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It is OK to turn a 14 section cast iron commercial boiler on and off every day?

elfie Member Posts: 264
have a 14 section cast iron boiler and during the night its not needed to heat a large facility and only need heat during the day (its a thermostatically controlled facility that has actuators to control water flow, so hot water heating loop is reduced overnight)

have heard its really not a good idea to turn a large commercial boiler on and off on a daily basis as the need to have heat arises (ie. best to leave on)  - alot different than a residential boiler which is designed for this type of use.

if you turn it on and off it tends to really strain the boiler and just to do it to gain energy use savings is not healthy for the boiler (ie. thermal stress).

anybody have thoughts on this?



  • STEVEusaPA
    STEVEusaPA Member Posts: 5,689
    Wasn't this....

    Asked and answered a few weeks ago?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • elfie
    elfie Member Posts: 264
    it it ok to turn a 14 section cast irom commercial boiler on and off every day?

    not sure it was really answered other than asking the boiler manuf.

    is it healthy to heat up and cool off a commercial boiler on a daily basis   -  is it true that the temperature variation and stress is not healthy for boiler

    have heard that its best to simply turn it on at beg of season and off at end of season

    thanks again
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Did you ask the manufacturer?

    I'm also guessing that the people who made the recommendations to leave it on all the time are not paying your utility bills...

    If you did ask the manufacturer, what did they recommend?

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jamie Hall
    Jamie Hall Member Posts: 18,924
    Always best

    to ask the manufacturer!  They are sometimes, however, somewhat cryptic in their answers...

    It seems to me, though, just reasonaing from first principles (and some knowledge of marine power boilers) that the real key to it is slowly.  If you can heat it slowly in some way, then it will heat -- and expand -- evenly, and there shouldn't be any problem with turning it off and turning it back on.  If the temperature changes are quick, however, then you can get uneven expansion and possible problems.

    Consider: a big marine power boiler may take anywhere from hours to several days to be safely brought from a cold start to starting to develope steam.  These are very different boilers, of course -- big water tube units at high pressure -- but it is typical to allow at least 10 hours as a minimum from a cold start to steaming... and as much as two days to cool down (using partial firing during that time to control the cooling) from full power to cold.  For what it's worth...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,689
    not to pile on......

    and I understand you came here looking for help.  In most cases, you will get many responses for help. But once Mark steps in and advises you, it's pretty much the final, and usually the best, well researched/experienced answer.  He just happened to get to you sooner, and the rest of us mere mortals benefit as well:)

    I, as well as many here, are always grateful, when he, and some of the other very experienced professionals, take time out of their busy days to respond.

    Keep posting ME, and watch out for those wildfires.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Thanks Steve...

    I appreciate the kudos. I'm just here to help.

    The fire is still 100% un-contained. 23 homes damaged so far. It is physically about 30 miles away, and other than having to see the debris falling from the sky, and breathing the smoke, we here in Denver are not threatened. However, if the wind shifts today, something like 3,500 additional homes will have to be evacuated.

    Talk about weird weather, and global climate changes, our February set new records (2nd highest) for snowfall, and moisture content. Then March came storming in like a Lion, and came to a dead stop weather wise. If this trend continues, this March will be the driest on record (trace on March 7).

    Spring has arrived early 100 miles West in the high mountains as well. Strange seeing Red Breasted Robins sitting in the pine trees while you are skiing on fresh snow.

    Mother Nature can be a cruel woman. April will probably break records (and trees) with devastating snowfall...

    As we say here in Colorado, "If you don't like the weather, wait 5 minutes. It WILL change..."

    To the original poster, setting a simple outdoor reset controller to run between 140 and 180 degrees F will reduce the thermal stress on the castings, AND save energy. These are off shelf controls that are simple to install and wire.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Owen
    Owen Member Posts: 147

    I have timers on all mine without DDC and set them so when boilers aren't running full on (not needed for heating) they come on just enough to keep the boilers at 180º or better. I check it with an instant read thermometer held in a stream of water out of a mud leg. Off times vary from an hour or so to three or more. I do this during the so called "shoulder months"
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Really Owen???

    If memory serves me correctly, you are running steam boilers, correct? For a broke school district, right? And you maintain 180 degrees F when the boilers are not needed? Does this include long periods of no occupancy, like spring break and Summer?

    Tell me why that is... What are your fears? Leakage or complete boiler failure?


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Owen
    Owen Member Posts: 147
    edited March 2012


    Steam boilers, yes. (Also have seven hot water boilers that I don't do much with.)

    Broke? That's what they claim but they have money sitting in the bank earning 1.8% intrest.

    That 180º is approximate. It is on the advice of the boiler contractor/vendor who installed some of them, and is intended to prevent wide thermal swings on the metal.

    It does include spring break, but not summer.

    And, yeah, leaks caused by wide temperature swings, and the old "more efficient to start steaming from 180º than..." whatever they would drop to if shut off completely.

    What are you suggesting? It's unnecessary? Wow.
  • elfie
    elfie Member Posts: 264
    thermal stress 180 degree boiler temp and shutting it down

    here's the concern when you want to shut down a hot water boiler for a couple days during the season transitions

    according to a boiler manuf (HB Smith), its preferable to avoid turning off a system unless its for several days.  these are not residential boilers and are not designed to be turned on and off (ie. should avoid thermal stress)

    further, if you do turn it off, make sure that boiler temp cools sufficiently to avoid thermal shock when all the 60-70 degree water comes back when boiler restart occurs (and its good to reduce burner so that the system warms up slowly)

    turning on and off a boiler frequently, can lead to condensation of sulfuric acid that is not good for a boiler. 

    keeping temperature at 180 degrees in my opinion is too high if the facility can survive on water temps of 130 - 150 (to me there is more risk of thermal shock and section cracking with water temps at 180 degrees) - temp differential on returning water should be no greater than 50 degrees.  as ong as return temps dont drop below 130-140 degrees, sulfuric acid condensation risk is not a problem.

    ideally, a mixing valve installed will control the temp of water circulating to the facility so the boiler temp can be kept at a constant temp all season long
  • Henry
    Henry Member Posts: 995
    Sulphiric acid?

    Unless the the boilers are oil fired, there is no sulphuric acid problems. If you have two or more boilers, they should run alternate each day with a proper modulating control from someone like Heat Timer. Most if not all of the timer on-off steam systems provide poor comfort to the occupants, have no real energy savings (usually too high a pressure) and do nothing regarding thermal stress and shock.

    But with large section CAST IRON boilers, it is important during the heating season to keep the boiler warm. A 30 degree differential from 180 to 0 PSI steam is a good way to save a boiler from thermal shock. We have had two schools that needed sections replaced due to thermal shock nearly each year! We added an aquastat to keep 180F during the heating season and, no failures since!
  • elfie
    elfie Member Posts: 264
    lack of sulfuric acid condensation in NG??

    not sure I agree that sulfuric acid condensation risk goes away when NG is used to heat up a hot water boiler.  if this were the case I would keep the boiler temp alot lower ( but manuf recommends keeping it above 140

    why do you keep your boiler at 180? would place stay heated if you used 160? (ie.as long a return temps dont drop below 135-140)
  • Owen
    Owen Member Posts: 147
    180 Degreesx

    That is the recommendation of the boiler fitter who does most of the contracted work.

    I believe part of his concern was condensation, but I don't remember if it was on the outside of the cast iron sections or whatever, but we discussed it and he was quite firm in his advice to not let them cool down much below that.

    One thing I do know, the temp inside the boilers depends on from where you take your sample; steam chamber, top of water, middle or bottom. It varies a lot. I just figure shoot for 180 from the bottom and that's close enough.
  • furnacefigher15
    furnacefigher15 Member Posts: 514
    Acid in the stack gas for NG

    The acid is not sulfuric, it's carbonic.

    Owen. 180 is high, but with no water movement, once the water is 180, it's just about as much energy to maintain 180 as it would be to maintain 140 or 150 or 160 or 170.

    Also, those cracking sections have less to do with the boiler, then the piping. Steel moves a lot more then cast iron when heated.

    Most fitters like to weld piping over a certain size, and on boilers with multiple tappings like a steam boiler, there needs to be a swing joint made using threaded fittings to allow the header to move without stressing the boiler.
  • Bob Vennerbeck
    Bob Vennerbeck Member Posts: 104
    curiosity on swing joints

    I've always understood one reason for swing joints to be desirable was that piping can be aligned during assembly without stressing the fittings or compromising required slope, level or plumbedness - are such joints actually intended to rotate on their threads to accommodate thermal expansion and contraction, or does the pipe making up the swing itself absorb the stresses by torsion? If the former, that's asking a lot of the pipe dope, and if the latter, it would seem to make sense to make the center of the swing long enough to absorb the torsion instead of using closely coupled elbows.

This discussion has been closed.