Boiler Replacement or Not - need advice

I am considering replacing my old boiler (1948 Amer. Std. Severn SC-29 - up to 216MBTU and 960 sq. ft. installed radiation - full capacity).



I currently have 578 of Amer. Std radiation and the burner is an older Beckett (maybe 20 years old) with a 0.85 x 80W nozzle. Piping for radiation is separate feed and return, single zone.



I used 900 gallons of oil last heating season (mid Oct 2008 to mid Oct 2009) keeping my house at 65 to 67 degrees mostly. Degree days in Pa. near where I live run around 5200 using 65 as the base number.



House is about 1500 square feet not counting the basement in which is located the boiler. Basement is about 640 sq. ft.



Feed and return (1 inch copper) are insulated except for the 1/2 inch feeds and returns to the radiation.



I did a heat loss calculation using a Slantfin program and got 61000 BTUh loss at design temp of 0 deg. and indoor temp of 70 deg, water temp 180 deg. ( I assume the water temp number is meaningless for the heat loss and was only required to calculate amount of radiation to install on a new job.)



If I calculate BTUh per deg day and then multiply by the number of degree days where I live, I get about 800 gallons of oil needed for my heating season.



I had a local heating contractor come and size and price a new boiler (Buderus or Burnham). His heatloss calc. came out to 63000 BTUh using 0 design temp and he predicted oil use of 822 gallons.



SO - my huge Amer Std. boiler only uses about 100 more gallons per year than a modern boiler is predicted to use.



I am wondering if I should just try to make the older one more efficient for another year or two. It is in very good condition overall, has a barometric vent damper (a few years old), and looks like it could last another 60 years.



Currently I have what appears to be a bad aquastat. At least when it registers 135 deg. cutoff it the boiler thermometer (at the front) shows 175. Plus the aquastat variable differential (L6006A Honeywell) is set at 5 (minimum) and the burner doesn't fire until

the water drops to 120 (boiler thermometer). The aquastat is installed in a well about 4 inches deep at the top rear of the boiler. The original aquastat had a much longer probe (about 14 inches). However, placing a small Taylor bimetal meat thermometer in the short well registers a temp about 5 to 8 degrees lower than the boiler thermometer. I have no insulation around the meat thermometer so I expect that could account for the slightly lower temp. This is why I think the aquastat is defective. It is about 10 years old.



The room thermostat runs the circulator, the aquastat will fire the burner when the boiler water temp. drops. There is an original low limit aquastat on the other side of the rear of the boiler which will shut the circulator off as the water temp drops but it doesn't seem to work properly. It is a beautifully made bi-metal device, probably Detroit brand (owned by Amer. Std) but I suspect it is getting tired. Originally the house had water heated by the boiler, now heated by separate electric water heater.



This was my parent's home and I've lived here since age two (off and on since 1949).



Anyway, I was ready to replace the whole thing with a more modern setup - cost estimate for Buderus G115 with net output 74k Btu or equivalent Burnham would take about 20 to 25 years to get payback if I only save 100 gallons of oil per year.

Is it possible I am getting some efficiency out of this old boiler. I assumed I could save 300 to 400 gallons when I started this process. But my heat loss is validated by the contractor I used as is my predicted usage.



I have observed the current boiler and have these large numbers.

Cold start from 65 deg ambient temp to boiler cutoff at 175 - takes about 45 minutes.

Firing rate of burner is 0.85 gallons/hr (based on nozzle size).

Firebox area is about 6 cu. feet (measured by me). New Buderus is about 1.2 cu feet and it has about 9 gallons of water capacity. I have no idea how much is in my Amer. Std. Severn, but I believe it must be a lot. Anyone have a guess?



When my circulator runs with the boiler at 175 the boiler temp will stay constant for a few minutes then drop rapidly to 110 degrees. This bothers me because it is well below condensation temp. The temp. will rise very slowly with the circulator running indicating the burner can produce enough BTU's to slightly exceed the ability of the radiation to shed heat. 578 sq. feet of radiation (from the Amer. Std. literature for the radiators I have - so much per section of 4, 5, or 6 tube 25 inch high) can output 85K BTU at temp of 180, 60K BTU at 150 (if the web searches I did are accurate).



Any thoughts on all this?



Also, to try to gain a little more efficiency - I will replace the aquastat and hope to get more accurate burner firing.



Will cold start be of any benefit - since this boiler takes a long time to heat. Should I increase the firing rate? Would this reduce the cycle time and gain anything.



I have gotten very interested in following this out since my potential savings of going to a new system seemed so slight. I think there is a lot of science in this which I can't appreciate.



Also, for the time when I do replace it - will outdoor reset or a condensing boiler be of any practical value? I've read that castiron radiators should use 140 degree water and maybe a +20 deg difference or more for best heat transfer. And condensing boilers are at their best at low water temps and light load.



Thanks to anyone for any insights.