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Lowered Max firing rate down On Mod /Con?

Danscrew
Danscrew Member Posts: 120
I lowered the max firing rate on my Weil Mclain WM97. Down to 5200 from 5430 and it seems like it is running better. It doesn't Modulate up past the target Temp then come down. It reaches the water set-point nice and slow . What are the pros and cons of doing this. It should be running more efficiently at this rate? What do those numbers actually mean. thanks Dan 

Comments

  • Zman
    Zman Member Posts: 6,965
    Fan speed

    It sounds like those numbers are the max fan speed. I am guessing.

    Your boiler will run most efficient with low water temps and at low modulation.

    If your boiler is a bit oversized, turning down the max is a good plan.

    Most mod/cons are pretty fast at getting to the correct output on their own so it is not always necessary.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Danscrew
    Danscrew Member Posts: 120
    Fan RPM'S

    I was thinking that also it lowers the RPM'S of the fan. So it turn it gives less air to the flame and lowering the gas consumption?. Which would slow down the makeup time of the supply set-point . Then it will find the load and modulate to that point. Correct me if I am wrong ? Tomorrow is the big test the temps are dropping to below 32 so I should see how it operates under a good load. Glad i am around to watch it.                     thanks for your reply Carl
  • Zman
    Zman Member Posts: 6,965
    Firing

    The fan speed is how the boiler modulates.

    The control board on the boiler is pretty smart. Usually the boiler will come up to a certain speed on startup, then it will modulate down based the delta T it reads between the supply and return water and the gap between the supply temp and the setpoint.

    Every manufacture does this a little differently. Although turning the max output down, is not a bad idea, I doubt there will be a significant energy savings.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Danscrew
    Danscrew Member Posts: 120
    Firing/ Modulating

    So is it save to say that the Fan is needed to remove the exhaust gases as the boiler modulates higher. The flame gets higher and the fan speeds up so when you lower the RPM'S on the fan it also lowers the amount of flame you are using? is this correct ? thanks Carl for all your help and knowledge   another question I don't see much Weil Mclain Mod /Cons on this Board? any reason for that they should be just as good as the others.  Dan 
  • Boiler wrestler
    Boiler wrestler Member Posts: 43
    fan speed

    The WM97 has a negative pressure gas valve just like the Ultra. The speed on the fan dictates the firing rate, i.e. when the fan goes faster it pulls in more gas and more BTUs.  I see no benefit to lowering the firing rate and would leave it where it was. The boier is smarter than,,... well it's just smart and doesn't need that kind of help.
  • Danscrew
    Danscrew Member Posts: 120
    Firing Rate

    I hear what you are saying. Then why did they provide a setting that is adjustable? the Book  says you can make an adjustment to stop it from a slight short cycling.
  • Jason_13
    Jason_13 Member Posts: 299
    edited November 2013
    Lowering fan speed

    I always match the maximum fan speed to the heat loss if doable. Why ever fire above the heat loss of the building. Longer run times is key to full modulation and long run times. I will usually set the fan speeds for heat and indirect water heater independently. I have seen fuel savings by doing this. Since most boilers I service, not my installs,  are way oversized this really seems to help.

    Slower temperature climbs seem to allow for better modulation.
  • Danscrew
    Danscrew Member Posts: 120
    Fine Tuning

    Jason

    You said you always match the maximum fan speed to the heat loss. What is the best way to accomplish this. Is it trial and error with the fan speed? to match the gradual increase in water temp to meet the supply set point with out over shooting the set-point ? thanks Dan
  • Danscrew
    Danscrew Member Posts: 120
    Spoke to Weil Mclain Tech

    I spoke with a Weil Mclain Tech. And he said no problem with lowering the firing rate.What  it does is lower the peak capacity of the Boiler. He said if lowering it helps it run better by all means do so.
  • ced48
    ced48 Member Posts: 468
    Running My Lochinvar-

    at 65 percent, just above the house's heat loss-Why run any higher?
  • Jason_13
    Jason_13 Member Posts: 299
    Lower firing rate

    I divide the heat loss by the boiler DOE output. The result is the percentage of the heat loss the boiler output is.

    Next I determine the max rpm's for heating. Than multiply the rpm's by the above as a decimal.

    Here is an example when I encounter a much oversized boiler on a service call

    Example:

    Heat loss 78000,

    Boiler input 150,000, DOE output 136,000

    78000/136000 = .56 (rounded up)

    Rpm's 5000*.56 = 2800 rpm's

    This produces a more gradual temp increase and reduces overshoots on temp.

    The same works when a boiler is larger than the IWH BTU requirement.
This discussion has been closed.