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Brian Nystrom
Joined: 26 Jan 2004
Posts: 4312
Location: Nashua, NH

1/26/19 10:07 AM

How would two units in series be more efficient? The temperature rise required is still the same and you'd be heating up the innards of two systems instead of one.

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April
Joined: 13 Dec 2003
Posts: 6496
Location: Westchester/NYC

1/26/19 10:35 PM

My guess? The first unit raise the cold water temperature to "warm" water so the second unit can raise it to the desired temperature?

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 4312
Location: Nashua, NH

1/27/19 9:24 AM

Yes, I'm assuming the same thing, but I don't see how that would boost efficiency, since the total water temperature rise required is still the same. It takes the same amount of heat to raise the water temp, regardless of how you do it. Using two heaters means that you have to heat additional hardware, which probably takes more heat than with a single, larger unit, reducing efficiency.

Such a system is bound to be more expensive than a single, larger unit, too.

OTOH, if you used passive solar to pre-heat the water and an insulated storage tank to hold the warmed water, then used an on-demand system to finish the heating process, you'd have a pretty efficient system. That's probably the only way it would be cost effective up here.

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KerryIrons
Joined: 12 Jan 2004
Posts: 2974
Location: Midland, MI

1/28/19 11:14 AM

Efficiency

I never said it would be more efficient. Thermodynamically, it would be no different than a single unit that had the wattage to do the same amount of heating. I've never costed it out but when somebody says that an electric tankless unit is not a good choice when source water is too cold, you could install two (CHEAPER) smaller units to get the job done and MAYBE that would be cost effective.

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Sparky
Joined: 08 Dec 2003
Posts: 17132
Location: Portland, OR

1/28/19 12:10 PM

Other than amortizing two pieces of hardware instead of one. Watts is watts pretty much.

The other possible issue is running two 40 AMP feeds just for water [assumption] instead of one if like us you are still running 60 AMP Main. But having said that, we run the Electric Stove, Clothes Dryer and WH simultaneously.

When we bought here, the guy had a 60 Amp breaker running out to the shop for his ARC welder. There was a Aux shutoff on the WH, and he probably told his family not use the Elec Dryer while he was welding. ;)

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Kramer
Joined: 12 Jan 2004
Posts: 109
Location: Richmond, VA

1/29/19 9:52 AM

Instantaneous electric just doesn't work

As a person who designs commercial plumbing systems for a living, just say no to instantaneous electric water heaters unless you have no other choice.

The watts to usable output ratio is just too high. A small hand washing sink with a 0.5 gallon per minute (gpm) aerator sure go for it. Anything higher than that go electric/gas storage type or instantaneous gas.

Standby tank loss is so small now that you aren't losing much by storing water in a tank.

A few companies even make a small thermostatic mixing valve (it is more residential so I'm a hair out of the loop) but it allows you to store water hotter at 140F and mix to more usable 120F as it leaves the tank and more or less gets you additional hot water capacity. A coworker installed one on her electric storage tank heater and she said it is amazing.


Last edited by Kramer on 1/29/19 10:11 AM; edited 1 time in total

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 4312
Location: Nashua, NH

1/29/19 10:01 AM

What I was told about electric on-demand systems by the tech I spoke with was "they work well in Florida". 'nuff said.

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