Thank you David England for experimenting and sharing what you have learned. As one who is also experimenting with self funded, DIY hydronics, I appreciate your efforts to share. I too have also seen the utility in utilizing an electric tankless water heater in my hydronic heating design, and I very much enjoyed reading about David's discovery process with an ineffective account of the tank-ed water heating with a thermal mass system.

In my present experiment, I have burned up a 208V electric tankless heater element. allegedly the heater I burned can only accept cold water, and has no shutoff to prevent itself from burning-when all else fails, read instructions, who knew? Live and learn. One pricey mfg specs warm water feed as acceptable ONLY for their thermostatically controlled models, but many mfgs offer very similar units which do not mention feed requirements in their specs. After burning up a galvanized wire & plastic heating element, I just ordered a thermostatically controlled heater made by one of those 'other mfgs', which has a copper surfaced heating element. To me, temperature (amperage) control is a significant improvement and I would expect this capability to be competitive across the market, and also be capable of accepting warm water....I bought a different heater than David, so I hope to report back on its effectiveness. anyone want to place bets on how many hours it will take me to burn the new heater up? For updates, check http://www.evinem.blogspot.com/2014/01/bed-rock-design.html

I'll admit I don't know a whole lot about modern high-efficiency boilers, other than their applications seem to mostly be found in buildings of a much grander scale than the average single family dwelling, or my 25sqft radiator. Most things hydronic tend to be of a different demographic from the people whom are hired to design and install the systems. The equipment is not designed for smaller, DIY applications, and smaller projects can expect to diverge from the cookie cutter shape, and contain a degree of adaptation-which is half the fun for creative people.

I wonder if tank-ed water heaters are better suited to low mass radiators (warmboard), where less energy is needed for initial heat up, but thermal buffering may be desirable to accommodate larger swings in peak demand. There is no way I can possibly justify a boiler for my 600lb, 25 sqft concrete hydronic floor radiator, so don't bother selling me one.

We regularly design hydronic radiant heating systems for 150 sf sun rooms and 1200 sf houses using water heaters, condensing boiler, condensing comb-boilers and electric boilers. We do not misuse appliances engineered for domestic hot water in space heating applications. Like "open" space heating systems the practice is unwise and thoroughly unnecessary.

Rather than blazing a new trail, most are simply falling into a hole and a potentially dangerous on at that.

The choice of heat source and emitter has to do with heat load and design water temperature. With micro-zones such as your a high-mass heat source, such as a water heater would be in order as long as expensive radiant floor components are not exposed to potable water.

I have a very simple question regarding using a new tankless water heater to heat my house instead of my current oil-fired water boiler. Can I simply install the tankless on the input of my boiler after disabling the oil fire, and using the circulator and the thermostats of the existing system?. So the water would circulate from the tankless to inside the boiler without firing the boiler then to the zones. This could be the least expensive way to convert to tankless water heater for heating the whole house. I need your input please. Thanks.

Heating up the thermal mass of your mothballed boiler turning it into a kludgey high-mass hydraulic separator takes a real toll on efficiency, since with a hot boiler you'd still have all of the standby losses. It's also not clear without an analysis of how the output of the boiler is plumbed, your room-by-room heat loads and your radiation type if you could get there at all with that type of hack.

While it's possible to use tankless HW heaters as modulating boilers with almost any type of radiation, you still have to design it into the system to have it function reasonably well. If the place is so chopped up into tiny zones that you need a buffering mass to keep from short-cycling the thing into an early grave, an electric hot water heater (not wired up) is a less-lossy and higher-mass hydraulic separator than an old cast iron boiler.

Start with a room-by-room heat load calc, and the amount of radiation in each room, that will determine your peak water temp requirements, and give a hint as to whether it's even remotely possible to get it to work with a tankless. But bear in mind, a tankless is not a boiler, and you have to treat them VERY nice to get reasonable lifespans & performance out of them in heating applications (as myriad radiant-slab hack designers have figured out the hard way.)