I have discovered that using a tankless water heater for both domestic water and radiant heating is an efficient and inexpensive alternative to a boiler. Keep in mind there are no building codes in my rural part of the country, as there may be in your ‘neck of the woods’. And the cheaper tankless heaters are usually not recommended or warranted by most manufacturers to be used in a radiant heating system and it will probably void your warranty. We plumbed the radiant heating loop of our vacation cabin to the domestic hot water, making an open loop system. We don't usually drink water from the hot tap - we heat it in the microwave or on the stove, but I wouldn't see anything wrong with that either, as the water is not contaminated or otherwise unsuitable as in a closed loop system. It just goes through (a lot more) tubing and a stainless pump. Fresh water gets introduced into the loop every time we use the shower or DHW. Originally I tried a 10 gallon tank type water heater, but it took a long time to heat the floor, even after I upgraded to a 220v/4.5kw element. So I decided to try a 7kw tankless 'point of use' heater, and was pleasantly surprised that it heated the cold concrete slab floor to 70 degrees in a reasonable time. The tank type water heater evidently was overwhelmed and thus could only provide tepid water, but the tankless heater put out a continuous supply of hot water which got the job done much sooner. The only drawback was if the floor heat was on, the domestic (tap) hot water would run just warm. This was OK, even for showers, but a dishwasher requires hot water (unless it has provisions for heating the water). And I prefer the hot tap to produce hot water. So the following year I bought a larger but relatively inexpensive 'Titan' brand N120 tankless (11kw), and (after enlarging the outlet orifice to 1/4" - which will also void your warranty) was happy to find it heated the floor in half the time while providing hot tap water! At 95%+ efficiency, I don't see why this wouldn't be feasible in Northern climates as well, providing your floor or slab is properly insulated , and electric rates are reasonable (electricity here is only 7-8 cents a KWH so is in line with propane costs). Once the initial heating cycle is completed, intermittent use keeps it warm. We were so happy with this arrangement that we have since converted our home as well. With the money saved from having to purchase higher priced ‘boilers’ we have purchased another backup heater for peace of mind. Time will tell how long they will last, but I suspect we’ll get a good return on our investment. Dave
