What should boiler high limit be?

Rav

I have a Weil-McLain series 8 CGM-5 hot water heating boiler.  Runs fine.  (We have a separate DHW heater.)



About two years ago the 30 PSI PRV started spurting out small amounts of water, infrequently.  When I checked, the pressure was at 25, when it's normally around 18.  I had a pro come out to check it.  He checked the PRV (thought there might have been some grit partially blocking it), and drained the overhead expansion tank since it hadn't been done in 3 years.  He also turned the high-limit down from 150 to 140, saying "the radiators are as hot as they need to be."



About 1.5 months ago I noticed that, although the booster pump would remain on and continue circulating water, the gas to the boiler would constantly turn on and off, since it was hitting the 140 high-limit.  I turned the high-limit back up to 150, and have had no problems with the PRV or pressure (reaches about 18), and it's not cycling the gas nearly as often.



However, it still continues to cycle the gas on and off some, 2-4 times each heating cycle (again, with the booster remaining on).  I am wondering whether the high-limit should be turned up to, say, 160.  I mention 160 because I have a note from some service person many years ago which says that the max should not be more than 160; I don't know why.



This boiler has only a high-limit adjuster.  No low-limit or differential adjusters.



Should I turn it up to 155 or 160 to reduce the gas cycling?  On the other hand, will I end up using more gas that way, since it takes ever more energy to heat water higher and keep it that high?  Thanks.

Steve Whitbeck

limit setting

I usually set the limit with cast iron rads at 160* ---  But you only need the temp needed to heat the house. With a cast iron boiler the limit should not be set below 160* so that the return temp is no lower than 140* to stop condensation inside the heat exchanger. The burner cycling on and off is more efficient than constant on with a higher operating temp.

icesailor

Limited Limits:

The control is doing what it is supposed to do. If the house stays warm at 140 degrees, that is a good system temperature. If not, raise it.

What type of radiation do you have in the building?

Rav

I should have mentioned ...

that I have old (1931-era) cast-iron radiators.

icesailor

High Limits:

In that case, you should really look into having a Taco "I" series 4 way mixer installed. You would not believe the increase in comfort and performance in your system.

I put one in an old house that was too hot in some areas and cool in others. The system was running at 170 degrees for the indirect.

With the 4-way, the whole house is the same temperature and is even. The radiators are just warm.  I went by the house the other day to do some work. The system pump was running, the thermostat was set at 63 degrees, the system temperature was 122 degrees and it was 16 degrees outside. All the radiators were warm to the touch. The boiler runs at 170 degrees when the system is calling but only uses enough hot water to maintain temperature at what the outdoor sensor decides.

The system runs how it was designed to run. As a gravity coal system with 4" mains and 1 1/2" risers to the big column radiators.

I have never found anything that for the money, will do so much. The only piping is to connect the boiler supply and return with two properly placed, closely spaced tees, connected to the valve, and adding a circulator to the system and having them pump together. The last time I was there, the boiler came on for a few minutes. I used a Wilo Star 3 speed and it runs fine on #1.