Pump It Up

New York City’s ambitious Climate Mobilization Act has mandated that all buildings larger than 25,000 square feet, including cooperatives and condominiums, must reduce their greenhouse gas emissions by 80% by 2050 – a stiff challenge. Over the past several years, I’ve become convinced that the only way to achieve this goal is through electrification.

What is electrification? It involves sharply reducing fossil-fuel combustion and converting everything – heating, cooling, transportation and more – to electric operation, while simultaneously “greening” the electric grid. Electricity must come from renewable resources, including wind, solar, geothermal and, quite possibly, nuclear as well. Electrification has already begun, but the pace must accelerate. It all needs to happen by 2050 – a mere 30 years from now.

Finding the Right Site

In late 2018, Ian Shapiro of Taitem Engineering and I approached one of the world’s largest HVAC manufacturers and asked the company to support a pilot project that would convert a central oil-fired steam heating system to individual air-source heat pumps in each apartment. Our idea was that until someone performed such a conversion and showed that it could be done at a reasonable cost and would yield significant energy cost savings, very few buildings would take the risk.

The project needed an appropriate test site – a building not so big that the job would be too costly but not so small that the results could not be applied to a wide range of building sizes. We turned to Valerie Corbett of Intelligreen Partners for suggestions, and she recommended what proved to be a near-perfect site: a 100-year-old, 10-unit, five-story co-op on the Upper West Side with an oil-fired, single-pipe steam heating system.

Luckily, Lisa Harrison, one of the residents and a co-op board member, was already interested in heat pumps, and she turned out to be an ideal liaison between the project team and the shareholders. But the layout of the building made installation trickier than usual, and the residents used so little fuel for heating and hot water that improving upon that would pose a serious challenge. These seeming disadvantages – difficult installation and the challenge of reducing energy costs – actually turned out to be advantages. If heat pumps were cost-effective here, they could be cost-effective in almost any building.

Because this was a pilot project, Cornell University agreed to perform a before-and-after air quality study, and the New York State Energy Research and Development Authority agreed to support monitoring of pre- and post-installation energy consumption. Since it’s important to size heat pumps correctly, Taitem Engineering was asked to calculate the heat loss in each apartment and draw up plans for bidding purposes.

Tightening the Envelope

To make the project as widely applicable as possible, the designers had to balance the need to minimize installation costs with the need to maximize energy savings, so they did two things: they figured out how to install the heat pumps without upgrading the electrical service, and they insisted on plugging up the building’s largest air leaks as an integral part of the installation.

Many people assume that an electrical upgrade is an absolute necessity for a heat pump conversion, but after careful calculation, the design engineers for this project concluded that it was not, despite relatively small capacity in the apartments. This has broad implications not only for other buildings but also for the local electrical grid itself. If electric utilities have to upgrade the service available in the street to satisfy increased use of heat pumps, the cost of doing so must be included in the overall cost of electrification.

Plugging selected air leaks, also known as air sealing, is mandatory in a heat-pump conversion; large leaks that are simple to fix should always be included in the project. The more air sealing, the lower the heating (and cooling) loads, the smaller the heat pumps need to be and the lower the electrical load on the building and the grid.

To make it easy to locate the air leaks, the engineers pressurized the building with what’s known as a blower door. This device, which is typically used for single-family houses, pressurizes and depressurizes a building to exaggerate the air leaks so they’re easy to find. It’s unusual for a blower door to be powerful enough for a 10-unit building, so for this project two were used.

Pumping the Energy In or Out

An air-source heat pump is a device that moves heat energy from one place to another – from indoor air to outdoor air during the cooling season and outdoors to indoors during the heating season.

These heat pumps have been around for decades, but for many years they had a bad reputation in colder climates because they would switch to expensive resistance heat mode when the outdoor temperature dropped below about 40 degrees. But in the last decade or so, a new generation of heat pumps has become so efficient that they can pull useful heat energy out of air as cold as 13 degrees below zero, or even colder.

Because they move heat energy rather than create heat energy via combustion, the best heat pumps can be much more efficient than a typical steam heating system. (There is heat energy in the air all the way down to absolute zero, which is roughly 450 degrees below zero Fahrenheit, but it becomes more and difficult to harvest that energy as the temperature drops.)

For an individual apartment, an air-source heat pump will usually consist of one or two outdoor units and multiple indoor units. The indoor and outdoor units are connected by refrigerant lines and power wires. Each indoor unit has its own hand-held remote control so that the user can treat each unit like an individual zone. This makes it easy to eliminate overheating, the bane of almost every multifamily building in New York City.

No construction project would be complete without a few surprises, and this one had plenty. Unexpectedly, in most of the apartments it was not possible to run the refrigerant lines in the ceiling. This meant that the lines had to be run along the ceiling against a wall, and a soffit had to be constructed afterward – adding a bit to the cost.

All 10 heat pumps were up and running for the last month or so of the cooling season, and residents have since shifted over to heating mode. Generally speaking, everyone seems to be happy with the heat pumps’ performance so far. “The heat pumps are great,” says Harrison, the co-op board member. “They’re quiet and cover a large area. Each unit is controlled separately, so they can be set to different temperatures and turned on or off as needed. There are also a lot of options, so each unit can be customized for individual preferences.”

Harvesting the Data

Now that construction is complete, the year-long data gathering process has begun, which will be the true test of the system. Will energy bills be lower, and will residents be more comfortable than they were in the past? Some apartments have already seen an increase in their electric bills, which is very much to be expected. The question is: Will the increase in the electric bills be outweighed by the decrease in oil bills for the building as a whole? I’ll let you know in a year. In the meantime, this building has already reduced its greenhouse gas emissions – the ultimate goal of the Climate Mobilization Act – and it will reduce them further in the years ahead.