Your Mod-Con Boiler Energy Savings Could Surprise You

I walked up to the ATM machine the other day, put the card in, punched in the code and got a rejection slip instead of cash. “Transaction selected is not allowed.” I pulled out the card, waited for the machine to reset and tried again. Got the same result.

Now, I knew that money was in the account, so I stepped back to gather my thoughts when I realized I was using the wrong card. This kind of thing is happening more often, like looking in the wrong cabinet or opening the wrong drawer. Funny how the mind starts malfunctioning as we get older. Things get a little fuzzy sometimes.

Back Thirty Years Ago

Something else that’s fuzzy is predicting energy savings when modernizing hydronic systems. Back thirty years ago, we tried to predict energy savings when replacing one standard efficiency boiler with multiple standard efficiency boilers. The idea back then was to stage the multiple boilers on and off based on building demand.

With water boilers, we used the system supply water temperature. As the temperature dropped below a target water temperature, the staging control would fire up another boiler. With steam boilers, we used the steam pressure measured at the common header. As the pressure dropped below the target pressure, the staging control would fire up another boiler.

It was assumed that the more boilers we could stage, the closer the energy input would be, therefore using less energy. The manufacturers had some charts to predict energy savings, but the estimates could be a little fuzzy.

When high efficiency modulating-condensing boilers came on the scene about twenty years ago, the typical difference in efficiency was about 15%. The standard efficiency boilers were around 80% while the mod-con boilers were about 95%. Simple math would predict that the new boiler or boilers would use about 15% less energy than the old boiler being replaced.

However, life is rarely simple. Contractors were telling me that homeowners were exceeding the 15% savings prediction. A church that had replaced a large on-off cast iron boiler with two mod-con boilers had the energy bills to prove that they saved 50% in the first year. The degree days for the two years were almost identical, so the savings were real.

Turn to the Trade Press

This got me to thinking (which can be dangerous). How could they be saving so much? What other factors besides the government ratings are affecting the actual savings? Back then, I used to read the trade magazines, like the one you’re reading now. I learned a lot about hydronics from the printed pages that came by snail mail and I actually looked forward to digging in to the latest edition that showed up in my mail slot each month.

About this time, an article came out with a chart/curve predicting the seasonal efficiency of on-off boilers based on outside temperature. The efficiency went down from the government rating as the outdoor temperature went up from the design outdoor temperature. Most of the boilers in America work in climates with a design outdoor temperature around zero °F.

Northern Minnesota uses a much lower temperature, while Atlanta uses something higher. The government rating is based on long cycles of combustion, like on a cold night at design temperature.

But not all of America has many nights like that. In fact, digging into weather data, I found that in the Cincinnati area, there are typically very few extremely cold nights. In fact, I found out that about 75% of the degree days were above 32 °F. The weather we had during the “Freezer Bowl” playoff game was not typical.

Comparing Curves

The light bulb in my head was starting to flicker with ideas. I took the efficiency curve of a typical mod-con boiler and the efficiency curve of an on-off boiler and put them on the same piece of paper. As you can see from the chart, the efficiencies are close at outside temperatures of zero °F, but quickly get farther apart as the outdoor temperature gets warmer. By an outdoor temperature of 32 °F, the difference has about doubled.

In the mild temperatures of the heating season, the difference can be as much as 40%. If your weather is like ours, with 75% of the heating season above 32 °F, reported energy savings can be well above the assumed 15%.

Two Things to Consider

Two things are affecting the amount of savings: condensing flue gasses and boiler mass. The more you condense flue gasses, the greater the efficiency will be. In mod-con boilers, flue gasses condense when the return water temperature is below about 135 °F. During mild weather in a typical hot water system using indoor-outdoor reset, that’s going to happen a lot.

Cast iron boilers have a lot of mass, with older cast iron boilers having even more. The reason the efficiency drops off with the on-off boiler is standby losses. These losses occur more often during the milder weather. The longer the boiler is off, the greater the stand-by loss. The greater the mass of the boiler, the more energy it takes to re-warm that mass before it starts warming the water.

Cast iron boilers with natural draft and no vent damper are likely to incur the most stand-by losses. I have been in many a boiler room with a commercial size cast iron boiler and an open window for combustion air.

The outside air comes right through the window, travels up between the cast iron sections, transitions the draft hood, and flows up the chimney. That process cools the cast iron sections when the burner is off. If the air is cold enough and the off cycle long enough, the stand-by losses start becoming a problem.

Fortunately for mod-con boilers, the mass is low and the outside air doesn’t pass through the boiler when the burner is off. There is very little cool down between cycles and a lot less mass to warm up. Don’t let your customers assume they are only going to save 15% in energy usage when they step up to a high efficiency boiler. Chances are they are going to save more.

Patrick Linhardt is a forty-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.