Rule to control a metered plug in switch that turns on/off my gas hot water heater

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1

Hello All!

I have my hot water heater plugged into a generic zigbee outlet. The outlet has power meter capabilities. The "turn on" rule is not an issue. I have it turning on at 06:30 am everyday. What I need is for the "turn off" rule to do the following:

  1. Time = 2300 every day
  2. Evaluate power level
    a. If power level >0, wait until the power level is=0 then turn off the switch
    b. Else, if evaluate power level =0, then turn off switch.
    The goal is to prevent a mid-heating cycle shutdown; to protect the electronics, burner, and venting of the hot water heater.
2

How about something simple like this:

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The trigger is conditional upon the heater being on. So that the rule doesn't have to run if you have purposely turned the water heater off,

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3

Or maybe this, since it sounds like you want it off at 2300 if there's no active draw...

Trigger: Time is 2300
If switch is (whatever metric ID's a draw running), then
Wait for event: switch is (whatever metric ID's the draw as over)
Switch = Off
Else
Switch = Off
End If

I was intentionally vague with my "metric" reference above, since it's not always as simple as it seems to determine when the power-reporting device is really reporting a draw or not -- it depends a lot on how you have that device configured and how/when it sends reports. In my experience, I don't think I ever see a no-kidding "0" draw, but maybe that's just me.

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4

A gas hot water heater with a plug?

This is a hot water heater with a tank?

I'm just curious.

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5

Here is how I would write it, just to add to the above list of very good ideas.

The "Wait for Condition" will not pause execution if the power is already 0.0. If not, it will wait until the power becomes 0.0.

I added a 5 minute delay to turning off the power to the water heater to allow it to finish anything its electronics might be doing after completing the heating cycle of the water if it was in process.

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6

TIL! I did not know that - thanks :+1:

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7

Possibly a newer high efficiency type that uses a Power Vent to exhaust combustion gases.

This type of heater does not have a continuous pilot light. So without AC powering the electronics, it will never be able to light the burners and produce heat.

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8

Just to be clear... a "Wait for Event" will wait until the hub receives the specified event. So, if the power was already 0, the hub would wait for a new event setting the power level equal to 0. The power would need to become non-zero first, and then back to zero.

"Wait for Condition" behaves as previously mentioned.

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9

It could be a gas heater with power/direct venting that needs 120V AC?

But if that’s the case, makes me wonder how much the electrical draw changes when the heater is heating/idle (since the gas burner is doing the work of heating the water).

Also, I realize the OP didn’t ask, but why turn off a water heater at night while a house is occupied?

10

This. I'm not sure it's saving any money to turn it off.

If it's smart could just turn it down.

11

Blockquote
Short answer: for a modern high‑efficiency gas water heater with an electric exhaust fan, turning it completely off for ~7 hours at night usually does not save meaningful money and can actually cost you more—or introduce other problems.

Here’s why, broken down clearly.

1. How these heaters use energy

A high‑efficiency (often condensing) gas water heater has:

  • Very low standby heat loss due to heavy insulation
  • Sealed combustion and an electric exhaust (inducer) fan
  • An electronic control board that expects continuous power and gas

When no hot water is being used, the heater consumes:

  • No gas
  • A tiny amount of electricity (controls + fan only when firing)

Standby loss is typically well under 1% of tank energy per hour.

2. What you “save” overnight

Assume:

  • No hot water use 12am–7am
  • Tank volume ~50 gallons
  • Temperature drop overnight: maybe 2–5°F
  • Energy saved by turning it off: a few thousand BTUs at most

That’s worth pennies per night, often less than $5–$10 per year depending on gas prices.

3. What you may lose (or risk)

A. Reheat inefficiency

When you turn it back on in the morning:

  • The burner runs harder and longer to recover fully
  • Condensing heaters are most efficient during long, steady burns, not cold starts

This offsets most or all of the standby savings.

B. Electricity savings are negligible

The control board draw is usually:

  • 2–5 watts idle
  • Fan only runs during firing

Seven hours of idle electricity is roughly:

~0.03 kWh/night → less than ½ cent

C. Wear & reliability (often overlooked)

Repeated daily power cycling can:

  • Stress electronic ignition components
  • Increase failure rates of control boards over time
  • Trigger occasional lockouts or error codes

Manufacturers generally do not recommend daily shutdowns.

D. Safety & convenience issues

  • Morning demand surge (showers + dishwasher)
  • Risk of lukewarm water
  • If gas valves or electronics glitch, you wake up with no hot water

4. When turning it off might make sense

There are a few niche cases:

:white_check_mark: You leave for several days or weeks
:white_check_mark: You have an older, poorly insulated tank (not high‑efficiency)
:white_check_mark: You’re doing maintenance or safety shutdowns

For normal nightly cycles → not worth it.

5. What does save money instead

If your goal is cutting water‑heating cost, these work far better:

:white_check_mark: Lower the setpoint (best option)

Reducing from 140°F → 120°F can save 5–10% annually with no downside for most homes.

:white_check_mark: Fix hot‑water usage patterns

  • Low‑flow showerheads
  • Wash clothes with warm instead of hot
  • Repair even small hot‑side leaks

:white_check_mark: If you have recirculation

If you use a hot‑water recirculation pump:

  • Put the pump on a timer or motion sensor
  • This can save substantial energy overnight

6. Bottom line

For a high‑efficiency gas water heater with an electric exhaust fan:

Turning it off for 7 hours at night usually does not save money and may slightly increase cost or wear.

Leave it on, set the temperature appropriately, and focus on usage reduction instead.

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12

One scenario might be, a tankless gas water heater is close enough to a bedroom that someone washing their hands in the middle of the night generates enough noise to disturb somebody.
Odds are they won't use enough water for it get warm anyway.

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13

Yes, seems like benefits are low. Of course their are also the many unintended consequences of power cycling a device that expects 24x7 power feed. Safety, stress, wear out, ...

14

I have a tankless gas water heater. It doesn't make that much noise. I have mine on a ZEN15 power switch, but not to turn it off. I only use it to monitor its energy use and power levels. My Rinnai pulls about 23 watts when running. That is just the exhaust fan. When not in use it really does not register.

In the summer it uses about $0.03 of energy a month. It goes up to a $1 or two in the winter. My tank is on the outside of the house and it has heat strips that come on when it gets below freezing. That is another reason I have it on a smart plug I have rules that let me know the strips are coming on in the winter.

15

If it is in fact a tank, not tankless, I have read, and not sure of the timeframe, but allowing the water to cool in a water heater can promote the growth of bacteria, and isn't super safe

16

I wouldn't say mine is noisy, but if you're in the same room with it you can definitely hear it start and tell when it's running.

Sleeping two floors away, it for sure doesn't wake me up. :laughing:

17

Yes the general recommendation is to keep water tank temperature at least 120 Fahrenheit and as high as 140 to prevent the growth of Legionella bacteria.

140 is safer from the bacterial growth perspective, but can increase the risk of scald burns if mixing valves aren’t installed at bathroom fixtures to bring the temperature back down a bit.

18

Newton's law comes into play here to:

AI Overview

Newton's Law of Cooling implies that while turning off a water heater for 8 hours does save energy, the savings are relatively small—typically 5% to 12% of the water heating bill—because modern tanks are well-insulated and do not lose heat quickly.

Here is how Newton's Law of Cooling applies to this scenario:

  • Rate of Loss: Newton's law states that the rate of heat loss is proportional to the difference in temperature between the water and the surrounding air.
  • Insulation Factor: Because water heaters are insulated, the cooling process is slow, often losing only about 1/2 to 1 degree Fahrenheit per hour.
  • Reduced Savings: Because the temperature drops slowly over 8 hours, the water stays relatively warm, meaning the "energy savings" (reduced heat leakage) are not as substantial as if the water cooled to room temperature.
  • The "Reheat" Negation: When the heater is turned back on, the energy required to bring the water from a slightly lower temperature back to its maximum setting nearly offsets the energy that was saved by not keeping it hot.

Conclusion on Savings:
Turning off a water heater for 8 hours daily generally results in minor savings, often estimated to be around $20 to $70 per year, or roughly $0.76–$1.36 per day. It is more effective for reducing energy usage to reduce the set temperature or to add a thermal insulating blanket to the tank.

Note: For gas heaters with standing pilots or older, poorly insulated tanks, the savings may be higher.

19

I made a basic rule to turn off the water heater fan off at 10pm and on at 5am. It won't light until the fan goes on It's worked perfect for 3 years.

20

There is also this to consider....

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