHW Member Posts: 66
    Hope everyone is OK and safe with the current weather. There has been quite a bit of coverage in the news over here UK.

    John - Have a good Xmas.
  • Constantin
    Constantin Member Posts: 3,796
    hot_rod said:

    My electric cooktop does the same PWM, but not 24/7, and I only hear it when I stand over it

    Depends on the appliance. Some have used solid state relays for the last 20 years to PWM several hundred times a second to maintain a given duty cycle without the "click on" followed by "click off" of the traditional relay-based approach.

    Ditto microwaves, if you buy a Panasonic (aka inverter) model or one of the many private label units they manufacture for other OEMs. My panasonic canister-style vacuum cleaner had a very funky Triac-based approach for the fan motor whose heat sink was in the handle. Resistive loads are the best case scenario for solid state relays but cost usually keeps them out of most appliances.

    One downside of heat pumps is that by virtue of allowing 2-way operation there is some system losses vs. AC-only operation. For example, Lennox' XC20 went from 20 SEER to 19 SEER in the heat pump equivalent when I looked at them. In TX, the more important question may be whether you can store enough LP on site to run the generator until the power comes back on.