Basic electric HW heater question

[CaptSpiff]

Just swapped out my Mom's 30gal electric HW heater this weekend. It's a simple dual element unit labeled GE, but made by Rheem. I got concerned when the install manual noted a 50A CB was required, yet the existing CB in the panel has been a 30A forever.

My calculator said that two 4500w elements (9000 total) would indeed draw 37amps, so I disconnect one of the elements for the "light off". Everything is fine, but when I start playing with my multimeter I realize that the upper thermostat controller only allows one element ON at a time. The Heater has upper and lower t-stat settings, but the upper t-stat sends voltage to only the upper element until it is satisfied. Then the upper stat shuts off the upper element, and sends voltage to the lower stat/element combination. The lower element then runs until its stat is satisfied.

The electric schematic in the install manual seems to back this up. So my question is: Do all electric HW heaters work this way, ie only let one element operate at a time?

Next question is: What is the best temp setting for the upper and lower t-stats (same or different)?

Third question: Why would the install manual's electrical table require a 50 amp breaker (romex wire for that costs a fortune per foot), when the WH will never use more than 19 amps (for one element)?

 

[Highbeam]

Every standard electric water heater that I've seen operates one element at a time. The elements can be different wattages but usually are 4500 each. A 30 amp breaker and 10 gauge is the typical circuit spec for a water heater. I suspect an error in the manual of your new heater.

 

[Clarkbug]

Most residential heaters only use one element at a time. Commercial units can operate both simultaneously.

You want to set the upper one a little lower than the bottom one. That way the bottom one does most of the work, and the top only comes on under heavy demand. This way your hot water storage is maximized.

 

[CaptSpiff]

Thanks Guys.
I've done a bunch of NG heaters, but this is only my 3rd electric, always same for same, and the first time reading the manual.

The manual has an electrical reference table for a dozen breaker choices for a dozen power levels.
I ASSUMED both elements operated simultaneously, so 9000w indicated 50A CB minimum. Clearly I was wrong.

Thanks for schooling me on the non-simultaneous operation.

I think this proves that life is more blissful when we do not RTFM.

PS. Left both stats on 120 (factory). She'd like it warmer. I'll go back and bump the lower one to 125 deg F on my next visit.

 

[maple1]

The manual for mine pointed out very clearly & emphasized that both stats should be set at the same setting. I couldn't understand that one - but did it anyway. Think I actually started a thread on here asking about it but don't remember much light being shed. Then I got to playing with them, now I forget what they're set at - I think I had the bottom one a little lower. My breaker is turned off anyway for 10 months of the year and I heat it with my boiler for those months - so it's been a while since I've looked at them. I think they're set around 115 - but I'm pretty sure they were a lot higher than that out of the box. I think if they were to be set different, the top one should be higher?

My electrician used a 20 amp breaker. 4500w at 240v is 18.75 amps, so it should be good. But I don't know what the manual spec'd, you've got me curious now so I'll have to try to find it & see what it says.

 

[HybridFyre]

The manual for mine pointed out very clearly & emphasized that both stats should be set at the same setting. I couldn't understand that one - but did it anyway. Think I actually started a thread on here asking about it but don't remember much light being shed. Then I got to playing with them, now I forget what they're set at - I think I had the bottom one a little lower. My breaker is turned off anyway for 10 months of the year and I heat it with my boiler for those months - so it's been a while since I've looked at them. I think they're set around 115 - but I'm pretty sure they were a lot higher than that out of the box. I think if they were to be set different, the top one should be higher?

My electrician used a 20 amp breaker. 4500w at 240v is 18.75 amps, so it should be good. But I don't know what the manual spec'd, you've got me curious now so I'll have to try to find it & see what it says.

That's cutting it a little close. Most electricians I know don't like to go over 80% of the capacity...which on a 20 amp breaker would be 16 amps...but since it doesn't run that much continuous it probably doesn't matter assuming it's the only thing on the circuit.

 

[Highbeam]

My electrician used a 20 amp breaker. 4500w at 240v is 18.75 amps, so it should be good. But I don't know what the manual spec'd, you've got me curious now so I'll have to try to find it & see what it says.

If so, he screwed up. The circuit ampacity is to be 1.25x the load rounded up to the next largest breaker so a 30. You had better hope he didn't stick you with 12 gauge wire as well. It should be 10 gauge on a 30 amp breaker.

 

[Highbeam]

Most electricians I know don't like to go over 80% of the capacity

It's the law for these types of circuits. Reference the NEC.

 

[HybridFyre]

It's the law for these types of circuits. Reference the NEC.

I figured as much I just never asked to confirm. I was just surprised to read an electrician did that. I've seen plenty of DIY homeowners do stuff like that though.

 

[Seasoned Oak]

THe new electric water heaters heat very fast and seem to be very efficient. I get HW in about 10 minutes from flipping the switch. I always use a 30A breaker and seldom install more than a 30 gallon heater in my rentals. Its getting hard to find them(30s) as the local suppliers seem to have 40 gal as the smallest ones. My 30 does just fine for 5 people.

 

[Highbeam]

THe new electric water heaters heat very fast and seem to be very efficient. I get HW in about 10 minutes from flipping the switch. I always use a 30A breaker and seldom install more than a 30 gallon heater in my rentals. Its getting hard to find them(30s) as the local suppliers seem to have 40 gal as the smallest ones. My 30 does just fine for 5 people.

Do you think a 30 gallon tank heater in a regular single family home is a negative for resale value. I mean, is it something a home inspector will flag as inadequate just due to tradition? It's not like you save money on buying it or really on operating it but people expect to see a 50 gallon unit.

 

[Seasoned Oak]

Do you think a 30 gallon tank heater in a regular single family home is a negative for resale value. I mean, is it something a home inspector will flag as inadequate just due to tradition? It's not like you save money on buying it or really on operating it but people expect to see a 50 gallon unit.

I think your standing loss is more with the 50 gallon. Iv already had 7 people staying here on a 30 ,so i dont know why i would need a 50 or even a 40. I have 30s in all my rentals and never had any " not enough hot water" complaints.

 

[Highbeam]

I think your standing loss is more with the 50 gallon. Iv already had 7 people staying here on a 30 ,so i dont know why i would need a 50 or even a 40. I have 30s in all my rentals and never had any " not enough hot water" complaints.

That's not what I was asking. I know it will work, but do you suppose it would actually be a problem for resale? a flaw?

 

[Seasoned Oak]

That's not what I was asking. I know it will work, but do you suppose it would actually be a problem for resale? a flaw?

No i dont think its a flaw NOT to oversize your hot water heater any more than than it is NOT to oversize your space heating system. Oversized appliances waste energy. That would be a flaw. Sizing is important with an appliance that has a lot of standing losses (like a WH)

 

[Clarkbug]

The manual for mine pointed out very clearly & emphasized that both stats should be set at the same setting. I couldn't understand that one - but did it anyway. Think I actually started a thread on here asking about it but don't remember much light being shed. Then I got to playing with them, now I forget what they're set at - I think I had the bottom one a little lower. My breaker is turned off anyway for 10 months of the year and I heat it with my boiler for those months - so it's been a while since I've looked at them. I think they're set around 115 - but I'm pretty sure they were a lot higher than that out of the box. I think if they were to be set different, the top one should be higher?

Having them the same isnt a bad thing at all. Power will always go to the top one first if its calling, then it will use the lower element. When they are the same, the top of the heater should always be at the setpoint, and the one at the bottom will get the incoming cold water. That way the bottom element turns on until you use up all the hot water (lots of showers, big bathtub, etc.) and then the top one kicks in to try and make sure your tap doesnt get cold water.

As soon as the top half is good, that element kicks off, and then the bottom one energizes.

If the top one was higher, it would run the system in the "quick recovery" mode more often, and will cycle between top and bottom more frequently. Ideally the bottom element does 90% of the work in your heater. So, I have the bottom one cranked up a little higher to make sure it stays the one working more, and then the top will kick in if the temps in the tank are really chilly.

 

[Highbeam]

Meh, it doesn't matter. Both stats the same, follow the directions.