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Getting pumps pulling in solar hydronic heating - and advice of sizing.

 
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Hi, It is coming up to winter in New Zealand, and I am trying once again to get my summer outdoor shower with no water storage (240m of coil gives 15m of hot water stored in the coils) to turn into winter underfloor heater with wood stove back-up.  My biggest problem is getting the system to fill with water and say fill of water.  Esp for example when I am trying to pump from the storage tank up over the top and then back down to the underfloor heating.  The pump is not meant to be run dry and won't draw water from dry, but how do you fill the system with water in the first place?  Have tried various ways pouring water in with little success.

I should add that we are 8km from the beach and the minimum winter temperature on record is -4C, and while -2C frosts and ice on puddles are common in winter, the pipes to the water troughs in the paddock have never frozen, so a drain-back capacity is not needed.

The second question is advice about pump sizing.

The underfloor heating will be a little over 300m of pipe in 50 coils, and will need to push back up about 1m to that top of the tank.  

The initial panels are 4 coils with each coil being 1m in diameter, using 60m of pipe, and 25 turns.  (so a total of 240m of pipe, and 100 turns.  These have been flat on the ground, but I want to rise them to about 70 degrees from the ground for maximum winter sun).
I will then also later cover the sun facing wall of a green house that is 1.4m high x 8m long with a bank of black alkathene tubes that are rising.  

 
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If you are hoping for a thermosyphon system, than the system needs a consistent gradient with the heat source at the low point, and the heat storage at the peak without high spots or air traps. One air bleed at the top to evacuate trapped air, no motor required.

If you have a positive displacement pump (gear, piston) than any combination of gradients can be forced to work with enormous grief and excessive delay while it forces the air through the system if there is a place for the air to escape the piping system into a storage reservoir, once a hydraulic lock has been achieved (no air in the circulating system) than system efficiency has been achieved, and a remarkably small motor is adequate.

If you have a slip type pump  (centrifugal, vane, screw,) than it is a balancing act consisting of horsepower vs air compression rebound , and sometimes air rebound wins over a fairly large motor,, multiple air bleeds are you friend in this case.

For a real world example, at my house where it drops to - 10 Fahrenheit I have 2, 150 foot runs, leaving a boiler dropping 5' below ground looping through 2 furnaces, 2 heat exchangers for domestic hot water and 2 clothes dryers, with a 700 gallon non pressurized tank at the high end.
A  (1) Bell and Gosset 1/25th HP pump 3/4" is adequate for both runs because efforts were made to ensure there were no high spots to entrap air and once the water is pushed through all entrapped air escapes via the tank vent. I pump 180 (F) degree water through and choke off the return line to equalize temperature (around 150 (F) once the system equalized) between the two runs.

Conversely I've seen industrial jobs that required a two HP centrifugal pump for less than 1000 linear feet of piping and were a constant headache with hours of time devoted to purging air every single time the system was opened.
 
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That is going to be a lot of resistance... I would try one of the grundfos pumps with the strength selector switch and definitely push the water into the coils through the pump not try to pull it. I would force pump water into the coils either with household hose pressure or using a wobble pump to draw it in like this one: https://youtu.be/fWg-AKPcYRk
You will need to make sure you have air bleed valves so no air gets trapped which is an ongoing process as entrapped air bleeds out of the water as it heats and cools... some things to consider...
Cheers,  David
 
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