White scum inside water jacket?

Warno, I wonder if its mineral deposits, it could be the anode though as well

Take a sample to a lab, usually when's anode breaks down there are deposits under the rod, on the garn the white film is mineral deposits

hard water deposits

As you heat water, the minerals in in tend to precipitate out as a factor of lbs/btu heated, and stick to the wall of the heat source. This is why tankless coils have a tendency to plug up and tank heaters have a procedure for cleaning.

On a closed loop system, the initial mineral content load in the water is relatively fixed compared to a system that runs on constant fresh makeup water. So it's not a problem that runs to infinity. It will run to its constant steady state as long as the system is run closed loop.

As far as how much of a problem or what to do about it, it's beyond my expertise. Doing nothing may be one of the options, after the problem has been more researched and explored. Mineral scaling by itself may possibly be benign as long as it does not plug up any passages, but corrosion would be potentially catastrophic, and is its own specialty.

I would expect this is either the nitrate treatment and/or some oil that was on the steel you used.
It is unlikely that there is any mineral content of any consequence in the system.
I believe that scum is usually due to poor cleaning/ oil residue.

You might want to check the PH if you haven't already done so. By trying to balance the nitrite level you may be raising the PH beyond reasonable levels. Do you know the hardness of your water? A light coating of calcium should not pose a problem as long as you are not continuously adding or changing the water.

You need to get a kit that tests higher ph levels. My heatmaster requires 8.5 to 10.5.

Yes open system and the nitrate level is suppose to be no lower than 730 ppm and the conductivity is suppose to be 100-4000.

Warno, Have you made any progress on the PH levels? I don't know much about required levels and it appears that Honda is much more knowledgeable, but 10.5 seems awfully high to me. Also, knowing little about what chemicals are available for boiler maintenance, my first thought would be to use citric acid which is available in spice sections at the grocery store, healthfood store or home canning sections. Also wanted to get this thread back to the top.

warno - check your y-strainer too, if you haven't, good chance it's coating over. Mine did.

The strainer might have kept the goo out of the HX, and is much easier to clean -

It's a good idea to test the water that you fill or refill with. High mineral content will coat the HX surfaces and lower the efficiency, depending on the thickness of the layer.

You might buy, or rent a DI unit to fill the system through after a good cleaning with a hydronic clean and flush product.

Sample the water and send it to a lab or company that specializes in treatment for boilers. You want to tailor the treatment or chemical use to your water condition.

Fluid quality is becoming a yuge issue in the hydronics industry. Manufacturers are tightening up warranty replacements when they see failures related to fluid quality, boilers, pumps, control devices, heat exchangers, etc.

An open system will require an on-going treatment program to assure a long and efficient lifecycle.

Here is some intro information regarding water quality.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_18_na.pdf

Warno, to your last question I would say no - I believe rust will start to form [soon too] on all those surfaces unpainted inside, leaving you a bigger mess than the white scum. Not sure what the scum is, i'm planning to run mine another month before doing what you are now. I'd bet if you got a good sample of it, Precision Chem could tell though -