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Transitioning to a Net Zero Carbon Home

Updated: Jul 15

By Henry MacLean, Architect and Sustainable Milton Member, SustainableMilton.org Milton Times, Sustainably Speaking, April 15, 2021


The purpose of this column is to promote sustainable solutions and practical steps for a healthier and more resilient Milton. Advocating for town-wide projects and programs in the face of global climate change is at the heart of this mission.


This April 3rd, high atop Mauna Loa Observatory in Hawaii, C02 counts surged past 420 parts per million for the first time in recorded history. Climate scientists and activists were quick to follow up that this current trajectory continues a path that will likely trigger irreversible planetary change, unless we act!! Part of this growing movement is focused on transitioning all our buildings now producing 40% of this C02 (66% in metro areas like Boston) to become net-zero carbon by 2050. In the recent Building Sector Report of the Massachusetts 2050 Decarbonization Roadmap, the State is calling for net-zero emissions and “nearly 100,000 homes installing heat pumps or other renewable thermal systems each year for the next 25-30 years.”

As we emerge in Milton from another heating season, the Mass Clean Energy Center’s (MA CEC) Air Source Heat Pump program has been extended to June 25th, 2021. The program is set up to offer rebates for renewable heating and cooling systems to MA residents. “These technologies offer a high level of comfort, are generally more cost-effective to operate than traditional systems, and reduce your carbon footprint.” In this state where heating and cooling comprise over 60% of energy costs and cooling needs very much on the rise, these quiet and discreetly designed systems are now cheaper to operate than their natural gas competition, given current and future trends in energy pricing.

Back in 1997 when I moved to Milton, one of the first things we did to help green our 1945 Crosby Colonial was swap out then 30 year old oil fired boiler for a new energy star efficient gas boiler. Over the intervening 24 years, I’ve been steadily improving the efficiency of the house, adding insulation, air sealing, updated windows and doors for existing and new areas and adding solar PV panels. This last winter, with our boiler nearing the end of its lifespan, we jumped into the CEC program and had a new 5 zone air to air heat pump system replace our 2 zone gas fired hydronic loop. Now comparing last winter’s gas usage to 2019-20, we’re seeing a 43% decrease in energy use, a 65% reduction in carbon emissions.

As part of the intake process, the CEC requires an in-depth analysis of the home’s current mechanical systems and envelope, listing R values (or resistance to heat loss) of existing walls, floors, roofs, windows and doors. The design/build team we selected also ran heat loss models to confirm the required loads, and a blower door test required by the CEC program to verify how air 1tight the structure is. The results, measured in air changes per hour (ACH-50) help determine if a home ventilation systems is recommended to provide enough fresh air for occupants.

Given whole house ventilation is recommended with an ACH-50 of 4 (out test number was 3.83) we decided to add an Energy Recovery Ventilation system (ERV) as part of our new 2nd floor centralized heat pump zone. The ERV also runs on electricity to preheat incoming fresh air with outgoing stale air, saving over 20 times the energy used to power it. We also took this opportunity to better insulate the added bedroom over the garage, typical to many Crosby Colonials, as well as the main attic roof rafters for a new warm mechanical and storage space.

With the $4,000 in rebates through the program, a 0% Mass Save 25K loan allowing alternate investment, deferred costs for a new gas boiler, and 15% energy savings to date, our new system will pay for itself in 14 years. This 7% ROI is before we factor the value our new system adds to the homes value, the free electrons from our 5.4 KW solar PV system once that 7 yr payback (14% ROI) is complete in 4 years. This PV system is currently maxed out for our home’s moderate exposure and provides 25% of our energy needs. But as we complete the task of getting to zero carbon by replacing our aging gas water heater and dryer with heat pump water heater and dryer, and gas stove with induction electric stove, we will also save another 50% of the combined energy those gas units now use. Once our panels are paid for, we can also get in line for the next generation of panels coming with double the efficiency and production.

Like many others in Town, we buy the remainder of electrons we can’t produce from Renewable Energy Certificates (RECs) through our selection of 100% green power. Upcoming tools of Community Aggregation which the Town of Milton is working on, carbon pricing legislation happening at the State and National level, and large-scale New England renewable energy now supported by Biden’s green agenda, will all help make this carbon-free transition attractive to Milton homeowners.

So as we move into spring and witness the alarming data from our NOAA scientists up on Mauna Loa, it may be a good time to visit the MASS CEC Weatherization Checklist, the best place to start in making a plan for a zero carbon home. You may be surprised at the savings, added comfort and personal gratification in joining this movement to “Restore our Earth”, the theme for Earth Day 51 now just around the corner.


Sustainable Milton (SM) raises awareness, educates and motivates residents, town government and businesses to reduce waste of all forms, to help create a healthy, vibrant future for all. Click “JOIN” at www.sustainablemilton.org.

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