Posted By dulatoag on 03 Oct 2014 09:07 AM
Good morning ... I have a 550sf living space, concrete slab radiant heated. I live in Upstate New York, so winter's are tough. I currently use a 40-gallon LP water heater to feed the system and it has been adequate. I'm going to try replacing the tank heater with a Takagi tankless ... specifically the T-KJR2-IN model ... 6.6 gpm, 140k btu. A rep at Takagi actually recommended a couple of other models, but the specs are the same as this model. I see some folks using this model for larger spaces (albeit in milder climes), so minus truly intelligent math calculations of which I'm simply incapable, I'm thinking/hoping this could work, and the model gets excellent reviews. Hoping someone here has some thoughts, experiences with these heaters. Thanks so much!
A house that size with any insulation at all will have a heat load under 15,000 BTU/hr, and may easily have a heat load under 10,000 BTU/hr. The min-fire input of the T-KJR2-IN is 19,500 BTU/hr, probably 2x your actual 99th percentile heat load and many times your average heat load. You may be able to tweak it into working without short cycling by adding mass or some sort of minimum timing scheme on zone calls, but's not very well adapted to the size of the load.
A 3/4-ton or 1-ton
cold weather mini-split is probably a more appropriately sized solution for space heating, and will have less than 1/3 the operating cost of a pretty-good tankless at recent years' propane prices. At -13F these things still have something like 70% of their full-rated capacity at +5F.
Before picking any heating solution it's important to calculate the actual heat load at the
99% outside design temp for your location. "Upstate New York" covers a fairly broad area, but there are very few locations that have an outside design temp below -10F. It's possible to size a mini-split correctly for sub-zero temps.
A propane tank hot water heater can work at almost any arbitrary burner size, since it's short cycling can be limited by the thermal mass of the water in the tank and the differential built into the controls. But the ~80% steady state efficiency of non-condensing propane tanks makes them not super-cheap to run. It's cheaper than a mini-split solution, to be sure.
I'm currently heating my home with a tankless Takagi (KD20) but the system als includes 48 gallons of buffer tank (with an internal heat exchanger for potable water) on the primary loop with ~7F of hysteresis on the aquastat to establish a minimum burn time (after tweaking flows) and my 99% heat load is about 35,000 BTU/hr, probably 3-4x yours. If you go the TK-Jr route it's worth buying the kit that makes it a sealed combustion unit, ducting in combustion air from the outdoors rather than drawing from conditioned space. You'll have to insulate the duct to keep room air from condensing on it during cold weather, but it won't be sucking air into your house from random places.