Part 2 of Mark Eatherton's series on the Largest Greenest Job.
Read Part 1.
The residential system is laid out as 22 individual zones of heating and cooling, which is a record for our company for the most zones of heating and cooling. There are essentially three floors to the home, including the basement, and there are 12 zones of hydronic radiant floor heating. The main living area floor is all radiant heating with chilled water fan coil units used for cooling. There are five bathroom zones of radiant floor heating on the second floor of the sleeping area, one of which is a zone that we lovingly refer to as the “human microwave oven,” also known as a radiant floor, walls and bench steam shower. Other than the radiant floor heating systems in the bathrooms on the second floor, the balance of the sleeping quarters heating and cooling needs are met by the ground source heating and cooling system, which is multi-zone with a variable speed blower system.
The owners expressed a desire to go with a total ground source heat pump system during our initial conversations with them, their representative and the mechanical engineer. After initial budgetary numbers came in, the owners decided that maybe they wanted to be a lighter shade of green, hence our final design of a hybrid system utilizing the state-of-the-art equipment. In the end, we sized the ground source heating/cooling system for the main floor based on its cooling needs and the ability to use that capacity to base load the space heating needs of the system.
The ground source heat pump system was sized for the most efficient load, that being air conditioning. As I have stated before, it is much easier to push Btus from the surface of the earth to its depths instead of pulling them from deep within the earth to its surface. The co-efficient of performance dictates that when cooling, the C.O.P. is around four or 5-to-1 and when heated is around 3-to-1, meaning that for each watt of energy consumed, three watts of heat are being moved from the low grade earth heat source to the domestic hot water pre-heat buffer/storage tank. We backed this up with a modulating/condensing natural gas fired heat source, so that when the capacity of the ground source heat pumps were exhausted in the heating mode, the mod-con boilers would blend in seamlessly.
I should also note that the homeowners do a significant amount of entertaining in the home throughout the year, especially in the main living area, so the potential for a simultaneous call for radiant floor heating in some parts of the home and a subsequent call for cooling are a likely occurrence. With that in mind, the water-to-water heat pumps are only capable of doing one or the other function at a given time. Therefore, it was decided that cooling calls would receive the priority of the ground source heat pumps output, hence, an additional need for the high efficiency boilers.
In the course of discovery, we queried the homeowners as to their wishes, wants and needs pertaining to the home's domestic hot water needs. They gave us a worse case scenario hot water use schedule that matched our potential output of the high efficiency boilers through the reverse indirect domestic hot water heating system.
The DHW system consists of the two 210,000 Btuh mod-con boilers coupled to one 120-gal. reverse indirect. This auxiliary heating system is supplied with preheated hot water from the ground source heat pumps water-to-water physical plant. The output of the mod-con boilers are prioritized to the DHW loads during simultaneous demands. All in all, the DHW system has the ability to heat 518-gal. per hour of 100°F rise water.
The snowmelt system provides coverage for the grand front entrance to the home, the rear parking area in front of the garage and the back walk in entrance to the home. The total square footage of the snowmelt system serves approximately 3,600-sq.ft. of active snowmelt area. The owner's mechanical engineer specified that the sub-grade insulation be at least 2” thick, 25 PSI of extruded poly-styrene foam. The snowmelt system is set up for automatic operation and can be manually started with a 12 hour twist timer. The homeowners have learned to use the twist timer whenever possible because snow melt systems work much better pro-actively then they do reactively.
Vital project statistics:
Living space: 25,000-sq.ft.
Total number of vertical bore holes: 22
Connected tonnage: 22t
Btuh equivalent of ground source heat pump: 211,000 heating; 264,000 cooling
Total natural gas mod-con capability (space and DHW): 440,000 Btuh
Snowmelt mod-con capacity: 500,000 Btuh
Solar PV capacity: 20 peak kW capacity.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.