TechPractices: Grove at Patuxent Woods, Patuxent City, MD

TechPractices are outstanding housing projects throughout the U.S. where innovative technologies are implemented. Builders and remodelers can use these examples as models for projects of their own.

Summary

What do you do when you're stuck with a site that can't handle the septic load? Well, in this project for seven affordable houses, it was possible to concentrate shared wastewater treatment systems in areas of the site with adequate soil conditions, allowing four houses to route their septic to a fifth lot, while two other houses shared a septic field. Other innovations worth considering here are pressure-treated permanent wood foundations to minimize site disturbance and eliminate the added cost of concrete work; optimum value-engineered framing that saves material; and geothermal heat pumps that tap into the earth's steady underground temperature, to reduce utility bills.

Details

This competition-winning design for a small development in a depressed part of rural Maryland contains seven houses on land that was previously considered unbuildable-every developer's nightmare. Percolation tests revealed that the soil conditions could not support development.

The development is clustered to reduce costly service runs, such as roads and sewer, by keeping the houses close to the road and preserving the far lot as common open space. A buffer lot between the clusters accommodates a community play area and one of the wastewater recovery areas, across the road. The road was designed for private maintenance so that it could be narrower, less extensively engineered, paved with pervious materials, and cheaper than county standards. Aligning the septic system with the road allowed a single excavation. Both the road and the septic is owned in common. The sloping site, coupled with the dry, sandy soil, made the wood foundations feasible. The combination of methods used to keep costs down is clearly innovative, but the individual technologies are tested, economical, and widely available.

A two-ton geothermal unit is installed in each 1,100 sq. ft. home. The vertical, closed loop system works with a ground-source heat pump that takes advantage of steady undergroundtemperatures. In a climate like Maryland which is neither excessively hot nor cold, a ground-source heat pump makes sense. The average temperature 200 feet underground is 57°F, within the acceptable range for heating and cooling heat transfer, allowing the soil to serve as a heat sink or heat source for both heating and cooling. In the summer, a heat pump water heater function provides domestic water heating from the air conditioner's waste heat.

Installation/Construction

Wood foundations have a few things worth considering: they eliminate the need for concrete on site and allow for fast construction, yet qualify for a conventional mortgage. Poured gravel is used instead of a concrete footing.

The geothermal system requires a well driller to bore 6 inch diameter holes 200 feet deep for vertical pipe installation. The holes are then sealed with bentonite, a good heat transfer material. A flow center is installed inside the house to recirculate the glycol heat exchange solution. The utility rebate program requires that home registration and inspection be coordinated with construction phasing, especially for the geothermal installation.

Benefits/Costs

The design team approached the developer with the suggestion that the project be designed for a state-sponsored competition for affordable housing. The project was one of two selected for recognition, which allowed partial state financing. Additional funds are provided by a non-profit county housing provider, a federal mortgage program, and a rural development grant. Costs are cut through efficient space planning, the wood foundations, the value-engineered framing system, and the narrow road. As a result, these houses average $72,500, which is 45% of the reported median house price for the area.

The shared septic system was the most economical way to allow this site to be developed. Homeowners will need some training in maintenance procedures to deal with the commonly-owned road and septic. The geothermal heat pumps are expected to reduce utility costs, especially when combined with boosted insulation standards required by the Southern Maryland Electric Cooperative (SMECO). SMECO offered a rebate of $3,500 per home through the Power Saver Home temporary rebate program for choosing a geothermal system. SMECO is now developing a full-service Energy Star Homes Program that can prescribe components and analyze building systems to help the builder meet Energy Star requirements.

Good for temperate climates, geothermal systems may not be cost-effective for southern regions where the ground is too warm. In the installation at the Grove, the transfer solution never climbs above 80° in summer, so the amount of pipe needed is not excessive. No furnace is needed because the liquid solution never dips below 40° in winter, and a cold winter results in energy savings the following summer. First cost of the geothermal system is somewhat higher than a comparable electric heat pump, but energy savings will eventually pay for increased installation costs. According to energy modeling comparing the geothermal system to an efficient air-to-air heat pump system, yearly electrical savings are $304. Increased maintenance intervals save another $75 per year. Without a condenser, there is no outdoor unit to get beat up or vandalized. According to the installer, the key to a cost-effective installation is designing the loop with the proper amount of wells and piping.

Code/Regulatory

Charles county offered the site to the non-profit housing provider with the understanding that the sewage regulations might be relaxed. The county agreed to allow sewage from four lots to be conveyed to a fifth and from one lot to the adjacent lot where percolation rates were acceptable. Still, the shared wastewater system caused delays, taking almost a year to be approved.

Feedback

The lending agency requires that houses not be built until a qualifying buyer is located. So far, four houses have been built, with a fifth under construction. The entire development should be finished up by winter 1999. The clustered development is different from the conventional, low-density zoning in the area. But it's a lot better than the slum conditions that preceded it.

Contact(s)

Do you have a specific question? Try the contacts listed below:

Bennett Frank McCarthy Architects, Inc.
7003 Carroll Ave.
Takoma Park, MD 20912
301-270-9480

Southern Maryland Tri-County Community Action Committee, Inc.
P.O. Box 280
Hughesville, MD 20637
301-274-4474
mike@smtccac.org

Southern Maryland Electric Cooperative
Power Saver Home Program
P.O. Box 1937
Hughesville, MD 20637
301-274-4338
www.smeco.com