Boiler water pressure is controlled by a fill-valve, whether auto or manual, and an expansion tank, whether ceiling-hung or bladder-type Extrol. Boiler pressure can also be affected slightly by the temperature of the boiler water or if the expansion tank is undersized.
Bladder-type expansion tanks (Extrol) are generally pressurized at 12# and air can be added/reduced at the Schraeder valve on the end of the tank. If you suspect the bladder has failed in the tank, reduce the pressure of the boiler to zero and check for 12# of pressure at the Scraeder valve. If you have 12# of pressure, the tank is good. If the pressure is lower than 12#, add air to bring it up to 12#. If it doesn't hold air pressure, the tank is no good.
Ceiling-hung tanks are not pre-pressurized, but are the same pressure as the system. Ceiling-hung tanks look like a long tube suspended in the joists. They may have one drain valve and possibly a second valve. Some tanks may never fill with water during their lifetime and some will. If there is too much water in the tank, there is no place for the hot water to expand and the pressure relief valve blows off the excess pressure. At this point the tank must be drained. Isolate the tank from the system and open the drain valve. If there is no second valve to break the vacuum, the tank will not drain, but many people think it is drained, put the boiler back in service and continue to have pressure problems. Installing a second valve is the easiest way to allow air to enter the tank and fully drain it. Another method is to push a small tube up through the drain valve and up to the top of the tank, where it will allow air to enter the tank and break the vacuum. You can EXPECT to get soaked if you don't have a bucket ready for the water. Be sure the water is not hot before trying this and open the isolation valve when finished.
If you have checked both types of expansion tanks and found them to be good, another possibility is that the automatic fill-valve is not holding at it's preset pressure of about 12#. It could be allowing house pressure to enter the boiler and cause the relief valve to blow off pressure. Similarly, a pinhole leak in the domestic water coil will also allow house pressure to enter the boiler. In both cases, the water pressure will steadily increase regardless of the water temperature. If you suspect either the fill valve or the coil, shut off the supply to the fill valve and shut off both sides of the coil and see if the problem is solved. Then narrow it down. Remember to re-open the fill valve when the test or repair is done.
If the boiler increases excessively in pressure along with a rise in boiler water temperature and decreases along with a drop in boiler water temperature, the expansion tank is probably suspect. If the boiler increases excessively in pressure regardless of the water temperature, the water coil or the fill-valve are suspect.
Boiler water temperature is controlled by the aquastat. It's not unusual for the boiler water temperature to increase 30 degrees after the stoker has shut down. This is because there is a pot full of glowing coal and is also the reason why there is a 40 degree spread between the low and high limit settings of the aquastat. If oil is used to fire the DF520, the aquastat settings can be adjusted for a normal 20 degree spread between low and high limits.
Without the 40 degree spread between low and high limits when firing with coal, the boiler will often be above its high limit and will not fire when the timer calls for heat, resulting in outfires. This is a common reason for outfires on warm Winter days or when using the boiler through the Summer. You can increase the spread to 50 degrees in the Summer with settings such as 130-180. Winter settings are usually 150-190 or 160-200.
In prolonged Winter warm spells, the timer will still call for heat every 30 minutes. With no heat being dissipated at the radiators, the boiler water temperature will continue to creep up until it exceeds the high limit. If the water temperature isn't reduced, there could be a long period of no stoking, resulting in outfires. In a normal Winter day, with constant demand, the timer isn't necessary and the boiler water will rarely exceed the high limit.
It is a common misconception that the aquastat high limit setting is what the boiler normally tries to maintain. Although the high limit is often reached, the boiler runs off the low limit setting and will continue to heat and circulate the water as long as there is a call for heat, but will shut down the stoker when the high limit is reached. This is a safety setting. If sufficient heat isn't removed by the radiators and the thermostat isn't satisfied, the boiler will eventually reach its high limit and will run at the high limit and 10 degrees below (caused by the circulator) until the thermostat is satisfied.
If the boiler is running excessively hot, the aquastat high limit feature could have failed or the high limit setting is too high to begin with. For example, if the thermostat was satisfied and the stoker/circulator had shut down and the water temperature happened to be just below high limit, the fire in the pot could easily raise the water temperature by another 30 degrees above the high limit setting. It is necessary to observe why and when the boiler water goes over the limit. An overheated boiler could also be caused by a failed circulator.
It's also possible that the aquastat and temperature gauge are reading uneven temperatures inside the boiler. This could happen if both return ports aren't plumbed as they should be and without the bypass loop, there could also be stratification of the boiler water.