HVAC DESIGN DESCRIPTION
In the 1970s and 1980s passive solar designs were a nascent industry and novel techniques for capturing, storing, and redistributing solar energy were numerous. Many conventional inefficient heating methods were set aside to realize the gains available from the sun. One such idea utilized in this home is a metal ductless heat distribution design, one that is standard in all conventional homes. Metal ducts of course are necessary where there is a very hot heating element such as an oil or gas burner, or an electrical heating element. The only such hot elements used in this home are electric baseboard heat that in practice serves only as backup for failure of the geothermal in cold weather, and the Russian fireplace which also radiates heat directly into the home. Electric baseboard is never used in practice.
Without such hot elements the diagram below shows that circulation of heated air from the geothermal heat pump, somewhat above room temperature, passive solar input, and the Russian fireplace are distributed through two very large sealed air plenums which are in fact the home flooring structures. The system is quiet, lacking duct noise, and flooring is slightly warmed, or cooled in the case of summer, by the recirculated air.
Now, a second requirement for metal ducts concerns home fire which the HVAC system could inadvertently exacerbate with enhanced air flow if it is operating during the emergency. In residential metal ducts the requirement is only that the duct not melt for a specific period of time. The HVAC is allowed to continue blowing air through the registers even though this could aid the fire intensity. In commercial metal ducts, the requirement meets the residential but also specifies that the fire detection system must deactivate the HVAC air blower.
In this home fire safety is recognized as a priority. Thus a separate residential fire & smoke detector is installed with a standard interface to the both the destratification air handler and the Water Furnace geothermal air handler to disable both air handlers in the event of fire or smoke detection.
As such this home meets the commercial level of fire protection from its HVAC systems and does so without metal ducting which has been the source of certain respiratory illnesses in the past. Residential home inspectors will require the information provided here to understand the air flow system.
Figure 1. below provides a schematic representation of the HVAC system, home insulation levels and unique air flow design. It does not represent the architectural design or actual scale
of the home but does show the functional elements included in the design. Four subsystems are shown with associated air flows in overlay. A fifth not shown provides emergency backup heat via baseboard radiant electric appliances. In actual use this backup is not needed unless there is failure of the Geothermal HVAC.
Figure 1 shows the interrelationship between the many aspects of a novel and unique HVAC system. Following is a description of each contributing component:
AIR HANDLING SUBSYSTEM - This system provides air flow to support all the sources of heat and cooling. The major components are:
RETURN AIR PLENUM (green shading) - A novel part of the design is use of the underfloor crawl space as a central return air plenum and heat storage area. This space is completely sealed from outside air, water and vermin infiltration. Continually supplied with warm return filtered air from the second floor ceiling it returns the air to the first floor for warming and balancing temperatures throughout the home. Fig. 1 shows the crawl space foundation walls are insulated from the earth by R-12 rigid board insulation while foundation coating and a French drain beside the footer insure water does not infiltrate the wall. A concrete slab covers the floor of the space under which is placed a vapor barrier and bead board Styrofoam insulation. These measures insure the crawl space is a clean energy efficient space through which household air is constantly filtered & recirculated. Also, this plenum provides a convenient space under the entire first floor for routing new wiring and pipes in the event future modifications are desired.
SUPPLY AIR PLENUM (rose shading) - The supply plenum is formed from the second floor joist system comprised of 16" high factory prefabricated and prestressed trusses using 2 x 4 framing lumber. The result is an open air cavity measuring 42' x 26' x 1-1/4' for a total air volume of 1400 cuft, minus the space occupied by truss members. This space is sealed and insulated with vapor barrier around the external perimeter. The air plenum distributes supply air to first and second floors from the geothermal HVAC system without the need for a metal ducting system. Since supply air is routed through structural members, a commercial level fire protection system is added to deactivate the HVAC upon detection of smoke. This prevents the fan system from supplying oxygen to any internal fire.
AIR DESTRATIFIER & FILTER UNIT - This low power fan unit continually draws in warmed air from the second floor ceiling, down flows it through mechanical and electronic filtration elements before discharging it into the sealed crawl space where it is returned to first floor rooms. As such it is an integral part of the passive solar assisted and masonry stove wood heating systems. In addition it increases home comfort by balancing air temperatures throughout. This fan is also deactivated during detection of any internal fire.
GEOTHERMAL HVAC SYSTEM - Installed in 2011, this system comprises a WaterFurnace™ Envision series indoor unit served by a 400' deep dedicated well grouted with bentonite. This unit provides 5 units of heat or cooling for each unit of energy required to operate it for an efficiency up to 500%. The highest efficiency fossil fuel system is natural gas which provides 0.98 units of heat for each unit of input energy for an efficiency up to 98%. Clicks buttons below for more information on geothermal.
PASSIVE SOLAR ASSISTED HEATING - The home design maximizes winter heating from solar input by orienting the entrance face due south and concentrating more glass on this face than on the other sides. Click the button below for more information on the installed passive solar assisted heating system.
HIGH EFFICIENCY MASONRY HEATER - The home includes central installation of a masonry heater design used for centuries in Scandinavia and Siberia, hence the common name, "Russian fireplace". This is the most efficient and ecologically clean wood burning appliance available. Click the button below for more information on the installed version which is Finnish in origin.