Electric Boilers: When and Where? [30 Mechanical Minutes]

HPAC magazine hosted another edition of 30 Mechanical Minutes, the free webinar series, on January 31st, 2024 focused on electric boilers. In this episode, HPAC editor Doug Picklyk was joined by regular contributor and hydronics expert John Siegenthaler to discuss how electric boilers work, their advantages and drawbacks, and some installation considerations.

This edition of 30 Mechanical Minutes, hosted on Zoom, was sponsored by Conforto, a division of Granby Industries.

Electrification

To begin, the discussion opened with comments about the recent 2024 AHR Expo that had taken place in Chicago just one week prior. The trend towards decarbonization and electrification was very evident on the trade show floor, with both the host and Siegenthaler noting the large presence of new air-to-water heat pump demonstrations.

But they both also commented on the growing number of electric boilers being shown, particularly products for the commercial segment of the industry. “Electric boilers were a hot topic at AHR Expo this year, and thus a great time for us to be talking about them here on 30 Mechanical Minutes,” said Picklyk.

In explaining just exactly how the technology works, Siegenthaler began by describing the electric boiler is a “relatively simple device.”

The units convert electricity through some type of a resistance element, similar to the elements in an electric water heater. “Typically, a residential electric boiler has a small pressure vessel, and there could be anywhere from one to as many as four heating elements,” says Siegenthaler. “While some of the large commercial electric boilers have more—I actually looked at one at the show that had, I believe, 15 different elements.”

All of these elements are controlled, and he acknowledged that in the past they were just turned on and off with contactors, but today most of the electric boilers control the current that goes through the element, and they do that with solid state devices.

“So an electric boiler, unlike a fossil fuel boiler, can basically regulate heat output anywhere from 0 all the way to 100%,” says Siegenthaler.

He also explained how the controls within these boilers have progressed, some can not only maintain a pre-set boiler outlet temperature, even though the inlet temperature varies, they can also control based on outdoor reset, so as it gets warmer outside the supply temperature is reduced.

The devices can also be relatively small compared to fossil fuel boilers because they don’t need to accommodate the combustion chamber and gas assemblies.

Panel Consideration

Something to beware of with electric boilers is the wattage that is being converted to heat. Siegenthaler provided a simple equation to convert kilowatts (kW) to Btu/h: just multiply kW by 3,413. For example, a 15-kW boiler is just over 51,000 Btu/h.

“That would be high enough capacity to heat a house, and it’ll be served by a single phase, 240-volt circuit, and that would require almost 63 amps,” he says.

This is why the circuit panel is one of the very first things a contractor will want to check, especially in a retrofit situation where some older houses, especially those already using fossil fuel heating systems, might have 100- or 150-amp services.

“I don’t want to say that you can’t install an electric boiler with 100-amp service, but if that panel is pretty much filled up with breakers, you’d probably reach the limit.”

For new construction, using an electric boiler with enough capacity to heat a 2,500 sq. ft. modern house will require at least a 200-amp service, and that’s especially true if an electric vehicle charger is being added to the home.

Typical Uses

Electric boilers can be used for any hydronic distribution system that you would supply with a gas-fired or oil-fired boiler including radiant floors, panel rads, hydronic baseboard, etc.

And an advantage of the electric boiler is no combustion, so there’s no issues with fuel supply or venting, and it doesn’t generate carbon monoxide (CO), so that’s one safety concern that’s eliminated.

In addition, Siegenthaler says that short cycling is really is not an issue at all with an electric boiler, plus they make very little sound. “You may hear a very small amount of sound coming from it, but that might be just the water moving through it.”

One of the unique potential advantages of an electric boiler over a fossil fuel boiler is the ability to coordinate their operation with time-of-use utility rates.

“You could easily set up controls that would allow an electric boiler to operate under the lowest-cost electricity,” suggest Siegenthaler.

Also, an electric boiler can take advantage of solar photovoltaic systems. “If you’re dealing with net zero houses and you have photovoltaics, some of the energy that you’re producing can go directly into the boiler,” he says. “And if you have net metering, where any surplus electrical energy generated by the solar array would simply go back out through the meter at the same rate that you buy it at, essentially the utility becomes a battery for you.”

Drawback

Aside from the high current draw, Siegenthaler touched on the thermodynamic efficiency concept called exergy, the idea that we can do a lot of things with electricity to maximize its thermal output. “We can run a compressor, for example, in a heat pump, and in effect, we get three or four times as much low-grade heat into a building compared to just dissipating the energy into heat with an electric boiler.”

Where’s the fit?

When asked where he’s seen electric boilers in operation, Siegenthaler pointed to net zero and low energy houses or apartments as a possibility. “We’ve used electric boilers in very low load houses, in areas that have low-cost electricity, or for clients that want photovoltaics and an all-electric solution.

“We’ve got a project in Minnesota that only has a 6-kW electric boiler that heats the entire house, and it does that with radiant floor heating in the basement and some panel radiators on the main floor. It’s not a large house, about a 1,500 sq. ft., but it’s extremely well insulated.”

Those homeowners asked, “Why not just put electric base board in versus a hydronic system?” His reply was about flexibility. They can use a low-cost electric boiler now, and switch to a more efficient heat pump down the road.

“So electric boilers definitely have a niche. We like to use them as backup to air-to-water heat pumps, or even geothermal water-to-water heat pumps.”

He then shared details of a research project in Northern Maine where they are looking at off-peak rate structures and want to grab off peak electrical energy when it’s available and store it.

“It’s kind of a unique situation. They have a lot of wind turbines, and when they have a strong wind event the local utility has more energy than it needs, so they’re looking for ways to dump some of that capacity without curtailing operation of the turbines. I’ve actually been told that at some times they have negative utility rates, which means the utility would pay you to take this energy,” he says.

The project is using a prototype heat pump unit that can generate high water temperatures up to about 175F, where most air-to-water heat pumps are limited to about 130F.

In this case they can be tied into thermal storage, but most heat pumps really can’t get the temperature as high as an electric boiler, which could easily do 180F.

Siegenthaler shared a schematic where a heat pump and electric boiler are piped in parallel, and that enables either heat source, or both heat sources, to operate at the same time (Figure 1, below).

Figure 1. An air-to-water heat pump and electric boiler piped in parallel enabling either heat source to operate, or both at the same time.

“If the heat pump wasn’t able to keep up with the load, or if the heat pump was down for service, the electric boiler could come on automatically.”

He adds that the controls in this system are designed to charge the thermal storage tanks up when the low-cost electricity is available, and then discharge those tanks back into the header, and ultimately to the hydraulic separator and the heat distribution system.

“The system is set up so energy can go into thermal storage from either heat source or it could go directly to the load.”

Some heat pumps can automatically signal the electric boiler and its circulator would turn on.

He adds that heat pumps are much more complicated than electric boilers in terms of controls and sensors, and he suggests that there’s a better chance that a heat pump could go down for some service requirement.

“So we like to build electric boilers into systems as a backup to make sure the building has heat, especially in remote locations where you know that service may be much more than two hours away.”

In that situation, he sees an electric boiler as an ideal and relatively low-cost option compared to putting a fossil fuel boiler in as a backup.

The webinar closed with great questions asked by the live audience. To see the entire webinar and find past editions of 30 Mechanical Minutes visit the HPAC YouTube channel @hpacmag, and be sure to subscribe so you don’t miss the next episode. <>