Raymond

Maine, USA - Ground-source heat pump

 

Summary
A two-storey single-family house with cathedral ceilings and large glazed areas was built in Raymond, Maine, where the annual temperature differences are significant. Instead of using town water, a water well supplies low cost heating and cooling via a geothermal heat pump throughout the year. Two reversible water-to-water heat pumps are run in parallel to share the overload from the common well pump in winter and summer. The heat is distributed via a hydronic radiant floor heating system and fan coil units. The overall measured COP of 4 includes the well pump and the circulators. The cost of space heating is one third that for an oil-fired boiler. The heat pump system provides also useful cooling. The performance of the heat pump has been continuously monitored since 1998.

 

System schematic ground source heat pump in Raymond, Maine, USA


Building and design values

 
Building type: Single-family house
Location: Raymond, Maine, USA
Year of construction: 1997
Number of storeys: 2
Heated floor area (m2): 400
% of total floor area (%): 100
Design outdoor temperature (ºC) Heating: -9 Cooling: 35
Design indoor temperature (ºC) Heating: 20 Cooling: 24
Degree days Heating: 4 230 Cooling: No data
Base temperature for degree days (ºC) Heating: 18 Cooling: No data


Heating and cooling

 
Application: Space heating and cooling and DHW
Heat pump type(s): Water-to-water
Heat pump installed capacity (kW) Heating: 24.7 Cooling: 24.7
Refrigerant: R22
Heat source Groundwater
Details (circulating pumps): Two heat pumps: 10.6 + 14.1 kW (3 + 4 ton) and two fractional horse power (FHP) circulators.
a) Type of ground heat exchanger: Water/refrigerant, standing column well
b) Borehole depth (m): Well depth 237
c) Pipe length (m): Column length 190
d) Heat transfer fluid: Water
e) Flow rate (l/h): Well water at 3.23 l/kW (3 gal/ton). Total 4,800 l/h (21 gal/min)
Distribution system(s): Radiant floor/fan coils. The heating circulators supply radiant floor and the fan coils for winter heat. The cooling is done in the summer only with the fan coils.
Supply and return temperature (ºC) Heating: 43/-1 Cooling: 5.5/-1
Auxiliary system: None required for geothermal radiant
Heat pump design: 100% heating. For DHW there is s desuperheater on only one of the heat pumps.
Supplementary system: Back-up for DHW is propane-fired boiler
Heat pump system completion date: October 1997
1 Return temperature depends on the instantaneous loading. Depending on the month of the year the heating temperature is varied.

Additional notes
The details of the well design:

This is a standing column well of about 183 m (600 ft) for 24.7 kW (7 tons) of heat pump capacity, about 26 m/3.5 kW (85 ft/ton). When the heating system is running the total measured power consumption for the well pumps, heat pumps, circulators, controls etc. is 8 kW. This gives a nominal 24.6 kW (84,000 BTU/h) heating.

There are several ways to configure standing column wells depending on the depth and flow capacity of the well.

In this case the geothermal well is geothermal only and there is no concern of drawdown, which might uncover the pump. As an economy move the pump was located near the top of the well with the return to the bottom of the well. This saves a heavy duty copper wire to the pump and simplifies the maintenance. Locating the pump at the top also works for combined geo/domestic heater wells where there is a high capacity flow rate.

Typically in Maine, for lower yield wells of the order of about 7.6-15 l/min (2-4 gal/min) the pump is located at the bottom for residential wells of up to about 152 m (500 ft).

In this installation the two heat pumps are run in parallel to share the overload from the common well pump in winter and summer. The common hydronic storage tank controls the water temperature from the separate heating and cooling aquastats on the tank.

Performance

 
Heating energy Heat pump Aux. heating system Auxiliaries 1
Energy input (kWh/year): About 16 000 - Included in “heat pump”
Energy output (kWh/year): About 48  000 - n/a
Energy cost (USD/year): 800 - Included in “heat pump”-
Cost tariff (USD/kWh): 0.052 - Included in "heat pump"
1 Circulation pump ground loop, fans, circulation pump heat distribution
2 On-peak tariff 0.26 USD: Monday to Friday 7.00-12.00 and 16.00-20.00, off-peak tariff 0.05 USD. Heating is essentially at the off-peak rate of 0.05 USD because the storage is concrete and because of the control strategy. The latter includes a time-dependent 2-stage thermostat approach for hydronic tank temperature control. It also includes the differential temperature off-peak heat storage in the basement concrete.

Cooling energy Heat pump Aux. heating system Auxiliaries
Energy input (kWh/year): 2 400 - Included in "heat pump"
Energy output (kWh/year): No data - n/a
Energy cost (USD/year): 240 - Included in "heat pump"
Cost tariff (USD/kWh): 0.11 - Included in "heat pump"
1 Average of peak and off-peak rates for cooling: level load average of 73% at 0.05 USD and 27% at 0.26 USD. Cooling is supplied whenever needed both at peak and off-peak times. The cooling is automatically disabled during the peak time. However, by manually overriding, cooling can be turned on during the peak time when it is really needed.

Coefficient of performance (COP)
Heating: 4.6 and 4.0, for 14.1 and 10.6 kW heat pump respectively
Test conditions: Heat sink at 38ºC
Cooling: No data
Test conditions: No data


Heat pump cost breakdown
Heat pump only (USD): 5,431 (two heat pumps)
Installation (USD): No data
Capital cost (excluding heat pump) (USD): No data
Maintenance: No data


Alternative system (if has been considered) Propane boiler


Fuel cost 3 000 USD/year


Payback Estimated simple payback of heat pump system over a propane boiler is about two years.


CO2 emissions No data


Operational experience and other comments
The performance of the heat pump has been continuously monitored since 1998. The radiant floor heat allows different water temperatures depending on outside temperature, which gives a COP ranging from 4 to 5. The overall COP, including well pump and circulators, was measured as 4.
In the four years of operation there have been no reliability problems. The only maintenance required involves periodic flushing of the supply water filter.

Off-peak electricity with geothermal radiant heat from the pipes in the concrete under the floor gives storage. This precludes heating at other than off-peak time.
The heating costs with the heat pump are three times lower than with an oil-fired boiler in this part of the USA.

Contacts

 

Contact 1:

John Logan
    Company: Water Energy Distribution
    Role: Owner and system design
    Address: 15 Wawenock Road
Raymond, Maine 04071
USA
    Telephone: + 1 207 655 7529
    Fax: + 1 240 414 8924
    Email: geomaine @ maine.rr.com

 

 

 
Last updated: 1 March 2004
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