This submission is sort of relevant to this thread:
I have a well-insulated (straw-bale) home, with slab-on-grade, containing 1/2" PEX serpentine tubing that allows it to supply radiant heat. (Please do not make my mistake: the 2" rigid insulation (typically polystyrene foam) that code specifies under poured slabs truly would have improved this system's performance, especially at the beginning of the heating season when the soil under the slab has not yet come up to temperature, and even at other times when there is water movement through the soil -- i.e., during most of the year.
Anyway, in addition to passive gain through our southern glass, I have 3 (gifted, old) 4'x8' aluminum-tube-on-aluminum-plate collectors, that I plumbed in to supplement the passive heat. With the slab, no water storage is needed for the active system -- the slab's thermal mass is much greater than any conceivable tank would have supplied.
The backup heat for the circulating fluid comes from two 1500-W electric water heaters (small, 2.5 gallon, heaters -- these actually do keep the house at 65 deg on cold cloudy days in January!) hooked back-to-back with a 240 V supply, thru a 2-Pole switching relay; so no house wiring heaver than #12 is needed.
The interesting part is the controls, whose details I understood by working out the logic tables. There are only a few logical states, and for each state it is simple to figure out what the system should do.
The control box is powered by a 24V transformer, contains 24V intermediate relays (NO SOLID STATE!), and switches the external 110V power to the pump and heaters thru slightly larger external relays whose coils also accept 24V. The little relays (in those days, 2010) cost $3-4 each, and plug into sockets also costing $3-5 each; they have a predicted lifetime of 1,000,000 operations, which would last something like 375 years. The (light gauge wire) connections within the control box were worked out using something they call a "ladder diagram", which is easy to read even if you didn't design it. And light-gauge wires, soldered where needed, will gladly supply current for the relays. (There are marvelous online sources for this unusual stuff.)
The general idea is, the basic heat source is chosen by the box from 2 sources: the external flat-plate collectors, when the sun is shining (they contain a snap-disk thermostat that sends a heat signal to the box); or else, from the 110V heaters . (So never are both sources in use at once.) The sun-heated water from the panels comes quickly up to the current slab temperature, typically about 85 degF; after that the slab passes what heat is supplied to the house, and cools or warms up to suit the conditions. When the sun is shining, the panels generally overheat the house, until at least one of the zone thermostats says "I've had enough", and switches off to decrease the load.
On the other hand, when the sun is hidden, a pipe-thermostat measures the temperature of the water coming from the boilers,, and switches them on or off to stabilize that temperature. In practice, that is the main way in which one controls the house temperature: one adjusts the pipe thermostat to request what is felt to be needed. (There are "zone" thermostats, like the ones regular houses have on their walls, which switch the zones on or off independently; then the water temperature reflects the merged needs of the 3 zones.)
I've been writing this discussion mostly while sitting at the computer, so I suspect many connections may be missing; I will try to supply them in response to people's queries.
There's a subtlety, the "Cloud/sun" switch. Its general idea is, if the weather is sunny, then sun on the panels _and_ passive sun through the windows will both store heat in the slab -- enough heat to carry the house thru the night with no backup. Otherwise, some backup will be needed probably at night, possibly during the day.
And now the bottom line is, I still have copies of the latest descriptive paper-work (operation description, logic equations, ladder diagram, other useful details), and will gladly send copies to any who want them. I'd like to receive a copying/mailing fee of $25, but will take individual needs into account.
Hoping this has been of some use to someone, I am
Jerome Brown in Southern Oregon
geodejerry@gmail.com (I can dispense with the $10 mailing fee on an email request)
8528 Bly Mountain Cutoff Rd, Bonanza OR 97623
(I can also attach requested photos of components and connections, for another $10.)
Now please tell me what's still needed!
...Jerry
