Percent antifreeze for radiant slab?

nibs

Need to figure out how much of the dreaded antifreeze to put in the system.
Based on the tremendous amount of experience you folks bring to the forum what is the consensus?
Slab insulated to R 20. Roof to R 50 and up, walls well insulated and partially bermed, big volume of thermal mass. House will be un occupied for 3 - 6 months in winter.
Coldest temp in the last 8 yrs, -25C- -19F. Average low temp -9C- +4F.
My question is....... we know that there is a lag, while the slab heats or cools, will 30% (-18C - -3F) glycol be sufficient, or should I go to 40% (-25C - -19F).
The boiler and related piping will be exposed to ambient air in house. ( I think my question just answered itself, it ain't the inslab, it is the boiler and plumbing I must protect) will post anyway to get feedback).

Tinman

I'm in Chicago, fairly similar. I've always used 40%.

leonz

We used 100% automotive antifreeze in the car wash my father owned and operated from October to May every year to keep the floor heated and the aprons ice free.

Is there any real reason you cannot drain the system as your home will be unoccupied for long periods? Are you familiar enough with the system to refill it with plain water and bleed any air out of it every year?

Have you thought about just using RV antifreeze in it when its drained of water in the boilers steam chest?
Closing the water feed valve and opening the vents and the boiler drain is a much simpler solution and if you have an air compressor you can flush the water out pretty quickly.

Zman

You need to use a product designed for heating systems. I would absolutely not use RV or automotive.
When the system is running year round and you are just trying to prevent a freeze up in an air handler or exposed pipe, 30% is acceptable. It sounds like you want to turn it off for part of the winter. I think your safe bet would be 50%.

hot_rod

The lowest % that will protect to the lowest expected temperature.

There are a few numbers with glycol, slush point and burst point.

Anything below 0° you are not moving much if any fluid with a centrifugal pump anyways, so burst temperature protects. 35- 40% is often adequate but protection level varies from brand to brand, read the label.

Actually above a 50% mix the protection temperature doesn't drop anymore, so 100% would just add cost, higher viscosity, and no additional protection.

Use PG if it could leak and be exposed or blend with potable water in an indirect for example. And use only hydronic glycols. Automotive fluids have silicates blended in, not ideal for hydronics.

EG like DowTherm, (PG is DowFrost) is still available and a better heat transfer fluid, but higher toxicity issues.

GroundUp

I have a similar climate here, and use 40% Propylene in all snowmelt and "shut down for winter" applications. Please never use automotive antifreeze in a heating system. It will do horrendous damage to the system and is toxic. My own snowmelt at home is 40% PG and is still pumpable at -35F air temp. My gauges don't go low enough to know actual fluid temp but the exposed lines in my basement will frost up for a minute until they begin heating up

delta T

Also make sure to check your boiler see and if you have an aluminum heat exchanger. If so, you MUST get PG with the proper inhibitors designed for use on an aluminum boiler. Regular PG will attack and destroy aluminum in relatively short order. Whatever you end up with, checking the fluid quality, PH, and glycol concentration is essential to maintaining your system, do it every year without fail.