Information about radiant heated floors and heat exchanges
Heat recovery from gray waterA significant amount of heat energy is lost through the waste water from showers, dishwashers, and washing machines. About 50-60 percent of this heat may be recovered through the use of heat exchanges between the intake pipe to the hot water tank and the outgoing sewage line. It has been estimated that the amount of heat recovered this way is roughly equivalent to 2 kwh for a 12 minute shower (J. Becker, the evolving world of heat exchangers, PMEngineer, 8/02) Becker estimates that the roughly 13,500 BTU are available in the waste water from a shower, and notes that with 60 percent efficiency, 8,000 BTU or 2.4kWH can be recovered. His article discusses Gravity Film heat eXchangers (GFX) and gives a number of case studies.
GFXtechnology has a discussion of how their system works and includes price lists ($300-400 depending upon size and effeciency and space considerations.) See also endlesshower for further applicatons of the GFX system and a discussion of how it works. (Becker's article seems based upon this discussion.) Although some of their systems require 5 vertical feet, their literature suggests that have created systems with 20 inches of clearance.
See also the Federal Energy Management Program discussion of green technologies and the list of ongoing projects from the DOE. As the DOE report points out, 14% of a home's energy is in domestic hot water, much of which can be recovered through a GFX system.
Combined house furnace and indirect hot water heating is diagrammed in an article taken from no regrets building by the LBL national lab.
Joe Fiedrich, president of Stadler Controls, discussses how to control the "flywheel" effect of radiant heating systems by using an external weather sensor. The Stadler system is used by others (e.g. Northwest Mechanical), as are Viesmann Boilers.
A review of the logic behind outdoor reset controls suggests that because heat loss is a direct function of the outside temperature - the inside set point, and because radiant heated floors have at least a one - two hour lag in response, it is useful to adjust the water temperature flooring through the floor as the outside temperature changes, before the inside temperature has changed noticeably. Control systems seem to be made by Stadler, Tekmar, and Johnson.
Tekmar has a very useful set of pages both for the technician and the end user about how reset controls work for boilers with a particular discussion about radiant heating. They are distributed in Chicago by Bornquist.
Increasing the thermal mass of the floor helps make the temperature more stable and increases the comfort of the floor. This may be done by putting sand between the sleepers, or using a more solid poured surface such as gypcrete. (An example of a do it yourself pour of gypcrete on radiant floors. See also another phototour of laying gypcrete on radiant floors )
A critical perspective on radiant heating suggests that increasing the thermal mass is not desired (not is radiant floor heat) because it is harder to contol. More positive set of questions and answers about hydronic heat and control systems are also available. As pointed out in a very useful discussion of radiant heating, one advantage of using something like gypcrete in a radiant system that relies partly on solar is, of course, that by increasing the thermal mass the flywheel effect keeps the house warmer when the sun is not shining.
The conclusion from this quick review is that it is very important to have an outdoor reset control. It also seems important to increase the thermal mass by using gypcrete or some kind of fin.