Help with Calculating EDR

saikosis

Hi. I've started down the path of switching my one-pipe steam system from oil to gas. I have contractor number one coming tomorrow morning, but my copy of "We Got Steam Heat" is still in transit. I was hoping to get a little help figuring out whether my current boiler is appropriately sized for my house. That will help me figure out whether I should try to upgrade what I have or whether I should install a new boiler.



I've consulted these guides for calculating EDR:

<a href="http://www.columbiaheatingsupply.com/page_images/Sizing%20Cast%20Iron%20Radiator%20Heating%20Capacity%20Guide.pdf">http://www.columbiaheatingsupply.com/page_images/Sizing%20Cast%20Iron%20Radiator%20Heating%20Capacity%20Guide.pdf</a>

<a href="http://www.usboiler.net/resources/toolbox/sizing-obsolete-radiation.php">http://www.usboiler.net/resources/toolbox/sizing-obsolete-radiation.php</a>

<a href="http://www.heatinghelp.com/files/posts/16023/Boiler%20Sizing%20Chart.pdf">http://www.heatinghelp.com/files/posts/16023/Boiler%20Sizing%20Chart.pdf</a>



The problem is that I get different results with each one.



I have nine radiators total. Eight of them look the same to my untrained eye, but some say "Gothic," others say "Thatcher," there's at least one "Corto," and another says "American Radiator." The house is from the 1800s, so it's likely that they are all pretty old. I think these are tube type radiators, although I'm not sure. The last radiator is a column type.



I'm hoping someone can help me size one of my radiators and then I can handle the rest. The big one in my kitchen has 23 sections. Each section has 5 tubes and is 17 inches tall. The tubes are 1.5 inches by 1 inch. If I include the height of the legs, the total height is 20 inches.



The guide from Columbia Heating Supply doesn't give a number for a 17 inch, 5 tube section. Should I be counting the legs when I measure the height? The closest height in the chart is 20 inches, which is the height including the legs, so maybe that's okay. The 20 inch, 5 tube section is 2.67 square foot EDR. That times 23 sections is 61.41 square foot EDR.



Typing in 17 inch height, 1.50 inch tubes, 5 tubes, 23 sections into the U.S. Boiler guide gives me 10.63 square feet per section and 244.38 square feet total. Using 20 inch height gives 287.50. Why is it so far off from the previous calculation?



Finally, the guide from here has me figure out if I have columns, tubes, or thin tubes. I definitely have 5 columns/tubes and the width is definitely 8 inches. I guess 8 3/4 inches is close enough, meaning I have tubes (style 2). Again, there's no 17 inch value given, so I'll round up to 20 inches, which is 2.6 square feet of radiation per section for 59.8 square feet of radiation total. That's close enough to the first number, but is it right?



My current boiler is a Utica SFH4150ST. The plate says:

Output BTU/hour: 170,000

Input BTU/hour: 210,000

Heating capacity: 170,100

Steam ft^2: 531



If I use the Columbia Heating Supply guide or the guide from here, I get about 350 square feet total. That means my boiler is a little too big. I think two radiators have been removed within the last decade or so to accommodate some renovations, so maybe the overage reflects that.



If I use U.S. Boiler, I get 1400 square feet total, which would mean my boiler is way too small. We get pretty decent heating in general, albeit with some hot and cold spots, so I have to assume U.S. Boiler's guide is just wrong.



If I go with the 350 square foot number, is the 531 square foot of the current boiler about right or is it over enough that I should consider a smaller boiler? Basically, what's the right amount of cushion? Note that I haven't measured any of the steam pipes; should I? Also, the boiler has a tankless water heater coil insert. Does that change my calculations at all?



Thanks for any help.

BobC

60 sounds right

I did a quick calculation and figured that a section of 5 tubes has

about 400 sq in so 17 of them would be about 6,800 sq in; if we divide

that by 144 it gives us 47.2 sq ft EDR but does not take into account

the interconnections between tubes and sections.



Two of your

charts give very similar numbers so I would discount your Burrnham

calculator and use about 60 EDR for that radiator.



If you use a

boiler that can supply as many sq ft of steam as you have in EDR you

should be ok. You don't have to add anything for the piping because that

is already factored in. For normal water heating you shouldn't have to

add anything unless you have very heavy hot water requirements.



Bob

Dave in QCA

Columbia Chart Wrong

The information in the Columbia Chart is helpful, but don't use it for trying to measure your radiators. The information in the chart is confusing and some of the radiators are mislabeled.



Radiators should be measured from the floor to the top edge of the radiator. Another issue to confirm is whether they are large tube or small tube. Large tube radiators are 2 1/2" per section. Small Tube are 1 3/4" per section. Measure from the big of the rib to the tip of the next is the easiest way to do it, or form the joint line of one to the next joint line is fine too.



I am assuming that you have large tube radiators and am also guessing that your 17" radiator is probably 20" from the floor. If that is the case, it is 2 2/3 sq ft per section.



Sorry, I just went back and re-read your post. Yes, you have large tube radiators, and yes, you are supposed to measure from the floor. You have two different ratings that are essential the same thing, but rounded off, (incorrectly) the actual rating is 2 2/3 sq ft per section. Or, as a decimal, 2.67 sq ft/section. One of your formulas has it listed at 2.6, instead of 2.67. The rounding is wrong.



I just went and tried to use the US Boiler chart calculator. I put in several different measurements for radiator for which I KNOW what the ratings are. I could not make it give an answer that was even close. WAY OFF!!

saikosis

EDR

Thanks, Bob and Dave. It's reassuring to have someone double check your math!



Why aren't the pipes factored into the equation? Is it assumed that the pipes are insulated well enough that the steam isn't condensing in them and therefore they don't contribute to the square footage? My pipes aren't insulated, but that is something I want to do soon. I have a feeling they were insulated with asbestos at one time but some previous owner took it out and never replaced it.



Once I have my total EDR, how do I pick a boiler? Let's say my total is 350 and a manufacturer offers two boiler models: one that does 400 and another that does 450. Will the 400 cover it or should I go up to the 450 for a little more wiggle room? Is there a rule of thumb to follow here?



Thanks again.

Dave in QCA

Piping Factored In Already

If you size your boiler using its EDR rating to the EDR of your system, the extra needed for piping and pickup is already factored in. That extra allowance is usually 34% and it is usually what is necessary for piping with insulation. Since yours is not insulated, you might need more. Dan recommends 50% in these situations.



If you're going to use a different piping and pickup factor, than you should ignor the EDR rating of the boiler and use the DOE output instead. If your radiators add up to 350 sq ft, then 350 sq ft x 240 BTU/ Sq FT = 84,000 BTU. 84,000 BTU x 1.5 = 117,600 BTU output required. Using the 34% factor it would be 84,000 x 1.34 = 112,560 BTU output required. Often, the output rating is also stated as DOE rating.

vaporvac

Insulate those pipes!

You should reinsulate thos pipes after the install. 1" fiberglass is the general reccommendation and there are quite a few posts on where to get it. Not only will a bigger boiler cost more upfront, but it will cost more every year in fuel. Plus, the steam will reach the rads more quickly..

saikosis

Thanks

Thanks for the help. I was able to identify all of my radiators: seven Thatcher Gothics, one American Radiator Corto, and one American Radiator Peerless. Using old manuals, I double checked the numbers I got from the guides I listed above. I came up with 343.33 square feet. With insulated pipes and a 133% pickup, that's 457.78 square feet or 109,866.67 BTU/hour. With uninsulated pipes and a 150% pickup, that's 515.00 square feet or 123,600.00 BTU/hour.



If I look at the Weil-McLain EG boilers, it looks like I'd be good with the EG-45, which has 125 MBH D.O.E. heating capacity. That covers me with or without insulation.



Does that sound right?



If I wanted to replace the two radiators that previous occupants removed, that'll add another 100 square feet or so of radiation. With insulation, I'm up around 140,000 BTU/hour. At that point, I might want to move up to the EG-50, which gets me to 145 MBH.



Does it make sense to upsize the boiler like that to account for future additions? One contractor told me that newer boilers sometimes have two operating modes to let you expand like that. Anyone know more about that?

Dave in QCA

You're mixing it up a bit

You method might come out with the right answer, but you're confusing a few things along the way, and that cause you to have a misunderstanding, or worse, an error in your calculations.



Using the numbers you have provided, 343 sq ft. If you size a boiler using the sq ft or EDR, you simply find a boiler that is the same or slightly larger than your system EDR. The extra 34% has already been calculated by the boiler manufacturer, that is, if a boiler has a rating of 350 EDR, what they are saying is it will match up with a system that has 350 EDR of radiation. Now, of course, they are assuming that the mains in the basement are insulated. Now, if you have radiators that are missing, and you're going to add them in the future, you would want to size the boiler for the future load. So, if you add 100 sq ft to your existing 343 sq ft, your at 443 sq ft. A boiler rated at such would certainly compensate for your lack of insulation an then some.



If you want to calculate a piping and pickup factor of 50% instead of 34%, I would recommend converting your load to BTU instead of EDR. When you start fudging with the EDR numbers it gets confusing because of the 34% already being included in the boiler rating. If you were never going to insulate the pipes, and your wanted to add the missing radiators, then 443 x 240 x 1.5 = 159,480 BTU. The number should be matched to the DOE Output of the boiler, sometimes called the Gross Output. If you wanted to calculate a 50% piping loss and pickup factor for your system as it exists, it would be 343 x 240 x 1.5 = 123,480 BTU. (which is the same number that you came up with)



If it were me, and I was sure that I was going to add the missing radiators, I would size the boiler for that future size. This will make up for the fact that your pipes are currently uninsulated. When I got around to installing that radiator, or even before, I would make sure that I got all of my mains and run outs in the basement insulated with proper pipe insulation. With a 34% pickup for your system with radiators added, you would need a boiler rated at 142,468 BTU output. The EG-50 is the best match in the WM line, as you have already concluded.



Two stage firing is available on some of the larger commercial boilers but I am not aware of it being offered on any residential boiler.

saikosis

Makes Sense

Thanks, Dave, that makes sense. It's much more clear what's going on when I do it your way.



What would you do if you weren't sure you'd add the missing radiators? How much is too much when it comes to oversizing the boiler? Let's assume I insulate my pipes properly. The Weil-McLain EG-45 does 392 square feet with 150 MBH input and the EG-50 does 454 square feet with 175 MBH input. That's a difference of 25 MBH input. What will that translate to in terms of gas usage? I think a cubic foot of gas has about 1000 BTUs. Does that mean means I'm burning an extra 25 cubic feet of gas every hour if I got the EG-50 instead of the EG-45? That's wasteful, sure, but is it a big waste? How many hours per day does the average boiler run? Are we talking a couple bucks a month or ten, twenty, thirty bucks a month?

Dave in QCA

you're welcome

i re-read my post and found a number of things that were confusing. It as been edited now. You got the same answer and came up with the same final conclusion, but your method was a bit confusing and I suspected that it was confusing to your as well.