Dan Holohan: Why we use 180 degrees for hot-water systems

Most hydronic systems have the boiler running up to 180° F with water returning from the system at 160°. This rarely happens in real life, but it’s the traditional way we do things.

Have you ever wondered why that is? I sure did, so I did some digging in the old magazines. There was once a wonderful magazine called, The Metal Worker. It came out weekly and carried all the trade news of the day, both good and bad. I have access to every issue of that magazine for 1899, which was an important year in heating history, and a very wacky one as well. Many of the oddball devices that I’ve seen in basements over the years — the things that made me scratch my head in wonder — appeared as new products in that magazine during that fin de siècle year. Reading through those 52 issues is like getting into a time machine. And there’s a certain peace to that. There is no uncertainty when spending time in the past. We know exactly what’s coming next.

That year was known as the year of The Carbon Club, an association of boiler manufacturers who got together in the spirit of what they called “cooperative competition,” a concept that would send you to the slammer if you tried it with your competitors nowadays.

The Carbon Club guys were brazen in their very successful effort to control not only the price ut also the supply of boilers. They did this for what they thought of as the good of the industry. They also did it because cast iron was scarce in 1899.

Oh, and they wanted to make huge profits.

What they did was in total violation of the Sherman Antitrust Act, which had already been the law of the land for a decade. The government was going after much bigger fish at the time, though, so the boiler guys must have figured they could get away with it.

And they figured correctly. In the July 15, 1899 issue, they told of a meeting where they got together to fix the prices of boilers across the board. They said they would drive out of business any price-cutting member who didn’t get on board with the plan. How would they do that? By finding the “selfish” competitor’s customers and giving them free boilers. That would drive the selfish ones right out of business. And as it turned out, those who were thinking selfish thoughts gulped and got in line. And that bit of lawbreaking was what launched the hydronics industry as we know it today.

The Carbon Club did a few other things that year and reported on how everyone was getting along. This tidbit appeared in the Dec. 23, 1899, issue of The Metal Worker:

“A meeting was held of the Carbon Club at the Murray Hill Hotel, New York, Dec. 18 and 19, with a large attendance of the members. Several applications were received from manufacturers and some new members were elected. The recommendations of the Committee of Boiler Ratings, which were discussed at the November meeting, were taken up, and after some minor changes, were adopted. This is virtually a standardization of the home heating boilers made by the members of the club, and with the uniform rating and uniform prices, many of the perplexities of the trade are removed. All boilers are now rated on a proportion of 100 for steam and 165 for water, with steam at 2 pounds pressure or water at 180 degrees at the boiler. The rating now includes all mains, returns and risers as heating surface, and the surface exposed in them must be added to the surface required in the radiators to determine the boiler power needed. It is only necessary for the trade to understand that the mains must be considered to avoid purchasing a boiler that is too small. If a boiler shows the 2 pounds steam pressure or 180° temperature in the main when at work, the rating will be considered verified by the manufacturers. The new list also divides boilers into two classes. A uniform rating has been agreed upon for tank heaters on a basis that they will heat 130 gallons of water for every 100 feet of surface that they are rated to carry, and their prices have been rearranged so that concessions are made to the buyer on some sizes.

What we have here is the agreement between all the boiler manufacturers of the time that a hot-water boiler be proportionally larger by 65% than a steam boiler serving the same building. You see that today when you look at the difference in the value of Equivalent Direct Radiation for steam and hot water. With steam, a radiator will put out 240 Btu/h per square feet EDR, but that same radiator, filled with hot water, will put out only 150 Btu/h.

They also agreed that no steam heating system from that day forward should need more than 2-psi pressure at the boiler to heat the building. This was a very significant decision because it put a stop to what was becoming a very dangerous situation. Contractors had been using boiler pressure as a competitive edge. They were sizing systems with as much as 60-psi pressure at the radiators. Higher pressure means smaller radiators and pipes, but the problem was (and remains) that all steam-heating systems have to start at 0-psi pressure, and at the lower pressure, the steam can (and did) suck the water out of the boiler (low-pressure steam moves much faster than high-pressure steam). This caused many of those coal-fired boilers to either dry-fire or explode.

The Carbon Club put a stop to that wackiness. They did it by establishing standards for sizing steam pipes. These standards allowed for one ounce of pressure drop over 100 feet of travel. They leveled the playing field for contractors, and it’s the reason why they can heat the Empire State Building on most days with just 1 1/2-psi steam pressure.

At that meeting, the members of the club also recognized that there is a piping pickup factor, which must be recognized by contractors when they size a boiler, lest they undersize a boiler, which would be very bad for the manufacturers.

They established a standard for heating domestic hot water, which stood for many years, and finally, they let the proof be in the pudding. If a contractor could heat the entire building with a boiler that contains no more than 2-psi steam pressure, or 180° hot water, then he had picked the right boiler for the job. If it couldn’t do it at that pressure or temperature, then he had screwed up somewhere and the problem was on him.

But now that the water was moving faster, they had to come up with a temperature drop that made sense. If the water returned too cool, it would damage the boiler. So they decided to use a 20° temperature drop, and that’s why we now design for a return temperature of 160°. Another advantage of the 20° temperature drop is that they were using this formula and 20 made the math easy to do in your head.

And that’s why we use 180° as a benchmark high-limit setting. They just picked it out of a hat at the Murray Hill hotel back when bushy mustaches were in vogue. It was a safe temperature because the residential hot-water systems ran on gravity and not on circulators back then. We didn’t see circulators until 1928. Gravity systems were open to the atmosphere and the Carbon Club members didn’t want the water boiling at the top of the system. That’s why they chose 180°. It was safe.

When the circulators arrived, their main purpose was to speed up the water in those old gravity systems, which had begun to corrode. Corrosion slows gravity flow and the circulators were the solution to that problem. That’s why Bell & Gossett called their first pumps “Boosters.”

Another advantage of the 20° temperature drop is that they were using this formula and 20 made the math easy to do in your head.

GPM = Btu/h

ΔT X 500

That triangle with the T is engineer speak for “change in temperature.” If the water goes out at 180° and comes back at 160°, we have a 20° temperature change across the system. If you multiply 20 by the constant factor of 500 (which we get by multiplying the weight of a gallon of water by 60 minutes) you get 10,000. Divide the Btu/h rating of the boiler by 10,000 and you get the required flow rate for sizing the circulator.

Some of the things we now consider holy in hydronics came from a bunch of men sitting in a room and making it up as they went along. They had good, practical reasons for the temperatures they chose at the time, but feel free to change those temperatures to save money on every job. The 20° temperature drop doesn’t make much sense these days.