Overbrook Farms

This case study follows the installation of geothermal in a residence, located in the Overbrook Farms neighborhood of Philadelphia.

Property Challenges and Requirements

I live in the Overbrook Farms section of Philadelphia. It’s a community that was established in 1892. The Overbrook Farms Steam Heat Plant centrally heated homes in the neighborhood from 1893 until 1973. Our steam system was then updated to use a natural gas-fired steam boiler in the basement. Many houses in the neighborhood heat with gas or oil, continuing to use steam or choosing to convert      their systems to use hot water radiators. Geothermal is a viable conversion option for supplying hot water to a radiator system, if a two-pipe steam radiator system was installed. If central cooling is desired, a forced air system will be needed. 

Our project faces many challenges. Our lot is not large enough to accommodate drilling equipment that is common to this area. To cool our home, we decided to use a high velocity system because its ducting takes less room and could more easily fit into our home without tearing out lots of walls. Prior to this project, we added a Unico high velocity system in the attic for cooling only to our second and third floor and want to repurpose it for heating and cooling. There is no space sufficient to run air ducting between the basement and attic, so a two-zone system will be needed (one for the first floor and one for the 2nd/3rd floor).

The lot size is 9,290 square foot (60′ x 154′). The house, built in 1919, has 4100 ft2 living space with an unfinished basement. The front yard is 60′ x 50′ and has three mature trees that we don’t want to disturb. The back yard is larger and has more open space. Access to the back yard is limited to equipment about 7 ft. wide unless we disturb fencing and bushes bordering our neighbor’s lot. Even then, we’d only gain an additional 4 to 5 feet.

Our system

We decided to go with Morrison Geothermal out of Duncannon, PA. They have IGSHPA certified installers and drilling equipment which can reach our backyard. Our house needs two 5 ton heat pumps and a closed loop system with 6 vertical boreholes in order to heat and cool the 4100 ft2 of living space. The borehole depth is limited by the small drill rig’s capability of 250 feet. The boreholes will be located 15 ft. apart within a 15 x 30 ft. field in the back yard. Double loops will be installed in each borehole to achieve better efficiency.

The system will have two zones: one zone on the 1st floor and a second for the 2nd and 3rd floors. The two zones will give us more accurate control of the home temperature. The first-floor zone will include a humidifier to keep the home from being too dry in the winter. The first-floor zone will have floor registers with supplemental vents into the basement. The 2nd/3rd floor zone will continue to use the 1.5 inch round high velocity vents.

Both heat pumps will be in the basement. The first-floor zone will reach the registers with ductwork placed in the basement. The 2nd/3rd floor zone must route an insulated refrigerant loop to the attic. We plan to route the loop through the back stairs and a closet. There will be minor repair work after the installation to hide the pipes. We could leave them exposed, but prefer to hide them.

The cost? The base cost for installation is $77,000. There is a variable cost for casing used in the borehole until bedrock is reached (expected at 40 feet). Casing is only needed to keep dirt from collapsing the hole and is no longer needed once bedrock is reached. The anticipated cost is $16/ft x 40 ft x 6 boreholes = $3,840. The Federal tax credit of 26% will help defray the costs.

Please note that your cost is likely to be significantly different, probably significantly lower for another residential property. Three major considerations have increased costs on our property:  the large size of the home, increased exterior wall area from the L shape, number of windows and the relatively little amount of insulation in the walls and the limited space to allow a larger drilling rig into the yard which could drill more efficiently and inexpensively. Your costs will vary significantly from ours; ask your contractor to help you estimate the expense of the project given your own site specifics.  

Insulation

We have already invested a significant effort to insulate our home. Geothermal heat pumps are not as powerful as a central steam system. To maximize comfort, this must be considered. We have already:

  • Insulated the attic and knee walls to R30 with batt and loose insulation.
  • Injected loose cellulose insulation into the walls from the inside. The holes were plugged by the contractor requiring some additional patching/painting to restore the rooms.
  • Used open cell and closed cell foam insulation during larger room remodels.
  • Removed window casement molding to spray Great Stuff Window and Door around the window frame (this made a remarkable difference in reducing the draft around windows). The molding was easily replaced and required minor patching to restore their appearance. 
  • Replaced door seals with a system from Conservation Technology, http://www.conservationtechnology.com/building_weatherseals.html.
  • Replaced windows with insulated glass.

Most of the work was completed by insulation contractors. I highly recommend the FLIR ONE infrared camera for detecting heating leaks. It’s the least expensive infrared camera system I’ve found. It attaches to your smartphone. I used it to find and fix insulation problems.

Installation

We signed a contract in April 2022. I’ll provide updates as we progress.

Other considerations

Your installer will help you with the topics discussed in this section. If you want to understand some of the background, you may enjoy my notes.

Heating and Cooling Load

Your installer will use Manual J to calculate your heating and cooling load to determine the size of the heat pump you need. Manual J considers many things such as home orientation with the sun, construction material, square feet of living space, number of people in the residence, appliance count, window space and insulation value. This gives you a more accurate measurement than just the home size. Homes built before the 1970’s didn’t consider insulation. My home was built when radiators were sized to leave the windows cracked open in winter because fresh air was thought to be required for good health.

I attempted to measure the heat and cooling load using the Manual J calculator at https://loadcalc.net/. The instructions for the site are a bit weak, but I was able to use it to get approximate numbers.

Manual J Btu
Cooling61,847
Sensible Load56,284
Latent Load5,563
Total Heating121,631

The existing steam radiators in the house have a calculated load of 134,920 Btu/h with a 400,000 Btu/h steam boiler. The radiators were sized in 1919 for an uninsulated home. Our home currently has R13 in the walls with some using closed cell spray foam. The attic is mostly R30+, with some sections, where access is difficult, limited to R22. The home is brick and stone. Most of the windows have spray foam insulation around them to stop drafts and all but a few dormer windows have been replaced with low-e argon insulated glass windows. I have more work planned to continue my insulation work. The frequently used rooms are well insulated and comfortable.

Ground Coupling Decision

To understand what types of  ground-coupling options could work for my location, I used USGS National Geologic Map Database Collection – CKAN. Looking at the Bedrock geologic map of a portion of the Philadelphia quadrangle, Montgomery and Philadelphia Counties, I was informed of the bedrock, water well yield and geology. My bedrock is Springfield Granodiorite Ssg. “Well yield is 10 gal/min with good water quality. Geology is described as dark-gray- to black-weathering, medium- to course-grained, moderately to strong foliated granodioritic gneiss composed of plagioclase, quartz, microcline, biotite, epidote, and minor hornblende. Accessory minerals include sphene, magnetite, apatite, and zircon. Portions of the Springfield exhibits porphyritic texture characterized by 2- to 4-cm plagioclase and microcline crystals with rare occurrences of alternating, centimeter-scale mafic and felsic layering.”

Although I could try an open loop system or a standing column well, there are limitations on my property. An excellent resource for explaining the different ground loops and their requirements is A design and Installation Guide for New York City. Some good references to inform my conclusions include:

  • Table 2.1 Typical Configurations – to get ground coupling spacing and depth information.
  • Nice tables identifying the capacity, system considerations and pros/cons of each type of system:
    • Figure 3.7 Analysis of Closed Loop Systems
    • Figure 3.9 Analysis of Open Loop Systems
    • Figure 3.11 Analysis of Standing Column Well System 
  • Table 4.1 ASHRAE Standard 90.1 2007 Minimum Efficient Requirements

From this, I determined that I didn’t have enough land for an open loop or standing column well. Although my water is of a good quality, my well yield isn’t enough for my Btu requirements of an open loop. The remaining option, closed loop, is the most common solution for urban homeowners. It doesn’t have the efficiency of water source systems, but requires less maintenance, has a longer lifespan, and can be configured to achieve excellent efficiencies. 

For a closed loop system on a small lot, we need to install a closed loop system with vertical bore holes. The number of bore holes is based on the heat pump size, requiring between 500 ft per 2.5-3.3 tons of heat pump capacity. Geology will determine the size needed. United States installations place two pipes, combined at the bottom with a U joint, into a 6-inch diameter borehole. To increase thermal transfer, a grout is used to fill the hole and provide contact between the tube and ground. The grout is pumped into the ground from the bottom up. Grout selection is made based on a trade-off between thermal transfer capacity and pumpability (thickness) of the grout. Thicker grouts have a higher transfer capacity but are more difficult to pump, increasing the cost. Interestingly, the maximum depth of the bore hole is related to the pressure the wet grout near the bottom of the bore hole exerts on the outside of the pipe, causing the pipe to flatten before the grout hardens[1].

Drilling

Drilling companies in the Philadelphia area appear predominately to use truck-mounted drilling rigs, which are typically 8 or 9 feet wide and 30 feet long. An example is pictured here: https://www.mainlinedrilling.com/contact.

Small portable drills are available that can drill up to 120m (393 ft) and are more appropriate for homes with limited access. You can save some money if a truck-mounted drill can be used. Work with your installer to identify the pros and cons of each drilling option.


[1] Geothermal Heat Pump and Heat Engine Systems, Theory and Practice, by Andrew D. Chiasson 2016.