The new GSHP (Tranquility 27) was put into operation last week, but a water flow problem has been noticed. The design is open loop, with water from the heat pump returning to the well. Downstream of the heat pump there is a Taco 5101G "Heat Motor Zone Valve," Hays flow regulator, and a Blue-White flowmeter. The well pump/controller is a Franklin Subdrive 75 VFD, with a rather small pressure tank. The control on the pump keeps pressure fairly constant within a very small range.

When the system is running, the flowmeter shows that the regulator is indeed spot on at the specified flow rate. After the system stops, the Taco valve closes, so that the well pump isn't constantly circulating water through the loop. However, I have noticed that there is a very small flow leaking past the valve. It isn't enough to show on the flowmeter, but the leakage drops the pressure in the pressure tank enough within perhaps ten seconds to turn the pump back on again. Closing the shutoff valve upstream of the heat pump ends that tiny flow, so I know that it is the Taco valve that is not closing tightly. I don't like the thought of the well pump turning on and off endlessly when there is no other water use in the house.

I couldn't find much on the Taco-HVAC web site. The valve design allows the head to be twisted off from the body. I may drain the line and take the head off to see if there is anything I can see, like some debris that won't flush out when the valve is open. If it isn't debris, the only other thing I can imagine is a defective valve. Does anyone here have suggestions as to what the problem may be?

Nope no other suggestions. As you said, check for debris first, then you may need a valve.
One this is likely covered under warranty, so you want to contact installer.
J

1) debris (79% likelihood)
2) valve does not close properly even without debris (20% likelihood)
3) something else (1% likelihood)

Just my guessed prediction based on my experience....

Taco 5101G’s normal flow range is specified as 1-4.5 GPM. http://www.usagency.net/taco/PDFs/ct100_3.pdf Does this mean the valve “leaks” 1 GPM in its closed position? Regards, Masoud

You have identified the major drawback to the vfd line, over sensitivity. A very small leak or drip will cycle the vfd on and off quickly. I agree that the seats on the taco line is very sensitive. Debris, sand, teflon tape, ect. will cause the problem you are experiancing. In my kneck of the woods the thermo-plastic valves comonly used on irrigation systems are preffered over the costly taco's.
Eric

This document:
http://www.taco-hvac.com/uploads/FileLibrary/10-3.2.pdf
says the 5101G normal flow range is 4.5-6 gpm. My flow regulator is 6.0 gpm, and that's what the flow meter reads when the valve is open. The table also says the valve is good for 150 ft of pump head, or about 65 psi. The Franklin Subdrive pressure regulator is set at 50 psi.

Edit: that URL doesn't work if you click on it. Go to www.taco-HVAC.com, and navigate:
Products
Hydronic accessories
Zone and flow control valves
Heat motor zone valves
Geothermal valves
Submittal data sheets

Click the only entry, sheet 100-3.2 to bring up the .pdf file.

A bit of irony - not knowing "better" back in 2007 when I deployed my own system, I used a cheap HoDepo irrigation valve to regulate open loop discharge. Later I read here and elsewhere that the Tacos are generally preferred. I figured eventually I'd have to swap mine out. I actually plumbed a 2nd cheapo irrigation valve parallel to the first against the day the primary cheapo fails.

I can state with certainty that the primary cheapo (1" grey bodied plastic thing with a black cylindrical actuator) continues to shut off completely - nary a drip from the discharge.

While I like the idea of VFD well pumps, I think I'd still want a small to mid sized bladder tank to keep the VFD from coming on in response to every piddling water draw - between typing these paragraphs I got up and rinsed a beer mug with a half cup of water (enroute to filling another). I don't think I'd want any pump, VFD or not, cycling in response to that or other small water draws.

The biggest problem we've had with inexpensive irrigation or plastic bodied solenoids is noise (buzzing) the second is a failure to close.
j

Followup comments:
Today I ran the heat pump for a while to put some heat into the house. We're not quite moved in yet, and I haven't had the heat on until I resolve the valve issue. After a couple of hours of operation (large superinsulated house and heat pump is only two tons), I turned it off. I noticed that the Taco valve wasn't leaking, or at least over several minutes the tank pressure hadn't dropped any that I could see. Perhaps it was indeed only something in there that got flushed out. I'll monitor this for a while.

Part of the reason I had the system running was also to pursue getting rid of noise propagation up into my office just above where the heat pump is. The heat pump itself is very quiet, but the noise of 6 gpm rushing through 3/4" pipe, and particularly the hissing sound of it through the Hays flow regulator, was obnoxious upstairs. I already had added pipe insulation to prevent sweating, but that didn't help the noise much. The plumber had each of the supply and return lines anchored in two places from the floor joists, using rigid copper straps. I disconnected those attachments and the noise upstairs disappeared. After searching for a suitable material for reattachment with sound deadening, I settled on a pair of rubber stoppers screwed into the joists, with the copper straps wrapped around the stoppers. That provided the support I wanted and still there was no noticeable noise upstairs. Progress.

Congratulations!!!
You should very soon be a happy geothermal user.
Eric

On open loop systems we oversize the flow regulators and install a ball valve down stream from it. This allows us to close the ball valve a bit, putting back pressure on the flow regulator and take away a lot of the flow noise.

Bergy

Another update: so far the Taco valve is holding shut, after another cycle of running it. I do have a shutoff valve downstream (both supply and return, so I can isolate the heat pump). I slowly closed the return valve until the hissing of the Hays flow regulator stopped, even while it kept the correct 6 gpm flow. I closed the return valve a bit further, until it started making noise of its own, then backed of to the original point where I just stopped the hiss from the Hays. I still have the chattering of the check valve. There is another at the top of the well, so the one just upstream of the pressure tank is redundant. The well guy said I could remove it. What I may do is remove it, take out the internals, and put the shell back in instead of a pipe nipple.

I have had a geothermal heat pump "Water & Energy Model 060 since 1996. The Taco valve fails regularly. It only takes a small piece of sediment to get caught on the valve seat just when the valve is about to finally close and it creates a leak. This eventually erodes the seat even though it( the sediment particle) might eventually float away. The damage to the seat is permanent and only gets worse due to the constant flow of water and well pump cycling. I've given up since you would need a pretty fine filter that would have to handle 14 gal/ min on this 5 ton unit. So when it gets real bad I cough up the money and replace the valve. They need a redesign. Anybody have any ideas. Haven't been to impressed with the quality of service in this area for Geothermal maintenance. One guy came when I had a problem, said it was the freon level was down. Hooked the tank up put a little freon on in , turned the unit back on and before he got out of the driveway the unit started short cycling again. He came back did a little poking around scratched his head and said he would have to come back. He never did. Second company came and said it had to be the computer board ($400) and it was the only thing that controlled the compressor. Well the zone valve controls the compressor. Too bad he couldn't read the schematic. So about $900 later with a new computer board and the unit still short cycling I convinced him to try one of my old taco power heads on the zone valve and voila, it's been running on the old power head to 6 years now. The door didn't hit him in the ass on the way out.

We have replaced all our "Heat Motor" valves with the Taco Geo-Sentury valve. Google it to see what it is.

Bergy