Zane Bridgers

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since Jul 28, 2014
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Recent posts by Zane Bridgers

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:

Many, many thanks for all the help!
11 months ago
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:

11 months ago
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...
11 months ago
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.

11 months ago
Hello -

I have the opportunity to buy a lot of 15 solar thermal plate collectors for $2k (~ $130/piece) They are black chrome 3x6 collectors from the 80’s, drain back design and still appear to be in pretty good shape.

These would ideally be for some combination of an indoor hot tub, radiant floor heat and DHW in a 500 sq ft house in Northern New Mexico.

Does this strike you guys as a good deal? I have to transport them quite a ways so I’m trying to make an informed decision.

I know a lot of the net zero home guys says it’s a better bet to use PV and a heat pump these days, but that’s for a new install and definitely less eco than recycling old panels. I also like warm floors!

Any help would be much appreciate. I’ll be posting on the planned build shortly!

Thanks so much for everyone's excellent feedback! I quote Frank's detailed response for future viewers:

frank li wrote:
I have been repairing and installing solar thermal hydronics for 14 years and rarely see a failed collector that is made of metals and glass.

I have installed seveal systems with used collectors, including my own and dozens of systems featuring new collectors, flat plate and evacuated tube. The tubes do perform better in very cold ambients, but this is michigan and when a person has a 30 plus year old flat plate system that fails or needs to be re-installed and fired up after a re-roof, it is common to hear that they couldnt possibly consider, not fixing or replacing it, it saves too much money!

That is from long time owners who originally had them installed and from people who moved into them.

If you do a pressure test on them and they hold air, they are likely good for another 40 years!
Look for corrosion and sediment coatings on the inside of the waterways. If the ways are clean and not corroded, they are a great deal at that price.

Collectors (4x8 and 4x10) were going for $250 from AAA solar at new mexico last time i checked.
A new Steibel Eltron or Heliodyne will cost you $1200 to $1400 each plus shipping, $900 or so for the smaller 3x6.

I have seen 150 deg F fluid flowing in flat plate collector loops under less than full sun and below freezing outside, overcast, not so much, but the colder the storage temp falls, the higher the scavenging of heat from the collector, so you will at least pre-heat on most days.

Just like pv, expect 5-10 percent energy output under moderate gray sky, and almost nothing on heavy overcast days, occasionally.

Location matters. And a reverse cycle chiller type heat pump will start at around $3500 for a high quality machine, you would still need storage, distribution, circulation, control, pv panels, wiring, mounts, integration materials like power electronics, transfer switching, etc. Depends on what you have for energy systems existing too.

One of the cool features i have seen are heat pump systems that use a transfer from pv to 240vac.
Of course you could just run it off of your house inverter, but some systems do not want to have much additional loading or pv capacity without renovation or upfit.

Still, heqt pumps are great solutions where appropriate.

The cool thing about hydronics is being able to integrate multiple sources of heat to storage and distribution.
There are relatively few components to a hydronic system and service intervals for failure are 15-20 years or more. Much of the system parts are common to the plumbing industry leaving collectors and controllers as specialty items.

Wood/biomass, electricity, gas and direct solar thermally produced heat can all be added to liquid storage easily and distributed from in a manner that allows new inputs and outputs efficiently.

Storage is easy and lasts 30 years or more. Very long lived if you have stainless, possibly lifetimes.

Pv prices are going back up... soon they will be back to 2007-2008 prices, is my guess.

270 square feet of collector surface is quite a few btu on any half sunny day as long as you have a low temp loads. Perfect for domestic hot water, space heating with radiant hydronics embedded in mass and keeping a hot tub heated with the least amount of fuel. Depending on what insolation (sun availability) and coldest temps and wind factors of course.

If those collectors look good inside and out, and the insulation and backing are in good shape, they are worth 10's of millions of btu per year for dozens of years on most of north america.

11 months ago
Hi there friends -

Here's the question:

Do you think an old oil residential cast iron boiler or a newer tankless heat exchanger coupled to a rocket stove would work better to raise hot tub water from ambient to 110*?

I understand these units serve different functions. The tankless heater exchanger is made to immediately raise the temperature of a small amount of water 30-50* while the boiler is meant to raise a lot of already hot water 20-30*. In a sense, I need aspects of both of these systems, but which do you think would work better.

Both systems would be paired with a 750gal per hour pond master pump (not thermo-siphon)


I built a ferrocement 500 gal tub and a 7" J-tube rocket stove with permalite-concete riser insulation. It feeds into an old 50 gal water heater tank which has a 10 gal compressor tank (not sealed - ie no pressure) inside with a 30 ft copper coil. This was based on a YouTube design, perhaps from a permies member though I don't recall.

The heater can get 10 gals of water scorching hot in 10 mins, but it's just not up to heating the whole tub. Therefore I am looking at re-purposing an old commercially designed heat exchanger. I understand the safety concerns with steam pressure etc. but so long as the pump is running, I don't see that as a real risk unless my rocket stove magically starts putting out another 100k BTUs.
1 year ago
Hello Folks -

We recently acquired 100 free 150w solar panels and accompanying inverters/shut-offs for a grid-tie system. We want to ground mount them on our 8 acre property. We don't have the money to hire a contractor at $22k and even quotes for just electrical were $14k.

I would like to do the work myself. I have the time, and am willing to do my homework, but I would be curious what other's experiences have been.

The structural stuff doesn't seem too daunting. I have a spec sheet for California ground-mount requirements for 105mph winds using 1.5" SCHED 40 steel pipe and have already drawn up those plans (who can have more stringent regulations than CA, right?) We need our plans approved by an architect to get a permit from the township, but again, this is doable.

The electrical is where it gets daunting. The panels are rated for 34.5Vmp 43.5Voc 4.35A. I've already planned out the panel arrangements - we plan to use 88 of the panels (two rows of 4x11 panels wired two series of 11 in parallel for 473Voc 9A per 22 panels. These four 473VDC lines would run to a j-box, solar breaker box in the nearby shed, then 100ft to the DC disconnect, inverters (Sunnyboy 550VDC 13A continuous), combiner, AC disconnect, meter and finally the main OCPD (breaker box). I won't get into grounds here. I feel I could figure out all the wiring until the meter, since everything can and will be dead up to that stage. At this point, before powering everything on, I could have an electrician/solar expert inspect my work and do the final hook-up.

This would involve me drawing up my own electrical plans, and even more challenging, finding an electrician willing to just do that final step. They all seem to want bigger contracts or nothing to do with solar.

For those of you who have been through this process, was it manageable? Worth it? Things to consider/avoid/re-consider? Even if you've only worked on DC off-grid systems, your experience is welcomed!
4 years ago
Thank you kindly Allen!

That design does seem pretty foolproof. Ironically, it reminds me of Geoff's design on youtube (I know I know):

Just to clarify the copper coil in water bath design. Is this for efficiency or safety? I know it's a big no no to have copper coil directly exposed to high heat, but in an open system like this, it seems just as logical to keep the boil-off tube, but just circulate water directly through the tank. I suspect therefore it's water-to-water heat transfer being more efficient than air-to-water...
4 years ago
I greatly appreciate your warning! While it makes intuitive sense that phase changes in pipes could result in a pipe bomb, I'm not sure I grasped the severity (just watched the mythbusters to convince myself).

Now as for this design, the existing gas water heater has a burn chamber and vertical flue (heat exchanger). I expect the risk with a direct rocket stove conversion is the increased fuel/hotter flame. If enough of the water turned to steam too quickly without an escape outlet, that would mean a big explosion.

However, there is one blow-off valve built in (likely inadequate for increased burn temps/solid fuel mass) so could't I simply drill a number of large diameter holes on the top of the unit as extra precaution? So long as it does not hold pressure, there is no immediate risk, correct?

Now, as for the actual rocket stove design, it needs to be far enough below the unit for the exhaust to heat and re-burn before coming in contact with the water. I suppose that's a calculation I can do with some searching.

What about the heat exchanger? Any thoughts on directing the hot gases around the exterior of the tanks verses through the existing central pipe?

Many thanks for all the support and wisdom!
4 years ago
Thanks so much for the aprovecho resource (turns out I have some friends interning there - a pleasant coincidence!) I had to drop the project for a while but I'm back at it.

I attained a free 50 gallon gas water heater. I took off the bottom to expose the natural gas burn chamber, which funnels straight up through the water tank.

It is this design:

Seems like an inefficent design to me, but they mass produce these things, so you would think they are fairly efficient. If I were to put my J-pipe below the flue and seal it all up, would that suffice?

The aprovecho water heater filters hot exhaust around the entire water tank which seems better (more surface area). In the existing gas heater, there is a gap around the tank filled with insulation which I could remove and then filter exhaust around. Then I would need to re-insulate the outside of the unit however. Anything I should watch out for?

Either way, I plan to dig the J-pipe/burn chamber into the ground and surround it with perlite cement mixture.
4 years ago