Yup, It's time to move on. Time to build this thing and hope it performs as well as the collectors do.

If it only works as a heat dump, so be it. That was the original plan, It if also provides preheated water to feed the water heater even better.
Space heating, probably not a do-able. The sun quality is just too poor during the time of year when you need it the most.
But time will tell.

An engineering approach would have provided for heat dumping or intentional undersizing from the get-go, with automated controls for the switchover to keep the tub from becoming dangerously hot.

Yes hindsight is so 20-20 perfect. This whole solar thing started with a timer on the tub and long runs of black garden hose, 400 ft laying around in the sun. And surprisingly enough it worked fairly well, but still need to use the electric heaters to trim.

Not knowing exactly how well flat plate collectors would actually work, I knew I'd have to address the over heating issues the next summer.

So it's summer and time to fix.

Does the house have a crawlspace? No and it doesn't matter, I'm only trying to store heat and later release it to the cold water feed of a propane water heater.

I only take a 5 minute shower every other day and a load of dishes 1 or twice a week so hot water load isn't all that much,
Space heat is from fire wood mostly, aka wood stove, I have forced air propane heat but didn't use it, until I ran out of firewood.
When I replaced the floor in kitchen, dining room and hallway with tile, I installed electric radiant heat which is what I'm using for heat these days.
I was thinking that I might add between joists hot water radiant heat to approx 250-500 sq ft of floor area. But the more I think about it the less likely it will be, solar output during the winter months was pretty pathetic.
In speaking with designers/installers I could never get any or even a straight answer to my 2 questions, will adding your evac tube collector work better that my used flate plate units and how many will I need to add to heat to my 1500 sq ft home? Best estimate was well it depends, even after giving a detailed description of the house structure, windows, doors, siding, and r values. I told them I wasn't interested in writing a blank check.
I figure the easiest way to solve the problem is to add collectors to the array until I reach a point of reasonable heat output during most times of the year.

Well, enough on this thread...

With your low hot water use patterns you're likely to be getting no more than ~30% efficiency out of a tank type propane HW heater. Using the solar as pre-heat for the tank will save you a little on propane, but going to an on-demand type of HW heater would likely save more. (and would be ridiculously not cost effective unless propane hits $6/gallon.)

There ARE no simple answers to the evacuated tube & how many it takes to heat the place, but if your clear wall R-values are less than R30 or so it's gonna be a big number. Used Craigslist flat panels work pretty much like new ones- there isn't a huge performance premium, but getting the most performance out of 'em takes some real design work.

Radiant hydronic floors are a way of getting the most efficiency out of solar, since it takes lower temp, and lower temp in the collectors means lower losses. Lower temp thermal storage also means lower standby loss. Radiant ceilings or high-mass radiators (but not crummy fin-tube baseboards) also work well at modest temps. Thin-line convective cast iron radiators from the 1930s-1950s show up at scrap yards all the time, and can be had for cheap (I've seen 'em everywhere from free, to $10/foot to $30/foot).

No matter how you're heating the place, going nuts on air sealing, and insulating the slab edge are still worthwhile from both a fuel savings and comfort point of view.

OK, I may have to just settle for a insulated heat dump. The hole is already dug, most of the materials are purchased and I need to fill this hole before the dog days of summer (all three of of them ) begin or a small animal or even me falls in.
A 6ft deep hole with rocks will definately leave a mark...

Personally, based on my limited understanding and calculations that once I obtain additional panels, I'll should able to store a significant amount of heat in the storage module. But like everything it's wait and see.
I may have made a mistake and will need to doublecheck my calculations,  but if I'm right or even close it's going to take 10's of thousands of btu's to heat this much dirt just to raise it just one degree. And that doesn't account for heat losses.
And if so, with the current array, we are talking weeks to see any serious changes in the the temperature.  But the controller will deal with the automatic switching.  I just need to do the plumbing and a siple reprogramming and it's ready for new storage module.

So my next class assignment is to find out how to calculate heat transfer of materials,  water to pex to dirt, heat transfer through dirt and back into pex +water.

example if I run 139 degrees in a 100 ft loop, how long will it take that heat to transfer the 6 inches to the output loop.

good questions I'm sure and I curious to find out and what the formulas look like.

Damm I knew I should have taken more physics and thermodynamics classes in collage, but too late now...

In closing I wish I could afford a 1500 to 2500 gallon tank but that's some serious coin, but it would make the plumbing easier.


OK then ttyl

BTW: hot tubs are notorious wasters of energy, even the so called new green models.
I compared notes with a fellow worker who purchased new energy efficient and his calculated energy consumption wasn't much less than mine.  Go figure.
When I first installed it,  a poorly insulated shell, my electric usage was up around 75 kw per day (for a single person), soon it was retro fitted with many layers of insulation, a new 6 inch foam cover and a brick surround.
But even then after all the energy improvements, the bill was still too high for a person on a strict budget. Enter solar, todays usage is 33 kw, and I have a tub waiting. A bit toasty at 103, but flick a switch and the solar controller will dump the surplus heat back into the collectors, once 101 is reached I go back to automatic and it's ready.

Once of the nicest things about solar thermal is once the equipment is brought and paid for, it's basically free energy to all who need it.

Thanks for the suggestion for Craigslist,  I'll have to monitor a 500 mile radius and see if I can pick up a 1/2 dozen panels for cheap.

If we could only produce solar electric panels for less than 0.15 per kw