I am changing my steam boiler from oil to gas. I currently have a HB Smith boiler with a Beckett blower with a .85 nozzle. I looked at the Weil McLain instructions on there website and calculated my radiators to be only 46,905.04 BTU’s. I also have a hot water loop in the basement with a ten-foot baseboard. The plumbers I know tell me I should use a 100,000 BTU boiler. Is that too much? I just want it to be efficient as possible. The system I use now seems to use a lot of oil for a small house, total 1066 square feet for the first and second floor. My roofline is steep so the second floor has a lot of un insulated roof on the walls but the attic floor is insulated.

Steam boilers are not sized based on burners nozzles etc.
The boiler is sized based on the amount of steam that is required to fill all of the radiators at the same time. This determines the volume of water that is required to be in the boiler. That is the only way to properly size this heating system.
What needs to be done is to measure the size of each radiator section. Then count the number of sections each radiator has. Then the style of radiator. Some are square, some are round etc. Once you have that you need to obtain a conversion table that tells you how much steam volume each section holds. A little math and you end up with total volume. or square foot of steam required to fill all of the radiators. Then all is required is to size the boiler based on steam volume it can produce.
To little and the boiler will not heat the house. To much and your heating to much water that is not needed to do the work.
Try http://www.oilheatamerica.com/index.mv?screen=home
or try http://www.heatinghelp.com/
You should be able to get the information you need at one of these sites.

Actually I did calculate the radiators the way you say.That is how I came up with the 46,905 BTU'S total for all my steam radiators. I was concerned that the 102,000 BTU boiler recommended to me was to big. But now I was informed that is the input BTU figure, the net BTU figure is around 62,000 BTU so that is closer to what I need, with a little reserve capacity. The next size down boiler from the same manufacturer has a net output of 47,000 BTU so I think that is cutting it to close.
The reason I mentioned the .85 gal per hour oil Nozzle was because I was informed that it was equivalent to an 85,000 BTU gas boiler. I don't know if that is correct but it seems it must be close.

The BTU's have nothing to do with the boiler size. Steam produces x amount of BTU based on a given volume. You can have a 1000 BTU boiler or a million BTU boiler and both will produce the same amount of heat at the radiator because the steam output is the same temp. But if you do not have enough water to covert to steam regardless of size. Neither will do its job and fill the radiator and produce its required heat output.

So when sizing the boiler you first look at the amount of volume the boiler will fill. This number should be at least the total size of the all the radiators combined. When this is converted based on the boiler sizing information you now have the min requirement of how large a boiler you need. The BTU sizing comes in when you determine how much over in size you want to go to produce the steam. Of course larger will produce steam faster then smaller. I would suggest a 20% increase of what is determined to be needed to be safe.

However just sizing the boiler does not fix costs of operation. Of course the smaller the unit the more efficiency it produces. But the delivery of the steam is one big factor.

The steam vents should open and close properly. Once the steam is delivered to the radiator the vent should shut down. Once all vents are shut down the boiler through its pressure control should shut off. Until the vent opens and lowers the pressure in the system. At that time the boiler will start up again and deliver steam back to that vented radiator and it then again shuts down. Repeating the cycle until the thermostat shuts everything off.

With this in mind you must lower the setting on the pressure control. Way to many times heating contractors with little knowledge of steam systems set them to high. I always suggest that they be set to the lowest setting. Many folks set them around .8 which is enough steam pressure to heat a 10 story building. Remember as the pressure rises within the system the water does not boil but gains latent heat. Once the pressure is released the water begins to boil and steam is generated. By setting the boiler control higher then needed all you’re doing is burning fuel to super heat the water to deliver steam when the radiator vent opens. Waste of money.

My suggestion is first determine what the steam setting is at on the boiler. This is the little gray box that has a screw on top. Turning this screw raises and lowers the on/off pressure settings. Lower is lower, higher is higher. Go as low as you can go. If you have a real large home, then set it just a little bit above the lowest setting.

Nozzle size determines the amount of heat energy the burner can produce. It also determines the efficiency of the burn. Depending on the type of firebox you have and the burner depth within the box will determine what size and type of burner nozzle that is required.

The burner service person should be checking the stack temp along with the CO output to determine what size and type of nozzle is required by trying different sizes and types. By type I mean that they come in solid, semi-solid and hollow patterns. Also how far the flame is thrown out and its final shape. This all has to do with efficiency. I find most times the burner person has no idea of how to do this. You need an old timer that knows oil and steam systems well.

The fire box is also critical to proper combustion. If the boiler had a burner change from when it was first put in, then the fire box may have needed to be changed as well. The size and shape works in conjunction with the nozzle to raise efficiency.

If the fire box is rough and worn out a new one is in order. The idea of the firebox with new burners is to create a radiant glow to help evaporate the oil droplets as they leave the burner nozzle. What happens over time is little droplets of oil that were not evaporated properly hit the back of the fire box and burn. This causes carbon build up and carbon is an insulator. Thus lowering the radiant heat output and lowering the ability of the burner to properly convert the oil to a vapor so it burns properly.

My suggestions are to look at the existing system. See if the controls are properly set. See if the firebox is in great shape or not. Fire up the burner and see if the firebox glows if you can see it. Make sure that the flame does not hit the back of the firebox. And make sure that the steam vents both on the radiators and return pipes are venting and shutting off once some steam vents out. Some vents take longer to shut off, the result is longer run times and more fuel used. If this is done properly the current system you now have in place is working the best it can. Typical efficiency ratings can be achieved in older units of around 82% Working properly, compare that with what a new one would do and see if the cost involved justifies changing it or not.

The bottom line is if the vents are not working right, and the settings on the boiler are set wrong, and if the fire box is not proper, or any combo of these things will cause any boiler new or old to run inefficiently.

Thanks for all the info. I actually have my pressure control on the lowest setting and new vents on my radiators. But now that I think about it there was a leaking radiator vent on a pipe in my basement it was plugged when I first moved in because the thread was messed up . I guess it was a return pipe. Would that make a big difference in efficiency? I will still be switching to a gas steam boiler this month anyway, but I will keep all this in mind with the new system .By the way my oil company guys always said the furnace was running great when checking the stack temp and CO.But when I look in the chamber it doesn't look in great shape.