Minimalist Solar Thermal Radiant Heating

Hello,

I love the permies community because it's based around practical, DIY friendly, baseline functionality. I could post the same question on a pro solar forum and get by the book, to code, answers, but the system would never get installed due to prohibitive upfront cost. So I'm looking for baseline functionality here as a backup heat source and warmer floors (we have a minisplit that currently handles the full heating load).

That said, I have 3 older 4x8' plate collectors that I helped take off a friend's roof, he said some may have pinhole leaks but they all function, and he's a conventional HVAC guy who doesn't want to deal.

My plan was to install a ground mount drain-back system with the 3 panels and a well insulated underground 100 gallon commercial water heater as the storage tank. I have a few questions:

1. Can I run the same water through the panels, storage tank, and radiant tubing or due I need a heat exchanger between the panels and storage tank? This would be a closed system, no DHW for now.
2. What's the easiest way to limit the temperature of the storage tank to say 190*F? I figured a thermostat controlled panel pump since it's a drain back system....
3. If the storage tank stayed below 200*F, any reason not to run pex-al-pex from the tank to the radiant tubing? I plan to run copper from the panels to the tank.
4. Can I run 1/2" pex ~100' from the storage tank to the house radiant tubing (also 1/2") or should I up-size to 3/4" or 1" and adapt down?
5. Has anyone insulated their own buried pex? Prices for pre-insulated seem quite high vs. buying standard pipe insulation at lowes depot...

I really appreciate the advice!

Hi, Zane

I can't help with your question though I though if others are interested in installing plate collectors these threads might help:

https://permies.com/t/57410/Solar-water-heater-plans-safe

https://permies.com/t/76915/Solar-Thermal-Plate-Collectors

Maybe others will speak that can answer those questions.

Hey Zane, cool project!
Firstly, the best diy solar community that I know of is:
https://www.builditsolar.com/
The emphasis there is on doing,with whatever you have on hand.
Secondly,  must your panels be so far from the house?
Even if they must, the storage tank shouldn't be, nor should it be outside, if possible.
Burying the tank doesn't add as much insulation as placing it inside an above ground  box of dry earth would.
Defunct water heaters are easy to come by, so consider more than one tank.

When it comes to PEX, I belive the temperature ratings given are at full DHW pressures.
I don't think a larger diameter pipe is helpful.
It will mean more water in the ground, where the heat is just lost.
I have read that pool noodles make for a good and cheap diy pex insulation.
Anti-freeze can be expensive.
In my opinion the weakest link in the proposed system is the lines that run underground, and they are the place where you eould want to use anti freeze.

The tank being underground was more of a drain-back than an insulation consideration, I was planning to put a LOT of insulation around the tanks, but our house is very small and there is no room tanks in the house, though perhaps in a small outbuilding.

Pool noodles is a genius idea. That probably give better insulation than the standard foam pipe insulation!

I wish I could get the panels closer to the house, but our south facing exposure makes that tricky…

Thank you very much for the considerations

I have a basic question: I was planning to use a drainback configuration with all water, no glycol.

I was going to use an old 120gal commercial water heater for storage, but I'm seeing that all designs spec a heat exchanger between the panels and radiant tubing. Why is this? Concerns about corrosion? Glycol used in part of the system? It would be much easier to pump heated water directly through the storage tank and then floor tubing, of course with a manifold in the circuit to bring the temp to spec.

Thank you! Funny how hard it's been to find an answer to this simple question!

The heat exchanger has a couple of functions, the main one is when you combine your DHW system and your heating systems to make use of the solar thermal storage tank. Mainly this is to add a layer of separation between DHW and a possible glycol leak. Another might be corrosion as you stated. In hydronic heating systems with iron pipes and cast iron boilers, the water in the system is sealed in, and what oxygen is dissolved in the water at the start is depleted by a small (tolerable) amount of corrosion, and minimal fresh water is required to be added over time to keep the system full. These small additions add a little oxygen, but not a significant amount. In systems that develop a leak, the automatic fill valve replaces more and more fresh water, often exacerbating the problem.

In a drain-back solar thermal system, the storage water is open to the air repeatedly, and might be a source of oxygen. Many DIY storage tanks are not tightly sealed either, so more opportunity for oxygen.

A third reason for the heat exchanger, might be to isolate elements of the system from leaks. If you had a leak in your solar panel, you could potentially pump your system dry, and into the yard. If you had a combined fuel/solar system, you would then have big problems, and not the backup you thought you had. (and probably at just the time you needed most for it to work.)

There is a guy near me that has a DIY storage tank in a crawl space under his modest sized home. He has a YouTube channel (David Poz) and a video about it.

Wow thank you so much Kenneth! That is the most helpful information I've received yet for this system design.

My plan was for the drainback and heat storage tank to be all in one - i.e. a 120 gallon water heater that is below the panels so water can gravity drain back into it, but I now see that leaving some air space in that tank is a constant source of oxygen contamination. Very interesting consideration.

My other thought was to use a very well insulated outdoor hot tub as the thermal mass storage with the water heater being a high heat reservoir. The hot tub would be kept around 110F and the water heater around 180F, then at any point if either the radiant floor or tub were desired to be warmer, a plate heat exchanger could transfer heat from the water heater to either the radiant loop or tub.

I really don't need a backup. The house's heating load is fully met with a minisplit which has been the only heat source for 4 years of cold winters. This is really just about wanting warm floors! And using less electricity of course.

Look up temperatures for radiant floors. They don't usually need to be very hot to do what you want.  Might be affected by your floor material and coverings. I know with my design I don't expect to ever need my temp over 110 degrees. But I'm going for a long slow heat, and leave it running for 6 months, with high thermal mass; once it's up to heat, it won't need much to keep it there. And lots of tubes designed in, with emphasis on the parts that will need more, near the edge of the slab etc. And lots of insulation involved.

Yours will be different. Figure out how much heat you want from it and how high of a temp you actually need.

:D

Very helpful, thank you! It's a acid stained slab, so no coverings other than some rugs. The pex is also very near the surface. I ran wider than recommended spacing because it wasn't primary heating for the structure and we were on a shoestring budget, I think it's 18" spacing, but there's still 500' of 1/2 pex in a 720 sq ft slab. It's well insulated, 3" EPS underneath, 4" EPS around footings.

That's an encouraging word for the hot tub scheme. I like the idea of getting two for one in the case of building my own storage tank. If I could maintain 110F in the tub, and it's ~350 gallons that's a lot of thermal mass to heat the building and when it's sunny and warmer, take a soak! Of course it requires a heat exchanger, which isn't the end of the world, but an added expense nonetheless.

Zane Bridgers wrote:That's an encouraging word for the hot tub scheme. I like the idea of getting two for one in the case of building my own storage tank. If I could maintain 110F in the tub, and it's ~350 gallons that's a lot of thermal mass to heat the building and when it's sunny and warmer, take a soak! Of course it requires a heat exchanger, which isn't the end of the world, but an added expense nonetheless.

When money is tight, one approach is to organize the plumbing so that adding on when funds are available are easy to do.

Money in good insulation may seem expensive up front, but most of the time that payoff is measured in years, not decades, and it means if money gets tighter, you may not have to sacrifice comfort to afford food.

Yeah I've plumbed it for the hot tub scheme. I plan to put 6", maybe even more EPS foam around it and on top. I could then pour pumice-crete around that and have a really well insulated tub that should match the performance of an indoor unit. I always insulate all I can upfront. For the solar thermal lines, the limitation was getting them in a conduit, and going from 4" to 6" conduit is a massive jump in expense and harder to source. I opted for 1/2" wall insulation and putting foam down in the trenches, hopefully it will perform alright...

Thanks for the wise advice!

I don't think you need a storage tank. Three solar panels will not overwhelm the thermal mass in a 720sqft slab floor. Do you see the three solar panels working so well that it heats up the floor and the house so well that the heat pump doesn't even turn on once all week in the winter, and then not only does the heat not turn on but it gets so hot that you have to open all the windows and you are still sweating buckets of water? If that can be verified I will send you $6,000 for those solar panel. The unit on the rooftop could also just be de-activated if it gets that hot.

A drainback tank that can hold the water in the three solar panels sounds like a good idea, do you think it's 10gallons of water? This would make it easy to turn off the unit in the summer, repair the system or tie it into a DHW system.  Ditto for a Heat Exchanger.

The windows in your house can act as a solar collector and then you can circulated the collected heat in those room with the pex-pipes in the ground to other rooms.

I’m thinking about a very similar situation. Have you pursued this? How did it go?

shouldn't need heat exchanger for the drain back part.  Here is a really good video on doing drain back systems.



Now I would borrow a stratifier design from this video.  Start at roughly 1 hour and 9 minutes in an go till you get to the stratifier discussion