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# Sizing New Steam Boiler and EDR

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Member Posts: 16
I am replacing a 30+ yr old Weil McLain (17500 btu) gas steam boiler with an Edr of 418. It also has a separate zone off the coil for a baseboard/hot water for a small den.  I have received a couple of estimates so far and one sized a replacement using the same size and the other a bit smaller at 150000 btu.

While I am waiting for a couple of other estimates, I started researching and found that EDR is the appropriate way to size the boiler. I went and measured all of my radiators. I have 10 Aero Convector radiators (circa 1930’s), 3 sunrad / cast rays and one traditional cast iron radiator (photos attached).

However, I am unsure of how to calculate the EDR myself, using the measurements I took, to verify.   I was unable to find a conversion chart for the Aero and not quite sure how to calculate the others, so that I may ensure the estimates are sizing the boiler correctly.

The new Weil EG50 has an EDR of 454 and the EG45 of 388.

I want to ensure I am getting the right size.

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your going to have to do some digging. there is information on this site to help with the sizing
• Member Posts: 16
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All of the past links to which contained those charts for the Aero are broken.
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When measuring the convectors for Edr, do you measure the actual l x w of the radiator or the enclosure it’s in?

for example, the large convector is 58” x 13” x 5.5”. (The enclosure is 66”x30”x6”)

Would you then multiply 58x13=754/12 = 63 sq ft for the edr of that radiator?
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rjfork said:
When measuring the convectors for Edr, do you measure the actual l x w of the radiator or the enclosure it’s in?

for example, the large convector is 58” x 13” x 5.5”. (The enclosure is 66”x30”x6”)

Would you then multiply 58x13=754/12 = 63 sq ft for the edr of that radiator?
That’s not how you do it, but, surprisingly, you’re not off by much.

Looking at the table on p 232 of Dan’s EDR book, the closest enclosure sizes listed are 5 7/8” X either 65” or 67” X either 29” or 32”, making the EDR somewhere from 61 to 65, but if the enclosure is 65” long, the element is supposed to be 62” long, and if it’s 67”, the element should be 64”. If the element is 58” long, the enclosure should be 61”, so, unless someone put a smaller element in your enclosure, you’re not measuring the dimensions in the same way as the manufacturer. Unfortunately, they don’t give any details about how to measure elements or enclosures, we might have to settle for an estimate.
Just another DIYer | King of Prussia, PA
1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
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I have another large one 45x13x5.5 in enclosure 54x30x6.  The rest are all in enclosures of 40x30x6. But the actual lengths differ from 17”, 21”, 23”, 25” and 30” by 13”x6”.

Would you be able to post an image of table on p 232 of Dan’s EDR book? That would make things easier to figure out.
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All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
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This may be what you are looking for. Pages 15-18.

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Now I’m not sure how to approach this. The Aero ratings assume you have the original enclosure with the corresponding element, but I don’t think any of these are. The enclosure above doesn’t look like any of the Aero enclosures. Even estimating the surface are of the elements won’t tell the whole story, because the output of a convector has a lot to do with the enclosure, because it creates chimney effect and controls airflow through the element. I’d have to appeal to some of the pros for a suggestion on how to deal with this.
Just another DIYer | King of Prussia, PA
1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
• Member Posts: 16
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awesome random.  Thank you.  Yes, hap hazard…Seeing this chart begs the question, what is the best approach to calculate Edr. Definitely the enclosure length does not line up, as some of my convectors are less than 1/2 the length of the suggested enclosure/panel length.
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edited September 2022
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This chart may answer your question: https://babel.hathitrust.org/cgi/pt?id=mdp.39015002121674&view=1up&seq=31&skin=2021&q1=aero Go to Table 27, on page 19. You should be able to interpolate. That book was published in 1938. From a 1931 patent entry for Aero convectors it looks like that's when they started being manufactured. Sometimes ratings change, but hopefully not in your case.

There may be other relevant catalogs on that site: https://babel.hathitrust.org/cgi/ls?q1=%22national%20radiator%22%20aero;field1=ocr;a=srchls;lmt=ft&facet=bothPublishDateRange:%221930-1939%22

This is a price sheet with dimensions for Aero Convectors, but no EDR: https://digital.hagley.org/Trade_Cats_1032013_045_9131#page/1/mode/2up

The catalog I uploaded has the following note at the bottom of the charts on pages 17-18: "For complete information request Catalog No. 273-A." If I were you, I would send a research request to the Smithsonian Libraries and reference this collection: https://americanhistory.si.edu/collections/search/object/SILNMAHTL_33015

They have 261 items from just the National Radiator Corporation in their collection. Send an email to The National Museum of American History Library at: NMAHLibrary@si.edu. I did this when I was sizing the radiators in my house, and they were very helpful. You might have to wait a little while though.

You should post pictures of the traditional cast iron rad as well. Is the cast ray the same dimensions as this: https://ocsind.com/wp-content/uploads/2016/03/15981-OCS-Catalog.Updated15-6.pdf I like hunting for old EDRs...
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edited September 2022
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I am curious to know whether the length of a convector enclosure matters when it comes to EDR. I thought it would, but the bottom of Table 27 on the first link I sent you says that "L=length of convector, not enclosure length." So it looks to me like only enclosure height matters. Maybe one of the other pros on here can answer that question. I think you have at least one 5B but there is only one height of 20 in. for that model in the table. 5B and 5C are pretty close though looking at the pdf I sent you...
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random, thanks again for the info. I’m glad someone understands and likes to dig for this stuff.

My cast ray measurements were spot on with the chart you provided.  As suggested, I am uploading a pic of the traditional cast iron rad too.

I’m including my worksheet too. Hopefully, this is fairly accurate. Some of my radiators were in-between lengths and I therefore rounded up to the larger size.

So, based on the charts and radiator measurements, I came up an Edr of 341 (furthest left column).

A pro that came by estimated 460-495 edr , but used the enclosure for the calculations.
• Member Posts: 469
edited September 2022
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BTW, I am homeowner and not a pro, so take this as you will. I just want to add that my situation is similar to yours. We have an oil steam boiler (megasteam) with tankless coil for our domestic hot water and a loop piped off the rear tappings of the boiler going to a plate heat exchanger which supplies a radiant loop in our kitchen floor. This was a terrible, awful design decision in retrospect. My advice to you: Don't do it! If there's any way to mothball the hot water baseboard in your den and pipe steam there instead, that is absolutely the way to go. One hole in the floor, a few lengths of cast iron pipe, a radiator, a vent and you're done. You are going to have to keep that steam boiler hot all the time just to supply the baseboard. Inefficient, expensive, wasteful, more polluting. Better to let the steam boiler do only steam and handle all of your space heating and do your domestic hot water separately if you don't already with either a gas tank, tankless, or heat pump water heater. Simple, cheap, efficient. We're probably burning an additional whole tank of oil every year because of this setup we have. Not worth it. I promise you. I don't know where you are, but natural gas and heating oil prices are going up in the Boston area...

EDIT: just saw your post, need to look at this...
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edited September 2022
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I am curious to know whether the length of a convector enclosure matters when it comes to EDR. I thought it would, but the bottom of Table 27 on the first link I sent you says that "L=length of convector, not enclosure length." So it looks to me like only enclosure height matters. Maybe one of the other pros on here can answer that question. I think you have at least one 5B but there is only one height of 20 in. for that model in the table. 5B and 5C are pretty close though looking at the pdf I sent you...
I’m not a pro, but I know a little about convectors. When there’s an enclosure involved, size does matter. If you look at the Aero ratings in Dan’s EDR book, there’s always a 3” difference between the width of the element and the width of the enclosure, so there’s only an inch and a half gap on each side, so most of the air is being forced through the element where it picks up heat. If the enclosure is twice as wide as the element, it becomes more like operating it without an enclosure, so it behaves more like a funny shaped radiator. An enclosure can either increase or decrease the EDR of a radiator. It’s safe to assume National designed their Aero enclosures to maximize the output of their convectors, so any deviation from the dimensions they used would decrease the EDR.
Just another DIYer | King of Prussia, PA
1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
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Ideally an enclosure that is much larger than the convector element should be baffled horizontally or vertically to seal off the space that isn't occupied by the element to force most of the air through the element.
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Unless you actually need that much radiation to heat the structure which is very unlikely, a little under is better than a little over. Even at that there is a pickup factor for the piping baked in to the rating of the boiler that is usually significantly more than the actual pickup factor.
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BTW, I am homeowner and not a pro, so take this as you will. I just want to add that my situation is similar to yours. We have an oil steam boiler (megasteam) with tankless coil for our domestic hot water and a loop piped off the rear tappings of the boiler going to a plate heat exchanger which supplies a radiant loop in our kitchen floor. This was a terrible, awful design decision in retrospect. My advice to you: Don't do it! If there's any way to mothball the hot water baseboard in your den and pipe steam there instead, that is absolutely the way to go. One hole in the floor, a few lengths of cast iron pipe, a radiator, a vent and you're done. You are going to have to keep that steam boiler hot all the time just to supply the baseboard. Inefficient, expensive, wasteful, more polluting. Better to let the steam boiler do only steam and handle all of your space heating and do your domestic hot water separately if you don't already with either a gas tank, tankless, or heat pump water heater. Simple, cheap, efficient. We're probably burning an additional whole tank of oil every year because of this setup we have. Not worth it. I promise you. I don't know where you are, but natural gas and heating oil prices are going up in the Boston area...

EDIT: just saw your post, need to look at this...

A better way to make the hot water for the radiant would be to use an indirect water heater as the heat exchanger, then it can also act as a buffer thank and reduce the amount of time the boiler has to run.
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mattmia2 said:

A better way to make the hot water for the radiant would be to use an indirect water heater as the heat exchanger, then it can also act as a buffer thank and reduce the amount of time the boiler has to run.

If you're a pro, you know more about this than I do, but it's my understanding that an indirect on a steam boiler, while doable, is not recommended. You're circulating that dirty boiler water through the indirect tank, and that can cause problems if its not done correctly, and a lot of installers don't know how. I suggested it to my installer, and even though steam is his life, he was against it. I also spoke about our application with an engineer at Ergomax which makes reverse indirect tanks, and he didn't think it would save much, if any, energy.
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rjfork, can you take more pictures from the side of that rad? There is writing embossed around the tapping at the top but I can't make it out. There may also be writing elsewhere on the sides and bottom tapping.
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Finished basement and the fact that room heated by baseboard is far from boiler would preclude that job. But thanks for that suggestion. In an ideal world, it’s probably a good one.

Can’t say I understand the pickup factor.

Hopefully, the Edr I calculated looks good.

I couldn’t make out the name either.  Perhaps u can make this pic out.
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A shell and tube hx might do a bit better with hot water from a steam boiler than an indirect but an indirect will be a lot more resistant to clogging up than a brazed plate hx.
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edited September 2022
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rjfork said:

Finished basement and the fact that room heated by baseboard is far from boiler would preclude that job. But thanks for that suggestion. In an ideal world, it’s probably a good one.

Can’t say I understand the pickup factor.

Hopefully, the Edr I calculated looks good.

I couldn’t make out the name either.  Perhaps u can make this pic out.
You might want to read one of Dan's books if you haven't already. We've Got Steam Heat is short and easy. Steam boiler ratings include a "pick-up factor" of 1.333 to take into account the heat loss from the steam pipes that supply your radiators. See the footnote on the bottom right corner of page 2 in that brochure. You don't need to worry about that anyway, just match the EDR of your radiators to the boiler's net AHRI rating as closely as possible. I'm getting 334.52 from your notes and using the chart I found, so looks like the EG-45 would work with room to spare. EG-40 might be too small.

As for your traditional tube rad, pretty sure that's an Oriole radiator made by the Republic Radiator Company in Baltimore. 2.33 sq ft per section was the standard for large tube type rads with 3 tubes as far as I know. That's my best guess. 18.64 for that one. I looked in the usual spots but couldn't find a single trade catalog except at, you guessed it, the Smithsonian. You should send them a research request for that and the Aero convectors and then post whatever they give you on here so others can benefit. When you view the radiator from the top, is the thickness of each section 2.5 in?

I think what mattmia2 and Hap_Hazzard said makes sense. You could narrow the enclosures that are too wide in order to get the rated output. Having convectors in exterior walls is not great from an efficiency standpoint because some heat is lost through the wall to the outside. I might suggest filling in the enclosure and replacement with a normal tube type rad brought forward into the room, but you have so many of them in this case, you probably don't want to go that far...If you're in mid-Atlantic, Steamhead is there...
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mattmia2 said:

A shell and tube hx might do a bit better with hot water from a steam boiler than an indirect but an indirect will be a lot more resistant to clogging up than a brazed plate hx.

You would think so right? But my installer didn't share my point of view. I pushed him on this, but he wasn't having it. We've been using the same brazed plate HX and Taco circulator for 20 years without any issues whatsoever. I'm not sure but the amount of water you need to heat up the indirect may be a lot higher compared to feeding a small radiant load.
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Random, thanks for the info and double-checking my numbers. I was leaning toward the EG-45 or Williamson GSA-150, recommended by one pro, which are fairly comparable in both btu & Edr.

As I said, another two pros wanted to go with what I currently had 175000btu/418 Edr, but one only took  raw measurements of just the enclosure and the other just multipled the # of rads by 30 for a ttl of 390 but upped it due to baseboard.

You guys r great, like treasure-hunting Indiana Jones, but for radiators!
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rjfork, another possibility if you don't want to pipe in a new rad is installing small ductless mini-split heat pump for your den. The payback on that might be even faster. \$300 tax credit too.

I also thought more about what the other two posters said about the convector enclosures, and I think they're absolutely right. The pdf I uploaded shows that the enclosure length is always only 3" longer than the convector length. The enclosure should be as narrow as possible and well-sealed so the cold air at ground level is all going through the convector. You can get a couple pieces of wood cut to fit snugly into the recessed space and up against the grill and then seal the cracks with fluetape.

This should maximize the draft and air circulation through the unit and heat the rooms more quickly. It also means your thermostat should be satisfied sooner. You can add aluminum foil taped together with high temperature aluminized flue tape to the back of the enclosures on your exterior walls to minimize the radiant losses to the outside. Here's one of ours:

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I made a mistake, it's actually 335.166666666666666666.
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I like the idea of narrowing the enclosure to direct the heat out more efficiently.  Is there a particular wood enclosure to build (eg 1/2” plywood, 1”x6” board, etc)?

One mentioned the split idea too.
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Don't know, never attempted. Getting a contractor to build a brand new enclosure can't be cheap though. Not sure what you mean exactly. I was thinking just keep the enclosures you have but stick two thick pieces of wood on either side of the convector, maybe 2"x6"x30" and put the existing metal grill over that. Don't see why that wouldn't work or be unsafe.

Have you been able to find a good installer?
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Ok. That’s what I was thinking too and would be easy enuff to build.

It looks like I’ve found one. Funny thing, a reputable company came back with using a 200,000btu furnace.
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You could use sheet metal horizontally or vertically too.
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rjfork said:

It looks like I’ve found one. Funny thing, a reputable company came back with using a 200,000btu furnace.

I'd steer clear of those guys. That's nearly double what you need. Did you try this? https://heatinghelp.com/find-a-contractor/ What area of the country are you located in?

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I wouldn't go up to the EG-45, I'd go with an EG-40. Let's look at the numbers. Base on the mentioned 335 EDR.

EG-40
125 input
82% efficiency
103 output (their number)
EDR of 321, which factors in the 33% pick up factor.
Add that back in and it supports EDR of 428 so for you it would be.
335 EDR with ~28% pickup factor. For me that's so close to the typical 33% it doesn't matter. I and several others on here also feel the pickup is excessive at 33%.

EG-45
150 input
82% efficiency
124 output (their number)
EDR of 388, which factors in the 33% pick up factor.
Add taht back in and it supports EDR of 517 so for you it would be.
335 EDR with ~54% pickup factor. For me that's a lot of overkill and will build more pressure, be louder, short cycle etc. I can't see an upside to this at all.

So it's either 5% "under" or 21% over, I know which one looks closer and would be going into my house.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
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I've got an EG-40 connected to 392sqft.

It was an EG-45 for a few years. I like it as the 40 much better.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
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EG-40 might be ok if he insulates his pipes and gets rid of the hot water baseboard loop. If there's any exposed pipe in a cold crawlspace or garage for example...at least in our case before the pipe insulation went on, one of the rads barely heated up at all when I went from a Megasteam 396 down to a 288 with an installed EDR of 264.
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EG-40 might be ok if he insulates his pipes and gets rid of the hot water baseboard loop. If there's any exposed pipe in a cold crawlspace or garage for example...at least in our case before the pipe insulation went on, one of the rads barely heated up at all when I went from a Megasteam 396 down to a 288 with an installed EDR of 264.

Insulates piping yes,
Radiators also need to be vented correctly which doesn't mean all Hoffman 40's or Gorton 4's it means a mixture and proper main vents.

The hot water loop should be fine.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
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edited September 2022
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Agree on venting. @rjfork this book is great for that: https://heatinghelp.com/assets/documents/Balancing-Steam-Systems-Using-a-Vent-Capacity-Chart-1.pdf

He's got 26 ft of baseboard. Doesn't the heat loss of that depend on flow rate, water temp, and room temp though? https://slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf

I'm seeing anywhere between 160-610 Btu/hr/ft. assuming baseboard is usually not more than 180 F. Multiplied by 26 ft, divided by 240 Btu/sq ft to get the steam rating that's an additional 17.3-66.1. Now its 352.5-401.3. EG-40 at 321 might have trouble with that. Yet another reason to ditch the baseboard...
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One pro let me know I needed to stay with current size 175k btu boiler due to the increased load of the hot water zone.  He Wouldn’t recommend going down one size. Not enough heat on the coldest day of the year he said.

If my Edr measurements are correct, is this true?
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@rjfork What @KC_Jones is saying was true for us. When we went down a size, there was less short-cycling, the system hardly builds up any pressure at all now, and the vents stopped whistling completely. It's really nice not being woken up early in the morning by a whistling radiator vent. I don't know if this has been a problem for you so far, but no baseboard + pipe insulation means you could potentially go with the EG-40. I think this is another reason why it's your best option. But it's your decision, and with the baseboard, I think the pro you spoke with has a point. You might need to go with the EG-45. I'm not familiar with forced hot water systems, but the EG-50 even with the baseboard seems like it would be overkill. That's bigger than what you have now, 454 vs 418.
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I just noticed what could be a major discrepancy from what I reported earlier, depending on whether the Aero is a model 5B or 5C.

I was using the attached Table 27.  However, I see that I incorrectly used the convector length at the top of the page instead of the lengths associated with the 5B. (My bad).  Using those numbers, the EDR jumped to 436 (highlighted on my calcs).  The EG-50 is 454.

The other table below shows the values for the Aero 5C, which are similar to the prior calculations I provided.

Not sure the difference between the two models, but there is a huge difference in the outcome.  It looks like I may have the 5B!!