The question comes up again and again: what type of valve should I use for my geothermal unit?
Now we are talking about a fresh water, “pump and dump” system. This is an application where ground water is pumped through the unit and then dumped back into the ground. Usually people who have this type of setup have two wells.
One well will have a constant pressure pump installed and supply water for the residence as well as the geothermal unit.
The other well is the “dump well” for the geothermal unit. It will have the discharge pipe from the geo unit, and this is where all the water goes after the geo unit has extracted or added the BTUs it needs for heating or cooling.
Typically, the discharge pipe will be lowered into the well so that the end of the pipe is below the water level in the well. This way the pump does not have to work as hard and uses less electricity. That’s good for everyone! See, the weight of the water in the discharge pipe is pulling the water from the one side, while the pump is pushing water from the other side.
This setup has to be configured properly to work though. You must maintain positive water pressure inside the heat exchanger of the geothermal unit. The weight of the water in the discharge pipe will cause the water pressure at the top of the pipe to drop into a vacuum (below atmospheric pressure). The lower the water level in the well, the deeper the vacuum will be.
Why is this important, you ask?
It’s because of the boiling point of water. At 53 F, water will boil if the pressure drops to .196 PSIA. That’s well below atmospheric pressure, but it would only take about 33 feet 10 inches of water column in the discharge pipe to achieve that. The water cannot be allowed to boil in the heat exchanger. This would hamper the geothermal unit’s ability to transfer BTUs, cause inefficient operation and ultimately damage the refrigeration system inside the unit.
Therefore, the flow control valves must be located on the discharge pipe of the geothermal unit. This maintains water pressure inside the heat exchanger at all times and ensures adequate heat transfer, regardless of the water level in the dump well.
How I found the system
I started servicing a two-stage ClimateMaster geothermal unit for a customer maybe about five years ago. It was setup with dual flow control valves on the discharge pipe. See Pic A. It had a Taco Zone Sentry, followed by a Watts Flow Setter on each branch of the manifold.
The reason for the dual flow valves is to reduce the amount of water that has to be pumped through the unit. This was a two-stage compressor. The first stage runs at 70 percent capacity and second stage kicks in the last 30 percent to bring it up to full capacity. Likewise, the flow control valves are set up so the one branch provides proper flow while the unit is in first stage. The second branch opens up and provides the additional flow needed when the unit goes to second stage.
This was all well and good, save for the fact that the Zone Sentry wasn’t closing properly and allowing water to leak through, wasting it down the dump well. I pulled the valve out and noticed the ball was a little chewed up. That wasn’t a big shock because it was the wrong valve for the application. I replaced it with a Taco Geo-Sentry valve. This valve is manufactured just for this purpose. They have a different type of ball in the valve that is better suited for freshwater applications.
I kept on servicing the unit annually without any issues until this last year. I noticed that the Geo-Sentry valves were now leaking water when they were supposed to be closed.
Where to go from here? Do I replace the valves with the same thing and expect to do it every three years, or do I look for a new solution?
I consulted the homeowner on the subject, and we agreed that finding a new solution was the best thing to do.
So, the search begins for a new valve. If it has anything to do with geothermal, my search always begins with a phone call to Bill. Bill sells these systems, and he has made it his life’s work to know everything about them. But better yet, he makes sure he stays available to help you out.
Just this last winter, a Tranquility 30 Geo unit I installed stopped working. The customer called me on a Sunday morning, one of the coldest days of the year. He didn’t have any heat, and he was having a fit. This is one of those guys who is very excitable, and everything must be “just so.” I got an earful that morning.
I promptly headed over to troubleshoot the system. I looked at everything I could think of, and I couldn’t for the life of me figure out why this unit wouldn’t run. It would turn on briefly and then turn off on low water temperature. I checked the pump thinking maybe it wasn’t working. To pull the pump, I had to stand on my head and play Twister. The manufacturer has it neatly tucked among the pipes and wires, ensuring that only the most stalwart of souls can remove it on a Sunday morning without muttering a few profane words.
Undaunted, I attacked the pump removal with vigor, encouraged by the endless stream of questions that the customer wanted answered immediately. He had brought his chair down to the basement and was sitting there, unsympathetically, watching my every move. Finally, against all odds, I removed the pump.
There was nothing wrong with it.
So now I had the pleasure of going through all the unpleasant contortions in reverse to reinstall the pump. All the while getting berated with an entirely new set of questions from the customer.
That done, and nowhere else to turn, I placed a call to Bill. I nearly dropped my phone when he promptly answered. When’s the last time you got through to your distributer on a Sunday morning?
I explained the situation, and he knew exactly what the problem was. This geo unit has a Grundfos Magna on-board loop pump. The pump operates off a Delta-T platform to maintain correct flow through the unit. It can vary its speed from 15 to 100 percent, and there lies the problem. When the unit cycles on, it always starts the pump at the speed it was operating at when the unit last shut off. Somehow, the pump had reduced its speed to 15 percent before it shut off the last time. When it turned back on, it started at 15 percent, but this was not fast enough to break the inertia of the water and start flow. Therefore the unit would shut down on a low temp error.
This was an oversight from the manufacturer, but fortunately Bill had a fix. He instructed me to program the control to the parallel pump option. This would be the option that you would choose if the unit is controlling two pumps in parallel. One of the features of this setting is that it does not allow the pump speed to drop below 50 percent. That solved the problem for this unit.
Everyone needs someone like Bill to call.
Back to the task at hand.
Bill said they have been using irrigation valves for this type geothermal application. That came as a complete surprise to me. The agriculture industry and the HVAC industry are not very closely related. Who would have thought?
I did a little bit of research and it turns out, these valves are very well-suited for this type of application. I settled on a DIG 24 VAC inline solenoid valve. The solenoid acts on a fairly large diaphragm — maintaining positive control over flow and ensuring a tight shutoff. The valve has a flow limiter built right into it which is downright nifty. The knob on the center top of the diaphragm body turns, tightening or loosening the spring tension on the diaphragm and regulating flow like a pressure-reducing valve. Plus, it has a little white lever on the front of the solenoid for manual operation. This allows the flow rates to be set and checked without having to wait on that eternal five-minute delay when cycling the unit on.
I presented my proposal to the homeowner and explained the function of the new valves. He is a mechanical engineer and likes to know all the details. He agreed that we should get the project done without further delay.
What is cavitation corrosion?
Cavitation corrosion is an interesting phenomenon that has the potential to pop up anywhere there is fluid movement. It has plagued many different industries from diesel engines to large aqueducts, and hydronics, of course. I will explain it in a minute, but first, let’s get a visual.
When I removed the flow valves from the geo unit, I had to pull everything apart to see what happened to them. My natural curiosity would not let me do otherwise. What I found was very interesting. I had seen cavitation corrosion before, but never quite to this degree.
Take a look at Pic C. This is a photo of the outlet end of one of the Watts Flow setters. Notice all the pitting in the ball? It begins right at the edge of the ball and is pitted back to what would be the centerline of the pipe, or the crown of the ball as it is positioned.
Now check out Pic B. This is a photo of the outlet of the Geo-Sentry valve. Although it is not pitted nearly as bad, you can see some damage on the leading edge. There was enough metal eaten away to prevent a positive seal when the valve shut off.
What you are looking at in those pictures is cavitation corrosion, and here’s how it works.
Take the Watts Flow Setter for example. The ball remains positioned as you see it to create a pressure drop in the piping circuit and regulate the flow. Water enters the inlet of the valve at 60 psi and exits the valve at close to 0 psi or perhaps a vacuum. Six gallons a minute are forced through the little half-moon opening left by the ball. This turns the water into a high velocity jet.
Bernoulli’s Law states that an increase in velocity demands that the pressure of the fluid decrease. That might be difficult to understand, but think of it this way. An increase in velocity increases the kinetic energy of the fluid. If the fluid pressure stayed the same plus and increase in kinetic energy, the fluid downstream of the valve would have as much energy as the fluid upstream of the valve. The result would be that the flow setter would have absolutely no effect on the flow rate of the fluid.
The area of the lowest pressure in this jet is right at the outer band the instant the water passes the lip of the ball. Right at this point the water would rupture, allowing little bubbles of vapor to form. One could say it was boiling right at that point. As the little bubbles of vapor move further away from the point of low pressure to an area of higher pressure, they implode violently, sending shock waves through the water.
But how can that damage metal, you ask?
It’s time to look through the microscope. If you looked at the surface of the ball though a high-powered optics, you would see that smooth shiny surface give way to jagged peaks and deep canyons. You would see one of these tiny vapor bubbles drift into a canyon and suddenly implode as the pressure returns it to water. The resulting shockwaves would tear away those jagged peaks before your eyes.
It happens on a small scale, but given enough time, it can cause an abundance of damage.
Some types of plastic are much more resistant to cavitation corrosion. That’s because plastic is more flexible. While the implosion shockwaves break the rigid metal peaks, the plastic peaks can bend and flex in the ensuing gale.
The new piping arrangement includes two solenoid valves, which are piped in parallel followed by a visual flow meter.
The flow meter and manual levers on the solenoid valve allow for quick and easy flow setting. The valves are also slow-closing to prevent water hammer, another destructive force. I am looking forward to watching this system over the years to come and see if Bill got this one right.