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ZNE

When David Knight meets with his clients, they’re asking about zero net energy a lot — on almost every job.

Knight’s Carmel, California–based mechanical engineering firm, Monterey Energy Group, performs energy compliance work and HVAC, plumbing, and solar design for about 1,000 custom homes throughout California each year. Every project starts with a conference call or meeting with the homeowner, architect, and builder to discuss project goals, budget, and timeline.

“Very often it comes out that their goal is to be truly net zero energy,” Knight says. “They truly want to produce as much energy as they consume.” Other times, he says, customers “just want a nice, efficient house, and they want to do what they would consider the greatest value.” They want to be comfortable, efficient, environmentally balanced, and healthy, and they want to be able to afford it.

Understanding a customer’s zero net energy goals requires further discussion, Knight says. Does a client want to zero out their electric consumption? “That’s easy to do, cost-effective to do,” he says. “It’s not going to ruin anyone’s budget. And I would argue it makes all the sense in the world.”

The conversation gets more nuanced — and much harder — if the client wants to zero out all energy consumption, including domestic hot water and space heating. “It’s not impossible to do, but it does cost more upfront, takes up more space.” In our latest video, Knight explains how he designs the mechanical and domestic hot-water systems on his projects to meet each client’s needs.

California energy code

Enacted at the beginning of 2020, California’s new energy code is frequently referred to as a zero net energy code, and it falls closer to Knight’s first option, zeroing out electric consumption. “A lot of people were thinking that the zero net energy code was mandating all-electric homes,” he says. “It’s not.” In reality, the new code is neutral for propane, gas heating and water-heating systems.

So rather than focusing solely on electric systems, Knight specifies systems based on the design, energy cost, and comfort goals of the project. In modern homes, for instance, there’s usually very little room left over for extensive ductwork. One option he frequently employs, especially along the California coast, is radiant floor heating. “Architects love radiant floor heating systems,” he says. “It’s silent, it’s invisible, it doesn’t take up much space. The clients like it because it’s very, very comfortable.”

Energy costs are another important consideration. California has very high electric costs, so trying to use grid electric power for domestic hot water and space heating will usually cost a fortune. Instead, Knight typically evaluates propane and natural gas heating systems against the amortized cost of solar panels powering a heat pump.

In forced-air systems, propane and gas furnaces still have a comfort advantage over heat pumps, Knight says. “On the furnace side, they just put out warmer air,” he says. “And so the comfort level tends to be better.” And with the climate being so mild in most of California, the payback of geothermal heating systems is rarely worth the expensive upfront costs.

Water heating that meets high expectations

Californians have high expectations for their domestic hot-water systems, Knight says. “They’re used to an unlimited amount of hot water that turns on in a few seconds from the time they hit the switch. And the biggest challenge for going all-electric is domestic hot water. Because there just really isn’t a domestic hot-water system that’s going to perform that way.”

“They’re used to an unlimited amount of hot water that turns on in a few seconds from the time they hit the switch. And the biggest challenge for going all-electric is domestic hot water. Because there just really isn’t a domestic hot-water system that’s going to perform that way.”

Whereas a propane tankless water heater can provide 200,000 Btus per hour of heating, keeping the hot water coming indefinitely, electric storage tank water heaters have a much slower recovery rate. “With electric, you can add enough storage to make it so that they can get through, but it takes up a lot of space, and we’re already always fighting for space,” Knight says. Even if a home can manage to fit two 50-gallon electric water heaters, homeowners will need to wait a couple hours for the tanks to heat up again if they use up the stored hot water.

“What we like to tell people with a gas unit is that we defy them to run out of domestic hot water,” Knight says. “We do not say that with electric units. And some people are happy with that, but many aren’t.”

In the end, the mechanical system design comes back to the project budget and goals. That means it’s incumbent on builders, architects, and engineers to both understand their clients’ needs and educate clients on their recommendations. “We give them their options and the relevant benefits and features of different systems,” Knight says. “I almost always say, ‘Look, given this house, and given what you’ve told us about your goals, if this was my house, this is what I would do.’” By accruing deep education and experience across a variety of mechanical systems, Knight assures his firm is equipped to design a system appropriate for his clients’ dream home.