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How much of a temperature swing for Gas-Fired Boiler thermostat?

Pilot1226 Member Posts: 16

I have an older Utica Gas-fired Steam Boiler, installed around 1994, Peg-C model. One-pipe system.

Here's a little background:

I bought the house in August of last year (2012). One of the first things I replaced in the fall was the old mercury switch thermostat and replaced it with a Honeywell 1-day programmable one.

It has 4 programmable "times" for the 1-day, so I'm looking for help in configuring it. I've already set the thermostat to "Gas-fired Steam Heat" mode buried away in the settings to prevent it from firing too often.

A comfortable temperature my family enjoys is around 68-69. Once the thermostat turns off, the radiant heat from the steam radiators usually heats the home to around 71-72.

When we leave in the morning, I currently have it set to 66. I've programmed the thermostat to increase to 68 around 4 PM in anticipation of everyone returning home shortly thereafter.

What I'd like to know (and what my heating tech couldn't say without a doubt) is how much of a temperature swing I could program for. Obviously, a larger swing places a larger demand on the boiler itself. It will have to run longer.

In a typical day, nobody is home between 8 AM and 5 PM. So realistically speaking this could be set as low as I can go to prevent problems with freezing, assuming the boiler could handle the extra load to bring the house back up to temperature in the evenings. So, I suppose I would say optimal level for me would be around 60 in the days when nobody's home and keep it around the 68-69 we're comfortable with in the evenings.

I live in an old house, built around 1930. Plaster and wood lathe walls, almost certainly zero insulation in it. There is insulation between the ceiling second floor (bedrooms) and the attic floor that appears to be in good shape.

Also some background information while you're reading this post:

I was having issues with surging on my boiler that looked like it was not properly maintained and I had a technician come out here about a month ago to take advantage of the "21-point tune-up specials" which included a combustion analysis. The analysis showed very high (500PPM or higher) carbon monoxide. I had the chimney flue swept the following day by a local company and had the heating company return to run another analysis, and it was still high. After discussion with the tech, I had him perform a "chimney flue base cleaning" which included a cleaning of the heat exchanger, etc. Following analysis shows the CO level down between 25-30 PPM, and within acceptable ranges.

So, I'm not sure exactly what they cleaned regarding the exchanger and whatnot, but they fixed the problem and that was worth it in the end.

Thanks for the advice, suggestions, and answers.


  • JeffM
    JeffM Member Posts: 182

    As a general rule of thumb, the higher the mass of the heat emitter the less you should set it back daily (as it will cost more to bring it back up to temp later). The consensus seems to be that while it's not a huge mass like a radiant concrete floor, a steam system with cast iron radiators won't net you big savings from deep setbacks. My house is 40 years older than yours but the construction is similar and my steam system also similar (though a newer boiler) - what I do is set it back 3-4 degrees during the day while we are gone, and a few more at night as we like to sleep cool. I actually like the timing of knowing the system will be running when I get up and come home in the winter as radiators will be warm to set bath towels on and such.
  • Pilot1226
    Pilot1226 Member Posts: 16

    Thanks for the reply.

    So you are saying, generally speaking, it will cost more for me to:

    7AM - 60

    2PM - 63

    5PM - 66

    8PM - 69

    Than it would cost to maintain 69 constantly, or...

    7AM - 66

    5PM - 69 is a better option?
  • Jamie Hall
    Jamie Hall Member Posts: 22,891
    There are a number of threads

    here on the Wall regarding setbacks for steam heat, and you will see a variety of opinions if you research them.

    It is hard to determine a consensus from them, but sort of a central range is that a setback of more than 3 to 5 degrees is not going to save you anything in terms of fuel usage; it may, in fact, cost you. 

    That said, a not uncommon arrangement is what Jeff suggested: drop the temperature at night about 4 degrees from the day or occupied setting.  Then, in time for you to get up and enjoy a warm bathroom and breakfast nook, set the system to recover -- perhaps, though, since you are not there during the day, to something a little less than the evening temperature.  Then in the evening, in time for you to come home, have it come up another degree or two to a comfortable evening setting.  Then back down to the night setting.

    Worth a try, anyway...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Pilot1226
    Pilot1226 Member Posts: 16

    Thanks. I'll go program it now. We'll do 69 from around 5 PM until around 7 AM when we leave or get ready to leave (anticipating that it will remain around 69 until we're out of the house before it drops below 68, etc.), and then let it sit at around 66 for the day.
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