Sun Power SPR-E19-320 to power element directly series/parallels issue

jimmyaz

BB. said:

http://www.instructables.com/id/The-inner-workings-of-Counterfeit-FOTEK-SSRs/

Another article.

-Bill

I am aware of this.  But I believe they just replaced it with a lower rating part, meaning if it's advertise as a 25A, it is only good for 10amps.  But 10amps is probably all I am going to need.

animatt said:

If I do my calculations correctly that would be like 17000 btus on a good day.
I can not remember water heater size. But would assume 40 gallons.
17000 bus would Raise water approximately 51.2f degrees. Neglecting heat loss.

You may have answered it above but I can remember. 

Why not just bypass thermostat and have element always connected to panels.
And have the appropriate pressure release valvE to drain to take care of events of over heating.

You may or may not have issues with having very hot water in water heater.  Depends on existing system.

Matthew

Trust me, I want it to be as simple as possible too.... The tank is a brand new tank, which has the stock relief valves.  So that's one safety, but just for the heck of it I want a thermostat safety as well... But it seem it's a lot of troubles (or just not possible on low budget).  My worry is that if there's no thermostat to turn off when the tank get too hot... the relief valve go to work, but what if there's no more water coming in, for say.... main water is turn off?  Don't think this would ever happen, but there's a chance.

I believe with the 5kw a day, it's a 40gal and the outside of the bottom of the tank get to about 90F and the water temp is around 122-130F, not sure how accurate my temp gun is shooting at water.

mcgivor

Like the way you think, why jump out of a plane without a reserve parachute, a backup safty is always a good thing no matter how redundant.

mcgivor

Some info on contact arc suppression 

BB.

You would also want an anti-scald valve (tempering valve, mix very hot water with cold water to prevent burns).

http://inspectapedia.com/plumbing/Anti_Scald_Mixing_Valves.php

-Bill

jimmyaz

Hey guys.  Back again with question.    Someone told me this... and I'm a bit worry?

"Also, be aware that DC through a regular element will corrode the element within 4 months and the tank within 1yr"

Is this true?  I know that here in Arizona the water is very hard and has a lot of mineral that will eat through the anode rod and tank.. but DC in water corrode more???

animatt

Not true. Also another thing going for you. You are using a 240v element 4500w if I remember correctly. 
And running it at a much lower power level.

This is running element at lower stress levels.

 If scale builds up on some elements they can burn out. As the scale insulates them, and they over heat.  Running at lower power levels reduces this risk.

At a certain low enough power level the elements can even be dry fried and not burned out.

jimmyaz

I think what the person trying to tell me is that DC voltage make stuff corrode faster, in this case the element and the tank....

BB.

If you have + charged and - charged pieces of metal exposed to water--Yes, DC is worse.

HOWEVER--The polarity does matter. For example, metal gas lines are connected to a rectifier and with a few amps of current. The think you want protected is connected to a negative power source (becomes the cathode). And you connect the + to a "sacrificial anode". This helps prevent the cathode from "losing" metal:

https://en.wikipedia.org/wiki/Cathodic_protection

If there is no exposure to differential voltages--Then there is no electrolysis.

However, just having two different types of metal (like steel and copper plumbing) exposed to water, they act like a battery and cause current to flow.

-Bill

jimmyaz

BB. said:

If you have + charged and - charged pieces of metal exposed to water--Yes, DC is worse.

HOWEVER--The polarity does matter. For example, metal gas lines are connected to a rectifier and with a few amps of current. The think you want protected is connected to a negative power source (becomes the cathode). And you connect the + to a "sacrificial anode". This helps prevent the cathode from "losing" metal:

https://en.wikipedia.org/wiki/Cathodic_protection

If there is no exposure to differential voltages--Then there is no electrolysis.

However, just having two different types of metal (like steel and copper plumbing) exposed to water, they act like a battery and cause current to flow.

-Bill

Haha, as much as I want to understand that... i couldn't.  I guess what I am asking is... is my tank going to rust out within 1 year (6x times faster running on DC than on AC?)

BB.

No--The heating element (typically a Calrod type element) and the outside metal is insulated from the input terminals. So, unless there is a short circuit somewhere, the + and - terminals do not connect to anything in the heating element.

http://www.veryst.com/project/calrod-thermal-analysis

-Bill

jimmyaz

BB. said:

No--The heating element (typically a Calrod type element) and the outside metal is insulated from the input terminals. So, unless there is a short circuit somewhere, the + and - terminals do not connect to anything in the heating element.

http://www.veryst.com/project/calrod-thermal-analysis

-Bill

Hi Bill,

Thanks... I didn't know element actually have many layer... thought it was just a piece of conductor.  stupid.    So as long as the element do not rust over time, then the tank shouldn't be effect. 

Wonder how would I know though?  What sign would show if the element needed to be replace?  the power wattage would drop?

BB.

Calrods either fail shorted (to outside steel wall) or open.

Given the low wattage in your system, I would not ever worry unless you stop getting hot water at some point decade or more in the future.

For water heaters, I have the tanks/fittings leak before I ever had a calrod element fail (in my small sample of 4 water heaters over 30 years).

-Bill

jimmyaz

BB. said:

Calrods either fail shorted (to outside steel wall) or open.

Given the low wattage in your system, I would not ever worry unless you stop getting hot water at some point decade or more in the future.

For water heaters, I have the tanks/fittings leak before I ever had a calrod element fail (in my small sample of 4 water heaters over 30 years).

-Bill

I haven't research to see what type of element is in my new Rheme tank uses... But I see most of the element sold for DC use are Stainless, is there a reason?  they are better protected? 

When i bought a small 22gal tank electric tank for my other home use... the stock element 120/1600w element, the guy told me the tank been use about 2 years.  But the element is super rusted, corrode, still work, but I wouldn't use it.

I guess what I can do is check the Anode road after 6months and see what happen... If anything, it would eat the anode rod first at a alarming rate. 

BB.

I have never seen a rusted element (but I do not take them out often either).

Just a quick look through Homedepot website shows; titanium, copper, stainless steel, and other unstated materials.

I would guess that most of the DC low voltage elements you are looking at are just "re-rated" 120/240 VAC elements. I am thinking there is a very big market for DC elements. I went through looking a DC units a few years ago, and the mostly matched up with their AC counterparts (i.e., lower power @ 48 VDC and low current matched a standard high power 120/240 VAC element).

If you want things to last--Look at the sacrificial anode in the water heater. Take it out when new and use teflon tape or teflon pipe dope to make sure you can unscrew it a few years down the road. There are a few choices available depending on your local water chemistry:

http://www.waterheaterrescue.com/Longevity/water-heater-anodes.html

I have never bothered to change/upgrade any of mine (perhaps I should have)--But we have pretty soft water and don't really have big problems (the iron water pipes last 50+ years in the older buildings I have worked on).

-Bill

westbranch

You should not use Teflon or any grease that isolates the rod from the tank wall ...this is what makes it a sacrificial ANODE...  there has to be conductivity for it to be ''used'' up.......https://www.plumbingsupply.com/anoderods.html

jimmyaz

Hey, I got another comment today from the internet about my VDC water heater... this one sound more serious...

"If that hot water elament brakes it will ark on your head in the shower.  And running high voltage DC into your hot water that is meant for AC can be dangerous I'm not trying to be a smart ass or funny I install solar for a living and have 20 years experience" 

So which one is more dangerous, 120VDC @ 9amps or 240VAC @ 30amps?

BB.

Both are dangerous around water. However, if you have grounded metal pipes (standard copper or iron water pipes)--I would be hard pressed to see significant current from the shower head to your body.

If you have PEX (plastic) pipes--I guess it would be possible for a positive electrode in the water tank to pass current through the water to the shower head/assembly (I don't think people ground water fixtures with PEX fittings????). Interesting question... Then the question is how hard is your water and how much current would flow through the water column in the PEX.

Otherwise, most common jets of water break up into blobs of water vs a continuous stream of water just inches from the nozzle. So--There is little chance for a current flow in the stream.

AC power is nice because it is very easy to make a ground fault interrupter circuit that can even detect/trip on 5 mAmps of current (under the 10 mAmps that is thought to be the minimum to interrupt heart function).

There are issues with DC sustain arcs better than AC and such... But if this was a big issue, I would expect to see GFI Breakers required for electric hot water heaters--Which I have never seen.

I am not an expert on this issue--Just my personal thoughts. I would not worry--But if you do worry--I am not sure how you can do much about it other than abandon DC powering of the element and use a GFI Breaker on an AC powered water heater element.

-Bill

jimmyaz

I have thought about using a MPPT inverter to convert the power to AC... but I haven't been able to find a NON grid tie inverter?  Or at least one that can accept 60-120volts DC... Most of the one I have found are grid tie... and this is not what I want to do.
 
I may gain in MPPT, but then again, losses 10-15% because of DC to AC conversion. 

BB.

This is about as close as I have seen to something that could supply AC power:

http://mwands.com/store/12-volt-suntaqe-pwb-inverter-controller

But I don't think that it is a good fit here (solar panel output has to be >> AC load for a large part of the day. Otherwise, it will just recycle "starting" until there is sufficient DC power for the element.

If you have metal piping (copper/iron)--I would not, personally, worry.

-Bill