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Old 11-22-16, 04:42 PM   #31
Mobile Master Tech
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Yep, any "hot fill" bottles will work. After studying (with odd looks from passerby) all different kinds of bottles in grocery stores, the Ocean Spray and PowerAde bottles have the most efficient inner volume to stacked volume ratio, and they are the same height as each other. The Gatorade ones and many others aren't shaped as ideally, but of course will do the job.

Change of plans due to a stroke of luck: I have fired my gas company! I found THIS Sanden CO2 inverter drive air-source heat pump water heater with an 82 gal stainless tank on ebay for $675! This was a slightly freight-damaged preproduction unit. No need to build my own GSHP now; using this and thermal storage is much easier.

It is self contained; the only connections needed are power, water inlet/outlet, and the tank temp sensor. It puts out 149F water until the tank is full regardless of outdoor temp or inlet water temp by varying its pump, fan and compressor speeds.

It produces 4.5kw of heating down to -15F and COP (including pump and fan) for most operating conditions is between 4-6. WSU did a lab writeup in 2013 showing a COP of 2.1 at 17F ambient, which is a 132F lift!

I installed the outdoor unit where it will get a boost from morning sun warming that side of the house, and the tank flowing through an EcoSmart ECOTB240 30 amp PWM electric booster heater. The EcoSmart gives flow rate and temperature, plus makes up any output shortfall and acts as a backup heat source, since my DHW and hydronic heating are combined. The Sanden works beautifully; you have to be within a few feet of it to hear it, indoors or out! The Ecoboost has not had much on-time on the few cold days so far and it hasn't had to go above 50% duty cycle, but I want to reduce it's use to almost zero with thermal storage and passive load averaging.

I suspect my Sanden is slightly more efficient than the one WSU tested 3 years ago, as I have measured it taking 975w with 50F outside/75F inlet temp, and 1175w at 45F outside/92F inlet temp. I relocated the temp sensor from mid-tank to nearly the bottom so it kicks on sooner, as it doesn't kick on with the sensor in the stock location until more than half of the water in the tank has been used. In either location the unit shuts off when the inlet temp from the bottom of the extremely well-stratified tank rises to 125F, so it apparently only relies on the tank sensor to determine when to turn on, and the inlet temp sensor to decide when to turn off. Inlet temp stays at whatever temp water the tank filled with until the tank is nearly full; once at 95F, it rises to 125F/shutoff within 20 minutes.

I've calculated that running it 9 hours per day (most of the heating season will fall within this) costs less in Time-of-Use electricity than my minimum monthly charge for gas service! I'm going to monitor use this season to see if I should upsize my planned PV system, or if I will just supplement with grid electricity.

Next steps are to add thermal storage for the tank and the ground under the house, as the Sanden's output is enough for the winter on average but well short for peak demand. Instead of a 325 gallon tote with individual bottles of sodium acetate trihidrate within, I'm thinking a 77 gallon plastic barrel completely filled with SAT; a pex coil for heat exchange arranged so it is no more than 4" from any of the SAT and a 9w Ecocirc pump to circulate tank water through it; and a magnetic stir stick in the bottom to keep the SAT from losing capacity to stratification. This could be completely removed or updated without disturbing the Sanden or Ecoboost, in case a future homebuyer is nervous.

Instead of my vertical ground loops around the perimeter of the house, I'm thinking of drilling holes for 5' long copper coaxial tubes down into the dirt under the basement, using off-peak heating capacity to keep the center soil around 70F in the summer and 76F in the winter for passive heating. I don't have water table issues and can put several in existing joist cavities at least 15' away from the perimeter, so I shouldn't lose much to the outdoors. Anyone have any input on either of these thermal storage ideas?

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Old 11-22-16, 04:58 PM   #32
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Wow, that sounds like a crazy good deal. Nice find!

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