We heat with a hot water heater not a boiler. Ours is propane and an on demand. We too only have the water on a low setting and do not need to heat the water to a higher temp. and then add cold.

Insulation between the floors. This would really depend on how warm you will keep the lower floor. If it will be the same temp as the upper floor then no need to insulate.
If however it will be kept cooler or only heated when needed you should insulate. Remember the concrete floor slows down the heating cycle and it will be inefficient to setback temps during the night.

Both Gomer and Jmagill have exposed serious misunderstandings about how these systems work; mixing does not "add cold" in a radiant system, most boilers today won't need mixing because you're probably on gas and you should use a mod/con boiler, and whether the 1st and 2nd floor are the "same temp" has nothing to do with whether you should insulate underneath.

with a 2900 square foot house there is no way a tank water heater is an appropriate choice. Gomer's house was incorrectly specified with a water heater at least. I forget if jmagill has a serious heat load or not. Gomer is spending more on fuel, right now, than he needs to and he'd be better off getting a low interest loan to replace his water heater with a more efficient heat source, tomorrow, as the fuel savings would significantly more than pay the interest on loan payments.

We rate tank water heaters at about 75% efficient when heating: this is much better than their energy factors indicate, but there are technical reasons to upgrade your expectations when heating with tank heaters. Testing on the matter bears out that we are in the right ballpark with that assumption. Tankless heaters should DE-rate to about 80% or so for ballpark consideration, if they are even rated for use in this kind of application, and if they aren't, you have no warranty.

*Well controlled* cast iron boilers with intelligent mixing valves can run 85% plus efficient... of course, most boilers are not well controlled, but with outdoor reset controls and intelligent firing, that's very achievable. If your contractor is not afraid of tekmar controls, or other outdoor reset options, they are worthwhile with cast iron.

Modulating/Condensing Gas boilers like most people outside of new england should be using (in new england the assumption is you're oil, unless you're in a big town), well controlled you can run around 95% efficient.

Note that there are condensing and even condensing/modulating water heaters. But they don't save money over mod/con boilers.

the fact is, 10% or 20% efficiency jump on any real heat load (in maine, the threshold of economic sanity is around 20kBTUs/hr under design conditions) will eat up any typical cost differential between a water heater and a boiler install in acceptable timeframes. You have a real size house. If you're in a real heating climate, do not try to do this with a water heater or you'll waste significant money on your fuel bills every year that you did not need to waste, just like Gomer is.

As for insulation, you insulate under radiant floors to avoid making them uncontrolled radiant ceilings for the floor below. In some cases it won't matter much. Say, a SIP timberframe house with a open loft above, the 2nd floor almost never calls and has a tiny heat load when it does. In that case, doing radiant upstairs was a waste of time. with no finish floor on the second floor, the situation is more conducive to a lack of insulation than most situations would.

But in a situation where the floor in quesiton actually calls for heat, insulation underneath radiant:

1. makes sure you do not have to elevate your water temperatures to meet the heat loads unnecessarily.
2. makes sure you have the output available for upward heat transfer to meet the heat loads.
3. makes sure you do not overheat the living area underneath. I've seen 80 degree basements in homes without floor insulation under radiant systems. what a waste.
4. doesn't take load away from the lower floor radiant! doing that lowers the temperature of that floor and makes the radiant improvement less noticeable.

I get calls every year from people complaining that their radiant won't keep up and/or who "had" to turn up their water temps, who had not insulated underneath their radiant. this is an important step in *almost* all situations. It's not because the heat is "gone", it's because you want the heat where you want it, when you want it, and without floor insulation you can't make that happen reliably.

Just to clarify - we will have an all electric system (avoiding using any propane/gas in the house). 

We also have a 10 ml vapor barrier in the second floor slab (between plywood and concrete) so I would think that would stop some of the heat flow between floors?

side track discussion there.

the vapor barrier will not stop any heat flow between floors. it has a negligible R-value.

Ok - take that back. The vapor barrier won't slow heat

Conventional atmospheric water heaters can work well on small loads e.g. basement radiant jobs. They meet the ICC code when doing dual service.

Whether dedicated or dual service, great care must be taken in sizing and controls. A mixing strategy is a must or the tank style water heat will condense itself to death.

This is why I prefer the condensing tank water heater with heat exchanger and proper controls for smaller combi loads.

Naturally the ModCon is the only choice for more demanding applications.

One more thing.

Rob is right!

any recommendations for electric boiler for radiant heat - I liked slant fin, plumber recommended allied. not sure if there is much difference.

also - seems we should insulate under main floor as per above. what is recommended as best insulator under floor that is also low voc?

The lowest VOC insulation is still rock wool.

If you're not allergic to cotton (in particular denim) cotton batting insulation would work. Minimum R for floors over heated space would be R11. Minimum R over unheated but still interior space would be R19. Over vented crawlspaces etc it depends on the climate.

you wrote about gas leaking from the windows...we paid almost $2500 about for that krypton gas and then had to store the windows for a while because I got too sick to keep the project going and it had to be put on hold. It's all gone - and that was from being stored (oh, by the window manufacturer by the way, who we're arguing with because he won't warranty any of the gas now).

I think it's possible that your krypton gas isn't "gone". In a properly constructed window, you would expect to lose Krypton at a rate of less than 1% per year. The warranty may have ticked away some, but the gas shouldn't be "gone".

They had been kept in storage for a year by the window company (long story) and were never crimped - the balloons that were attached were all deflated.  We read about using infrared to see how much was left -- nothing.  It is tough to measure, yes, but the lack of crimping was a big prob.  Plus altitude...