We have a group home with a 8+ year old Lochinvar 300,000 BTU LP gas outdoor furnace. I suspect that the installer just shot from the hip on size (retrofit for ceiling electric). It is supplying 4 zones 60 to 100 feet away in the attic with 18"x24" heat exchangers (if memory serves). These are in the AC plenums. We just had all the zone piping upsized to the correct diameter (1 inch HePEX, you don't want to know what was there). It is using Taco 007's for the zones and a 009 for the furnace loop into the garage where the header is.

I happened to check it today because the weather is cold, 5 to 15 degree outdoor temps and a good wind, and this is the first time I got to see it since refitting. All 4 zones were calling for heat, and the furnace would fire for approx 2 minutes and then shut off for 2 minutes. This says OVERSIZE to me, and it will only be worse with less demand. Is there anything to do to improve things? I also noticed that right before it would shut off at the upper temp limit the pressure gauge would fluctuate +10 to 15 lbs and I would hear a banging or bumping in the pipes. What gives?

Install a indoor outdoor control or something similar. This will regulate water temp in boiler. Contact your local boiler expert.
You could be correct in that then boiler is way oversize or the heat exchangers are way to small. From your dimensions of the 4X 18X24 is not enough radiant surface for a 300,000 boiler.

Depending upon the $$ factor you might be better off with a smaller boiler.

As hvac1000 said you really need to get a hydronics expert out to your location. A 300,000 btu/hr boiler may be correct, there is WAY too many questions to be asked from this web site, such as what is the total square feet of the building, etc. etc. By what you stated in your post, I think the unit is ramping up to it's set point temp, and shutting down because the unit can't pump any more water (heat) out of the unit... because it is either not piped correctly, or the wrong size pump. A unit this big needs to be piped primary and secondary... and the correct pipe size depending on the BTU/hr of the unit. Which in your case the primary loop near the boiler must be 2 inch!

Thanks for the advice. Yes, the header loop is 2" copper, but I believe it is plumbed incorrectly. The 009 is in the supply side of the loop BEFORE the 4 take-offs. This puts it in series with the 4 (parallel) 007's. When all 4 zones are calling, the last 2 zones (by header loop flow) actually back feed through the back side of the header loop and get no hot water. I can tell this by the temp of the back check valves on the zone supplies. They are only about 80 degrees while the first 2 are 170.

We got the installer back out today and they are going to move the 009 to the back of the loop, effectively putting it parallel with the zone circulators. Hopefully they know what they are doing and there is no possibility of back feeding through that circulator. Or do we need a back check valve in line with it?

Tip! Always pump away from the boiler. Not to the boiler.