This is a standard hydronic heating question that can only be answered by a careful analysis.
First you need to know the design-day heat load of every room in the home. (The heat loss in that room at 68F on the 97th+ percentile binned hourly weather data for your location.)
Then you need to know how much radiation you have in each room, and calculate/estimate the water temperature required to deliver the necessary heat to the room based on the radiator/baseboard manufacturer's specifications. The room with the highest temperature requirements then defines the operating temperature of the system.
Then you need to verify that the hot water heater's controls can be set up to deliver that temperature + several degrees F (since the temperature will be lower along the flow chain, in series-plumbed rooms in a given zone.) One also needs to verify that the return temps at that outgoing temp will be low enough that it doesn't trigger a high-temperature safety in the tankless unit's control system. (In older Takagi gas-fired units the limit on the temperature of the return water occurs around 130F, which is easy to exceed if the system needs 160F water on design day in your worst-case zone.) If the numbers all work, and the maximum output of the tankless exceeds your whole-house BTU requirements, yes, you can make it work with a tankless (and more efficiently than with a right-sized cast iron boiler of similar combustion efficiency, if set up reasonably.)
Most tankless hot water heaters have plenty of thermal output to cover the heating loads for small-medium sized homes, even in cold climates. But they're fairly high-head compared to heating boilers, and it's usually better to set them up as primary/secondary loops with separately pumped boiler(tankless) and radiation loops connected with a hydraulic separator (or buffer tank) if the heating loops are long or high-flow. (I'm running my heating system with a tankless heater, using a similar architecture.) Set up as primary/secondary it's usually possible to set the boiler flow & output temp to get it into reasonable modulating mode for higher operating efficiency than bang-bang on/off operation at max fire. In a primary/secondary the output temp of the tankless can be much higher, and flow much lower than what the radiation requires, to deliver the same BTUs to the system. (Mine is micro-zoned with as many as 5 zones or as few as one gulping off it at at time, and the tankless modulates up/down based on the temp of the water in the return path, which is determined by the VOLUME of water being pumped in the radiation.)
Gas-fired on-demand tankless heater are little more than low-flow finned copper heat exchanger water-tube boilers with minimal insulation with a different set of modulation control parameters. Oil fired on-demands are somewhat similar in character, need to use stainless heat exchangers rather than copper (to tolerate the higher acidity of the flue exhaust) and usually aren't exactly "tankless"- they tend to have non-modulating ~150KBTU/hr burners, and high-volume (~5 gallon) FIRE-tube heat exchangers. The high thermal mass of the water in the heat exchanger keeps the output temps within a range as the burner turns on/off during the flow. They're distinguished from other oil-fired hot water heater by their burner/stored-volume ratio, and the sophistication of their control systems. See:
http://www.toyotomiusa.com/products/waterheaters/OM-148.php#
and
http://www.toyotomiusa.com/products/waterheaters/OM-180.php
(Look at the cutaway views of the heat exchanger- looks like a 6-8 fire-tube arrangement, with the combustion chamber & fire tubes all immersed a stainless steel bucket of potable water.)
So rather than "tankless", they're really "mini-tank" units with sophisicated burner controls. While they're lower-mass (resulting in higher cycling-efficiency) than their cast-iron cousins, they're not nearly as low-mass as a typical gas-fired finned copper water-tube type on-demands.
Most existing baseboard/radiator heating systems in the US were designed for 160-180F water, but it's not rocket-science to design them for 120-140F water- it just takes ~50% more baseboard than at the higher temps. In super-tight SIP/ICF construction with pretty-good windows and decent insulation levels the amount of even low-temp baseboard or convector radiation required won't be outrageous. Many manufacturers will now specify output temps at 120F now, whereas only a decade ago you might only get numbers at 180F & 160F (or 140F, if you were lucky.) It's fairly linear though- extrapolating downward from two higher numbers gets you close to reality for it's 100-120F output. |