Pat Linhardt
A Heat Generator -- with a quantity of mercury stored in its base.

Mercury Rising

Jan. 20, 2022
I had seen one of these many years ago, and with a little research found out it originally stored in its base a large quantity of mercury.

At the end of last month’s column/podcast about finding an expansion tank, the current clue was a stray ¾” line that disappears through a foundation wall last seen in a crawl space. On the other side of the wall is the original house’s basement (or so we think). It was a cheap tease to get you to read the next column (this one) and a stall to get to the other side of the wall. Turns out that the homeowner is still in Florida, so even I’m curious to find out the conclusion of this little mystery.

When I visit a jobsite with a hot water heating system I always want to find out where the expansion tank is located. As I explained in the column Location-Location-Location, the relationship of the expansion tank and the circulating pump has an effect on the air control in the system. Simply put, we want the expansion tank connection on the suction side of the pump for best air control. That’s why I, and you too, should always find the expansion tank and its connection to the system.

Historically, the expansion tank was above all the radiation, not near the boiler. It was also open to the atmosphere, with an overflow pipe in case the water expanded too much. That all made sense to me for years as I found tanks in attics or second floor closets, but what I found recently made me realize you can learn something every day if you dig a little deeper.

Heat Generators

I had seen one of these many years ago, and with a little research found out it originally stored in its base a large quantity of mercury, that slippery metal used to make a silent switch in the old thermostats. Mercury, the planet of the mind in the realm of astrology. What in the world is it doing in a hot water heating system?

The object in the photo is called a Heat Generator and was invented by a guy named Mark Honeywell back in 1907. It connects to the bottom of an open expansion tank in a gravity circulated hot water system. Its purpose or function didn’t make itself apparent at that time and I had quickly moved on to the next task at hand.

Twenty years later, as I am involved in this expansion tank mystery, another one appears at a job site. The homeowner told me this was the seventh oldest house in Loveland, OH, which was first settled in 1795. She also told me that there was an expansion tank was on the second floor, hidden behind a cabinet door. She solved that mystery without me even looking, but I had suspected, given the age of the home.

This time I resolved to get a better understanding of something that is loaded with a metal that is toxic and outlawed in our industry. I did my research on, a website started by Dan Holohan to improve the understanding of steam and hot water heating. In the Museum section, I found lots of info.

Full disclosure, most of what I know about hydronics was learned from Dan’s books, articles and website. He was writing for a magazine like this when I started in the industry in 1983, became the voice of the industry and my inspiration for figuring this stuff out. [Dan Holohan wrote a monthly column for CONTRACTOR Magazine until the late 90s – Ed.]

How it Works

What we have to remember about the very early jobs is that boilers were manually fired and there weren’t electrical circuits. No switch to shut the fuel off as a safety. The water got hot because someone built a fire, kept it going by adding fuel when needed and controlled the draft to maintain a steam pressure or a water temperature. In this case, it is water temperature.

With the rise in temperature, water would safely expand into the open expansion tank. But what could happen if the water got too hot, say above the temperature to make steam? Seeing an opportunity to prevent a dangerous condition and make a buck, Mark Honeywell patented the operation of the Heat Generator.

Water steams when it reaches the temperature of 2130F or so, depending on elevation, pressure, etc. In a typical two-story house with the cast iron radiators full of water and an open expansion tank above those, the static pressure on the boiler will be about ten pounds. At ten pounds of pressure in the boiler, it takes about 2400F or so to make steam. In modern boilers, we have safety devices that shut off the burner at 2100F or so. In a manually fired boiler, the water temperature gets as high as the guy shoveling coal decides, as long as he’s monitoring.

What if he goes for a coffee after getting a roaring fire going, then gets distracted, letting the fire and temperature get out of control? With a Heat Generator installed, the mercury in the bottom starts rising up in the stand pipe and “causes a pressure upon the water in addition to the atmospheric pressure, and that of the water above the mercury.” In other words, the water temperature can go well above the normal boiling point because it is under extra pressure from the mercury, which is much heavier than water.

In a manually fired steam system from the early 1900’s, the non-electric safety for an over-fired steam boiler was the boiler return trap. I explained its operation previously in the column Push It Back In. If the guy shoveling coal lets the pressure get too high to cause a dangerous condition, the boiler return trap jumped into action.

Back in the day, the safeties for steam and water boilers were based on physics and not electric circuits. If you run across a Heat Generator, be aware that it may contain a toxic metal and handle accordingly. Just like the boiler return trap, a Heat Generator can be removed from an automatically fired system, since its function was long ago replaced by the temperature high limit, often a Honeywell aquastat.

Patrick Linhardt is a thirty-seven-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.

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