on demand electric water heater, 120 amp

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Stevenfyeager

Senior Member
Location
United States, Indiana
Occupation
electrical contractor
I have to upgrade a 200 amp house to a 320 amp because the homeowner installed a 120 amp instant, tankless water heater. It trips their main. They have to turn off their furnace every time they want hot water. He fed it with 3 40 amp breakers, and (3)#8-3
cables. (Actually, he installed a 100 amp subpanel, then in that panel, he installed the (3) 40 's.) This is my first electric, on demand water heater to deal with. For one of the two 200 amp panels I will install, I'm thinking a small one and have the heater only on that panel. The other panel would be a standard 40 space panel. Does that sound reasonable ? Any suggestions ? Thank you.
 
Stevenfyeager said:
For one of the two 200 amp panels I will install, I'm thinking a small one and have the heater only on that panel. The other panel would be a standard 40 space panel.
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I had the same issue with an existing home. They added a tankless water heater that required four 40a circuits, overloaded the existing service, and repeatedly tripped the main breaker. They asked me for a solution, and I suggested a service upgrade.

I changed the meter base, added a second 200a disconnect, and ran SE cable to a new 200a M/L panel for just the four 40a breakers. No neutral required, which dumbfounded the inspector, but his supervisor confirmed my contention none was required.
 
jeff48356 said:
I would suggest the same thing. Electric units use ridiculous amounts of power. I can't imagine the cost to operate it versus a gas-fired unit.
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Power isn't the issue with operating cost - energy is. It still takes same energy to heat same amount of water by same number of degrees.

Unless you get into high demand usage rates your energy bill possibly goes down because you are not heating stored water and losing at least some of that heat when not using any hot water.

That said, I don't think the energy lost is significant enough to many of us to justify the larger service expense, higher maintenance the instant water heater will likely require, and likely more limitations on available hot water demand.
 
kwired said:
Power isn't the issue with operating cost - energy is. It still takes same energy to heat same amount of water by same number of degrees.

Unless you get into high demand usage rates your energy bill possibly goes down because you are not heating stored water and losing at least some of that heat when not using any hot water.

That said, I don't think the energy lost is significant enough to many of us to justify the larger service expense, higher maintenance the instant water heater will likely require, and likely more limitations on available hot water demand.
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It’s been a while since I ran the numbers, but my cost of energy from nat gas is significantly lower than electricity.
 
retirede said:
It’s been a while since I ran the numbers, but my cost of energy from nat gas is significantly lower than electricity.
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That will vary depending on location. Most places in recent years gas will cost less. 10-15 years ago electric was in the competitive price range or even less than gas in some places. Here we do get lower electric "winter rates" that does make electric heating more affordable than if we didn't get those winter rates. Water heating is year around load though.
 
kwired said:
That will vary depending on location. Most places in recent years gas will cost less. 10-15 years ago electric was in the competitive price range or even less than gas in some places. Here we do get lower electric "winter rates" that does make electric heating more affordable than if we didn't get those winter rates. Water heating is year around load though.
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I remember that - 2005 gas hit $13 per MMBTU. Lately, it’s been $3-4 with some spikes in the $5-6 range. When fracking was in high gear, it was under $2.
My net electric rate is between $0.15 and $0.16 year round.
 
retirede said:
I remember that - 2005 gas hit $13 per MMBTU. Lately, it’s been $3-4 with some spikes in the $5-6 range. When fracking was in high gear, it was under $2.
My net electric rate is between $0.15 and $0.16 year round.
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Not sure what my rates are now, but found a rate schedule I saved from 2012 on my computer.

For my house/shop which are all on same meter my rate then went like this:

Monthly charge of 25.50
first 200 kWhr per month @ $0.1175 (year around)
next 900 kWhr per month @ $0.098 winter $0.099 summer
next 4000 kWhr per month @ $0.060 winter $0.099 summer
anything beyond that is $.069 winter $0.099 summer

Rates have probably risen maybe 5-8% since then but AFAIK still similar structuring

I live in rural area and am supplied by an individual transformer that doesn't supply any other customers - If I were in one of the towns or other areas with multiple homes supplied by a transformer I could have been on a different rate schedule that had monthly charge of 14.25, some lower rates in the tier levels, but summer rates were mostly about the same - that is when their system is running at high demand.
 
Run away

Run away

kwired said:
Power isn't the issue with operating cost - energy is. It still takes same energy to heat same amount of water by same number of degrees.

Unless you get into high demand usage rates your energy bill possibly goes down because you are not heating stored water and losing at least some of that heat when not using any hot water.

That said, I don't think the energy lost is significant enough to many of us to justify the larger service expense, higher maintenance the instant water heater will likely require, and likely more limitations on available hot water demand.
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Kwired is getting close to the truth here. Now with that said, Tankless Technology be it gas or electric will be seen to be some of the most ridiculous, unreliable, problematic,and time wasting folly that ever was. I could write pages on what a bad idea it all is.
 
Got called once to a place where I knew the occupants had little money, had mental disabilities, and just overall lower living standards than many - but otherwise nice people. They lived in a old mobile home - built in 1970' or even 1960's.

Gas water heater went bad, they called me to see if I could connect new water heater. I get there and in outside access door that once had typical 30 or 40 gallon gas water heater inside has a briefcase sized unit hanging on the wall - electric. Can't recall how many kW it was anymore but the mobile home only had a 60 amp feeder supplying it and a 100 amp service disconnecting means. I told them it would be expensive to make things right, that thing needs more power then you even have available to the house.

Don't know what they ended up doing. This been a few years ago, I do know they live somewhere else now - but don't know if they ever had hot water before leaving old place or not. Old place has been removed completely by now.
 
I have discouraged numerous potential clients from installing electric tankless water heaters. I always suggest they use gas instead and of course if they don't have gas service, tanks of propane will be required.

The funniest one was a guy who lived in the middle unit of a condo with 100 amp service who bought a tankless and showed it to me when I went to look at the job. It was 2 x 40 amp. Took a while for him to understand that not only can he not install that in his existing panel, but also there was no way to give him extra capacity.

Personally, I would never have one because I like my hot water hot regardless of the flow volume.
 
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