Electric tankless will require an upgrade as well ... on the electric circuit. A typical electric tank takes a 25 to 30 amp circuit. Electric tankless can be high as 120 amps depending on the needed capacity. Capacity in tankless terms means the heating power required to raise the desired rate of water flow (gallons per minute) from the starting point (lowest tap-water temp experienced during the winter season) to the desired output temperature (typically at least 120ðF for a kitchen with a dishwasher). If your house has a large-enough existing electric drop from the pole, then you'd only have to run a larger circuit from the breaker box. Otherwise you'd also have to upgrade the service drop from the pole.

Have to be careful mixing radiant loop with potable water, concerning germs and such. Personally the only way I have seen it done on TV is with an indirect water heater, where the boiler heats water, the water passes through a coil in the water heater tank and transfers the heat to the potable water, then the coil water returns to the boiler. Sounds like you could use a tankless in place for a boiler. The thermostat (and whatever zones you have set up) could activate the pump and the tankless would start heating once the flow starts.

Thanks Jake the Wonder Dog. We have the tankless set at 120. It's usually just me and my husband here most of the time and 2 college age kids here on holidays and during the summer. Yes it does have service valves and we were able to clean the little inlet filter which didn't have anything in it. I'm pretty sure we will have no problem flushing the tankless. I watched a video from Rinnai. I just wasn't sure how often we needed to do it with our water hardness value. I see a submersible pump is needed. How powerful does the pump need to be? We have a Harbor Freight near us so I can look there for one. So with the information I gave you do you think every 2 years would be good for flushing the tankless? Thanks for your help.

updating to let you know that we did install a whole house booster pump and it has been very positive. Overall, our water pressure thoughout the house is MUCH improved and becase the demand is greater at the faucets, the heated water arrives quicker. The kitchen, sink, however, is still considerably slower and cooler. It's so weird, and I really cant decide what the actual probelm is (our house is on a slab, so no way to do anything with the pipes at this point) so we are going to install a small, 6 gallon, hot water heater under the sink so that I can have instant hot water. Warm water does not work at a kitchen sink - you defintely need hot water. The line will be tied in with the regualr hot water so if I am using lots of hot wter, by the time I use up my 6 gallons, the 'regular ' hot water will be there. I will come back to update after we get it. I know all of this sounds like an expensive overkill, but it is worth it to me to get the level of performance I orginally wanted.