Water Heater scheduling question

[spacejamz]

So I installed this Rheem water heater yesterday and I am messing around with creating a schedule for it....

Rheem Gladiator 50 Gal. Tall 12 Year 5500/5500-Watt Smart Electric Water Heater with Leak Detection and Auto Shutoff XE50T12CS55U1 - The Home Depot

The Rheem Gladiator 50 Gal. electric water heater provides an ample supply of hot water for households with 3 to 5 people. The Gladiator water heater is unlike any other water heater because it provides

www.homedepot.com

I will have temp go down to 110 degrees at midnight and then back to 125 at 8AM...

Does it cost more to just keep it at 125 or have the system heat the water from 110 to 125 at 8AM? I have no idea how to figure out which is cheaper...does the heating element work any harder in either scenario (to possibly shorten the life of the element)?


worst case, I could set one day to keep it at 125 and the next day to go from 110 to 125 and then use the built in reporting to see which used more kW...

 

[Paperdoc]

No, that won't cost more; it will save money.

The electric heating element should always work very simply: its control system turns it either fully on or fully off. All your timed temperature settings are done in an app on your computer or phone, I'm guessing, communicated to the water heater by WiFi. And a simple heating element is "100% efficient" - that is, ALL of the electrical energy supplied to the element IS delivered as heat into the water. Water heater efficiency depends on the insulation and heat loss from the tank, not on the efficiency of the heating element. So, no matter what temperature the water in the tank is at, turning on the element will always deliver to the water the same amount of heat per minute.

How does this same money? Well, when the water temperature is hotter, the rate of heat loss from the tank by conduction through the tank insulation is higher because the temperature DIFFERENCE between the water in the tank and the air outside is higher. When you let the tank temperature cool down during the night this heat loss into the air surrounding the tank is lower, and hence the heating element does not spend as much "on time" keeping it to the (lower) temperature setting. Sure, when daytime comes the element must stay on longer than usual for its first cycle to re-heat to the higher temperature setting, but the energy used to do that is LESS than it would have used to keep the higher temperature all night.

 

[jmagg]

I've often wondered if programmable thermostats and fluctuation in temperature in a single day actually save energy, considering a setback to cooler temps affect surface temps (walls, floors, furniture etc) which take longer to warm up than air. Especially in very cold outdoor conditions. I've had heating and cooling contractors advise leaving the stat on a constant temp, and others recommend a program. Its tough to study, since there are many variables.

 

[PowerEngineer]

It's really not that complicated. As Paperdoc explained, the loss of energy as heat whether from a hot water heater or a home is proportional to the difference in temperature between inside and outside. You will lose less energy during those times when you have reduced the inside temperature and therefore will consume less energy. Now whether or not you'll see a noticeable difference in your energy bills is a different question...

 

[spacejamz]

It's really not that complicated. As Paperdoc explained, the loss of energy as heat whether from a hot water heater or a home is proportional to the difference in temperature between inside and outside. You will lose less energy during those times when you have reduced the inside temperature and therefore will consume less energy. Now whether or not you'll see a noticeable difference in your energy bills is a different question...

Wouldn't there have to be some break even point where the raising the temperature back is going to cost more that just leaving at a constant temp (not that raising it from 110 to 125 would even come close to that point)?

Hypothetically, what if the temp was set to 60 degrees and the water heater had to get the temp back to 125? would that use more energy than just maintaining 125 for a period of 6 hours (assuming it is winter and the temp would drop down to 60 in 6 hours)...

 

[BoomerD]

So I installed this Rheem water heater yesterday and I am messing around with creating a schedule for it....

Rheem Gladiator 50 Gal. Tall 12 Year 5500/5500-Watt Smart Electric Water Heater with Leak Detection and Auto Shutoff XE50T12CS55U1 - The Home Depot

The Rheem Gladiator 50 Gal. electric water heater provides an ample supply of hot water for households with 3 to 5 people. The Gladiator water heater is unlike any other water heater because it provides

www.homedepot.com

I will have temp go down to 110 degrees at midnight and then back to 125 at 8AM...

Does it cost more to just keep it at 125 or have the system heat the water from 110 to 125 at 8AM? I have no idea how to figure out which is cheaper...does the heating element work any harder in either scenario (to possibly shorten the life of the element)?


worst case, I could set one day to keep it at 125 and the next day to go from 110 to 125 and then use the built in reporting to see which used more kW...

Damn...I keep my water heater set at 140. No kids in the house, and while me and the missus are both older, we're not quite to the "infirm" stage where water that hot is dangerous.

 

[PowerEngineer]

Wouldn't there have to be some break even point where the raising the temperature back is going to cost more that just leaving at a constant temp (not that raising it from 110 to 125 would even come close to that point)?

Hypothetically, what if the temp was set to 60 degrees and the water heater had to get the temp back to 125? would that use more energy than just maintaining 125 for a period of 6 hours (assuming it is winter and the temp would drop down to 60 in 6 hours)...

No, not really. Energy loss (and therefore the need to energy replacement) is driven by the difference between inside and outside temperature. Energy loss is at its highest when the inside water temperature is at its highest (i.e. 125 degrees). What "leaving it at a constant temperature" really means is heating the water back up (i.e. replacing lost energy) every time during those six hours when the temperature drops below your setpoint.

Let's assume an outside temperature of 30 degrees. The amount of energy used to keep the water at 125 degrees is going to be the product of a whole lot a factors (e.g. type and thickness of insulation) times the average 95 degree temperature difference over those six hours (125-30).

Now if we let the water temperature drop to 60 degrees and then heat it back up in time to reach 125 degrees after six hours, then average inside water temperature over those six hours will be less than 125 degrees - let's say that it's 80 degrees. So the amount of energy used in this scenario is the product of all those same factors times an average 50 degree temperature difference (80-30).

 

[mindless1]

You'll definitely save power by having the dual temp set up but not a lot because we're only talking about 15F difference. Unless you're a night owl you might consider setting to drop in temp even earlier, keeping in mind that at midnight it's not going to drop instantaneously to 110F, may take hours to do so.

 

[Paperdoc]

As I said above and PowerEngineer agrees, the temperature setback will save energy consumtion and hence cost. Exactly how much I cannot say. But I will comment further on how large a night-time temperture setback should be. From an energy saving viewpoint only, the larger the better. BUT you also have to consider the recovery time factor. That is, once the program changes back to the higher daytime setting, how long does it take to re-heat the tank full of water to that higher setting? Surely you want to have that process finished before someone wants hot water in the morning. You would have to do some experimenting to see how long the re-heatuing stage takes versus how low the set-back night-time temperature is. Especially with an electric water heater which probably has no indicator of whether the heating element is turned on or not, that could be tricky to do.

 

[mindless1]

^ A typical electric heater takes about an hour for a 30F rise, so about 30 min for the 15' difference spacejamz mentioned. Gas would be about half that, but there are a few variables so it's best to consult the manual.

This specific heater linked, spec's recovery as 25 GPH / 90F rise making it closer to 20 min for 15' rise.

This particular smart heater will blink the temperature selection LED while it is heating, or otherwise you could use a clamp ammeter.

EDIT: To clarify about the clamp meter reading, you need to put it around one of the conductors, not both of them enclosed in the same power cable sheath or else with current flowing both directions, it will cancel out the reading so you must measure after the conductors split apart. To further clarify, when the heating element is on it will conduct several amps while the heating element off will conduct closer to zero amps.

 

[Paperdoc]

Good info there, mindless1.