Water quality

Is it plain city or well water? That is a high TDS number, did you check the meter calibration? Some meters come with three calibration fluids to adjust the meter.

Look at the boiler manual and see what they recommend for fluid criteria.

Now if the system had a cleaner, or other treatments in it, that could skew your readings, it takes a good high temperature flush to get the old additives and cleaners out.

You can have low hardness tested with a softness test kit, but high TDS as some of the sodium from back wash will be in the water if it was softened.

Absolutely pure water, which doesn't exist in nature would be H2O.
You may recognize some of the elements in the bubble below.

But as water passes through the earth, pipes and treatment plants it absorbs ions. In typical water the ions and minerals are usually what the water passes through before it gets to your faucets. Water is often called the universal solvent. Most water comes from a hole in the earth, so the minerals it touches are absorbed in the H2O.

So one opinion for boiler fill water is to strip all those ions out and create pure water. DI or RO are common ways to remove ALL the ions, softening only removes the scaling ions and replaces it with the ions in the brine, hence the term ion exchange.

So when they blend hydronic chemicals they start with a pure water base and add chemicals the provide protection. These chemical additives are film providers, O2 scavengers, ph buffers, etc. It could be a dozen ingredients or more. So your pure water low or no TDS now has good chemicals added, and it will raise the TDS, but with good stuff instead of scaling or aggressive ions like chlorides etc. And the ph will also change from pure O2, which tends to be a low ph maybe 6.8 or so.

If you do add pure, low ph water it will buffer up then ph by absorbing some of the metals it touches in the system, within a week it should be ph in the mid to high 7's

So once a treatment is added to a system it needs to be tested, with the tester that the chemical providers supplies or suggests. Or send a sample back to the chemical manufacturer for their analysis.

Remember also, as the system depletes the chemical package in the inhibitor chemical, like non barrier tube causes ) it may need to be boosted from time to time. You need a test kit from Sentinel, or one they suggest the supplier should make you aware of that.

So to make a long story long, two opinions in the industry, use pure, low TDS water alone. Or use adequate water and add the chemical treatments to handle any bad effects from water less that pure H2O.

Personally I feel RO or DI water and a small amount of conditioner. I'm concerned mostly with the film providers that will coat the copper, brass, steel, aluminum, stainless, composites and what ever other materials are in the system, so they behave with one another and the fluid within the system.

The film providers are like anodizing or applying a very thin, micron thin rust so to speak to protect all the metals form fluid attack..