On demand hot water for radiant heat

I don't know what that is. Can you elaborate?
Thanks.

It basically is a tankless hot water heater. you never run out of hot water. it is heated electrically vs a tank with gas heat.

Doesn't a thermostat by nature only heat the water as you need it? Our boiler only uses a few gallons of water anyway so I don't see why a tankless water system would be efficient. You are talking about radiant heat, right, not your water heater?

Wrong application.

On demand type of water heaters are designed for sink usage. Maybe shower for the larger models, but know of none sized for whole house heating.

Whole house heating will have the "boiler" on for a long time periods. The on demand types don't have that kind of duty cycle rating.

The thermal inertia of a slab is way more than what they are designed to see. They are designed to see almost no thermal inertia.

It wouldn't make sense. The water heater is a reservoir of hot water which is being used up, and only slowly re-heated when it has been depleted. The advantage of the tankless heaters is that they provide a virtually 'endless' supply of hot water.

However in the radiant heat system most of the water is not in the boiler, but it is stored in the piping system and continuously circulated. The heat is stored in that water and in the slab itself. In that sense the system is already practically tankless. Also, the tankless heater is designed to provide relatively hot water for your household, and that temperature would be too much for the radiant heat.

Mixing up things and out of context.

Sure a slab heating system is basicly "tankless", but there are other attributes to the whole system.

Thermal interia is like mass. The larger it it, the more energy it will take to move it.

A tankless water heating system for a "sink" will have a much smaller BTU rating than anything designed for a whole house heating system of any kind. Whether forced air, or water distribution.

Then the duty cycle rating, meaning how much of the time it's designed to be "on". A very small BTU rating of a tankless sink system would be on for days/weeks or more to heat a whold house.

Again, mixing up attributes and are out of context to the application.

Maybe reverse logic. Why not take a whole house slab heating system to heat the water for a bathroom sink application? It would be on for less than a few seconds and turn off and on for the duration of sink usage.

Tankless water heaters are designed as previously described, to provide immediate and unlimited access to hot water, without having to store it. As a result, they are supposed to be more energy efficient. I'll leave it to the professionals to describe the technology, but I am surprised that so few here seem aware of them.

There are combination units that can be used as an all-in-one replacement for your hot water heater and your boiler. Like anything, there are advantages and disadvantages. On the disadvantage side, they are computerized and thus more complex to repair. From interviews I have done in the past with radiant companies that install these, they state that "it is tough to find anyone that knows how to repair them because their design is so complex."

Some homeowners put them in to save on energy costs; others to have access to greater amounts of hot water; others to save space in the garage or furnice closet.

Several of the advertisers on this site have experience installing them.

Cathye

Sizing, not type is what I'm talking about.

Maybe this: take a small room heater. Portable. Does fine job of heating a 12x10 room, but it will take forever to heat the whole house, where a properly sized whole house will get it up to temp much faster.

An Eichler boiler is an 'on demand' tankless water heater. The gas burner lights when the small electric recirculating pump starts. Modern efficient units have no pilot light. When you put your hands on the pipes (you better know what you are grabbing), you can see that there is only a small temperature difference between the water going into and out -of the boiler. Combined with little water volume, radiant heat is fairly efficient compared to forced air. It's the original Eichler that is quite inefficient. They were built with virtually no insulation anywhere. The dirt under the slab is actually a fair insulator.

The new 'on demand' whole-house or point of use water heaters can take a large volume of water and heat to the maximum temperature, continuously. The BTU's used are enormous. This is efficient when compared to keeping a tank of water hot 24 hours a day.

Hi everyone,
First time poster here but I can give some insite to what's available on the market.

Randy is right that the original style boilers and slabs are highly inefficient. New technology has made the method of delivering hot water highly efficient though. The old AO Smith boilers were at best 60% (ballpark guess) efficient. Newer replacements are closer to mid-70% efficient. With a standard tank type hot water heater it costs money to heat and store hot water when not in use.
Manufacturers like Noritz, Rinnai, and Takagi are tankless and heat water on demand. When the t-stat calls for heat it signals the circuit board in the unit to fire the burners to heat the water. The pump pushes the hot water through the coils in the slab and heats the house. Although I can't verify this, I've heard reps say that the tankless heaters are in the 90+% efficient range. They're rated at the same BTU input but burn and control the useage at a much higher efficient rate.

Hope this helps.
Dave

Hi,
This is a great forum for the homeowner; however, some of the information presented is often incorrect. Regarding the last posting, the original A.O. Smith boilers installed in the Eichler Home were 80.2% efficient. Everyone who posted in this section and noted that tankless water heaters are the wrong application as a boiler subsitute in the Eichler Home is absoultely correct. You can't compare the efficiency of a tankless water with a boiler. The government establishes efficiency standards for boilers only, so don't be fooled when anyone quotes efficiency factors/ratings for any tankless water heater. They are meaningless with regards to the annual fuel used to heat your home. All new water heater and boilers installed in your home must be permitted and inspected by the City/County having the authority in the jurisdiction. Call your building department and ask them if you can install a tankless water heater as a substitute for a boiler and in all cases the answer is no. Why? They also understand it is the wrong application of the appliance. Contact me for the best source of information regarding this subject and a complete explanation regarding this subject. Have a great day.