I initially started this as a reply to another thread here:
but thought it best to start my own so as not to threadjack too badly.
I was just at a customer's home this morning. The house is 7 years old with a 93% 2-stage bryant propane furnace and a 3.5 ton single stage AC currently serving their HVAC needs. When the house was built, the HO put radiant tube in the entire basement as well as the rooms on the upper floors with hard surface flooring. It's a total of 4 zones (5 if the garage is included - served by a hot water heater with radiant floor currently) of hydronic and the forced air is also zoned for the main and upper floors.
I'm thinking a water to water unit makes sense, but I'm trying to balance things out. The entire house isn't going to be able to be heated by radiant so I'll need to have a lot of space heating by the forced air unit and the higher temperatures required by a water coil on top of the furnace compared to the lower temps of the radiant floor adds a bit of a wrinkle to everything.
I have a perfectly serviceable furnace to use for backup, otherwise I very much like the use of a boiler as both auxiliary and DHW production. I'll offer that as an option, but I don't known that it's a necessary expense at this point for the HO. I'm looking into the Waterfurnace Synergy3D as a potential option - it seems like it might be a good fit, but that would be doing away with the furnace and then we're probably looking at electric as auxiliary backup, which really isn't a big deal I suppose as the current furnace doesn't have a VS blower either.
I'm pretty sure I can do it a number of ways - I'm trying to minimize the amount of equipment that will need to be purchased while maximizing the efficiency of the system.
I think the biggest decision I need to make, or at least one of the options, is whether or not I want to keep the furnace in the equation. If I keep if, auxilliary heat is taken care of, but it saddles me with a non-variable speed motor (could be swapped for a retrofit, but that's not the same) and a split geo unit (which I think excludes the Synergy). That makes the forced air side of things pretty simple, but leave the hydronic problem unsolved.
For the Hydronic I'm considering (1) Synergy3D or equivalent dual capable unit (2) Water to Water Geothermal (3) condensing wall hung boiler (4) High efficiency water heater.
I suspect something like the Triangle Tube smart multi energy tank might place a key role in allowing a tie-in between domestic hot water production and space heating.
The controls side becomes a whole 'nother problem to solve. There will be 2 forced air zones which have overlap on the 4 hydronic heat only zones. I certainly see where two pieces of equipment greatly simplify that equation, but again, trying to be cost effective as well. I would probably prioritize forced air heating as that covers the entire home and radiant floor would be for comfort rather than actual heating.
Still very much in the planning stages but really appreciate the assistance and any guidance or advice.