Lets talk about water heaters!

paradisePWoffrd said:
The real question is: Does it consume less energy overall vs a storage unit?
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So the one posted above 75a @ 240v, 18kW. Assuming it runs at max amperage while in use (there arent any specs to say otherwise), for 1 hr per day; that is 540kWh per month.

Looking at a random 50gal unit (Shop Whirlpool 50-Gallon 6-Year Regular Electric Water Heater at Lowes.com) the spec says 4622kWh/yr or ~385kWh per month.

With that being said, it doesnt look like there is any savings with an electric tankless, but convenience has a cost, so it could be a wash.

An alternative, you could install multiple POU units. Something like this: Marey Electric Tankless Hot Water Heater 110 Volt - 2 GPM - Eco Store Depot. That is 4.4kW. So you have 1 in bathrooms, 1 in kitch. Claim 2 hrs per day use between them. Thats ~265kWh/month.
 
First off, 50 gallons of water doesn't just up and go from 120F to 65F immediately. Case in point: We had to do a little project at the house on Saturday morning that required turning off the well pump. We shut off the water heater too, just in case. We turned the well pump back on, but forgot about the WH. Twenty-four hours later, we took a 20 minute, hot-ass shower from a 20yo, 40 gallon electric tank. It wasn't until Cyd decided that she was going to shave her legs that we realized the water heater was turned off. I've taken hot showers after the power has been out for two days. The power draw once a body of water is that hot isn't really a big deal.

Mixing valves get employed when you have a huge one-time demand that can't be met otherwise. Want to put a 125 gallon soaking tub in the master bath? Sure, no problem, but understand that you're not going to get an electric water heater bigger than 80 or 90 gallons. So buy a normal 50 or 60 gallon unit, crank up the thermostat, then use a mixing valve to bring the discharge temp back down to safe levels.
 
Fabrik8 said:
Lets just neglect the massive amount of water wasted while taking a bath from a 80 or 125 gallon soaker tub. ;)

And then you take a shower to wash your hair...
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Dont care, had sects?
 
shawn said:
That 50 gallon tank will support a lot more load than the tankless, too. The tankless in the link only does 2.6gpm.
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Correct, which is a shower and sink at the same time, at best. Other than needing to run multiple runs of power, IMO, the POU tankless units are nicer than a single, if doing electric. Now finding a 110v POU with a cycle time and flow rate that can support a shower or tub, good luck.
 
Mixing valve option my work in my next house. Especially if natural gas isn't a option. Thx for da edumacatin. Gas is cheap now and will be for a while. If ya got it, use it.


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Ron said:
Ok...so which is greater, the energy required to heat 80 gallons of 50F water to (pick a number) 190F in a period of 20 minutes and then maintain it or....the energy required to heat 50F water to 190F in a period of 8 seconds and maintain it until shut off.

Of course heater ambient, temp setting, input temp, demand duration etc etc etc alter the curve. Ive tried like hell to justify a tankless. But you just cant make the numbers work on an electric tankless. Ive tried. Now if you want to value space and showers longer than 45 minutes (how long my daughter can go on our 80 gallon) then sure. Filling up a hot tub? Maybe. But if you re a high demand user you also have to worry about Duke reclassifying you. 35kW for a period of 10 seconds 3 times in a given month is typically enough to get you bumped up to high use/light business class rates. Then you $/kwh goes 1.72x...
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There's definitely less energy usage for shorter duration water draws (IE 50 gal VS 20 gal). Really just comes down to how much you use hot water, I suppose. Being on somewhat of a small spring anyway, I'm a water miser. Some quick math:

50 gallons heated
416.5 lbs
50 Incoming T
125 Outgoing T
75 delta T
31,237.50 BTU req'd


20 gallons heated
166.6 lbs
50 Incoming T
125 Outgoing T
75 delta T
12,495.00 BTU req'd

My house has a ~30kW peak demand between my water heater, electric baseboards, fridge, lights, spring pump, washer/dryer. (EDIT** but I never use all these things at once)

I will disagree with your statement about business usage rates. In Duke territory, commercial customers get better rates than residential by at least 2 cent/kWh. We pay about 6 cent/kWh at work, as a heavy commercial user.

The most expensive electricity in Duke territory (my utility is actually BREMCO, but they buy their power from Duke) is TOU-D, or Time of Use Demand. Some residential rate payers opt for this route, but I've known several people get reamed for it in the winter months with electric heat. So I'll stick with my normal residential service.
 
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The point is, if you heat more water than you use, it will use more energy.

Less heated water = less energy. Period.

The only way to use less electricity than a standard electric resistance water heater OR tankless water heater to heat water is to use a heat pump WH with a COP >1, like a AO Smith Voltex or a Stiebel Eltron.

But no one on this website would care about those anyway :fuck-you:

Those units are about 300-400% efficient.
 
You are ignoring a critical variable and that is rate of heat change.
Sure it takes the same number of BTUs, but when you use electricity to generate those BTUs and since your voltage is fixed your ampacity increases dramatically with that speed change.



Regarding your Duke comments, you are comparing apples and motorcycles. Yes commercial services pay less per kwh than residential...sometimes. However because of grid demand factors and rate classes when you approach commercial levels in a residential service area your rates escalate quickly...

I literally have about 40 companies power bills sitting in my desk let me scan one and clean it up (remove identifying info) and Ill share
 
My SGS (Small General Service) rate at my shop is literally twice as much per Kwh as the residential rate for my house, and they both come off the same 50KVA transformer sitting on my property.
 
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So looping back around to the boring parts:

This really comes down to whether we want unlimited hot water I guess, and whether that is worth some extra money to make that happen. Well, and the time delay would be shorter because it will be much closer to the kitchen, etc.

The only other decision is whether the cost of gas versus electricity is going to be worth worrying about, and I don't think it probably is.

We're just two people in the house, so it could go either way. Her baths seem to kill the tank faster than my lazy showers, and I've never really had much problem with the tanked unit until one of the elements went out.

So I guess it comes down to convenience, and whether this is a convenient time to add that convenience.
 
I see their kW-peak and monthly kWh, but what was their kWh rate?

Generally the entire southeast has pretty dern cheap electricity rates and demand charges, with the exception of some (increasingly fewer) local co-ops that produce their own power - compared to everywhere else.

You can get into electricity bill analysis paralysis, but the electric tankless units are feasible to run in the southeast, both in regards to energy cost and incoming groundwater temperature. It's a matter of preference if the home or building's incoming service can accommodate the ampacity required for tankless water heating. A minimum of 100A electrical utility service size needed for these units.

The attached pic is how my unit turned out. The smaller tankless units don't require T&P valves also, which is pretty neat. Saves a ton of space in the closet and makes for a simple 1-man install or replacement.

I'm sure I'll get some electric and plumbing code nazis out for not installing blah blah blah, but whatever. It very well works for my application and, unlike a storage-based WH, it never runs out of hot water.

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Just had my rinnai installed the end of December, only did it for the endless hot water for the jacuzzi tub. Noticed about a 10dollar a month drop or so in power bill but again too soon to tell, especially since it's winter and my gas bill is more, (never been over 100bucks tho in all the years I've lived here) as far as not having instant hot water, it's not that big of a deal. Worst part is just waiting at kitchen sink to get hot water to was hands. Storage type use to be almost directly under kitchen so was very fast whereas now the tankless unit is on back of house.
I think NG is still the cheapest energy available and I've heard the electric tankless units are not very efficient but I don't know. The endless water and the ability to hook up my pressure washer to endless hot water outside are my big pros of the swap. I wouldn't do it to "save" money. Just my $.02
 
Then bold print doesn't say anything about their kWh cost, just their demand schedule.

Anywho - thread hijack. Back to water heaters!
 
Yes, back to water heaters.

I went under the house, and grabbed the serial number after I checked the elements. They both tested good, 12.6 ohms which is dead on for 4500W elements according to my math. So it's likely a thermostat issue. I took off the access covers and it looks brand new under the covers.

So after decoding the serial number, it's a 2011 build date, January. What? It's technically 2 months out of warranty, and just over 6 years old.

I'm completely shocked, because looking at it I assumed it was at least 10 or 15 years old. I was ready to write that thing off and replace it for liability.


I think this helped make me think it was older than it actually is:

IMG_20170307_180352391.jpg



Looks like it's going to get fixed and I'll get some more life out of it. That is, unless it's got a non-removable pile of sediment in it. Time to buy an transfer pump and see what happens when I flush it.

The new heater should be timed about right for the kitchen remodel and all of the re-plumbing that goes along with it.
 
A gas water heater will save $2-300/yr over an electric, all else being equal. That is just based on consumption. If you only have gas heat and are paying $10/mo in the summer for a connection fee, you can come close to heating your water for free during the warm months.
 
Shawn I'd be all over natural gas WH if it was available in my area!

Example: Nat Gas is about $1/therm (or 100,000 BTU). To get the same 100,000 BTU from electricity (or 29.3 kWh), it would cost $2.93 at 10 cent/kWh.

So for the same amount of energy, nat gas almost 1/3 the cost of power. Unfortunately propane is my only option, so electric tankless it was. And for the at least $400 difference in initial cost, not sure that a gas unit would ever pay for itself - especially running off of propane.
 
Ordered a pair of thermostats and a nice brass ball valve for the drain. And a transfer pump to defy gravity and drain out of the crawlspace.
 
Fabrik8, if you're doing the maintenance work anyhow you might look into a new sacrificial anode for it, too.
 
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