There are three types of solar radiant heating systems.

    First is heating the water directly, this is difficult because the solar heat is not available when the heat is needed.
    Second is to store the heated water until it is needed. We have installed a few of these systems, the cost of the storage system makes this type not practical for radiant floor heat most of the time.
    Third is to install a gas or propane combo heating and hot water system  and supplement it with a solar panel. These systems work very well and are very cost effective.

You do not need a southern climate to take advantage of solar
but you do need a home with a very low heat loss and a carefully designed system.
In a solar assisted system the heat loss of the structure is the most important part to consider. 

  Smaller homes that are extremely well insulated and protected from solar gain in southern climates could benefit from combining solar and radiant, but they would need the structure to be designed around the heating and cooling system. In other words you design the heating and cooling system to take advantage of the available climate and geo features and then design the structure within those limits.
  Solar radiant heat should be used in a slab it can not be used on a staple up system, because radiant staple-up requires at least 120 water.
  See page 98 as one example for solar cooling.

                       I see no value to using a gas water heater with a solar coil in it.
  How is the tank going to heat water with solar? The tank is very well insulated and stays hot all the time by gas, in fact it is much higher than the solar furnishes. The only time the solar will do anything is if the tank is cold or cool. So it will reject heat at any other time. The only solar system that would work would be a cold solar tank in front of the heater. I believe you would be wasting money by using a gas or electric water heater with a solar coil, although it could work on the wood boiler if plumbed properly.