Based on the nicest looking mechanical rooms in the portfolio of homes we visited, our Design Development conversation, and our architect’s and builder’s recommendations, we went with Jim Godbout for our mechanical subcontractor. As a preference, we wanted radiant heating in order to avoid forced hot air with its space wasting duct work, noisy operation, dry air generation, and dust ball production that we grew to hate over the years. Current green builder consensus is to use ductless mini-splits to reduce the cost of radiant heating installation. Mini-splits eliminate the noise of conventional forced hot air systems by locating the noisy compressor and condenser outside the house, and do away with the bulky ductwork by pumping refrigerant directly to wall mounted blowers – albeit ugly – inside the house. However, they still produce an uneven heat distribution as you don’t have to be a heat transfer specialist to see that radiant heating which makes the whole floor the heat source will produce a more even heat distribution than a mini-split with a point heat source. At the end of the day, we just couldn’t accept a forced hot air solution with mini splits after observing that every house with radiant heating we visited seemed more comfortable. Plus subfloor heating has been around for centuries from the hypocausts in Turkish baths to hot water pipes under floors in Frank Lloyd Wright’s buildings in 1930’s.
For Lily Pond House, Jim Godbout designed a radiant floor heating system everywhere except panel radiators in the bedrooms. In contrast to forced hot air, which is a convective system that uses air to distribute the heat, radiant heating system uses mostly energy waves to distribute heat from warmer (i.e. floor) to cooler (i.e. people) objects, is more efficient, and provides a more uniform temperature distribution with less dry air issues. For the living room, entrance, and basement, Crete-Heat foam insulation panels (3″ R-15) were installed in the cement slab to secure the Viega PEX radiant tubing in place. For first and second floor common areas and baths, radiant tubing were applied below sub floor with Low-E Tab insulation under radiant system. For the bedrooms, we went with high efficiency low temperature Biasi steel panel radiators.
Viega PEX radiant tubing over Crete-Heat insulation
Radiant heating needs low temperature water (85º-95º for radiant tubing, 105º-120º for radiator panels), and saves energy by having to heat to the lower temperature. Design Development called for propane fired Viessmann Vitodens condensing boiler (one of the highest efficiency (98%) boilers with excellent reliability) for delivering the low temperature water to the radiant PEX tubing and Viessmann Vitocell for domestic hot water. Before the Construction Documents got finalized, we decided to install solar panels on the second roof in order to complement the passive solar design towards a net-zero home. Our solar energy subcontractor Revision Energy came up 8.1 kilowatt-hours (kWh) photovoltaic array with 27 LG Mono X NeON 300 watt solar panels. Maine has a progressive solar energy policy with “net energy,” which is the difference between the energy used and the energy generated over a month. If Lily Pond House panels generate electricity in excess of our monthly usage, the unused kWh credits go into a “bank”. Our utility CMP invoices based on net energy for the month, taking into account any accumulated kilowatt-hour credits from the previous 12 months, which is better than the net billing in Massachusetts where the utility purchases the excess electricity at the wholesale price. Our solar panels are expected to generate roughly 10,000 kWh of clean electricity annually, and offset roughly 9,000 lbs. of CO2 emissions annually. Currently, our electric utility charges $0.15/kWh. Assuming that electricity costs stay constant over the next 25 years as we are also ignoring the cost of capital, we will recover our solar energy capital investment in the first half of the life span. In other words, we will have free electricity for the second half and beyond after the first half. Another way to think about this is that we are locking the price of electricity at $0.08 for the next 25 years, which translates to $18K of savings if we had purchased the same electricity from our utility at the current price. Currently, Maine does not provide the State incentives that, for instance, Massachusetts provides, which substantially reduces the period for recovery of capital.
Radiant heat transfer aluminum plates for PEX sub floor installation
In The Third Wave, futurologist Alvin Toffler coined the term “prosumer” when he predicted in 1980 that the role of producers and consumers would merge in the future. Given our decision to become a prosumer of electrical energy that we’ll be able to generate abundantly in the summer and bank it for winter use, our mechanical subcontractor changed the heating design to Daikin Altherma – an air to water heat pump with backup electric coil. Altherma air-to-water heat pump uses a sustainable energy source – heat from the outside air. The heat extracted by the Altherma outdoor unit is transferred via refrigerated lines to the indoor Altherma heat exchanger, which uses the intake to heat and distribute the water to the radiant heating tubes in the floors, and low temperature radiators. Altherma delivers about 4 kWh of usable heat for every 1 kWh of electricity it uses, giving it a high coefficient of performance (COP) of 4. For the domestic hot water, the design changed to Voltex Hybrid Electric Heat Pump from A. O. Smith. Voltex reduces water heating cost by 2/3 by extracting the heat from the ambient air. In our application, the water heater is placed in the unconditioned basement so as not to rob off the heat from the conditioned space. In addition to the heat pump, this model also has heating elements that help the water heater recover quickly during periods of high demand.
In older New England homes, it is not uncommon that Air Changes per Hour (ACH) can record between 12 to 30. Maine Uniform Building and Energy Code is currently based on the 2009 International Residential Code (IRC), which calls for 7 ACH. The Department of 2012 Energy Air Leakage Guide ratchets up the target to 3 ACH for our climate zone, which is yet to be adopted in Maine. Our architect specified 0.6 ACH – the Passive House standard as the target, which dictates fresh air ventilation. The ventilation design called for Venmar Energy Recovery Ventilator (ERV). Using the energy consumed by a compact fluorescent bulb, ERV provides cost effective fresh air ventilation while recovering the heat exhausted out of the house. In the winter, the heat and humidity of the outdoor fresh air is transferred to the intake air stream, thus not drying out the inside air during the heating system. In the summer, the heat and humidity of the outdoor fresh air is transferred to the exhaust air stream, to keep the inside air humidity low. We shall see.
Lily Pond House 