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boiler runs every ten minutes for three minutes at a time

The duty cycle of a system is really a function of your heat loss and net boiler heat output.

Say for example you have a building heat loss of 50k BTU/Hr. You will have to input this amount of heat energy each hour to make up for the loss, and therefore maintain constant temperature. Lets assume your boiler has a net heat output of 100k BTU/Hr. Because this is not a modulating boiler, it can only put out its rated 100k when firing, or nothing at all when off. So in order to put the needed 50k BTU/Hr into the house it has to run only half the time, or at a 50% duty cycle to provide an AVERAGE heat output of 50k BTU/Hr (100k + 0)/2 = 50K. Under these conditions the duty cycle will ALWAYS be 50%, but the actual on and off times will depend on the thermostat hysteresis (differential). If the thermostat is set for a 1F degree delta T, the boiler will have to fire on and off in short cycles to maintain constant temperature without overshoot and undershoot. If you increase the thermostat delta-T setting to 2F, the boiler will fire longer each time and also stay off longer, but will still fire only half the time at the the identical 50% duty cycle.

Now you change the burner nozzle so that your boiler fires at a lower rate. It only produces 70k BTU/Hr into the system. Because your heat loss has not changed, you still need 50k BTU/Hr to maintain temperature. To generate the average 50k BTU/Hr needed, the boiler must fire longer and stay off for a shorter time, which results in a higher duty cycle of 70% ( 70k BTU/Hr * 0.70 = 50k BTU/Hr.)

The closer the boiler output to the actual heat loss, the higher the duty cycle will be, until the limiting case where output and loss are equal and the boiler fires 100% of the time.


  • angelo_3
    angelo_3 Member Posts: 5
    hot water heating boiler

  • Couderay
    Couderay Member Posts: 314
    Boiler cycle times

    Need more info.on your system.Is it a new install,or a retrofit.Did you do a heat calc. How did you size boiler,any pics on the system.More info you can provide the better.
  • angelo_3
    angelo_3 Member Posts: 5
    boiler constantly runs

  • william_5
    william_5 Member Posts: 62
    three mins at a time

    angelo what are the aquasts set at ? what are the modle #s
    what kind of burner is installed? and do you know the firing rate? does it have a coil? steem or hot water?

  • Perry_3
    Perry_3 Member Posts: 498

    Please stop typing in all caps... It's not polite - and this isn't an emergency.

    I am sure there are several people on this site that will be able to provide good information. I found them most helpfull.

  • Jamie Hall
    Jamie Hall Member Posts: 21,553
    Still more info needed

    and another question: is the thermostat one of the newer ones? They have various settings for various types of heating systems. Check and make sure it's correct.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Patchogue Phil_29
    Patchogue Phil_29 Member Posts: 121
    Where are you located?

    Is it REALLY cold outside where you are?

    Are thermostat(s) calling for heat a lot?

    Does this boiler have a hot water coil? Maybe the hot water somewhere is dripping or leaking?
  • angelo_3
    angelo_3 Member Posts: 5
    boiler constantly runing

    the boiler model#is wbv-04 perless. there is a coil in the unit plus a 50 gal hot water holding tank.I have the aqustat set at 160 low and 190 high.The thermostat was graded by a plumbing supply clerk and I also checked the manufactures technition to make sure it was proper for my system. I have the thermostat set at 68.And the outside temp the past few days has been in the mid 20.
  • realolman
    realolman Member Posts: 513
    that might not be all that unusual

    What kind of aquastat is it?

    How big is the nozzle?
  • angelo_3
    angelo_3 Member Posts: 5
    boiler runs constantly

    could my boiler be over sized?The house is 28000 sq ft the vboiler has 155000 btu.
  • Bernie Riddle_2
    Bernie Riddle_2 Member Posts: 178
    Heat loss

    I take it you mean 2800 sq ft lol. Did they just replace the old boiler by trying to match it BTU for BTU with your old one? If so that might have been a mistake. Did they do a heat loss calculation? Thats the only way to get it right.

    The good thing about the WBV04 is if its oversized, I think it can be down fired by switching to a smaller nozzle size. That might stop the short cycling.
  • realolman
    realolman Member Posts: 513
    I should have kept track

    of who posted this, so I could give him credit... I did not write this, but he posted it in response to me when I was quizzing this forum about a similar situation as yours.
    I found it very interesting, so I kept it. My burner ran 7 minutes of every 13 in the example I showed. This is what he wrote:

    Since your present duty cycle is about 7/13 at an outdoor temp of 20 then, it looks like your present "design" outdoor temp is about -30 using the .85 nozzle.[-{13/7*(80-20)-80]. If you go to a .65 nozzle, then you are probably looking at a "design" temp of -5.

    If you want to increase the ON and OFF times you can reduce the differential in the boiler and/or lower the boiler setpoint temp and increase the hysteresis in the thermostats. But you have no control of the duty cycle which is governed by the outdoor temp for each nozzle size.

    I thought his post was very interesting... I wish one of these learned guys on here would explain it further.

    I lowered the nozzle size in my burner along with the static plate and a low firing rate baffle, and got longer run times, but more importantly to my thinking, I lowered the stack temp 200 °... from 650 to 450.
  • angelo_3
    angelo_3 Member Posts: 5

    thnks for the many responses to my problem I reset the aquastat to 150/190 and set the thermostat to 68.It seems to have fixed the constant runing again thanks angelo
  • realolman
    realolman Member Posts: 513

    You're the guy who wrote that in the first place aren't you?

  • No, but I was the one who commented on your nice graphs and data acquisition system. Do you still have it set up and can you post another graph with the smaller nozzle?
  • realolman
    realolman Member Posts: 513
    That's right I shoulda remembered...

    thanks again... I'd be happy to post one. If it doesn't display right , hover your cursor over it till a zoom button appears, then click that.

    This is during a recovery from a night time setback, so I guess that kinda throws a monkey wrench in the duty cycle, but I do like the 450 degree stack temp better than the 650° I started out with.

    The two lines between 70-80 have nothing to do with temp, but show if the two zones are calling for heat. The blue zone doesn't have night setback.

  • Charles J. Hills
    Charles J. Hills Member Posts: 29
    Better understanding

    I read this site alot because I learn alot. My question here is while there was a lot of technical language that I don't quite understand right now how much of the cycling can you contribute to heat loss via a drafty house ? Is there a test or calculation table a homeowner can use to realize his situation ?
    Charles J. Hills
  • zeke
    zeke Member Posts: 223

    Just noticed you. I own up to that equation. It derives from simply the result of two equations:




    k= proportionality constant for your house, the fundamental assumption being that the overall house load is proportional to the outside to inside temp difference.

    Q=the power of the boiler full on

    @=duty cycle

    Ti= room temp

    To=outside temp during test

    Td=design outsde temp, i.e. the lowest outside temp that can be sustained w boiler full on or @=1

    If you divide the two equations, k and Q drop out and you get



    which is essentially the formula I showed you.

    By the way, this is an estimate and if you use it, make sure that that you do the test on a sunless day, or better still, at night to eliminate any solar input.

    You also could use this information to get a new Q to change the design temp,Tdnew, since the Q is proportional to the nozzle flow rate.It will obtained from

    (Ti-Tdnew)/Ti-Td)=Qnew/Qexisting=new nozzle flow rate/existing noxxle flow rate
  • Latest graph

    Looks like you have it optimized pretty well. Seems to be running at about 66% duty cycle at 32F outdoor temp which would probably interpolate to a design temp of around 10F for 100% firing. How cold does it get where you are?

    Unless I am reading it wrong, the boiler ran constantly for 53 minutes coming out of setback, reached the thermostat or aquastat setting and the started to cycle. Are you set up with constant circulation? Boiler water temp seems to drop pretty fast during the off period. What are your aquastat settings?

    What type of thermostat do you have? Is there an anticipator setting or other means to adjust its hysteresis?
    I would think you could increase the cycle time by this adjustment unless the cycling rate is being determined by the aquastat differential setting.

    It's really great to have all this data in front of you on the graph and be able to analyze what is really happening
  • realolman
    realolman Member Posts: 513
    The boiler temp

    does drop off quick. I don't know what, if anything to make of that. You can see the temp drop of the zone. This is on a B&G series 100 pump, and there are actually 3 "zones" on it that each have a balancing valve on them, so I can sort of regulate the flow, even though all three zones are circulated at once. The other zone goes only to a sun room and has it's own pump and no setback thermostat.

    Mostly it works pretty well, but I'm not real crazy about some aspects of it. I have two big ol' radiators in the basement, that can be valved around or through, but that doesn't work all that great sometimes..It gets pretty hot or maybe not hot enough down there. I think I'd like to do something with them next year. Maybe TRV's or another zone. I don't know.

    The aquastat setting is 180 off 170 on.. my sensor is on the first inch or so of pipe out of the boiler and is not down in the guts of the thing like the aquastat, so there is some difference in readings. I don't know if I'm all that crazy about the programmable thermostat either ( Honeywell CT3451 ). It doesn't have an anticipator on it per se, but you can set the number of cycles per hour. That's another thing I'm not too sure about.

    I am in the mountains of western PA... although it gets plenty cold here sometimes, this winter was pretty mild. Last month or so was pretty much of a booger, though.

    Although it's too late to make a long story short, when oil went over 2 bucks a gallon last fall, I decided I needed to look at my boiler. The first thing I did was stick a thermometer in the stack and was shocked by the temp. Over 650°. Ever since the 80s I been messing with computers, so I started hooking up sensors and what not to the boiler. Many of the results I got were not clear cut ( at least not to me ) , but through the generosity of guys on this forum, I have learned a lot.
  • realolman
    realolman Member Posts: 513

    alot for that. I'm not exactly sure why, but that is very interesting to me.

    The knowledge and generosity of guys like you makes this "Wall" place a wonderful thing

    Thanks again.

  • It looks like the aquastat is what is causing the cycling because it seems to be bouncing between the 170-180 degree limits that you have set. Can you increase the differential to 20F so that the limits are 160-180 degrees? That would most likely double your cycle times. Is there any reason you have the limits set so close (10F)? Are you getting your DHW from a coil in the boiler?

    Does your circulator run continuously, or is it controlled by the thermostat?
  • realolman
    realolman Member Posts: 513

    It has a fixed 10° differential. I've been thinking about trying some other kind of aquastat

    It has a tankless coil, but I quit using it when I found out how much oil it used over the summer.
    I disconnected the aquastat for the DHW.

    As long as there is a green or blue horizontal line between 70- 80° on the graph, one of the zones is calling for heat, and the aquastat differential IS going to be what controls the cycling. On the graph, I don't think the house was recovered from the night time setback yet.

  • Thats what bothers me. The thermostat is not satisfied but the water temp is reaching high limit and shutting down the firing. It almost appears that you dont have enough radiation in the zone with the thermostat to dissipate anywhere near the amount of heat energy the boiler can produce. Looks like you need more than 180 degree water to meet the building heat demand?
  • realolman
    realolman Member Posts: 513


    I'll hafta look into that.
  • One more thought

    You mentioned that there were three balancing valves in each of the zones fed from the B&G 100. I wonder if you have these valves throttled back to the point that you are overly restricting the flow. You have a delta-t of about 35 degrees between output and return on these zones which is pretty high. Why not open the valves and see how much the delta-t drops. I'll bet it reduces the amount of cycling.
  • carol_3
    carol_3 Member Posts: 397

    A thermostat comes out of the box to do exactly what you're describing. Unless you change it, it's set for 6 cycles per hour. That means it will bring on the heat every 10 minutes (60 minutes divided by 6 = 10 minutes). The equipment runs for as much of the 10 minutes as it takes to reach set point (sounds like for you it was 3 minutes) and then it shuts off until the beginning of the next "cycle." If you want it to come on less often and run longer, your room temp will vary more, which is less comfortable and doesn't save you money. But if you want the longer cycle, set the thermostat for 3 cycles per hour (comes on every 30 mins). If your stat doesn't have a cycle rate adjustment (that is if it's not electronic), setting the anticipator to match the amp draw of the load gives you 6 cycles per hour. 1.2 times the amp draw of the load gets you 3 cycles per hour.
  • zeke
    zeke Member Posts: 223

    Looking at your data, I agree for the most part with Mike- your major problem is either lack of sufficient radiation (as evidenced by not reaching the setting on the room thermostat) surface or insufficient "average" temp of the circulating water. The delta T should be reduced by increasing the flow as much as practical and/or increasing the high temp to, say 190-195 deg , thereby increasing the "average" temp of the circulating water and thus increasing the heat transferred to the rooms. For example if the return is increased to say 165 then the "average" temp will go up 7 degrees and increase the radiation by nearly 10%. Similarly, increasing the high temp to 195 could gain another 10%.

    And as a further observation, if the outside temp goes from 32 to 10. you may need an additional 30% more heat transfer to sustain the load, which may not be available. I wouldn't change the 10 deg differential as yet since it will reduce the "average" temperature, which is not what you now need.
    I think, for your present thermostat setting for the 3 zones, your radiation is only marginally sufficient.

    For the moment, I would make only the aquastat and flow changes and go from there.
This discussion has been closed.