On a sand point well, I know water can be pumped from the well. Does it
work to reverse the process and put water back into the ground via the
sand point? The subsoil is mostly coarse sand. Flow rate should be less
than 8 gpm.
On an open loop system it is possible to take water from one well, run it
through an exchanger and then put it back into the ground via a separate
well. If the sand point works, then it won't be necessary to run a drain
to the pond.
The engineering is the sticker. How fast are you going to pull water
out and how fasst will it perculate back? You could easily pull more
out than will perculate back in.
You don't say what your goal is. Water loop for a heat exchanger?
I have an open loop ground source heat pump which operates as you
describe. You can put the water back - that makes it an injection well.
There are a number of ways of screwing up an open loop system and our
well driller and installers found several of them.
Open loops are hugely problematical and I would go with closed loops if I
had it to do again. I'll assume you have a good reason to reject closed
loop systems. I repeat - I wouldn't do an open-loop system again and I
wouldn't recommend one to my worst enemy. I had a special case where I
did not want to kill a lot of very old trees so I went to open-loop. I
should have killed the trees.
Here are some things I learned the hard way.
Flow rates should be approximately 1.5 to 2 gpm per ton of unit capacity
- varies from manufacturer and type of unit - so your flow rate is likely
OK for a 3 ton unit.
We had an iron bacteria infection because the well driller did not
disinfect the wells after he was finished drilling. Also, the installer
did not disinfect after installing the pump and discharge pipes. Iron
bacteria occur on all ground surfaces and it is impossible to avoid
transferring it into the well so disinfection is key. Add a jug of bleach
to each well when finished drilling and after installing the pump and
pipes - after doing anything to the wells that involves something
touching the ground and going into a well.
Once started iron infections are said to be permanent - I have
disinfected like crazy doing all the things several experts recommended -
came back in 3 months. I will now have a permanent disinfection process
and filter cleaning problem. Avoidance is the only cure.
Our injection well started the first winter with water levels 30 feet
below ground with the pump running - there was about a 5 foot rise above
static level. By January the injection well was overflowing. It would
have been a big problem if the overflow had entered a neighbors property
but ours is a river lot and it just went down the riverbank. The cause
was air locking of the injection well ground formation. Air locking
occurs when the installer uses a pressure reducing valve (PRV) to adjust
pump flows to the desired rate during the calibration process. PRV's hiss
and that sound is air or gasses coming out of solution and those bubbles
air lock the well after a while. The negative pressures that occur in a
PRV also cause calcium and other minerals to come out of solution and
those will plug the injection well and the ground formation. SO don't
permit the use of PRV's, no way no how - most installers use them and say
they never cause any problems but they most certainly did at our place.
We proved it was air locking by putting a pump down the injection well
and reversing the flow out instead of in. Nothing happened for 2 days and
then there was a shoulder high geiser for about 15 minutes. After that
the well rise was almost normal again - 6 feet rise instead of the
original 5 feet. The extra rise is due to calcium deposition but
allowable for now.
To avoid the need for PRV's you should use orifices at the end of the
discharge pipe in the injection well. Size the orifice so the flow rate
is what is needed. The pump will be a bit larger than required so an
orifice restrictor will be required to throttle the flow. The flow rate
is calibrated so the discharge water after the heat pump is no lower than
38 degrees. If you take too much heat out of the water then the heat
exchanger would freeze - that would happen if the flow was too slow. If
the flow is too fast then there might be extra wear on the heat pump
internals. We started with an orifice a bit too small - 3/16 inch
diameter - water temp was too low - drilled it larger just a bit - water
temp rose but still too low - drilled a bit more - temp rose, repeated
until temp was 38 degrees with heat pump in full heating mode. Took 5
tries. A lot of work but the well did not overflow this winter. Also
eliminating the loud hissing noise in the basement was a bonus.
Make the discharge pipe in the injection well as long as possible so the
orifice at the end is as far below water level as possible. The
submergence will avoid any negative pressures in the orifice. Putting the
restrictor at the end of the discharge pipe and deep under water keeps
the pipe under pressure which avoids calcium and other minerals coming
out of solution. That is why the pump should be slightly over-sized and
restrictor used to throttle back the flows. If the pump had only the
exactly correct capacity the discharge pipe would run under negative
pressure and the well screens and ground formations would become plugged
after a while - maybe a few years but could be much sooner. Depends on
water hardness and how extreme the hydraulics get.
In our province there are no great ground source suppliers and only one
good well driller. I asked several senior ground water engineers who they
would recommend and they responded instantly "none of them". I didn't use
the best well driller because he charged twice as much as the other
bidder - big mistake - if I counted the value of my time I would have
been money ahead. I am a civil engineer and got a lot of free consulting
from groundwater experts so I was able to bebug my system myself but I
shudder to think what is happening to normal folk out there. My first
ground source installer was only medium recommended by other prople but
he turned out to be a crook - police and tax colectors turned up looking
for him and he took off. He had been trying to get large deposits but I
only gave him a few $thousand. Lost about $2,000. Next installers were
good but they insisted on PRV's with promise to remove them if they
caused any problems - when well overflowed they fixed it with me working
with their guy and sent me a bill for over $2,000 - they reduced it to
Be very careful who you hire and don't give them more than a small
deposit. Don't pay the well driller at all until he proves that the well
will deliver the required flows with only around 5 feet drawdown in the
pump well and maybe 6 or 7 feet rise in the injection well - maximums.
Injection levels rise more than the drawdown value because it is harder
to pump water into a well than out. So the injection well should be
larger, deeper and have longer screen than pump well - unless they easily
exceed capacities. Note that capacity is flow at normal changes in level
up or down. It is not just a flow value.
The wells should be 250 feet apart for best efficiency. Mine are only 135
feet apart so I had one well cased 25 feet deeper than the other so the
groundwater flow between the wells would be partly vertical. We have
frcatured limestone so a little vertical separation cause a significant
increase in groundwater flow path because the vertical fractures are
smaller than the horizontal ones. Sand layers are more homogeneous so
vertical separation will only help a bit and the horizontal separation
becomes much more important.
When the wells are finished, as part of the capacity proof, measure the
static level in each well before starting pumping. Pump to surface
initially. The pump well should drop around 5 feet and the other well,
just standing there with no flow in or out, should drop no more than 6
inches or so in reaction to the pump out after an hour of pumping. This
reaction should be minor to prove that the wells are not hydraulically
connected to too great a degree and water will not short circuit between
the wells. It is OK for some of the water to flow between the wells, just
not a lot. The water will be picking up heat as it goes through the
After proving that the inter-connection is OK then pump from one well
into the other - discharge pipe MUST be well below static level before
pump starts - no splashing allowed at all. Check that drawdown is in the
5 foot range and rise is a couple of feet greater, say 7 feet. Lower
values are better, higher by a bit is OK but higher by a lot is bad. Bad
depends on the local experience but most homeowners don't pump at 6 gpm
for hours on end like a heating or cooling system would so you may not
find much to go by. DO NOT take the drillers word for what is OK - he
just wants to get paid and clear out. Consult with state or provincial
groundwater departments to get some idea of what sort of reaction to
expect. The values I gave you are for my area where we have fractured
limestone rather than sand layers to draw from.
Make sure your well casings are PVC and the sand screens are stainless,
best grade, don't skimp on quality. Wells need maintenance or they stop
working. More so for heating and cooling where you pump thousands of
times as much water as simple domestic users. Maintenance is generally
just chlorinating whenever working on the wells and once or twice a year.
If you notice drawdown or rise problems then you can regain capacity. If
the injection well overflows then stop pumping, either relocate the pump
from the draw well to the injection well or get another pump of equal or,
better yet, more capacity and reverse the well flow until a lot of air
comes out. Took me 2 days but sometimes it is a matter of minutes,
depends on many factors. If that doesn't work and you suspect mineral
deposition has plugged one or both wells then acid treatment would most
likely restore capacity. Industrial strength acid is required which is
very dangerous to handle so you must get professionals to do it. Try well
drillers or geotechnical engineering companies who specialize in
groundwater and who are familiar with acid treatment of wells. If your
well casing is plain steel then acid would destroy it. Same goes for the
screens - anything but the best grade stainless will be destroyed by the
acid. Includes clamps and all other parts - stainless or PVC only.
Polyethylene is NOT acceptable. Poly pipes and pumps must be removed
before acid treatment because they can not tolerate acid.
If your driller uses inferior materials, when you need to acid treat the
well at a future date - it is inevitable - then the well would be
destroyed and need to be replaced entirely at huge expense. Even
chlorination would eat at steel casings and cause them to rust which
would then become the source for iron bacteria.
The above rant and rave shows why closed loops are better than open
loops. Closed loop may need a cleaning flush every 5 or 10 years. Open
loop has no end of hassles. If you don't use the best well driller around
you will be sorry and the best guy knows how he compares to the
competition and charges much more - the only good guy in our area charges
double and triple and is kept busy. There is no way an open-loop system
is cheaper to build than a closed loop and the on-going costs and hassles
are huge. The majority of the problems are related to the injection well
so if you can eliminate the injection well by discharging to a pond it
would help. It would cause concerns about groundwater quantities because
huge amounts of water are used. If you are close to neighbors who reply
on wells there may be concerns about drawdown affecting them. Neighbors
could also get warmer water during air conditioning season.
So 3 tons needs about 3x1.75 = 5.25 gpm, cooling it by about
0.8x36KBtu/h/(5.25x8.33x60Btu/h-F) = 11 F.
Why not just pump some water out of the river and dump it back in cooler?
My 1820 farmhouse has a hand-dug well with water 9' below the surface and
a nearby creek.
The ground loop can be the most expensive part of a water-source heat pump.
Can a $400 18'x48' EZ Set inflatable pool be the heat source for a $5K 3-ton
Climatemaster Tranquility 27 heat pump with a COP of 5? It might be solar-
assisted where I live in PA, with an average 30.4 F January temp. We might
add a little ground water if the ice ever reaches the bottom.
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