I just bought a house that has in floor heat roughed in but no heating system. There is a forced air furnace but we would love to hook up the in floor heat to make the whole house comfortable. I have had two quotes for natural gas boilers and both were around the 10,000 mark which seems a little ridiculous for secondary heat source. The garage is also roughed in and I was wondering if one could heat both a garage and house with a hot water heater or would it create too much demand? Keeping in mind that I only want to run the garage at about five degrees. Any thoughts?

You're going to have to supply some basic information like size of home, heat loss analysis and your location.

If you know what is in the floor (e.g., PEX diameter, spacing, and circuit lengths) and the other required design parameters, you could use our free DIY hydronic radiant floor heating design software to answer your questions yourself:

Borst Hydronic Radiant Floor Heating Design Software

It is always good to get educated about hydronic radiant floor heating design even if you do hire a designer/installer.

knowing design temp of the floor is good as well. Tube diameter and space on center tells a lot about what you can use.

Yes, the heat loss analysis and the desired indoor temp set point will determine what the floor temp needs to be in order to get the required upward heat flux. If the company/person that did the design/installation did not know what they were doing, it is possible you could have circuit(s) that are way too long to have a successful floor heating system (i.e., the hydraulic friction could be too large for the flow rate required to maintain a reasonable temp differential in the circuit and floor area). So it is important to determine what is in the floor before proceeding. If one knows what they are doing, it is possible to test each circuit to determine the hydraulic characteristics necessary to design the system if this is possible given what is in the floor.