How effecient are tankless water heaters in hydronic floor applications?

[2zwudz]

   In a hydronic heated floor application how effecient are tankless or on demand  water heaters compared to normal hot water heaters that most use?  I can see where if the hot water is not needed it doesn't draw power to work but when they do work do they draw alot of power? 

Thanks Mark

[NM_Shooter]

What are the BTU ratings of the tankless water heaters?  My boiler for my house is ~85,000 output.

-f-

[MountainDon]

I've seen tankless ratings of well over 100,000 BTU. If fact if gas the piping sizing is critical to assure enough gas at the burner. Ditto, electric, as far as sizing the service

I don't really know anything about using them for hydronic heating systems, but somethings rattling around in my brain saying that some of them are suitable. Maybe that has to do with the output?

The flow rate has to be sufficient to activate the tankless burner/heater element.

As for actual efficiency I'm not certain what the typical tankless burner efficiency is. Gas boilers can reach 90%.

[glenn kangiser]

Some have successfully used them.  Some say they have had to use a pump on both sides of them to get them to work.

This guy recommends a regular tank type heater.

http://www.radiantdesigninstitute.com/page64.html

[TheWire]

I have a gypcrete radiant floor in my 3000 sq ft log home.  I custom built a boiler with 4 2500 watt water heater elements in it.  I can heat the house with 1 element to about 32 degF outside and 2 elements keep things cozy to about 10 degF.

2500 watts is about 8500 btu/hr.  I like to keep the heat as constant as possible instead of cycling on and off.  With 4 elements and the ability to run 1 element on half power (120volts/1250 watts) gives me the ability to adjust the heat input to keep the heater from cycling.

I live in an area with lots of hydropower and 4.5 cent/kwhr power so the electric works well for me.  The concept of a high power electric or gas boiler cycling on and off seems to go against the slow constant heat of a radiant floor.  But the mass of the floor would also help even the cycles.

Cheers,

Jerry

[Okie_Bob]

Hey Jerry, where do you live that you can buy power so cheaply? If I could get it for 4.5 cents I'd be one happy camper.

[considerations]

Wisconsin, I betchya

[ScottA]

Gas fired tankless water heaters are used to replace boilers all the time. I don't like the electric versions because the load is so high that it usually requires a upgraded service to the house.  Besides anything that runs on electricity is going to cost more to operate in the long run IMO. Tankless heaters can also be ganged together for higher output if needed. The upfront cost is usually less for a tankless system vs. a boiler as well.

[TheWire]

Here is a link to an article on "Synergy of demand water heaters with solar and radiant floor heating" http://www.chandlerdesignbuild.com/files/demandWaterHeaters.pdf  He has some good ideas about inter-connecting systems to get the most energy into the house.

http://www.chandlerdesignbuild.com also has lots of other neat articles on custom/creative building.

Jerry