Trying to improve efficiency of 2200sf house radiant system with 5/16" PEX Wirsbo QuikTrak above subfloor radiant system. I-joists below the floor currently have R-19 sitting on top of the bottom chord of the joists resulting in a space above the insulation and below the subfloor. We allowed for a space in the event of a leak. No leaks in five years of operation. I know you leave a space between the insulation in below-floor staple up applications, but am wondering if it make sense to spray closed-cell foam directly to the underside of the floor to direct more heat upward. The unfinished basement below is unheated and remains a constant 55 deg in the winter (possibly b/c it is gaining heat from above). The system runs off a Bradford White Combi 2 HWH w/ heat exchanger inside. Also looking for a more efficient heating appliance recommendation. The heating load is about 40k btu/hr. Thanks for any recommendations

There's no point to spraying foam in there- it won't make the floor appreciably warmer, as long as there hasn't been massive air leakage at the band joist leaking through that cavity you left between the R19 and the subfloor. Snugging the batts up the the subfloor would slow down any thermal-bypass currents like that.

Putting 2" of closed cell on the band joist & foundation sill to air-seal the basement would be a more effective use of the foam. Insulating the foundation walls with 2" of foam all the way down to the slab might even be less foam than doing the whole floor in some designs, with far greater benefit.

There are several condensing tank HW heaters out there with side-ports for hanging a heat exchanger. (The A.O. Smith Vertex units come so-equipped.)

Since you have a heating history with the Combi-2, your location and an annual fuel usage number can make a closer estimate of design condition heat load. (40K may be on the high side- it would have to be about -15F outside for my ~2200' + ~1500' basement 1920s antique with known insulation gaps to have a similar heat load.)

Dana,
What are your thoughts on a wall hung condensing boiler for radiant floors and hot water. Or is a tank type heater a better bet for price, simplicity and performance ?

There is no simple answer to the question- it's design-specific (to the system & loads, and the required water temps.)

If the temp requirements of the system are a lot lower than domestic hot water temps you can squeak several percent more condensing efficiency out of a mod-con, provided it isn't short cycling itself to death on a million low-mass zone calls in a micro-zoned system, in which case going with the thermal mass of a tank may make more sense.

If the design-day water temp requirements are above 130F you'd be cutting severely into condensing efficiency with a condensing tank maintaining it at a non-condensing temp all season, but you'd still be fine with a mod-con under outdoor reset control, even if it's only getting ~85% combustion efficiency during the fleeting hours of the cold weather extremes, and the average efficiency would remain high.