Electric tankless add on to tank water heater

Worth it to add a small electric tankless water heater in line after my electric tank type? This way I can use timers to only run the tank heaters during the day when solar is strong and have them set to be off at night and early morning. They (two) come on at least once or twice during the night and then early morning bathroom sink use turns them on again before I have good solar. This seems to be a better solution than adding more batteries. Since they seems to be power hogs, I thought I would run them off my grid AC since they only come on for a very short time. This way I will not have to constantly watch to see what is running so as to avoid an overload of my three inverters. With two inverters I had to be sure the clothes dryer was not running while the oven was on and well pump kicks in at same time water heaters decide to run. Anybody have one and do you like them?

Makes me wonder about putting the small electric tankless before the big tank? Basically preheat the water going in? It might be enough to just keep the large heater from running much, if at all.

Lt.Dan said:

Makes me wonder about putting the small electric tankless before the big tank? Basically preheat the water going in? It might be enough to just keep the large heater from running much, if at all.

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My thinking was that during the day, use all the solar I can, this preheats the water but maybe not as hot as I would like to shower in. Then when there is no solar, use the grid AC for the brief 4 to 5 minute shower and not even use my batteries up. I can't justify $3000 for two more batteries that might or might not take me through the night and early next morning. Trying to think of some strategies to get me there while spending the least amount of money......watching those Will Prose videos all last summer hit my pocket book hard......but independence is worth it.

As long as you will use the hot water.
It's a great storage medium for energy.

timselectric said:

As long as you will use the hot water.
It's a great storage medium for energy.

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Oh it will get used, just trying to not have to keep it hot all night and early morning. Would be hotter than cold well water, and minimal grid use to heat it up the rest of the way. I quickly found out if no one is home to use the appliances during the day, we waste all the energy we could have made. Never realized we used so much early morning and later in the evening. Can't justify extra batteries.

Do you have enough amps left in your service to drive a electric tankless?

If you have a smart water heater maybe you can program it to have a higher set point during the day. They can soak up more excess power and store more energy this way, though they will also have more standby losses during the day. If you have excess power anyway those losses are irrelevant…

SolarScott said:

Oh it will get used, just trying to not have to keep it hot all night and early morning. Would be hotter than cold well water, and minimal grid use to heat it up the rest of the way. I quickly found out if no one is home to use the appliances during the day, we waste all the energy we could have made. Never realized we used so much early morning and later in the evening. Can't justify extra batteries.

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What type of water heater do you currently have?
If electric and powered by your system.
Maybe (if you have the room) add a second tank And a circulation pump.

zanydroid said:

Do you have enough amps left in your service to drive a electric tankless?

If you have a smart water heater maybe you can program it to have a higher set point during the day. They can soak up more excess power and store more energy this way, though they will also have more standby losses during the day. If you have excess power anyway those losses are irrelevant…

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I probably could not use my inverters..but my grid AC in could run them. May try to use timers on the big tanks instead.

timselectric said:

What type of water heater do you currently have?
If electric and powered by your system.
Maybe (if you have the room) add a second tank And a circulation pump.

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50 gal 240 electric, two. I think I will try and flip off the breakers a little after sundown and see how warm the water is the next morning. if this works, timers may be all I need. I keep them at the highest setting and that is probably why they come on more during the night.

SolarScott said:

50 gal 240 electric, two. I think I will try and flip off the breakers a little after sundown and see how warm the water is the next morning. if this works, timers may be all I need. I keep them at the highest setting and that is probably why they come on more during the night.

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As long as the tank is well insulated. It should keep warm all night. Increasing the temperature helps, as long as you don't have kids that could scald themselves. (A mixing valve would solve this)

I don't recommend using an on-demand hot water heater unless it is at point of use near where hot water is used. The only real advantage is to avoid pushing a long run of cool water through a hot water feed before hot water shows up at use point.

The peak power demand is very high. Almost impossible to feed from a battery powered inverter. If you do have a large enough inverter to feed an on-demand hot water heater you will waste more energy in inverter idle overhead power consumption over the energy heat loss from a hot water heater tank. You can wrap an insulating blanket around hot water heater tank to reduce heat loss.

As discovered by many folks with solar hot water heating feeding the input of a conventional hot water heater, injecting hot water into cold water input of conventional hot water heater damages the enamel internal tank coating with repeated temp shock that fractures coating resulting in premature tank rusting and tank leakage failure.

I opted for an 80 gallon tank, timer comes on from 4am to 7am, and from 4pm to 8pm. Plenty of hot water, if we need more I just throw the timer switch override. I will be adding a 50 gallon preheater tank in the future to further reduce the power used, probably heat that with the woodstove during the cold months, solar when my system grows up a bit. The bigger tank seems to hold the heat very well in spite of the short heating times.

Did my test last night. Turned off the breakers to both water heaters at 7pm. This morning the water was still hot enough to brush my teeth and batteries still at 52 volts. I think timers and water heater blankets are going to save me $3000 in batteries. Shouldn't even need small point of use heater now.

Don't use water heater water to brush your teeth. There is bacteria in water heaters.
Use cold fresh water to brush your pearly whites ^^
Same with making coffee - never use the warm water heater water for this same reason the bacteria.
Use cold fresh water.

On-topic: I use a normal insulated electrical water heater tank 200 liters but on a timer which turns it off from 6pm until am in the morning. The water still stay warm in the tank until morning.

LydMekk said:

Don't use water heater water to brush your teeth. There is bacteria in water heaters.
Use cold fresh water to brush your pearly whites ^^
Same with making coffee - never use the warm water heater water for this same reason the bacteria.
Use cold fresh water.

On-topic: I use a normal insulated electrical water heater tank 200 liters but on a timer which turns it off from 6pm until am in the morning. The water still stay warm in the tank until morning.

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Unless you keep your tooth brush soaking in Hydrogen Peroxide, it has way more bacteria on it than the water in the tank...in my opinion.

I would never brush my teeth with a water heater.
It's too awkward to hold sideways.

timselectric said:

I would never brush my teeth with a water heater.
It's too awkward to hold sideways.

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Jokes don’t get any better than this

timselectric said:

I would never brush my teeth with a water heater.
It's too awkward to hold sideways.

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It works the same way for eating extremely large ears of corn. Just turn your head sideways and move your head, not the corn or water heater.....

Looks like you already came to the same conclusion i would have recommended.. just timer switches should be enough for you, and they're CHEAP!

The water heater blankets aren't a bad idea but they have to be pretty cheap to justify themselves in my opinion. Newer water heater tanks are insulated pretty well to begin with.

If you have 'excess' solar power during the day the next best investment is probably thermostatic mixing valves so you can store more energy in the tank, which also equates to 'leave it off longer for same result'.

Vigo said:

Looks like you already came to the same conclusion i would have recommended.. just timer switches should be enough for you, and they're CHEAP!

The water heater blankets aren't a bad idea but they have to be pretty cheap to justify themselves in my opinion. Newer water heater tanks are insulated pretty well to begin with.

If you have 'excess' solar power during the day the next best investment is probably thermostatic mixing valves so you can store more energy in the tank, which also equates to 'leave it off longer for same result'.

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Just when I figure out how to go all night on battery without switching to grid, now I'm having hell with the inverters. Have solar monitoring going now to try and see what is going on. It never seems to end. Thank God I like to tinker.

I really like the method of pulling every bit of excess power right from the array no matter how small and bypassing the charge controller and batteries to heat water. It is efficient, cheap and in the world of electronics relatively easy. I can't believe no one else is doing this in the PV community.

Reading along had me think of the Rheem Marathon water heater. It's basically a conventional unit built like a thermos bottle. It's expensive, but also has a lifetime warranty.

efficientPV said:

I really like the method of pulling every bit of excess power right from the array no matter how small and bypassing the charge controller and batteries to heat water. It is efficient, cheap and in the world of electronics relatively easy. I can't believe no one else is doing this in the PV community.

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Interesting. Any ideas, links or videos on how to do this safely?

RCinFLA said:

As discovered by many folks with solar hot water heating feeding the input of a conventional hot water heater, injecting hot water into cold water input of conventional hot water heater damages the enamel internal tank coating with repeated temp shock that fractures coating resulting in premature tank rusting and tank leakage failure.

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That's very odd.

For one series water heaters are very much "a thing" in buildings, even residential.
Second, and more importantly, that doesn't actually make sense unless people are doing odd things.

The 2nd water heater should still be connected to power/heat and set to not below the safe level (50c, ~120f) iirc) or you risk "fun" bacteria. In the case where the first tank is not currently heating worst case is however cold the water from the utility is, but that's no different than effectively normal operation. Since this level of cold water must already be designed for that should never be an issue. If the incoming hot water is within say normal-ish operation (60-65c, 140-149f) even in the case that you completely empty the capacity of the first tank AND it has heated to maximum you're not looking at too much of a difference in temperature delta and some people run their tanks that hot anyways so again, no real difference.

The only scenario I see causing more thermal shock is running the first tank extremely hot which risks damage to both tanks. Per literature from the people who make, install, and service tanks it's less the thermal shock at high temperatures but more that it accelerates hard water deposits which cause damage. Not sure the mechanism behind it or if there are perhaps wrong.

Regardless, if you installed a thermostatic valve between tanks you could limit the incoming temperature, and resulting thermal shock, at least in the second tank.

How risky (bacteria wise) is it for the first tank to possibly sit below 50c for hours when hot water is used overnight? No idea. I *feel* like having the 2nd tank *always* be hot enough mitigates that but I'm no biologist.

The question that comes to mind is are we seeing "touched it last syndrome" (often seen in IT work, you fixed the malware the user opened so clearly the broken keyboard is your fault). Have people been having issues with new tanks, or with a tank that they converted for re-use that may already have had damage, or maybe the anode rod was never replaced.

Yes, calcium and lime deposits increase greatly with hotter temperatures. I have to empty and vacuum the tank every few years as the sediment accumulates above the lower element, up to about 24" in the tank. I take out a few full buckets of lime pellets each time. I am considering adding unions to the tank fittings So I can remove the tank and turn it horizontal to powerwash the inside to clear all of the sediment out. Keeping it cleaned out has enabled me to run this heater since 2004, I have replaced the lower element about 5-6 times (I swap in the upper element and put the new one in the upper position). Both elements are about 3800 watts or what I can find near that - (I removed the 5000 watt elements, they are really unneeded.)