black scaling when spa heater in operation

[mmeyer]

I could really use some help understanding the problem I'm having. I have been unable to solve this puzzle. I have a 18,000 gallon in ground pool and a connected 1000 gallon spa that was built in 2010. I use a SWG system to chlorinate. I live in Seattle and temperatures are around 40F in the winter and it takes the spa about an hour to come up to temp. I've been using it once to twice a week. The heater is only used to heat the spa, I use a geothermal heating system to heat the pool in the summer (the same system heats the house in the winter).

After about 10 years, I noticed black scales in the spa after heating the water. I assumed my heater element was corroding and replaced the spa heater with the same RayPak 406A 399 BTU cupro nickel propane heater last February. This did indeed solve the scaling problem for about 10 months and now it is back.

I noticed that my calcium hardness level was at 600. I had read that calcium oxalate can be reactive with copper while heated and cause black scaling. I drained roughly half the pool water and replaced it. Calcium hardness is now 280. I also flushed and descaled the heater following a video tutorial I found here on TFP (

). However, this also failed to solve or even improve the scaling problem.

My questions are what is the cause of scaling and is there anything I can do to solve this?

My most recent testing levels using Taylor K 2005 test kit
Ca Hardness 280
Total Chlorine 4
Free Chlorine 4
pH 7.8
Total Alkalinity 100
Cyanuric Acid 40
Salt 3600

I want to thank you for taking the time to read this and help me solve this riddle!

 

[mknauss]

Can you collect any of the black particles?

 

[newdude]

Is there anything after the heater that may have trapped old flakes which are finally working their way through ? Valves, actuators, aerators , etc ?

It could be a chunk of the old exchanger that’s now breaking apart. With any luck the problem fixes itself.

 

[mmeyer]

Marty:
I can definitely capture the flakes. I even brought them into my local pool service but they didn't have a clue.
I was wondering if there was a chemistry lab that could figure this out, but I didn't know where to start.

Newdude:
I was hoping the for the same thing. I just turned on the spa jets (which I never use) and a bunch more came out but didn't solve the flaking problem. Also, the new heater did completely resolve the problem only to recur about 10 months later. I suspect, I have some bad chemistry afflicting the heater element.

It's interesting that the new Raypak 406A heater came with a galvanizing element to prevent internal corrosion. I cleaned it with muriatic acid to remove the scaling. The scaling was white. I know the heater element gets coated in calcium oxalate. If I understand this correctly, the heat plus copper creates the black flakes.

 

[mknauss]

I assume the material is hard. Does it react in acid.
Your water chemistry as reported is not in scaling tendency. I have higher CH and TA with a heater and no scale or particle. Might your water have minerals of some kind?

 

[mmeyer]

The material is more soft than hard. It sinks to bottom of spa but is stirred up easily. It doesn't leave any stain.
I'm not sure about minerals in the water but is certainly possible. I use Seattle City water which is pretty good but I have added pool salt in the past.
I was also wondering how and why my CH went up over time. Is it just pool plaster?

 

[mknauss]

CH goes up with high CH fill water used for evaporation losses, or the use of calcium hypo for chlorinating.

Do you have a SWCG? EDIT - I see you do - I removed the odd color on your signature so it can be read easier.

If the material is soft, it is not mineral I suspect. Likely biofilm in your piping. You might investigate doing an Ahhsome purge of the spa piping. If the material only ends up in the spa. It will require you to isolate the spa and dump that water after doing the purge.

 

[mmeyer]

That's a great suggestion and easy enough to do. I'll get started on that and hope for the best!

 

[Darin]

If you believe that it is not organic then could it be manganese? Where i am from my private well has very hard water that had a lot of manganese in it. As iron turns your toilet tank red, manganese would turn your toilet tank black.
Are you on a private well or public water source? Just a possibility.

 

[mmeyer]

It's Seattle city water. Pretty much snow melt. Not much in the way of minerals or even calcium.

 

[mmeyer]

I want to provide a little follow up. I did perform an Ahhsome purge in the hope the scaling was biofilm. However, I was mistaken in that the material is more like sand in consistency.
In the photo, you can see what it looks like. It comes straight from jets (i.e. spa heater). If I understand this correctly, calcium oxalate turns black or dark when heated. Calcium oxalate builds up normally inside the heat exchanger of the spa heater and is apparently a normal even desirable condition. Problems arise when there is too much which could damage the heat exchanger.
Although the calcium hardness level in my pool is normal (my CSI level is -0.50) and I've tried everything I can think to reduce the scaling, I'm not sure if I'm doing harm to the spa heater or if this will eventually disappear with time.
I would appreciate any thoughts. Thank you!

 

[mknauss]

That is over my chemistry knowledge --- calling @JoyfulNoise

 

[JoyfulNoise]

It can’t be calcium oxalate unless you are adding a source of the oxalate anion ( C₂O₄²⁻ ) to your water, for example, using oxalic acid to clean with. Your fill water which comes from your municipal supplier better not have any oxalate in it as that would be highly toxic to the human renal system (and animals as well).

Why do you keep mentioning calcium oxalate?

 

[mmeyer]

JoyfulNoise: thank you for taking the time to explain that!!!
I must have seen it mentioned as a way of creating black scale. My calcium hardness levels were at 600 for a while and thought it might be the reason for the black scaling.
I am not adding any oxalate or oxalic acid.
Any ideas on the scale or where I could get it analyzed?

 

[ajw22]

Have you gathered up some of the black stuff and put some muriatic acid on it to see if it bubbles? If it bubbles it is calcium, if not it is something else.

 

[mmeyer]

That is a great idea. I'll do it tonight!

 

[mmeyer]

Allen:
That was very helpful. The black scales bubbled away with muriatic acid in seconds.
I guess I simply have a scaling problem. The calcium likely formed calcium carbonate which when heated changes color. Ultimately, this makes the heat exchanger less efficient and perhaps will damage it. Mostly, I'm tired of looking at the scale in the hot tub.
I'll go back and try to do a better job descaling my heat exchanger unless someone has a better idea.

 

[ajw22]

Using the chemistry posted in post #1…

With 50 degree water your CSI is -0.21

Raise the water to 100F and CSI becomes +0.23

In the heat exchanger if the water is 160 then CSI is +0.67 and into scaling territory.

Lower TA to 60 and CSI becomes +0.41

Then lower pH to 7.6 and CSI becomes +0.21 and I think your scaling will go away.

Lower your TA and pH is the answer.

 

[ajw22]

What pump do you have?

Is it a VS pump where you can increase the RPMs?

 

[JoyfulNoise]

Calcium carbonate scale is rarely white. It can be many colors depending on what impurities it picks up. Certain forms of copper oxide can appear dark brown or black and so the scale may have picked up some metallic impurities.

You should be very careful about descaling the heat exchanger. Unless you verify that there is calcium build up by pulling off the manifold and inspecting the copper tubes, then there’s no reason to descale. All heat exchangers will build up a thin layer of scale on the internal surface that is protective by nature. Stripping that scale down to the bare copper metal will only do more harm than good.

I would suggest you simply flush the heat exchanger with clean water to remove any loose particulates and then make sure the manifold is clean. Then put it back together and leave it alone. Your water shouldn’t be scaling out calcium at the levels you have posted but if you want to reduce CH, then drain the spa and refill it with fresh water. If you do that a couple of times then the entire pool CH will start to come down.