You can look at "warmboard" but it's costly. It's much better than "staple up" radiant heat that goes under the floor. The warmboard serves as the subfloor but has grooves for laying radiant tubing already built in. They design the layout for you (or so I'm told) but the board itself is around $190 for a 4x8 sheeet. This is a better option that pouring mortar or gypsum since it is lighter, and it replaces the need for your regular subflooring (the warmboard is around 1 1/4 inches if I recall). You place your PEX heating tubing right in the grooves. We opted against it in Pennsylvania because we felt we would still need to run ductwork for a backup heat and for A/C, so it was cost prohibitive to do radiant. Your basement radiant heat would run the tubing right in the concrete.

An electric hot water heater typically will not make sound when it runs. I would think this system you have is a closed loop, meaning you fill the tank and loop the incoming water is shut off and the water is moved thru the loop in the floor via a in line water pump, controlled via temp sensor(s). I am only guessing here... Note: just because the circulation pump is off doesn't mean the resistance heating strips within the water heater are off. (The pump you should hear, the strips you will not) Your electricity rate is pretty average, but being on balanced billing it can be upwards of 11 months of usage on your end until you realize the increase. Unless your balanced billing works differently? I had a customer of mine in which it took 11 months of the balanced billing to drop after I installed a new more efficient air conditioner. So if you use a lot of energy in any one month on balanced billing the energy company may rack this up and charge you for it pronto < I can only assume it would be this way because the energy company wants to get paid and possibly alert you to a problem. With all of that said, was this water heater sized properly for the 'heating' load you are putting it under? The colder it is, the heavier the load. At some point, the load 'could' be so great in which the elements that heat the water do not shut off. The only true way to know the heating elements are off in an electric water heat is to hook amp probe around electric lead and see what the amp draw is. If it reads 0 they are off. Because you are in an all electric house, there could be issues with your other systems. While rare this does happen >>> an electric resistance heater the strip breaks and shorts to ground. This causes the strip to remain on indefinitely, only turning off due to heat limiting controls of the appliance in question. The other outcome, even more rare than above besides this is an electrical fire. We all know these occur as well.

so I pulled up a small piece of floor to see what I'm dealing with. looks like one layer of subflooring with spaced by firns, leaving air between floors. suggestions? someone said to leave alone but will I still be able to heat home sufficiently?

As nice as it sounds to have heated floors in a cold climate, I worry that the slab over pillars will make this VERY difficult to achieve (ie....very expensive). To do this properly, you have to incorporate a HEAT BREAK into the slab. Yes. INTO the slab. Example: if this were on the ground, your HIGHLY EDUCATED slab expert (who is fully trained in thermodynamics...ahem...fully trained) would design a system where the heat block was installed FIRST (sitting on the ground) and then the concrete poured OVER TOP. As in closed cell polystyrene blocks (several inches thick). The foam blocks would STOP the transfer of heat DOWN into the earth (nothing like trying to heat the GTA through your heating system!). When working with pillars, this gets complicated because you STILL require the heat break. But WHERE to put it. Now you have to put it ON TOP of the slab. Then you have to use something sitting ON TOP of it to prevent it from melting when exposed to the heating system/coils/tubing. There are multiple systems that sit on top. They are fast, easy but expensive because they are fast and easy. They can/will raise the floor height by several inches....and that has to be incorporated into the design because the STAIRS will be pushed out of code. And once that heating system is in place you have to be aware that your FLOORING choices have just been GREATLY reduced. Example, that beautiful hardwood you have chosen MAY NOT be allowed over ANY form of radiant heat....at all. Whew! It is SOOOO much cheaper to get rid of the old boiler and upgrade the HVAC system. Much, much, much cheaper. And the operating costs will be MUCH lower...because old systems are usually inefficient.