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Modulating Pressuretrol - how should it work and be optimized

elfieelfie Member Posts: 264
below is a modulating pressuretrol device (with bellows)

how should this be set?  ie. does the modulating pressuretrol change the burner firing rate depending on the pressure in the boiler?

this boiler always fires at the same high fire rate with no variation as pressure builds in boiler.

i am concerned that frequent start up of a cold boiler with a high fire rate may be thermally stressful to boiler (ie. steam boiler used only a few times a week) - and that a slower warm up may be of benefit.   It has been recommended to install a low fire hold device.


  • Mike KusiakMike Kusiak Member Posts: 604

    The modulating pressuretrol will adjust the firing rate to maintain the steam pressure at the adjustment settings. Its hard to see, but you might have a 0-15 PSI control set approximately at 4 PSI.

    If this is the case, the burner will run at full output until it reaches near 4 PSI, at which time, the firing rate will be reduced to maintain constant pressure in the range of the main setting plus the differential. The differential setting determines the range of proportional control. A low differential setting will keep the pressure close to the main setting, while a higher setting will allow more variation in steam pressure.

    Here is a link to the instruction sheet on a Honeywell modulating pressuretrol:
  • modulating pressuretrol

    this has some extra contacts which could control a 2 stage gas valve or some sort of electrical valve on a steam system.

    only one problem, it is basically the same pressuretrol [insensitive at low pressures] we all find fault with. the same modulation could be done with a pair of vaporstats.--nbc
  • Mike KusiakMike Kusiak Member Posts: 604
    Control System

    Its not a on-off device, so using a vaporstat would defeat the modulated firing. The bellows controls a potentiometer which allows infinite variation of the firing rate. The actual burner control is done by a servo motor, which proportionally adjusts the air and gas to the burner.

    Most likely, the modulation is not working because the steam pressure never reaches the proportional control range.The control could be set lower to reduce output at a pressure lower than the current  setting. It all depends on what pressure the system was actually designed to operate at.

    A low fire hold would be a good idea to limit thermal stress from a cold start. An aquastat could be installed to force the burner to stay at low fire until the boiler is near boiling temperature.
  • SteamheadSteamhead Member Posts: 11,384
    Refresh our menory

    what is this thing used with- what burner, boiler, type of system? 
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • elfieelfie Member Posts: 264
    edited February 2012
    modulating pressuretrol operation

    here is a closeup of the modulating pressuretrol

    the settings are approx 4psi and additive is almost B which is about 3.5 psi (1.8 per increment: ie. -  min to A, A to B, B to C)

    the cutin control to right is set to cut burner off at 4 psi ( its for an over sized 12 section HB Smith boiler with a Powerflame burner), so the additive setting seems meaningless.

    i guess the settings mean the burner always starts at high fire and then reduces when it hits 4 (and in this case the burner shuts off before low fire comes on)

    maybe a better setting selection would be to set modulating pressuretrol at 2 PSI and have additive set to A which I believe means low fire would kick in at 2 PSI and then burner would cutoff at 4 PSI, which is where the pressuretrol on the right is set to. 

    seems if the modulating pressuretrol is well managed it could keep the burner on longer and therefore limit those energy losing purging process cycles.

    it would appear that the modulating pressuretrol may not be used to act as a low fire hold device.

    any idea how much it might cost to install a low fire hold device (ie. water temp sensor style)
  • Mike KusiakMike Kusiak Member Posts: 604
    Proper adjustment

    Yes, with the present settings the system will never work properly, since the cutout pressuretrol at 4 PSI will shut down the burner before the modulating range is reached.

    Adjusting the modulating pressuretrol to 2 PSI or less seems reasonable as at that pressure setting the burner will begin to downfire, maintaining the pressure at probably 2 to 3 PSI and no more. Therefore you will never reach the cutout pressure of 4 PSI and the burner will run constantly without cycling until the thermostat is satisfied.

    The differential adjustment is really a "gain" setting of the control servo loop, and determines how much variation in pressure is allowed above the main setting. Ideally you want to set this as low as possible without creating instability in the system. If the differential is set too low, the control loop will become unstable and the servo motor will "hunt" for the proper setting, moving constantly. If this happens, increase the differential slightly until the system stabilizes and the firing rate motor settles down.

    The modulating pressuretrol will not provide low fire hold, as it calls for full fire until pressure rises to the main setting. If not already present, a temperature sensing aquastat must be added to the boiler to force low fire until it heats up to near the boiling point, say 190F or so. Then at that point, the burner will go to full fire and begin generating steam.
  • Also,

    You may see a firing rate adjustment knob on the boiler.  You can set how high high fire is with this knob.  If it doesn't allow you to get low enough in the burners range, it can be replaced with another rheostat that gets you lower.  Getting the modulating controls set up properly with, hopefully, a boiler that at least has some modulating range left when you get the boiler turned down enough on high fire, will save the owners a bundle on the fuel bills (probably 15%) and add years to the life of the boiler.  The few times I have seen modulating controls installed, they were never even operating. 
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • elfieelfie Member Posts: 264
    edited February 2012
    modulating pressuretrol - differential adj

    thanks for your info, very helpful

    if i set the differential lower than wouldnt this reduce the amount of time that the lower fire rate is on (ie. main setting at 2 psi with differential set below A would I guess mean that low fire would be on up to about 3 psi),  isnt the idea to get the low fire to take over alot sooner

    also, the boiler is not thermostatically controlled so when the zones dont require heat, the boiler just maintains pressure.

    thanks so much
  • Mike KusiakMike Kusiak Member Posts: 604
    edited February 2012
    Differential range

    I am not sure I understand your question. Reducing the differential setting does not decrease the time the firing rate is lowered. Instead it may actually reduce the firing even more, so that the pressure will never even reach 3 PSI.

    It appears that your Powerflame burner has a 5 to 1 turndown ratio. This means that the lowest firing rate is only 20% of full fire. Because it is a modulating burner, the modulating pressuretrol can set the firing rate anywhere from 20% to 100% of full fire, depending on steam demand.

    For example, if the zone valves are all off, the pressure may quickly rise to 2 PSI, at which time the pressuretrol will rapidly reduce the firing, perhaps all the way down to 20% to prevent it from rising further. If a zone valve now opens, the pressure may momentarily drop below 2 PSI, now the pressuretrol will increase the firing just enough to maintain 2 PSI. The whole purpose of the modulating system is to maintain a constant steam pressure, regardless of the load or demand for steam.

    The differential setting detemines how close to the main setting the pressure is regulated. So if the main is set at 2 PSI, and the diff at 0.5 PSI, then the pressure should never rise above 2.5 PSI unless the demand for steam was so low that even at 20% firing rate, the pressure continued to rise. Then the firing would remain at minimum until the pressure rose to 4 PSI at which time the cutout pressuretrol would shut down the burner entirely.
  • elfieelfie Member Posts: 264
    edited February 2012
    modulating pressuretrol - how does differential setting impact burner?

    i think its getting more clear

    so basically, if the mod. pressuretrol is set correctly, the burner should run longer (i.e. less burner on/off cycles) - and much less system purging

    and the burner should increase and decrease as it maintains the pressure at around 2.0 to 2.5;  and then when all the valves are closed the pressure will gradually rise to 4.0 and the burner then cutsoff.

    at 2.0 the high fire burner reduces to low fire and with the main cutoff pressuretrol set to turn off burner at 4.0 what difference does it make what the mod pressuretrol differential is set at?  thanks for your patience on this question.  how does the differential impact the burner level in regulating the pressure?

    right now our system and pressure gauge is on a roller coaster and doesnt have burner modulation - how tragic that this has been going on for yrs.
  • Mike KusiakMike Kusiak Member Posts: 604
    Control range

    Yes, if the modulating pressuretrol is set correctly, then it will reduce or eliminate the constant cycling of the cutout limit pressuretrol.

    That's the benefit of the modulating system. The burner always fires at only the minimum necessary to meet the steam demand at that time.

    Setting the modulating pressuretrol diff setting as low as possible will result in the minimum firing rate for the given steam demand. For example, if the main is set at 2 PSI and the diff at A (1.8 PSI) then the firing rate will begin to reduce from 100% beginning at 2 PSI, but not reach the minimum 20% rate until the pressure reaches 3.8 PSI.

    On the other hand, if the main is set a 2 PSI and diff is 0.5 PSI, then minimum fire will be reached at 2.5 PSI and the burner will be at relatively lower firing rates for more of the time, resulting in  tighter control of the steam pressure. Only when the demand is so low that even at minimum fire of 20%, more steam is still being generated than can be condensed, will the pressure rise to the 4 PSI cutout and the burner shut down completely.

    So with the lower diff, the boiler will spend more time closer to 2 PSI than to 4 PSI, which is the result of the firing rate being reduced to a greater extent by the tighter control range.
  • elfieelfie Member Posts: 264
    modudulating pressuretro - a gradual decline

    now i follow, its a gradual burner rate decline between the main setting and the differential point.

    maybe i am the only one who didnt understand the modulating pressuretrol ???

    quite possibly if everyone understood this modulating pressuretrol and set it correctly, the energy crisis would be alot less

    of course if you have a grossly oversized boiler, the modulating pressuretrol will help but not it may not solve the problem

    thanks so much.
  • ArtVandelayLEEDAPArtVandelayLEEDAP Member Posts: 9
    I'm with Elfie...hoping for a little more clarification

    Here's my question:

    Lets say I have 2 pressuretrols:

    The operating pressuretrol is set for 5 PSI limit with a subtractive Diff set at 2 PSI.

    In this scenario, my burner cuts-in at 3 PSI....what would be the best setting for the modulating pressuretrol?

    Also...does the modulating unit pressuretrol automatically "take over" after cut-in? Is this where the main setting for the modulating unit comes in?

    Right now, I'm thinking that I should have the modulating pressuretrol's Main set to slightly below the cut-in of 3 PSI....say 2.5. This way the modulating function takes over immediately at cut-in. I would then want to have an additive Diff that brings me close to the operating limit of 5 PSI, so a Diff that corresponds to around 2.4 PSI. With these settings, the modulating pressuretrol will control the firing rate over a range between 2.5 PSI- 4.9 PSI. (i think). Is this sound?

    Also, what if we are seeing an older pressuretrol that does not have the additive Diff, and there is only one scale plate instead of two? Does the setting on the single scale plate mean that the modulating control takes over upon cut-in, and then modulates up to the Main setting?

    I'm typically seeing these on a Cleaver-Brooks CB-400 boiler....

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