Using a Water Heater for Radiant Floor Heating Instead of a Boiler

  When I starting installing radiant floor heat over thirty years ago, I was using my own wood boiler. But soon started using other boilers also. The boilers that were available were gas and oil boilers that were high volume boilers holding a large amount of water. In the eighties cast iron boiler manufactures all begin making low volume boilers. The volume went down as the price went up. It soon became apparent that low volume boilers were very inefficient with radiant heat. The problem was very large firing rates combined low volume (5 gallons) boilers, combined with low flows (1 or 2 GPM) of the radiant systems.  The boilers were engineered to heat baseboard heating systems that require large flows through the boiler and high water temperatures. Many cast iron boilers were destroyed by low water temperatures of the radiant systems. One solution I found was the Bock 50 gallon water heater. Another was to use a tank type heater in conjunction with the boiler. When I started Radiant Direct I started using the gas Polaris and the oil Bock. This was very successful and these systems are still in operation today, with many satisfied customers.
   Today the boiler manufactures are making new boilers, they are designed to modulate the fire, trying to match the flame to the heat requirement. The vessels are now made of stainless steel. The water content is even lower (1 or 2 gallons), they are designed to run at the lower water temperatures (120 to 130). They also require large flows per min. that radiant systems do not produce. So the manufacturers require added pumps, simply re-circulating water through the boiler and a lot of electronics trying the keep a very small SS heat exchanger from over heating, kettling and stress cracking. Kettling is where the water flashes briefly to steam, this separates the oxygen from the hydrogen, this causes corrosion by the oxygen and a noise with vibration that will cause stress cracking in the stainless steel welds as the steam collapses back into water.     
   The bottom line is that, in my opinion trying to heat one gallon of water with a 100,000 Btu fire in a very small stainless steel vessel, produces unnecessary stresses that will shorten the life of the boiler considerably.  And that after paying $4,000.00 for a new boiler and having to replace it in a few years, is not a good investment.
   The company who built the first American built SS modulating and condensing boiler, the Munchkin by Heat Transfer Products, although they still build the Munchkin, they are also building the new Phoenix combination water heater that is approved for both heat and domestic water heating.
   The Bock oil water heater, the Phoenix, the Polaris and the A.O. Smith Vertex are the best choice for combination heaters.   
   The energy efficiency of the boiler or tank hot water storage system is derived from the dual use, minimizing the standing water losses and typically offsetting electric usage with natural gas. The Department of Energy estimates that systems utilizing one heat source for both heat and water can reduce domestic energy consumption by
   U.S. Code Acceptance. Most systems are UL (Underwriter's Laboratories) and AGA (American Gas Association) certified for heating and domestic water heating. However, check with the local building permit department before installation. Since the water in the coil mixes with the domestic hot water, some inspectors might request a pump timer because the water could become stagnant. This really has no bearing as any plumbing system in a home that is left unused for any period of time for any reason would surely present the same problem.