Energy savings preheating well water with a heat exchanger before going into electric hot water tank

Roadrunner0

I plan on building a drain back heat collector and copper heat exchanger in a 55 gallon plastic drum.. Going to put 300 feet of PEX tubing in my attic and use a small boiler pump to circulate the water inside it.. It will sump into a 55 gallon drum to preheat the water going into my hot water tank which comes out of the ground at 45 to 50 deg. F.. I figure if I can get it to 80 deg. F + it will cut my water heating energy usage in half..

BB.

Welcome to the forum Roadrunner0,

Drain back water heating systems are common. Making sure you have the pipe angles set to drain (no freeze/burst pipes) and that they purge the air (possible air vent at high point of system)... And lots of plumbing and possible pump maintenance.

Watch the pipe length and diameter vs the volume of water you will be pumping (GPH) with your PEX heat loop... Long pipe runs and lots of bends/angles with smaller diameter pipe can actually take a fair amount of power (pumping energy) to move the water through efficiently. Ground source A/C system can need relatively large pump motors (1 HP +/-) which ate into the energy savings of ground source heating/cooling (energy saved with ground source--a good percentage was taken by circulating pump).

With PEX--I remember that standard PEX is for use inside walls/under floors... Make sure that you don't expose any to sunlight (attic vent, etc.)... Over time the UV in sunlight will damage the plastic (unless formulated with UV protection).

You might want to do some calculations/estimates of how much energy you can harvest from "attic heating" vs just a roof top collector exposed to direct sunlight... People used to just throw coils of black irrigation pipe (UV Protected) on their roof and pump water through that.

I had read this guy's account of building a drain back system... Very interesting and he listed his mistakes and fixes over the years:

https://www.arttec.net/Solar/BarnHeat.html

-Bill

Roadrunner0

Thanks Bill,
I just want a preheater for the well water as this will cut the net heating from 70 deg. to 35 deg. which is sure to cut the water heating cost.. It is just the wife and myself so we don't use a lot of hot water to begin with.. In the winter I plan to have a second direct loop that will be above our wood furnace on the ceiling of the basement run at full tap pressure which will need a check valve and expansion tank but it will still only be a preheater to take the chill off the water.. Painting the red PEX with black spray paint avoids the UV degradation but the attic is pretty dark so no worries there.. I do see that the small pump uses .75 amps at 120 volts so 90 watts but with 24 solar panels I usually have a 4kw surplus from 10 am to 6pm so it will not be a problem.. I have looked at the option of adding another Radian and having it feed the water tank in a grid zero support mode or adding a small 1 DC element tank to run off surplus but the cost is higher and more solar electric dependent.. I got shell shock when I saw that  5 and 10 gallon water tanks are now more then what I paid last year for a 40 gallon tank..