Need a Crash Course on Open Loop Systems

Hi all,

First post on this forum. Like the thread title says I need a crash course on open loop systems. I am very experienced with close loop but have to admit I have never installed an open loop.

First some background on my experience level to give you an idea of my skill level. I installed my first geothermal system in the early 1990's around the time WaterFurnace introduced their Premier AT line of equipment. This is going way back, they had a program where if a contractor bought 12 units in a years time they would give you a Mac Classic PC. I know I am showing my age. After loop school in Fort Wayne I tackled my first geo system ever. Large custom home with 4 units totaling around 18 tons. Horizontal common loop with a couple of 3/4 hp circulators. After a few more projects I got the Mac Classic in about 6 months time.

For the next 10 years I was heavy into geothermal installs. Several custom homes like the one I described above. A good deal of commercial ranging from 10 tons up to my largest of around 200 tons in a two story office building. VFD drives on the pumps, nice energy management control system... all the cool stuff for that time period.

My job changed and for the last 15 years or so all I do is replacements and a couple of nice custom homes a year. Nothing difficult. So when asked by a building contractor if I could install a geo unit on an open well I said: sure, I can do that... how hard can it be after some of the other projects I have done. Thing is closed loop is easy, open loop if just running the geo unit is also easy. This project will use the well for domestic water needs along with my unit..tricky stuff.

I haven't talked directly to the well contractor yet because I am still in research mode and don't want to look too stupid. The builder has told me it will have a 5 hp variable speed pump. Not sure if he is planning on a jet or submersible. He also will be installing (3) 100 gallon bladder tanks with 30+ gallons drawdown each based on your typical 60/40 psi.

And that is all the info I have at the moment. I understand the basics of open well systems and the normal pressure reducing valve on the inlet of the coax. My unit is a 6-ton dual stage and using 1.5 gpm/ton to run some math.

So my first question is based on the information above what would be the ideal design money not being an issue?

I look forward to comments and maybe a good thread to help me wrap my head around the details of this project.

regards,
Axis

 

Why open and not closed loop as the energy to run a large pump will be higher than a closed loop?

 

I'm only an owner of an open-loop, not an installer. But one thing to remember with open-loop is that you need a way to get the water in (ie. the main well), and a place for the water to go. In our case, that's a second well on the opposite side of the house. Our original open-loop ran the discharge line out into the woods and a ditch, but that's not code now. So when we had a house fire, and I knew I wanted geothermal again, we had to get a second well drilled (we reused the original supply well). No pumps on the discharge, but depending on your terrain, drilling that second well may be mighty expensive.

For the supply, you'd probably want a constant-pressure submersible pump. Constant-pressure so you don't have to mess around with pressure switches to avoid short-cycling the pump (there are other options), and submersible because I have yet to hear a jet pump that wouldn't get super-annoying whenever you were heating/cooling.

It wasn't an option for us, for a variety of reasons, but I'd not discount going closed-loop. You'll at least want to cost both out to get a clear picture.

 

Depth of the water you are pumping is important in open loop. You mentioned a 5HP, which makes me think the well is very deep. Even with a variable speed controller or a Cycle Stop Valve a 5HP will only drop down to maybe a 3HP load. It will be very expensive using a 3HP load when all the heat pump needs is 9 GPM. 9 GPM would only require a 1/3HP if the water was less than 50' deep. Might want to consider closed loop in areas where the wells are deep. You will also need a place to dump the 9 GPM flow. This can be dumped on the surface or back to another well. It can even sometimes be dumped back into the producing water well. Mine uses a 1/2HP well pump drawing about 1/3HP load, and dumps to an earth tank where the water was going anyway for livestock.

BTW, you either need a variable speed controller OR three large pressure tanks, not both. But a Cycle Stop Valve and small tank would do a better job than either of those. You might also be able to install a low pressure and less horsepower pump in the well, which could fill a cistern with a pressure pump for the house and a small circulator type pump for the heat pump. I am all for open loop when there is a place to put or use the dump water, but you got to keep the well pump horsepower low to be able to afford to pump that much. A 5HP for house water is not that big of a deal. But for a heat pump it would be expensive.