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# Using large bore well to improve heat pump efficiency

08/08/2022 9:13 AM

I have a large bore well,24 inch diameter tile,surrounded by pea gravel 1 foot thick all around,surrounded by clay/gravel earth.The water temperature is around 55F year round.The the depth of the water is 37 feet,volume of the water is around 725 gallons,inside of the tile excluding the amount of saturated gravel and surrounding clay/soil earth.Would it benefit me to use a heat pump to utilize the heat sink/source to improve efficiency of the system?

I realize this is a far cry from the efficiency of a ground source heat pump,or a well to well system,or a deep well system,but I wonder how much heat it can source or sink?

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#1

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 9:34 AM

Isolated, would sink or source around 3Mcals/water degree C. Now for heat exchange with its environment, too lose data to calculate. I would just experiment with a known heatsource, then take on advice the capacity and duty cycle of your heat pump. First glance, nope unless your heat pump now is forced to work at really low air temp and ice blocks it all the time. S.M.

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#2

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 10:39 AM

You would need a much larger water supply, this would only last a short time and recovery would be in the hours, not be worth the time or expense.... The heat pump is made to operate in a narrow range of temperatures and running it outside these parameters is problematic, therefore you need a body of water large enough to exchange the amount of btu's that would be needed at the maximum design load...You could use the water in the well to make a 2 ea 6 ft deep buried tank system where you pumped water from one tank to the other, and then reversed tanks at intervals....probably need like about 30k gallon tanks though....

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#3

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 11:11 AM

This well has a large contact surface area with the surrounding earth,which is saturated gravel and clay.The effective area is like a 2 foot diameter cylinder buried 18 feet deep, from the top of the well,37 feet of standing water column,so the surrounding earth will absorb a lot of Btu's from the water.The well has a flow thru of 3 gallons per minute,adding to the Btu absorption,taking heat away,so this is not stagnant water,it is being exchanged with incoming fresh cold water.

The surrounding pea gravel increases the effective coupling area to over a four foot diameter,and I do not know how thermally conductive saturated clay or clay/gravel is.

I realize that a heat pump is designed for a certain amount of Btu's and the expansion valve will only atomize a certain amount of Freon.The condenser and evaporator are sized also according to the Btu of the system.I know it is no benefit to lower the Freon below it's condensed state,it is the latent heat that is important.I am simply trying to reduce the compressor load and reduce the energy consumption of the compressor.

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#4

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 3:06 PM

ok let's start over...is this an installed existing water source heat pump? What is the tonnage?

"Most heat pumps require two to three gallons per minute of source water flow per ton (12,000 Btu/hr.) of heating capacity. A typical residential system demands between six and 12 gallons per minute of sustained water flow whenever it's operating."Sep 28, 2000

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#5

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 4:42 PM

Perhaps it would be better and simpler to install a radiator in the air return and circulate the well water through it to assist cooling in the summer,which is the biggest load. Not use in winter time.This is a pad mount,not a split system.

I have 2ea. 24"x24"returns on each of the 3 ton units,I have two 3 ton units on the home,which is double the normal area for that size system,so air flow restriction is not a concern.

As long as the water temp is below the room temp,it would help.I don't expect it to do the whole job.

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#6

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 5:56 PM

I don't think it would be worth the trouble....but it would assist the cooling if done properly...I would go for about 2 gal per min per ton...Ideally you would want the water around 40°F, but you want the coil temperature to be below dewpoint at least...These coils are not cheap...

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#7

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 7:21 PM

I am counting on the surrounding gravel and earth to absorb a lot of Btu's from the water.The diameter of the bore hole before inserting the tile was 4ft.The tile was 2ft diameter,then the gap between the tile and the earth was filled with washed pea gravel.

This gravel became saturated with water as the water level rose to 37 ft.This total volume is approximately 3500 gallons,or 28000 pounds. The surrounding earth,composed of clay/gravel mix will also become saturated.The gravel will absorb more heat than the water alone,and radiate it into the earth,so the water is not doing all of the work. A lot of the heat will be going into the earth,as well as the small flow thru of the water.

I do not know the thermal conductivity of the surrounding earth but I expect it to be pretty high.Clay has a thermal conductivity of about 5 times that of water.

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#8

### Re: Using large bore well to improve heat pump efficiency

08/08/2022 8:44 PM

Well if you did both systems that would be 12 gal per min, that would be less than 5 hrs of water supply...but if you are dumping the water back into the well the temp would slowly rise....if you are cycling 20 min on and 20 min off, and the water temp rise is 10°F, that would be 240 gal of 65° dumped into the 3500 gal reservoir...less than a 1°F rise per cycle, so you would need to dissipate that amount of heat in 20 min....seems possible...

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#9

### Re: Using large bore well to improve heat pump efficiency

08/09/2022 2:25 PM

I intend to circulate the water through the well,allowing the surrounding environment of the well to absorb the heat.By returning the water to almost the same level there will be very little head loss,only pumping and pipe losses.

The returning water will be routed thru a curved pipe attached to the inside of the tile to make it circulate in a circular direction around the tile,enhancing the heat exchange.The return line(warm) will be slightly above the entry(cold)line,and the heat will rise up from the warm side.

As the warm water cools,it should sink to the bottom by convection.

This should ensure that the warmer water stays above the cold intake.

Looks good on paper,but making it work in the real world is always the problem.

As far as the radiator goes,I am thinking of an automotive radiator,which is designed for high flow.Plenty cheap at salvage yards.The old ones are the best,made from brass,but the new aluminum ones will do in a pinch.

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#10

### Re: Using large bore well to improve heat pump efficiency

08/09/2022 4:22 PM

Bear in mind that humidity control is just as important as temperature, an air conditioner works by keeping the evaporator coil below dew point so that as the air temperature is decreased the relative humidity is decreased as well, this requires a steady stream of condensation from the cooling air....The ideal humidity level for comfort is ~50% RH, just dropping the air temperature will raise the humidity level....If on the other hand you are in a dry climate then this is not a concern, most modern air conditioners have a humidity control that is controlled by blower speed, the higher the incoming humidity, the slower the fan works, this allows more moisture to be removed and vice versa...

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#11

### Re: Using large bore well to improve heat pump efficiency

08/09/2022 5:20 PM

I will simply be pre-cooling some of the air going into the evaporator.The A/C should take care of the humidity since this is before the evaporator coil.Humidity will be lowered when the air hits the evaporator coil,and condensate drained out.The radiator will be placed behind one of the 24X24 filter boxes.Since these systems have twice the normal filter area(2 ea 24X24 filters on each 3 ton system) with an oversize return air duct this should not be a problem.

It will simply reduce the cooling load on the system.

Whether it will be enough to offset the pumping cost is yet to be determined.

I will perform all of the work myself,so I only have materials cost.

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#12

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 12:01 AM

Yes I was thinking the same thing, I guess it depends on the distance from the well to the return....anyway I was also thinking that the pump should come on before the AC unit to get the coil down to temperature before the blower kicks on...the increased return size is a good idea, it slows the airflow making the cooling effect more efficient, and also the filtering more effective...the humidity control might still be an issue at cooler times of the year, by adding this cooling you are effectively increasing the btu of the system and could possibly be oversizing the AC unit, this shortens the run time and the time that humidity is being removed...an AC unit is sized to run a certain amount of time, generally 20 min cycles, that's why running a heat load calculation is so important when installing AC, to get the tonnage correct...of course if you have a variable speed blower, this is not a problem...

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#13

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 12:15 AM

Is it at all possible to get a few litres of really hot water from say thousand litres of tepid water about 10 degrees centigrade higher than ambient ?

These heat pumps need power to start with, though the over all savings are palpable. They need a lot of fluid to be moved and for underground water, as in your case, perhaps intermediate storage even !

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#16

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 8:02 AM

I will be using the 55F water to supplement the cooling of the heat pump,not for primary heat dissipation.It will be warmed by ambient inside air, around 73F and returned to a large heat sink(The well) which is capable of absorbing many BTUs from the water.See the dimensions of the well,volume of water, and surrounding material above.

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#14

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 12:39 AM

Is the well used for your drinking water?

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#15

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 7:56 AM

No,it is not used for drinking.I am on a public water system now,and it was bored many years ago,but is still used for lawn and garden,car washing,etc.

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#17

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 12:02 PM

HiTek,

I suspect you are located in a major city along the Eastern Seaboard of the USA (based on your stated location in your posts).

Unlike what SE stated early in these posts, a heat pump is designed to work with a fairly large range of outdoor unit temperatures. But, as you know, its efficiency (SEER) is highest when the outdoor unit's temperature is close to the desired indoor air temperature. This is why ground source (including water source) heat pumps are much more efficient than air source. Your desire is worth pursuing.

Probably you will need to have a competent and experienced HVAC contractor look at your system. I recommend using a closed loop, probably with water and with freeze protection heat tape. If you use an open system, pumping water from the well and returning it, keep aware that in the winter you will returning colder water and in the summer hotter water, so thermal stratification will be a problem (unless you reverse the top/bottom flow directions).

Any good system should have a minimal need for maintenance or user interaction.

--JMM

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#18

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 12:36 PM

If you are going to take what I say out of context, at least get it right....This statement of operating in a narrow range of temperatures is for a water source heat pump, and specifically the condenser cooling water....In any case this is not relevant as the water is not being used in this manner....If you had read the thread, you would know this....your map also seems to be way off...

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#19

### Re: Using large bore well to improve heat pump efficiency

08/10/2022 4:44 PM

Not intended for winter use,as the water temp will be lower than the ambient air inside.

I will simply drain the system for the winter.

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#21

### Re: Using large bore well to improve heat pump efficiency

08/11/2022 9:16 AM

"I suspect you are located in a major city along the Eastern Seaboard of the USA "

According to his location:-

He is in

Croatia.

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#20

### Re: Using large bore well to improve heat pump efficiency

08/11/2022 6:02 AM

The bigger bore will help heat exchange to the earth around, and the mass of water could help deliver enough heating or cooling power at start-up and provide some storage of heat or coolth. e.g. Solar thermal has high efficiency for re-heating a borehole, or cooling could be powered by PV during daylight. What is the annual heating/ cooling balance? Insulating the house would bring the resource closer to the demand.