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Open Loop vs. Closed Loop

In initial meetings, many of our customers have stated that open loop systems are more efficient than closed loop systems.  In most cases they’ve heard this from another contractor. This statement is misleading and doesn’t apply to operating costs.  I’d argue that generally speaking both systems are equally efficient.  The most important factor is which one is the best fit for you. Let’s take a look at both…

“Open loop” and “closed loop” refers to the source.  The source side is the location where the heat pump will be extracting or rejecting heat to.  The load side is the home we’re trying to heat or cool.  Let’s start by looking at the pros and cons for each geothermal source.  Remember that efficiencies for ALL water source heat pumps exceed that of any other heating system today:

 

Open Loop Source (“pump & dump” or “once-through”):

Refers to running domestic well water through the system and discharging somewhere

 

Pros:                                                                                Cons:

Less expensive install costs                               Increased well pump usage

(In most cases  )                                                  Water quality concerns

Consistent entering water temperature            Additional water control valve required

Slightly more capacity in late winter                  Discharge water location / design required

 

 

Closed Loop Source (Horizontal pit or trench, vertical bore, horizontal bore):

Refers to circulating the fluid out through the ground and back through the heat pump in a continuous loop with high density PE pipe

 

Pros:                                                                               Cons:

Control over water / brine quality                     Typically higher initial install costs

No scaling or build-up concern                        Requires yard space

Less maintenance                                            Lower entering water temperatures (heating)

Zero water usage from well

Zero Energy Consumption from well pump

 

If a contractor tells you that open loop systems are more efficient, most likely they either don’t know how to, or don’t want to install a closed loop.  It’s much easier to set a unit, pipe water to it and be done.  Furthermore, designing a closed loop requires the installer to know more about the product they are selling.  So do they really know how to commission a system and verify proper operation (See our Blog topic ”Selecting a Geothermal Contractor”)?

 

Lets get back to comparing these two source options.  From the list above, open loop systems have the benefit of a consistent entering water temperature (EWT).  Your well water temp doesn’t change much year-round.  The output heating capacity of the system is dependant on the EWT.  The unit is extracting heat from the entering water, so if the temp of that water is higher, it has more capacity to heat your home (and consequently, it will also operate more efficiently).  Closed loop systems circulate the same freeze protected fluid (brine) forever.  This brine is extracting heat from the earth and EWT’s can get down to 30 degrees in late winter.  For example, let’s look at the spec book and examine the heating capacity and efficiency of a 4-Ton unit and assume it’s late winter:

 

Open Loop                       Closed Loop

EWT:                                    50 degrees                        30 degrees

Heating Capacity:              47,800 Btu/hr                     37,500 Btu/hr

Power:                                 3.22 KW                             2.97 KW

COP:                                    4.35                                    3.71

 

So the open loop system is providing 10,300 more BTU’s than the closed loop system due to the elevated EWT.  Open loop system is 435% eff vs. the closed loop operating at 371% eff.  The thing that’s missing is the well pump.  The well pump runs 100% of the time that the heat pump is running (obviously turns off when the heat pump shuts down).  A residential variable speed well pump will draw ~ 5 amps or more at lower flow rates.  This equates to another 1.2 KW and takes the total COP down to 3.12. (or 312% eff).  At this rate the closed loop system is more efficient with less operating costs.  It’s the turtle that wins the race in this example.

 

Our experience has been great with both open and closed loop systems.  We have learned that open loops require a little more attention in the weeks directly following install.  It’s good to check a new system frequently.  Inspect the flow meter to verify correct water flow.  This is important in the spring when sprinkler systems come on.  Sprinklers or other large water demand on the well, can reduce the water flow rate through your heat pump.  Sprinkler system start-up can also send quite a large amount of sand or other sediment up the pipe.  You’ll want to check the sediment trap at the unit location and clean when needed.  Once dialed in the system typically runs good with simple filter changes every 3 months.

 

The install cost is another factor worth discussing.  Just because you already have a well doesn’t mean the open loop will always be the cheapest to install.  Finding a good way to discharge the water can also cost money.  One method is to take a backhoe and dig a big hole, backfill with washed rock, and then see if the ground will take it all back in.  This works well in sandy soils but could still cost you $1,500 in backhoe time and labor.  If you have a surface pond you could discharge in there pretty cheap.  We have had customers who want to drill both a supply and discharge well for an open loop.  The cost of that would be equal to or more than simply installing a ground loop.

 

At the end of the day both systems are going to far exceed the efficiency of anything else you could install.  Air-source heat pumps have come a long way.  However, they’ll never achieve the eff of a geothermal system because it’s just too difficult to extract heat from air that’s 20 degrees (winter in Idaho).  It’s ~ 30 times easier to transfer heat using water than with air.  Water source heat pumps are moving onward as well.  We already have systems rated up to 500% eff.  The next generation has new design features that will significantly increase even this number.

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