My Furnace is about 10 years old, it is a Weil_Mclain. Ever since I had it serviced, the 30psi pressure relief valve has been leaking water on the floor that I just noticed. What would cause this? I was thinking that this could be caused by installing a
larger than required Oil Burner Nozzle? When I checked to see how much water was in the bucket this morning, it was about
2 gallons, which is way more than what I saw when I first noticed the water on the floor. I popped the valve thinking that it might help, but it did not. I was not very happy with the cleaning job and requested for them to not send the same guy when I next get the furnace serviced.

Assuming that this is a hot water, oil fired furnace you most likely have air in the water lines. If work was done which opened the water lines then air was not purged from the lines. Call service company back.

Assuming that this is a hot water, oil fired furnace you most likely have air in the water lines. If work was done which opened the water lines then air was not purged from the lines. Call service company back.

Correct Hot Water Oil Fired Boiler

Lines were not touched, just cleaning of the furnace, new oil filter and new oil nozzle for the burner. I do not hear any noise that would suggest that there is air in the lines. I need to check the temperature when the furnace shuts off. Might just be that the Watts 30 PSI valve has gotten old and needs to be replacement.

I just checked this morning and the amount of water released is about a quart and one half in about 4 hours. The gauge drips down to about 10 lbs after a water release, need to see what pressure goes up to before it releases. The light does not indicate that the boiler is low on water so it must be replacing the water okay.

UPDATE: I just observed the Boiler after the Burner turned on. The water temperature was 167 degrees and the water pressure was 10 pounds. The pressure quickly moved up to 30 PSI and it started to drip and went on up to about 36 PSI.
The burner shut off at about 181 degrees as it was set too. Don't know the why of the problem. Circulators seem to be doing their job without any air in the lines that I can tell. I still wonder if the wrong oil burner nozzle could be the culprit, the temp sensor is in the water jacket for the domestic hot water. The PSI gauge is not in the same location. Just looked at the pressure and it has dropped down to around 12 psi.

Above my expertise but while water expands when heated, air expands a lot more. Check pressure and temperature release specs of T&P valve but sounds like air trapped in lines. If you have radiators you should have bleeder valves at each. If not radiators (baseboard heater) connect garden hose to water input valve. Open drain valve and let water run for a while to hopefully blow out any air trapped in lines. Do each branch separately.