Electric tankless add on to tank water heater

[efficientPV]

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.

 

[Bob in Tucson]

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.

 

[Crashcourse]

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.

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

 

[Mithril]

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.

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.

 

[Hogheavenfarm]

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.)

 

[RCinFLA]

There are a couple of adaptors available to solve the problem.

There are 'J' ports that can be installed in place of water heater drain spigot that directs incoming water upward for solar hot water to be injected.

Another method, my preferred solution, is to replace the inlet port pipe that extends down to bottom of tank with a blocked off end with drilled holes on the sides of inlet pipe near the end of inlet pipe to allow incoming water to shoot out sideways, so it is not directed at bottom of tank.

This is a real problem as I learned on my installation. Commercial hot water heater lasted about two and half years before it rusted out and began leaking.

The end of attached video gives a pretty good explanation on how the dual electric heating elements work to yield the most available hot water from a limited tank capacity.

 

[Hogheavenfarm]

Mine was a stupid design, the drain valve is plastic and appears molded to the tank, and has a 45 degree angle in it , along with a plastic "screen" filter, so opening up the drain valve does nothing as the tank is full of sediment. I have to drain the tank by unscrewing the lower element. I wanted to replace the drain valve with a straight pipe with a "non gate" valve (no idea what they are called) which would allow pressure flushing of the tank, but was unable to because of the design. It is also too tall to replace the anode rod as there is only 12-18" clear above the tank. (I know they make folding ones now, my daughter showed me this weekend). I dont mind rolling it out of the garage every few years to power wash it out though.

 

[RCinFLA]

Most of the new hot water heater have a non-intuitive plastic drain valve.

The 'knob', which looks like a normal spigot valve, is only to loosen the clamping on the center plastic screw-in plug that is actually the spigot valve. You loosen up the knob a couple of turns until you can use a large blade screwdriver to open the center spigot valve plug post.

 

[efficientPV]

A neighbor gave me his 40 gallon tank after he installed a HPWH. The internal plastic dip tube which forced cold
water to the bottom had broken off so cold water mixed with hot at the top of the tank. Cold water is now introduced
at the drain port. This is a high lime location, honestly I filled a 5 gallon bucket with sediment. He also gave
several used heating elements coated with lime. These quickly clean up after sitting in vinegar. That sediment with
calcium and magnesium is great for the garden.

I keep my tank on a 15 inch high insulated pedestal. Typicially the bottom of tanks are poorly insilated with three
formed feet which touch the outer wall. The extra height is essential for draining the tank at the end of the season
and I do need to tip the tank. Removing the valve or pioe makes a bigger opening to clean. Washing machine hoses are
used to connect my tanks which make them easy to move. It is a low pressure system and the six gallon tank has to turned
upside down to drain.

Many heating elements are high density (because they are cheaper to make) meaning they get too hot and deposits easily
form on them. A good building products store will offer low density which are about 50% longer. If you can hear your
water heater when it runs it has high density elements. That popping is bubbles forming on the surface and that also
causes the scale to fall off filling the bottom of the tank. Low density elements will not build up as much scale. I operate
the elements at lower voltage which achieves the same result. Lower wattage elements can still be high density, just shorter.

Solar applications see wide changes in tank temperature. System should have an expansion tank to accommodate expansion. My
garage laundry system has a 3/4 inch stack pipe to accommodate some expansion. This is topped with a pressure gauge because
I couldn't find anything else handy to cap the pipe. The detached garage is feed water with a garden hose. In my locality
if I want water at the garage legally, it would have to have a separate billed water meter. Consequently, That hose is
turned off to avoid it splitting when I am not there. I can see the pressure climb an extra 50# in this closed system as
it heats up. Expansion can add an extra quart.

Legionella....... I'm on municipal water so I don't worry about it. My tanks cycle thru wide temperature ranges and have no concern.
I know othere with water feeds from their roof providing lots of neutrients. I won't drink their water with stuff floating
in it, won't even shower. They seem to be fine with it. Most important is how long water remains stagnant in a tank.
This video is the most logical explanation I have seen.

 

[Shimmy]

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.

I have solar hot water with 2x120-gallon tanks. We had miserable water hammer when we moved in, and eventually figured out that it was the swing-gate style check valve that was causing problems. The solution was to replace it with the horizontal pilot-hole style check valve and expansion tanks and it works like a charm now. Specifically for the solar hot water, our flow between tanks is bi-directional; solar hot water flows into the second tank and make-up is pulled from the first tank.

For Legionella bacteria, they are always in domestic water at some level. The biggest problem areas are dead legs on water pipes where water can sit for extended periods of time at around room temperature. It takes a pretty high temperature to reliably kill the bacteria quickly; I think around 60C needs 8 hours. So, if you have an environment where they can thrive and then they pass through a high temperature system that can kill 90% in 2 minutes you are still left with a high bacteria load. Best recommendation is to keep everything on the hot side above 122F/50C and everything on the cold side below 77F/25C. When we have been away for a week or more I always make sure I run the showers and sinks for several minutes as a precaution.

 

[Mithril]

I have solar hot water with 2x120-gallon tanks. We had miserable water hammer when we moved in, and eventually figured out that it was the swing-gate style check valve that was causing problems. The solution was to replace it with the horizontal pilot-hole style check valve and expansion tanks and it works like a charm now. Specifically for the solar hot water, our flow between tanks is bi-directional; solar hot water flows into the second tank and make-up is pulled from the first tank.

For Legionella bacteria, they are always in domestic water at some level. The biggest problem areas are dead legs on water pipes where water can sit for extended periods of time at around room temperature. It takes a pretty high temperature to reliably kill the bacteria quickly; I think around 60C needs 8 hours. So, if you have an environment where they can thrive and then they pass through a high temperature system that can kill 90% in 2 minutes you are still left with a high bacteria load. Best recommendation is to keep everything on the hot side above 122F/50C and everything on the cold side below 77F/25C. When we have been away for a week or more I always make sure I run the showers and sinks for several minutes as a precaution.

Thanks for the first hand experience!

Personally if I was going to do a "solar thermal battery" I think figuring out proper safe design would be key. Based on some testing I've seen modern tanks can *hold* heat overnight (or over-day) with little issue, but that was more centered around "time of use". I do feel like this claimed issue of tank failures is something else, likely either already used tanks (and perhaps moving them which subjects them to shock) and/or running the "solar" tank really hot (80-90C) at max. Not sure if anyone makes an "in tank" (vs at entry) UV light, but if so it wouldn't need to run at as high of power I think.

Just spitballing here. Series system, detect if the first tank is going below 50c and use a recirculating pump? Not sure if that would work with a thermostatic valve between them (top of the tank can be hotter so exit water could be able the valves limit). Do any hot water tanks have dual thermostats (not top bottom, but dual feed)? I wonder if any of the heatpump tanks can be set with time of day temperature limits or use wifi control based on SOC. Beyond that we're getting to maybe using a heat exchanger? :-/

 

[Shimmy]

Not sure if anyone makes an "in tank" (vs at entry) UV light, but if so it wouldn't need to run at as high of power I think.

Just spitballing here. Series system, detect if the first tank is going below 50c and use a recirculating pump? Not sure if that would work with a thermostatic valve between them (top of the tank can be hotter so exit water could be able the valves limit). Do any hot water tanks have dual thermostats (not top bottom, but dual feed)? I wonder if any of the heatpump tanks can be set with time of day temperature limits or use wifi control based on SOC. Beyond that we're getting to maybe using a heat exchanger? :-/

You are over thinking it. The risk is so small as long as you have water flowing through your system; a little bit of chlorine is plenty to manage the residual risk. I did some work for a hospital that was having problems because not all the rooms were used every day, and they had a domestic water storage tank that didn't get enough flow through it (it was too big for their normal water needs). That became a very complicated resolution... but for a house it is a non-issue.

If you over-circulate the water you just end up cooling it off... which actually increases your risk.

 

[Mithril]

You are over thinking it. The risk is so small as long as you have water flowing through your system; a little bit of chlorine is plenty to manage the residual risk. I did some work for a hospital that was having problems because not all the rooms were used every day, and they had a domestic water storage tank that didn't get enough flow through it (it was too big for their normal water needs). That became a very complicated resolution... but for a house it is a non-issue.

If you over-circulate the water you just end up cooling it off... which actually increases your risk.

I'm thinking about how I see some people talk about their setup, and they might be over simplifying compared to what they do.

Hypothetical system:
2 tanks in series; tank A is the 1st tank, gets cold water from the city or a well with chlorine system - tank B is the 2nd tank gets water from tank A, possibly with a hydrostatic valve to limit inlet temp.
Tank A is only connected to solar as a diversion load ONLY, and is set to a higher than normal limit temp. Due to being solar only it will never heat over night, and will also not heat on days with low solar input.
Tank B is connected to the "normal" grid power, possibly also to solar. It is connected in such a way it always has power, and the thermostat is a lower but safe temperature (say 50C-60C).

I guess the question is how long at 25C-50C is relatively safe with chlorinated water? Per some stuff for pools I'm seeing "days".

 

[Nobodybusiness]

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.

Do they have such a Thing where during the day the Thermostat could be set to say 190 when you are overproducing electric and then when the sun went down the thermostat would drop to like 110 where we normally keep it?

I guess you might be able to do something like that on a WiFi connected WH but what about a standard 50 gallon AO smith one with 4500 watt elements in it?

 

[timselectric]

Do they have such a Thing where during the day the Thermostat could be set to say 190 when you are overproducing electric and then when the sun went down the thermostat would drop to like 110 where we normally keep it?

I guess you might be able to do something like that on a WiFi connected WH but what about a standard 50 gallon AO smith one with 4500 watt elements in it?

Nothing "off the shelf" that I am aware of, but maybe.
It wouldn't take too much to diy a controller for that. If you are comfortable with that sort of thing.

 

[Nobodybusiness]

Nothing "off the shelf" that I am aware of, but maybe.
It wouldn't take too much to diy a controller for that. If you are comfortable with that sort of thing.

Yea I have never tried it. I though maybe someone developed something like that already.

I mean I could just bypass Tank thermostat and put power to the elements directly but with no way to read thermostat that seems dangerous.

Have to think about this a minute.

 

[timselectric]

Yea I have never tried it. I though maybe someone developed something like that already.

I mean I could just bypass Tank thermostat and put power to the elements directly but with no way to read thermostat that seems dangerous.

Have to think about this a minute.

Two thermostats and time clock, is all that it would take.
A standard 40 gal tank already has two thermostats. Set the lower one to 190 and the upper one to 110. Then wire the time clock in place of the diverter. Or just have the time clock turn off the lower element.

 

[Shimmy]

Tank A is only connected to solar as a diversion load ONLY, and is set to a higher than normal limit temp. Due to being solar only it will never heat over night, and will also not heat on days with low solar input.

The problem is that the water heaters have a high temperature safety/cutoff switch (as well as the over temp/pressure valve). You really don't want your water heater getting anywhere near boiling. My thermal solar tanks are specifically designed for a maximum temperature of 165F; I think normal ones are closer to 140F.

Before I understood how my system worked I thought it could be better. The only improvement I could actually make though is a tankless instant system downstream of the mixing valve. That would only come into play when the tank temperature drops below about 104F.

Slightly ironic to me... I had to turn on my electric water heater today. Only happens when the PV generation is under 20kWh-- normal day for this time of year is 45-50kWh.

 

[Oldtimmer69]

I had to build my system to cover the 4.5hr window i have from November to February and im WAY to big for this time of year. On nice clear days I'm full by noon. I need to figure out what to do with all of it.. Add a second hot water heater and only run them during the day? Maybe add a diverter to feed a mini split also?

If i set up a passive water heater, or go with any DC loads im really making it worse not better. Unless i can use the dc instead of sending it to the inverters.

I've tried to turn off the batteries when full but that shuts everything down. It would be nice if i could just run everything off PV when batteries are full.

I've read so much im now confusing myself.. Lol. Everyone has so many different systems and ways to manage energy.