I'm building a 20x50 garage with a second story but the main floor will be concrete and would like to put radiant heat in it and I wanna use a on demand hot water heater fired by natural gas is there anyone out there that nows about these being used instead of a boiler

Yes. They are water heaters., not intended for space heating. Missing outdoor reset and all oversized for residential garage heating.

I don't thing I totally understand your reply sorry

Lets start with your location. Where will the thermostat be set and when. Bathroom. living space?

For a garage HR floor heating system, both hot water tanks and tankless hot water systems are commonly used very successfully. The Navien NCB-180 is a tankless system that has been very popular with our DIYers who also need DHW in their garages too:

Navien NCB-180

You first need to accomplish a heat loss analysis before designing the HR system. For a garage, using our DIY heat loss analysis software is way more than adequate:

Borst Heat Loss Analysis Software

Then you can design the HR floor heating system:

Borst Hydronic Radiant Floor Heating Software

Please be sure to carefully read the instructions for the associated software and please don’t hesitate to contact us if you have any questions or desire any additional guidance.

We are Navien approved licensed contractors here in Minneapolis and design systems all over the US. Still have not found a residential garage that warrants 180mbuth boiler.

Nor can we recommend them where trained personnel are not available for the mandatory qualified technical maintenance outlined in the installation manual.

Obviously folks build garages that are smaller than their homes in Minneapolis... Why would anyone do that? A garage/shop has to be at least twice the size of home.

We have helped many DIYers install all sizes of Navien systems. Just be sure you get a permit before doing the installation work and get it inspected afterward to ensure it was done properly and is safe to operate.

The only way I thought of how to build a garage with radiant, and unfortunately I came up with this process too late for me to do it, was to have the concrete foundation crew pour the foundation WITH the garage apron included in the foundation as one piece. Yes it would be rough looking but only then could you thermally break the slab from the outside effectively.

If you don't care about the aprons sucking heat out of your floor, my comment doesn't matter. But I'd love to hear ideas here (I posted a thread earlier).

In the end I don't mind because I don't use the garage every day as a workshop so the time value to heat on demand being too long made the effort pointless somewhat.

I don't use the garage every day as a workshop so the time value to heat on demand being too long made the effort pointless somewhat.

Radiant heat is fine for such a garage, you just don't want high mass radiant heat. It would be interesting to see more discussion about buildings specifically designed to heat rapidly. Perhaps semi-insulating, low mass, radiant reflective surfaces.