Author Topic: Hot Water elements corroding  (Read 5767 times)

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adobejoe

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Hot Water elements corroding
« on: January 04, 2015, 07:50:42 PM »
I have excess electric generation from our 10 KW wind turbine and 3.3 KW solar PV. I use the electricity in my radiant floor heating system. The system has a propane hot water heater (Takagi) and I also installed a conventional domestic electric two element water heater in series. The system uses a fluid (Cry-otek) which is a type of ethylene glycol and water as heat transfer fluid. Anyway, the upper and lower heating elements in the tank seem to corrode through fast, like every year or so. To replace them is a pain because I must drain then refill the full 40 gallon tank. Any ideas for a better heating element, standard 220  V AC units? Ideas?

dnix71

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Re: Hot Water elements corroding
« Reply #1 on: January 04, 2015, 09:50:40 PM »
Water tanks normally have a sacrifical anode. What kind of SA are you using, magnesium/aluminum/zinc? Is the plumbing iron, PEX or copper?

The only 2 things that would make an electric element burn out prematurely are overvoltage and lack of flow. If you don't move enough water the surface of the element will spall as the water/glycol mix boils. If the system is pressurized, that raises the boiling point. Moving fluid fast enough prevents boiling also.

Also, what keeps the system topped off with water? Is there an air purge?

adobejoe

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Re: Hot Water elements corroding
« Reply #2 on: January 04, 2015, 10:34:40 PM »
The tank is a standard GE 40 gallon with 4500 watt heater elements. Not sure what or where the SA is located. Plumbing is PEX except for copper, maybe 30 ft of 3/4". Voltage is off a standard 40 amp double pole breaker panel. Water flow varies 1.5-2.5 gpm. I try to set the thermostat to heat to about 130 DEG F. System is pressurized to about 20 psi and there is air purge and small expansion bladder. Water makeup provided. Our well water is slightly hard.

OperaHouse

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Re: Hot Water elements corroding
« Reply #3 on: January 04, 2015, 11:53:49 PM »
Are the elements burning out or corroding.    One thing I would be worried about is the heat density.  Most hot water heaters develop a lot of sediment on the bottom of the tank from scale falling off the heating elements.  Water literally boils off the heating element leaving a deposit.   Through thermal cycling that scale falls off and collects on the bottom. I mention this only as an example of how these heating elements are made at the extreme end of what can be tolerated.  Given that your solution is not water the situation may be much worse because ol a lower heat transfer,    I don't know the actual chemistry.  It may be that the solution is breaking down chemically into something corrosive.  My gut feeling is I wouldn't want to put any more than half the rated power into these elements.    In a normal water heater only one element is on at a time.  Have you changed that arrangement?  Do you have the ability to circulate  the liquid in the tank when heating even if it is not going externally.

Frank S

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Re: Hot Water elements corroding
« Reply #4 on: January 05, 2015, 01:47:25 AM »
A big killer in radiant floor systems is not enough constant flow while the boiler elements are on.
 Like was mentioned spalling of the elements is common and in particularly hard water this is multiplied. filtration of replacement water is very important.
 you haven't mentioned the size of your re-circulation pump or how many total feet of tubing you have embedded in your floor or the square footage you are warming.
 A friend of mine has a radiant system throughout his house with 3 different types of boilers #1 the main boiler is diesel fired boiler # 2 is built into his fireplace system which is a Russian chimney design boiler # 3 is a 100 gallon  electric
 there are 7 different zones  each are valved and allow constant bypass flow when a zone is tuned to a lower setting or turned off. each boiler has its own circulation pump as well





 His system was first switched on in 1998  except for the new digital switching controls the system is still original
 He attributed the key factor to having used triple distilled water  when installed and if any replacement water added it to was distilled   
I live so far outside of the box, when I die they will stretch my carcass over the coffin

OperaHouse

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Re: Hot Water elements corroding
« Reply #5 on: January 05, 2015, 07:51:59 AM »
I worked with an engineer for a short time that had a solar house.  He had abandoned the system because it had developed pin hole leaks that caused an air bubble to form on the roof and pumping would stop.  He said it was because the liquid ate the solder.  This seemed odd because car radiators were soldered together. At the time I wasn't that interested, but he had access to people at one of the nations top ten R&D centers for advice.    Stuff happens that isn't always obvious.   He also had foamed walls for insulation that disintegrated.  The thing I did learn from his experience is the risk you take when you are an early adaptor.

A picture of the heating element would be interesting.

My understanding of a closed system is that once the corroding elements are used up (like oxygen for rust)  The system fluid doesn't need to be changed.
« Last Edit: January 05, 2015, 07:57:11 AM by OperaHouse »

adobejoe

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Re: Hot Water elements corroding
« Reply #6 on: January 05, 2015, 10:59:58 AM »
Thanks for the input. A couple of takeaways. One, I am not using makeup water as distilled water, so maybe that is part of the problem. I sometime keep elements "on" even when not circulating fluid, so I can change that and and only flip breaker on when I am definitely circulating.  These are 240 V elements, so maybe I could just feed with 110 V on each and see what that accomplishes?
The corrosion breaks them right in half and a piece drops to the bottom of the tank--there is a lot of sediment and scale in the bottom.

gww

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Re: Hot Water elements corroding
« Reply #7 on: January 05, 2015, 12:49:51 PM »
Isn't ethlene glycol toxic and don't most propolene glycols have an anti corrosion additive?
gww

joestue

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Re: Hot Water elements corroding
« Reply #8 on: January 05, 2015, 01:32:27 PM »
I worked with an engineer for a short time that had a solar house.  He had abandoned the system because it had developed pin hole leaks that caused an air bubble to form on the roof and pumping would stop.  He said it was because the liquid ate the solder.

I believe it.
i've disassembled a number of used brass car radiators.. and it is difficult to find any solder in them. all of it is corroded into brown gunk.

bart

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Re: Hot Water elements corroding
« Reply #9 on: January 05, 2015, 03:57:19 PM »
Isn't ethlene glycol toxic and don't most propolene glycols have an anti corrosion additive?
gww

I'm fairly sure on my Whirlpool Hot water tank that each element is fed one leg of the 240 v in. That would make each element 110v. And it only uses one element at a time. Also have hard water and have had no problems with burning out elements...yet. Been in use about 5 years.

bart

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Re: Hot Water elements corroding
« Reply #10 on: January 05, 2015, 04:07:48 PM »
Wrong dang quote.

kitestrings

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Re: Hot Water elements corroding
« Reply #11 on: February 02, 2015, 11:39:27 AM »
Quote
The tank is a standard GE 40 gallon with 4500 watt heater elements.

Quote
I'm fairly sure on my Whirlpool Hot water tank that each element is fed one leg of the 240 v in. That would make each element 110v.

If the tank(s) is unmodified, the conventional arrangement is that the two elements are interlocked, meaning a the maximum draw is 4,500 watts.  The normal arrangement is that 240V is supplied to a high-limit cut-out, then to the upper thermostat.  If the upper element is calling for heat, only the upper element is on.  Once satisfied, the upper thermostat opens and simultaneously closes (SPDT) a terminal to the lower element.  The lower element stays on until its thermostat is satisfied, or the upper element calls for heat again.  This information doesn't solve the corrosion/burnout issue, but may clear up some misunderstanding.

~ks

bart

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Re: Hot Water elements corroding
« Reply #12 on: February 02, 2015, 06:03:56 PM »
   Just checked the wiring diagram, in plain sight on top of the tank, and you are correct.
Thank you.