[Mobile Master Tech]

I've been away awhile, time for a system review and a few changes! Life....

I'm choosing a combo active/passive system. The passive part requires warm soil under my home, approx. 73F instead of its' natural 62F. This will cause heat to move into the house during winter temps but not during the summer, as I would never have the AC set lower than 74F. I have about 2200sqft of uninsulated basement wall and slab, which will be bare or covered in hard flooring or drywall for an approximate U-value of 0.6. A 70F interior temp and 73F soil temp will move about 4,000btu/hr into the house instead of my current loss to 62F soil of around 10,000btu/hr. This passive "heater" reduces its output gradually and automatically to zero as the house approaches 73F. That's a difference of about 11.5 million BTU per month. My max winter usage including DHW and cooking in the last couple years has been about 20 million BTU/month. This continues to go down as I convert to electric cooking, seal gaps, replace windows, insulate, etc.

The active part is collecting heat from the back of PV panels (heavy gage aluminum foil & pipes silicone'd to the back of the panel) and injecting it into borehole PEX loops surrounding the house, then pulling heat from the ground as needed for DHW and hydronic heating using a hacked GSHP. I've decided to do away with the boreholes in the middle of the basement and just let the perimeter heat work its way to the middle over time. There will be more than enough BTU's throughout the year with a 73F heat sink cooling 300+ sqft of PV panel. By the end of winter when I've taken quite a few BTU's out of the ground, the days will be getting longer to provide more.

One Laing variable speed sealless ECM pump taking 25-60 watts will circulate 30% ethylene glycol (enough to prevent burst damage to any temp, freeze protection to 0F) through the closed system: ground loops, then the collectors, then the flat plate heat exchanger for the hacked 2 ton GSHP, then back to the loops. Since the ground loop temp will be around 73F, putting the collectors before the HX means the GSHP may often see inlet temps over 80F, making it very efficient as it charges the buffer tank for the day. A diverter valve will bypass the collectors when their temp is below the ground loops but the GSHP needs to run, or if the ground under the house ever gets too warm. If the collectors have heat to give, but the GSHP doesn't need to run, the fluid just runs through the GSHP HX unchanged. If the GSHP ran continuously it will put out 17 million BTU/month, well more than the 8-9 million BTU max shortfall I project and nearly as much as my current usage.

The output plate HX of the GSHP will use a smaller 15 watt Laing pump to circulate water from a 330 gallon IBC tote made of HDPE, available nearly everywhere for $50-100 and occasionally free. The HDPE can handle 230F continuously, unlike the Linear LDPE totes that can only handle 150F continuously.

I decided against a large water tank as the buffer for my hydronic floor heat/DHW. Water is nearly free, but the ready made tanks or even the materials to build a wooden one cost more than cheap PCM's. My purpose is to pioneer a DIY, easy for others to reproduce, low cost system made from materials that can be sourced anywhere and are cheap and preferably recycled.

A 330 gallon tote can hold enough heat to ride through overnight cold snaps and low PV output if it contains Phase Change Material. Most PCM's and their containers are too expensive. Candle wax (paraffin) is a great PCM, has other prepper benefits and can be bought recycled for $0.40/lb or new for $0.55/lb from firestartersonline.com. Their wax has a melting point of 127F. For slightly more, you can buy waxes in bulk having any melt temp up to 150F from many places to avoid shipping. Goodwill, other thrift shops and some stores throw away their damaged candles. An enterprising DIY'er could get a lot of wax free. Candle manufacturers may have cheap or free scrap wax. Wax used for taper and pillar candles melts at 133-145F, better for DHW than the wax for tealights, votives and container candles which melts at 127-133F. Wax is stable, holds around 200J/g latent heat and isn't any more of a fire risk when encapsulated underwater than a wooden house: set it and forget it.

Sodium Acetate has many uses, including flavoring your salt & vinegar potato chips: it's cheap, easy to make/buy and known as Hot Ice. The stabilized Trihydrate (SAT) form holds over 250J/g latent heat, melts at 136F and is much more dense and thermally conductive than paraffin. It is hard to keep stable, but it appears doable.

Ecorenovators, please post if you have info on keeping SAT stable or good sources for SAT, paraffin or any other worthy PCM's in the Atlanta area! I can make my choice of PCM the last step.

PCM containers: free! The food packaging industry has given us special "heat set" PET plastic bottles, used for hot-fill of food products, juices etc, allowing in-bottle sterilization with no preservatives added. They are good for 185F fill temp and some even go up to 205F, far higher than non-heat set PET's maximum of 150F. Their perimeters are rigid but they have flexible panels to allow for expansion/contraction of contents, so they won't collapse or fatigue as a PCM changes volume during phase change. They have some of the lowest permeability of any plastic, and are impervious to diesel, oils, etc. PET isn't recommended for gasoline, mineral spirits or straight naptha, but you can put nearly anything else in them. Caps are made of PP (polypropylene): no problems there.

I have settled on 64oz juice bottles shaped like Ocean Spray's and the 32oz PowerAde bottles-very common, very strong, very compact stacking and I create my own supply or pick them out of recycle bins. Both have the same 3.5" cross section so it won't take too long to add/release heat from the PCM and are stable to stack. Both will easily fit through the 6" top opening of an IBC tote. They can be stacked in a pattern holding 234gal of PCM in a 330gal tote, while allowing space for circulation around the bottles. This allows me to use only a 20F temp swing from 125F to 145F with plenty of margin on either side of the melt temp to allow for supercooling. This tank will store over 200,000BTU using paraffin and over 300,000BTU using SAT. That's $800 or less for the paraffin, even cheaper for SAT. Using PCM's would also negate the need for any kind of mixing valve to regulate output temp (KISS principle). If using paraffin, rolled or folded heavy gage aluminum foil can be stuck in the bottle, making a thermally conductive path to the PCM in the center.

I will suspend a 30 meter(98ft) corrugated 25mmOD stainless pipe coil having approx. 24sqft surface area in the top few inches of the tote as my water heater for DHW and hydronic heat. Carlsonhx.com's calculator says a 20sqft stainless plate heat exchanger can use 135F input water to heat 2.5gpm of water from 68F to 130F-good enough for my purposes, since my inlet water temp will never be below that. My open loop hydronic system brings incoming water through the floors first, which would pull heat from the house if the water is colder. While I wished for a hotter shower at a hot water supply of 120F, I am happy with my shower and my floors can keep up with cold weather at 125F. Using a PCM that can maintain tank temp at 130F or above gives me some wiggle room and is hot enough to prevent Legionella bacteria.

A plate exchanger with pumped water will be more effective than a corrugated exchanger relying on convection to move fluid around it's exterior, but my coil has 20% more surface area than Carlson's largest HX and I could always add a circulation pump to move water around the tank. The first 4.5 gallons already in the coil will be at tank temp anyway once a hot water draw begins. Insulation will be 4-6" of reclaimed polyiso foamboard, plus 1" of reclaimed ductboard on the sides and top since it has a foil radiant barrier, all held together with UL 181B rated foil ductwork tape.

Search my other posts-other concepts, controls, parts of the system and construction methods are detailed there. This has been and will be built by "an army of two...."

I built a fence this year when it was unseasonably wet. I dug extra deep on a couple post holes, so I know my water table is at minimum 8ft below my slab level even with biblical amounts of rain. I believe it will be 20ft below or more, giving plenty of soil under and around the house to hold heat with. Time to get back to it!