Zane Bridgers

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!

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.

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.

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 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

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!

Thanks so much for the links to those heat pumps! I am definitely going to check them out!

I have started a thread on our actual build: https://permies.com/t/77046/sq-ft-Passive-Solar-House

Many, many thanks for all the help!

Greetings -

I posted this at Green Building Adviser (see the thread here: webpage ) and the consensus was that this is 1970's design and not relevant to modern building techniques. I'm curious to learn what the community here thinks.

It seems to me there are sort of two factions: passive solar enthusiasts and net zero enthusiasts, the latter being more focused on super-insulated, tight envelopes with minimum windows, PV + heat pump + HRV.

Another point made was that full southern glazing is excessive - the target should be closer to 9-12% southern glazing unless there is a surplus of thermal mass to soak it up. I love glass. I love the outdoors. I get that it's a terrible insulator, but if we can add more by adding thermal mass, I am intrigued.

The final question is whether insulated concrete forms count as thermal mass or not? Seems like it would be better than stick framed for sure, but how much better. My uncle builds with pumice-crete a lot which is locally sourced.

The original thread follows:

My partner and I are building a small (500 sq ft) house in climate zone 5B (NM/CO border). When we lived in Colorado, we had friends who built commercial passive solar greenhouses using south-facing glazing and water tanks for thermal mass. Using their formula, we calculated that we would need approximately 1500 gallons of water to meet their design specs for a self regulating high desert (7k ft elevation) house of this size.

The design specifics of storing that much water in an already very limited living space is an obvious practical constraint. As such, we were considering a couple of strategies, including:

1. A dark colored in-ground hot tub in the living room floor with a reinforced plexiglass cover. This could yield ~500 gallons of water thermal mass.

2. A dark 4" stamped concrete slab floor over 3" rigid foam. Since water is 4x as efficient for thermal mass, call this 1200 gallon concrete slab equivalent to an additional 300 gallons of water.

That brings the total up to 800 gallons, so to hit the target we would need an additional 700 gallons of water storage. Some potential options are:

a. Call it good enough, supplement with solar thermal radiant floor heat and a heat pump and propane water heater. I'd like to avoid the expense of a boiler. We plan to do this regardless.

b. Add another in-ground tub for rainwater storage, same design as above but with a fixed cover.

c. Add a covered rainwater cistern below the slab, possibly tied into solar thermal panels, either outdoor or built into the floor. This could easily start to get elaborate and expensive.

Another major consideration for all of the above are building code considerations. While the thermal mass is desirable, we would prefer not to jeopardize our ability to pass inspection.

I also asked on this forum about solar thermal plate collectors and got some great info and design considerations. See here: https://permies.com/t/76915/Solar-Thermal-Plate-Collectors

Frank - wow! What an incredibly thorough response! Thank you so much for taking the time to write all that up. I will keep this information on hand throughout our remaining planning and build. I think I will embed your response in my first post as well. It's a really great point about combining heat sources using hydronic systems. I've been intrigued by the same benefits myself.

Peter (& Frank) - I too am very intrigued by the combined heat pump / solar thermal setup. The guy with the panels actual has an air-water heat pump he'd throw in for $300, but it's a 3.5 ton unit and older so probably lower efficiency. I don't need more than a 1 ton. That has actually been the issue for me in general. All the air-water or water-water heat pumps seem to start at 24k btu (2 ton) and are significantly more expensive than air-air (I assume because of much lower demand, economies of scale). That makes it hard to justify. Cheaper I'm afraid to just have a separate backup air-air heat pump than tie them together for my purposes.

I do however love the idea of adding a wood fired boiler (forced air with exhaust gas through the embers as per the new high efficiency designs) tied into the solar thermal. Whenever it's really cold or time for a soak, fire that puppy up.

Unfortunately, I have decided to not purchase these panels on the grounds that solar thermal is likely one of the final steps in the building process, and the risk of damage in transport, theft during storage, and tied up funds makes it impractical despite being a good deal for a number of identical panels. I did notice other panels for sale in the Denver area, so I would still love to pursue this when the time comes.

If anyone else is within driving distance of Phoenix AZ and has a need for a system, I can get you guys in touch!

Thanks so much everyone - I will be posting a link to a thread specifically on our planned build momentarily...

Hmmmm, sounds like mixed feelings.

I would think that if space was not a limitation, I could make them make the same capacity for cheaper. The only problem is with construction of a house about to begin, finding the time is going to be tough.

I guess there’s really two considerations. One is whether solar thermal is really worth it at all in this day and age. I’ve read both sides. The other is whether buying 40 year old panels at this price point is wise.

I think the math favors PV+heat pump, except that we are really fixated on this in-ground hot tub, and I’m not interested in heating it with electricity or propane, so that leaves wood and/or solar thermal. The hot tub would also serve as the water storage and therm mass. I am also attracted to the idea of warm feet in the winter.

As for the panels, I worry about leaks as well. That said, this guy has worked in solar his whole life and says he bought these for a project that never happened. I’m not sure there is any way to look for signs of leakage (discoloration, etc.) but that would be big bummer. The guys swears these panels don’t degrade, but in my experience all metal corrodes and degrades with time, so that makes me a bit suspicious.