Old floor heat system

Is this floor going to be anywhere near 10°F?

50/50 sounds strong, are you reading the instructions clearly?

Remember the more glycol the less heat transfer.

Rarely do folks use distilled water for closed systems. Once the water is in there, you don't change it very often. I was told that the oxygen that is dissolved in that water, and any harmful minerals in that water will do their damage within the first few days. (which is minimal). Then the potency of those corrosives will be spent. Basically leaving the closed system water inert.

Every time you need to open the system for a pump replacement of other repair, you have a real involved process to put distilled water and antifreeze back in it. Are there any special problems with your particular city water?

I'd run a cleaner first, add it to what you have. Flush with clean water. Use pre-mix glycol 30%.

Or if you mix glycol on site it should be DI water as the manufacturers recommend.

If you really need antifreeze protection?

Some RV antifreeze is methanol based, some is PG propylene glycol. Unless you have an empty container it is hard to know.

The stuff sold as "RV Antifreeze" is not propylene glycol, which is what you want to use. The most cost-effective form I've found is Cryo-Tek AG, which the last time I bought it was about $200 for a five gallon bucket. Which may change your calculus, RV antifreeze was about $3.50 a gallon last time I bought that.

Getting the old fluid out means pumping out or blowing out with compressed air what's in there and capturing it. RV antifreeze can be poured down a drain, but you want to measure how much is in there. Because the propylene glycol is so expensive you really want to know the capacity of the system so you can order the proper amount.

Once you've got the old fluid out, you have to put the new fluid in. This will be mixed to a certain ratio with water. You need to make sure the system is empty before you start, otherwise it will be too dilute. You'll need a utility pump to pump the mixture in and pressurize the system. Purging air can be difficult depending on how the system is set up.

not all glycol is created equal. The more expensive products are lower temperature and have better inhibitors. Usually you can blend them down to a 35% mix and have adequate burst protection.

If you have stainless or aluminum in the system you need to use AL or multimetal glycol.

Food glycol, technical glycol all starts from the sane base stock, either petroleum based or bio based from corn , canola, etc.

Then additives are added based on the final use, whether it is hydronic, food additive , aircraft deicing, etc. So you want to use the correct product and blend

We had a glycol supplier clear up a lot of questions a few months ago. We have also had Dow, Rhomar and Nobel do glycol webinars for us in the past

Overnight WILL cause issues. You still don't need 0°F Freeze protection. 30/70 Will give you 10°F Freeze protection with -20°F Burst.

There will be several other frozen and broken pipes long before that floor gets near -20°F.

I would go with DI water for this project. But i yield to @hot_rod as the expert on this part of you system design. I only worked in one small are of New Jersey. Bob has experience from around the world.

Awful lot of poor information out there regarding glycol, and nobody seems to have mentioned that CryoTek -100 is only 55% concentration straight. You can mix that ~1.5-2:1 with DI water (NOT 50/50), or just buy the 45% CryoTek in the light blue bucket from Menards that's already premixed with the proper inhibitors and pump it in straight. No need to reinvent the wheel, but you absolutely do not want anything below 35% concentration for an application like this. 40-45% is even better.

Honestly, if you flush the system with water and then blow the lines out to the best of your ability then dump in pure CryoTek -100, you'll probably end up pretty close to 45% anyway. DI is ideal, but you'll never get the system dry.