Water Source Heat Pump Deep Pump Sizing

[Greg-VA]

After reading (lurking) forums on well sizing and geo thermal heat pumps, I have not found a central source of expertise for proper sizing a deep well to a GSHP. Standard single speed pump coupled to a good size pressure tank is old tech, but dependable. The newer variable speed pump solutions sound expensive and maybe too short of a track record. I am willing to spend a little more on electricity use if less on more costly fancy electronics and repairs for the long term cost of ownership.
So I found this forum to be the most helpful, on many topics, and thought that I would start with the expertise here.
I have a 405ft well with static level about 15ft from cap (cap sits 14in above ground and about 70ft form house). It is 6in diam. well. I will be using a Addison (www.addison-havc.com) water source heat pump - 3tons. Specs list 8.6GPM at 4.7PSI/10.9Ft.HD.(?). It is an open loop system - well to GSHP to lake.
Well supplies domestic water use to house - 3.5 baths, 1 kitchen, 2 outside faucets. Well was hydro-fracted at 405ft due to only 3GPM. Drillers siad I had a good 12-15GPM.
I have not found anyone locally who will/can size this system. Most guys just say throw a 1HP of their choice and it will be fine.
Any suggestions a system solution or the right methods I can use to size this greatly appreciated.
P.S. I am the builder of this house and the owner, have a hand in everything!!! and still fightig the county to NOT hook up to their water 1000ft away! What a water bill a GSHP that would be!!
Thanks to all for suggestions!

[sammyhydro11]

Greg,
it sound like this well supplies both the geothermal system and your home. The pump that i reccomend would be a 7 gallon per minute 1 horse power pump. It will start off giving you a good 15 gallons per minute at 40 psi but wont take out any more than what the well produces because the gpm will eventualy level out to the production of the well as the water level drops. That would be the correct pump for that well and what it produces for water. My question for you is how long does the heating sytem call for that 8.6GPM??

As far as the flow for the geothermal unit,i'm not too sure what type of valve you could put in to regulate the water to that 4 psi. Maybe valveman could help you out there.

SAM

[Bob NH]

Water requirements for a ground source heat pump are so different from the requirements for a household supply that you should find a way to make them both work efficiently.

You don't want a household water supply pump working at 150 ft of head to be wasting energy pumping water through the GSHP system.

Because the GSHP will be discharging to the lake, the most efficient way to do it is to run the discharge pipe to below the surface of the lake, and use a large pipe. I would use at least a 1 1/4" pipe for suction and discharge to keep the head loss at a minimum.

A 1 HP pump for the GSHP is WAAYYY to big. The problem is that most water supply pumps have far too much head.

You need to determine how much drawdown (final water elevation) you will have when drawing water for your GSHP. Then, you need to determine the elevation diference between the lake surface and the drawdown level in the well.

You also need to determine the elevation difference between the highest point in your GSHP and the surface of the lake to be sure you don't get cavitation in the heat exchanger.

With that information, an engineer or a knowledgable pump supplier can select an efficient pump.

For example, at 10 GPM and 20 ft of head, you need only about 0.05 horsepower in the water to serve your GSHP system. At a terrible efficiency of 20%, that would be a 1/4 HP pump.

If the drawdown is too great for a centrifugal pump located on the surface, the best solution might be a submersible with an 1800 RPM motor to get the head down to a reasonable level.

You could serve your household water supply needs with a simple jet pump or a mustistage centrifugal pump with suction connected to the discharge of the GSHP system. That pump should be selected based on the head and flow that the GSHP pump will deliver, and your requirements for household use.

It might be possible to put two submersibles in the 6" well. That would let the two pumps operate independently, perhaps with interlocked controls if the well won't support both flows at the same time.

Neither pump should be 1 HP. A 3/4 HP submersible such as the Goulds 10GS07 will deliver 12 GPM at about 70 to 80 psi, depending on drawdown, for your water supply.

[sammyhydro11]

That 10GS07 will only continue putting out that 12GPM @ 60 to 70 psi as long as the wells drawdown stabilizes at roughly 60'. Most rock wells get more than 50 percent drawdown before the pumping water level stabilizes. So the pump need to be sized according to that. I guess what he realy needs is a pump test. I did a pump test on a well this past summer that was 380' deep and produced 55GPM. The drawdown didn't stabilize until 230'.

SAM

[Bob NH]

That 10GS07 will only continue putting out that 12GPM @ 60 to 70 psi as long as the wells drawdown stabilizes at roughly 60'. Most rock wells get more than 50 percent drawdown before the pumping water level stabilizes. So the pump need to be sized according to that. I guess what he realy needs is a pump test. I did a pump test on a well this past summer that was 380' deep and produced 55GPM. The drawdown didn't stabilize until 230'.

SAM

He needs to do the drawdown test at the expected pumping rate and daily demand. There is no point in testing for drawdown at 55 GPM if the requirement is 5000 gallons per day for the GSHP with a peak hour demand of maybe 900 gallons, which is about what he says he needs.

[sammyhydro11]

Bob,
i was simply using that pump test that i did last summer as an example to show people how much drawdown you can get from rock wells before they stabilize.I know his well dosen't produce 55 gallons per minute.

SAM

[speedbump]

I like the two pump idea. My friend down in Punta Gorda has that same system in a five inch well. One is a 4" and the other pump is a 3". One for the house the other for sprinklers. His water is so nasty, he has a whole house RO, that's the reason.

I don't like to see one pump do it all on a system that is so varied. House use would be around 5gpm at the peak times for the most part and the GWHP is using a steady 9gpm most of the time in winter and summer if it cools also. This is at or near the wells expected max flow.

A cistern might also be considered, for the home only, then let the pump keep it full and keep the GWHP satisfied too. This would guarantee enough water for the home while that GWHP is cranking away. Then the little 1/2hp 10 gpm submersible would be the perfect pump for the job.

bob...

[Greg-VA]

Bob & Sam,

Thanks for the replies! Much to contemplate and learn from all the info. I am trying to find my well report that had to filed w/the county. It has specs on drawdown, GPM, etc.
There is a spring "Home Show" in Richmond, VA on the Jan 19th. I will attend this, as there will be a number of trades that I need to contact for info and estimates. Trades also offer show/spring discounts at these. I am sure that I can find several local HVAC and/or plumbing trades experts to do estimates after an onsite survey.
When I get this info, should be about 4 weeks from now, I will post back their "solutions" for review and comments here.
I love problem solving and learning from everyone in the different trades. Makes me more knowledgeable and better understand how to relate and coordinate the various trades - hence, a better builder!
A GSHP certainly does cross-over and combine several different and/or related trades. I expect several different solutions.
Thanks again, and I will post back on this in several weeks.

[bravo454]

It appears that the flow requirements listed are for a closed loop system in which there is a small circulating pump to move the fluid in out of the ground loop. I have an open loop 5 ton GWHP similar to the one you indicated and it is connected directly to the the house well water supply set 40/60. The water is then dumped into the river that flow in front of my house. This system requires 8 gal/min and has been in operation for 18 years. The rule of thumb for GWHP operation for an open loop system is 1.5 gal/min per ton. The flow rate is nearly double for a closed loop system.

[speedbump]

The rule of thumb for GWHP operation for an open loop system is 1.5 gal/min per ton. The flow rate is nearly double for a closed loop system.

I was always told 2.5 gpm. for well systems. Why less for closed loop systems? That doesn't make sense to me. It would seem like the closed loop system's water would be warmer than the well water due to friction. Of course if your heating that would be a good thing.

bob...

[sammyhydro11]

Speedbump,
i just got done working on a geothermal unit where the pump wasn't installed right. The guy has a well that is 600' deep with a twin pipe pitless adapter. The heating system discharges water back down the well through the 1 1/4" on the pitless. I guess originaly the well was only 125' but the guy decided to go with the geothermal system and the design called for the well to be drilled to 600'. They did this so the water would stay cool in the summer and the cool water below wouldn't be influenced by the warm water being discharged. It ran off a Sub Drive 75 that converts the 230 volt single phase to a 230 volt 3 phase and brings the motor up to the speed of a 3H.P. motor.The pump was a 18GS10. It works good but the guys that installed it hung it on 1 1/4" polly pipe and didn't use any wire guides. Also they installed the polly pipe to the pump on a slight angle so when the pump hung in the hole it was resting on the inside wall of the well rubbing every time the pump kicked on. The motor had 2 holes in it and the wire was bad. I couldn't believe it! I thought the unit was interesting but was amazed by the lack of good craftsmanship on the installation.

SAM

[speedbump]

I'm trying to imagine how many times that motor stopped and started to wear holes in a stainless motor. The wire I could see, but the motor? Wow. It's probably a miracle it didn't cycle it'self to death before the holes showed up.

bob...

[sammyhydro11]

Speedbump,
the whole pump and motor was right up against the inside of the well. The top portion of the wet end had just as much wear as the bottom portiion of the motor. I think every time the pump kicked on the bottom portion of the motor would smack the inside of the well and kickout causing the wet end to hit up top. These guys just pulled this up 9 months ago and replace the braided wire with UF cable. I just can't believe that they didn't pick up on the fact that this pump was sitting in the well like it was because of the way the polly was fastened to the pump. Needless to say i installed the pipe so its was perfectly vertical and also put in 2 TA's above the pump.

SAM

[speedbump]

Sounds like these guys should stick to drilling and leave the pump installs to the pros. It's these types that give the rest of us a bad name.

Sounds like you got them fixed up though Sam. Keep it up, your customers will definately appreciate your efforts.

bob...

[bravo454]

The reason for 1.5 gal/min versus the 2.5 gal/min open versus closed system is that the open systeem is using the water only once and will stay at a constant temp throughout the year. The closed system due to the thermodymnamic properties of the earth and the tubing used to bury in the ground, the fluid temp will drop in the summer and rise in the winter as the demand for cool/heat continues for long periods of time. The flow rates need to comensate for these temp changes in the closed system.