Hello ...

I know this topic has been covered in other forums, and perhaps this one. Last one I saw here was 2010, so I'd like some more current opinions, though I admit I didn't scour all the search results for this topic. So apologies in advance if this is redundant.

I have a 550 sf addition, hydro heated concrete slab ... single zone, closed system. I live in upstate NY, so it can get quite chilly. I started off with a 40 gallon electric water heater and it quickly became cost prohibitive. I converted over to a propane (we have no access to natural gas) 40 gallon heater. It's an atmospheric heater, and in a live and learn journey, I've heard not so much fun warnings about killing my family if something with the venting goes awry. It's vented properly, but I've reached that weird age where I'd rather limit those types of concerns. So I'm considering alternatives, such as a direct vent propane heater, etc. I've always considered the tankless option, have read some very mixed reviews about it, and figured I'd consider this again.

I'm looking at a Rinnai RL94iP model, 9.4 gallons per minute, 10,300 - 199,000 BTU. But I'd consider any relatively economical brand/model. I know some of the concerns are that if overworked it will limit the life of the device. I'm not too concerned about voiding warranties, etc. I'm interested in saving cash on propane annually, saving on some space, having a direct vent unit, and of course keeping my slab warm.

Any thoughts and/or personal experiences would be greatly appreciated as the cooler temps quickly approach!

Darren

Some type of heat pump would generally be a good replacement for propane. But practical matters (like few choices for air to water units in the US) may get in the way.

Propane isn't usually a lot cheaper per delivered-BTU that electricity- what is your are all-in deliver price per kwh (divide the bill by the kwh), and what do you pay per gallon for propane?

Upstate is pretty big place, with quite a variety of climates and outside design temps- got a ZIP code?

Even if is a sub-code R /U 550' addition it will likely be within the output at -15F outdoor temps of a Fujitsu AOU-12RLS2-H, and depending on exactly where you are, maybe even within the output of the AOU-9RLS2-H mini-split at your 99% outside design temp.  A mini-split would have the same order of magnitude installed cost of a condensing Rinnai. (You could also save a bunch with a 90% DIY approach, and pay a certifiedt tech to do the pump-down &refrigerant fill, and the final commissioning & test to keep the warranty valid.)

I you then used a HW heater and a floor thermostat to keep the slab at a cushy 70F or slightly warmer and heated the room a couple or three degrees warmer than that with the mini-split you'd use less than half the power of a HW heater alone, and have it pretty cushy overall.  Your actual efficiency will depend on just how warm you keep the slab vs. the room, (which determines how much of the heat is coming from the water heater) and the average winter outdoor temperatures, which determines the average efficiency of the mini-split.  At -15F the mini-split uses a bit over half (~55%) of the amount of power that a resistance heat source (like a hot water heater), delivering the same amount of heat, but at +15F it's  only ~40%.  At +25F it's about 35%, and at 45F it's ~25%.  If your winter average temp is +20F and the slab is delivering only 10% of the heat at that temp the combined efficiency would still be less than half what it would be if you were heating it only with the water heater.

If it rarely/never drops below -18F in your neighborhood (or you're willing to heat with only the HW heater for those hours) a Mitsubishi MSZ-FE12NA or MSZ-FE09NA are viable options too.

If your 99% outside design temp is  -4F (-20C) or above, there are numerous other mini-splits that would fill the bill.

http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf

Your actual design heat load is probably barely over the minimum firing rate of the Rinnai. There are OK/worse/worst ways of heating slabs with a tankless water heater, and most installations I've seen are pretty abusive to the tankless, over pumped and over-cycled.  If you go that route it's a good idea to take the time to set it up reasonably, if want years of service out of it, which may require a different pump.  Tanks are far more tolerant of crude slab-heating designs than modulating tankless heaters.

Dana1, I'll be honest enough to admit I didn't quite understand most of what you just unleashed, but I'm sure it's awesome info if I could wrap my noggin around it. I liked your last paragraph though! I live in south central NYS ... Binghamton area (13862).

Again, using my own logic ...closed loop system. I retain the electric 40 gal tank, to be used for storage only. I have a pump attached to the system that obviously circulates the water through the PEX tubing. I'm thinking the tankless heats the water coming from the tank, pump pumps it through the floor back to the tank. The water temp should come back to that at roughly room temp once the system gets rolling and circulates through for a bit. I'm thinking, and maybe not thinking intelligently, that since it's not always fresh, cold water coming through as in an open system that it would decrease demand on the tankless heater (less degrees to heat the returning water). In the end I have no clue. I love the idea for good or for ill so I'm stubbornly trying to convince myself of its merits - how's that for intelligent reasoning. I figure if it can't keep up, I could put the tank back in, and use the tankless for domestic water if I haven't already killed the unit. I'm wondering if any folks on this board are using tankless and are having positive experiences, or really negative ones that will convince me I'm being an idiot.

Thanks!

Personally, I'd look into something like a Fujitsu AOU12RLS2H compressor with the refrigerant->water heat exchanger portion of a Fujitsu Waterstage (called the Indoor Unit; would probably need to be imported). But obviously such hacks are both speculative and non recommended for any normal user.