Furnace runs cooler

Hi Folks! -- My first post. Here's what I got... Weil-McLain cast iron, oil fired, hot water radiators, hot water coil, about 12 years old. Replaced aquastat this morning because the circulator pump relay was shot. I set the HI/LO to 180/160 with offset at 15, just like the other one. Till then, when calling for heat, the water temp would rise to 190-ish, shut the burner off till the circ pump pulled the temp down to 175 or so and cycle the burner on again. This process would repeat with the burner on for about two minutes and off for about 20 seconds until the thermostat was satisfied.
With the new aquastat, water temp wanders between 155 and 170 with the circ pump on and the burner never shuts off. Once room temp is OK, pump and burner shut off immediately and residual heat rises the water temp to 190. Using hot water cylces the burner as usual. The radiator temps are about 10 degrees lower than before, understandably.
Is this related to the aquastat?
Me-thinks it's a new issue. I got a glimpse at the flame, and it was nice and bright yellow as before, but seemed a little ragged. Would a new nozzle (same size) help? This one's about 4-mos old.
Any help/advice will be appreciated. Thanks --- EdP

Welcome YardJockey:
A four month old nozzle should be just fine.
I think your problem may be air in the water system. Try the bleeder valves at each radiator; bleed them until you can't hear water running through them. Also try the main air vent above the boiler and make sure its working.
Glenn

Thanks for the reply, Glenn. I'll try that. Tomorrow. (The flyers just beat Buffalo, and I'm going to bed!)
But I've been thinking... One time when I was troubleshooting the original problem (fried circ pump contacts) I measured 103 volts being delivered to the curc pump WITH THE CIRC PUMP DISCONNECTED. If the circ pump was not running at peak, could that have an impact on the boiler operation? That is, shutting off after two minutes and coming back on after twenty seconds?
And now that the pump is getting full voltage, would that keep the water temp in the 170-155 range?
None the less, I'll give your suggestion a try. Thanks again --- EdP

Glenn --- Well, Tomorrow came and went! But Happy Thanksgiving, anyways!
I drained 1/2 gallon from each of six radiators, starting with the one upstairs that was the furthest from the boiler. I did not drain from the cast iron baseboards in the kitchen. They are nearest to the boiler, and too much activity going on for the holiday. After the draining process, the boiler temps continue to wander, but increased to 165 - 180! I am totally flabberghasted! The temps at the radiators have increased, as well. I will go through a second draining process, though I am not sure what to expect!
Now, can you or someone explain to me what's going on? Why did changing the aquastat lower the boiler water temp? Could it be a faster running circ pump? (Read previous post.)
Why did boiler water temp increase after draining radiators? I didn't notice popping or spitting water when I opened those individual valves, nor was the water in my bucket laced with bubbles.
There must be a reason --- Thanks --- EdP

Hello YardJockey:
Listen closely to each radiator (maybe even with a stethescope) and see if you can hear water running through, like it would in a stream (splashing and such). If you can't hear the running water the air is gone.
You mentioned a ragged flame; could you adjust your air flow to the flame?
The speed of the circulating pump should not affect the boiler temp but a faster pump would raise the temps of the radiators because the water doesn't have as much time to give up the temperature. An aqua-stat simply senses the temp of the water and they could vary 1 to 3 degrees but you shouldn't notice that difference.
Glenn

Thanks, Glenn --- Yes, I have a stethoscope, and I will use it. And yes, I can adjust the flame. Earlier this summer, there was a lot of black smoke coming from the stack. I opened the "air intake" (I forget the right nomenclature) and the smoke cleared up. Right not on a cold day, I see only white wispy vapors coming from the chimney. Maybe it needs a little less air. I will let you know. --- EdP

Glenn -- I drew about two litres from each of the radiators, including the nasty baseboards with the valve spout pointing up into my face! It seemed like one or two upstairs gave up some air, then delivered a pretty steady flow. I backfilled manually to maintain 20 lbs pressure. I adjusted the damper, but that was spotty. Outdoor temps have been moderate lately, so the call for heat has been intermittent. I continue to have nice wispy white "smoke" coming from the stack and the boiler temp at the gauge on the boiler continues to range from 165 to 180. The "HI" is never satisfied, so the burner is never interrupted. Which, once again, was not the case prior to replacing the aquastat. When I use an IR gauge, the nearest radiator sits at about 155 degrees, tho that might be subjective. I think I can do a little fine tuning on the damper. I ain't gave up yet!!
Thanks again for the help and advice. --- EdP