Sizing new boiler: discrepancies among my calculations and bids

jv100

I've gotten three bids for a residential oil-to-gas conversion plus the addition of an indirect water heater to replace the stand-alone. I'm a homeowner who is new to steam, but I have been studying Dan's books and the wealth of information on this website (thank you, all!).   I'm having trouble figuring out why the suggested boilers are so different from each other and from my calculations.



Here's the system:  Brick/stucco house built in 1928, 915 sq ft living space. It has 8 radiators: 6 are American Radiator Corto, 2 are SunRads (one recessed).  One of the Cortos is in the attached garage which is below 2 bedrooms.  The basement, where the boiler is, is unheated.  A bit of the original asbestos insulation is still on the steam mains, but much is lost.  I will put insulation on those mains, but I might have trouble doing so in the crawl spaces.  The windows were recently upgraded and are double-paned, except for a French door that is single-paned and lets lots of drafts through.  Attic is not well-insulated. The oil tank is in terrible shape and the boiler is 20 yo and was clearly not well-maintained, so I think it is time to upgrade.  This will be my first winter in the house. 



I measured the radiators and made EDR calculations as per "The Lost Art of Steam Heating". I came up with a total of 303.25 sq ft EDR. I calculated the size boiler I thought I'd need by multiplying that number by the pick-up factor (I used both 1.33 and 1.5) and by 240 to convert to BTUH, then dividing by 1,000 to get the MBH.  I came up with 97 MBH (pick-up 1.33) and 109 MBH (pick-up 1.5).  I compared this number to the DOE Heating Capacity in boiler specs.

 

The 3 bids I've gotten listed the following boilers:

Bid 1) Burnham IN-5: DOE Heating Capacity 115 MBH, Steam 358 sq ft

Bid 2) W-M EG45: DOE Heating Capacity 125 MBH, Steam 392 sq ft

Bid 3) W-M EG50: DOE Heating Capacity 145 MBH, Steam 454 sq ft

Bidder number 2 told me that I have 360.36 sq ft of steam; the other 2 bidders didn't tell me what they calculated (but they did all measure the rads).  My current oil-fired boiler is a W-M 68 series (DOE Heating Capacity 179 MBH, Steam 560 sq ft).  All 3 bidders said that is over-sized.



So, my questions:

Did I make a mistake in the way I calculated things?

Do I need additional pick-up or BTUs for the indirect hot water heater?

Am I missing some other factors that the bidders did factor in? 

Why was bidder 2's EDR measurement 20% higher than mine?

Why such differences in sizing from the 3 bidders?



Any thoughts and ideas appreciated. Many thanks!

JV 

jv100

fogot to mention...

...the rads are one-pipe.

Rod

EDR Chart

Hi- In this case I think I would re check my numbers when I calculated the EDR by calculating the EDR using a difference source data.  I've attached an EDR calculation sheet and here is a link to the Burnham Heating Helper which also has EDR data figures.

http://pro.usboiler.net/PDF/htghelper.pdf

I would calculate  the EDR using the info on the sheet and then use a second copy of the same sheet using the Heating Helper figures. That would give you your present sheet plus two more which you could then correlate to see what differences there were.

On the other items I think your new windows would give you a reserve that would take care of the water heater without adding extra output to the boiler. However, as I'm a homeowner, my experience is rather limited in this area so I would go with what ever the pros recommend.

Keep in mind that insulation also makes a BIG difference!

- Rod

jv100

Great idea, Rod!

Thanks for providing the links to those other sizing charts.  I'll definitely repeat the calculations.

I just came across this article here at Heating Help that mentions "modern tube radiators" in addition to "thin-tube radiators" and column radiators. (http://www.heatinghelp.com/article/163/Older-Steam-Heating-Systems/1294/Controlled-Air-Venting-of-One-Pipe-Steam-Systems).  I used the thin-tube numbers from chart in The Lost Art of Steam Heating, but maybe that was incorrect.  So much to learn!

Thanks again.

JV

Rod

Insulation and Boiler Sizing

A little more info that might be of help to you.You might want to take a look at this link on insulation:

http://www.heatinghelp.com/article/325/Piping/300/Why-you-should-insulate-steam-pipes

 My mains were well insulated  and I just had my near boiler piping bare and was presently surprised at the noticeable difference it made insulating the bare piping. Insulation is a large "bang for the buck". Read back a few weeks -  "Crash" did a great post with a lot of info on insulation.



From what I can see there seems to be a big tendency to keep adding multiple

"fudge" factors to the boiler size. You have to be cautious about doing this as it results in a oversized boiler with the corresponding higher fuel use. If you haven't read it yet the following link is to a great article by Dave Bunnell, "Boilerpro" on boiler sizing.

http://www.heatinghelp.com/article/323/Boilers/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell

- Rod

jv100

I'm with you on the insulation, Rod.

IIt's amazing that you saw a difference even with just the near-boiler piping. Thanks for the link.  I'll make sure my pipes are tucked in snugly!



Thanks also for the link to the article about oversizing. Looks like a "must read".

JV

AlexR

look for the baseboard + steam article

Read with caution, as I'm also a homeowner and not a pro:



1) it seems most installers like to oversize boilers.  Maybe double check your measurements and ask a friend to do it to.  You don't need to do the MBH calculations; just compare the EDR rating on the boiler to what you got from measuring the radiators.  The Boiler's EDR rating will include a pickup (Burnham used a 1.33 factor, if I recall correctly from when I did all the math)



2) I'll second Rod's comments about insulation



3) Here's something to chew on for the hot water heater: when you radiators were installed, they were sized (or oversized) to the heat loss for the house.  Now that you have better windows and might choose to insulate your attic, they're probably oversized.  So even on a design day, your boiler won't run all the time.  It will run for a while to fill the radiators with steam and then start cycling on pressure because the boiler produces more steam than the hot radiators condense.  So you can probably get away with *not* increasing the boiler capacity to account for the hot water loop because the boiler's "extra" capacity (compared to the radiator's steady state) will cover it.    One of Dan's articles covers this with respect to a baseboard loop off a steam boiler.  Furthermore, even the IN5 is oversized for your radiators, so have some extra capacity to start with.

Dave in QCA

Another Radiator Sizing guide

First, I'd say that I agree with all of the comments posted.



The terminology for the various types of radiators continues to cause confusion.  Here is a side by side chart that shows generic ratings for the various styles and sizes of radiators, i.e., column type, large tube, and thin tube.  It was printed by ASHRAE and I have always found it to be very helpful and accurate.



Here it is again.

jv100

EDR has pick-up or not?!!

The EDR issue is really confusing me.  If I understand Dan's books correctly, he says to figure out the EDR of your building, then add a pickup factor, and that is the DOE Heating Capacity number.  But I have read on The Wall that EDR includes the pickup. How can this be? Consider the specs listed for the IN-5 in Burnham's brochure: "DOE Heating capacity: 115 MBH; Net I=B=R: 86 MBH; Steam: 358 sq ft".  I'm assuming that "steam" value is the EDR of my radiators only.  Multiply the steam number by 0.240 (to convert to MBH) and you get 86 MBH, which is the Net I=B=R value for this boiler.  The Net I=B=R number is defined as having a 1.33 pickup allowance included in it, with the caveat that additional allowance should be added for unusual piping, etc.  Going back to the steam value, if that is multiplied by 0.240 (MBH conversion) and then 1.33, we get the DOE Heating Capacity value of 115 MBH.  So how does the EDR have the pickup??  Maybe when talking about boilers people say EDR when then actually mean Net I=B=R? 



I'll give that article about Dan's that you mentioned a close read.  Good point about the change in heat loss in the house over the decades and how that would affect the capacity of my system.

Maybe I'll ask my math whiz son to double check my numbers.  He was being a snarky teenager this morning and I was on my way to being a snarky parent in return.  Giving him an important job might still the waters, at least for a little while!

Thanks for posting!

JV 

jv100

Great article, Dave in QCA!

It's so helpful because it actually describes the size of the sections of the various radiators.  Using this article, I confirmed that my radiators are what he calls "large tube" with sections 2 1/2" on center.  The EDR for those radiators corresponds to what is listed under "thin-tube" in The Lost Art of Heating.  So the total EDR I came up  with for my rads should be correct. 

Thanks!

Dave in QCA

Not Complicated, really....

OK, I'll try to untangle it for you.       When you are speaking in terms of EDR, it means the sq ft of surface area of your connected radiation.  EDR= Equivalent Direct Radiation.  This does not include piping losses or anything else, just the total of your radiators.



If you want to know the size of boiler that you need in BTU, you have to do the calcs.  Lets use 358 sq ft because Burnham happens to have a boiler, the IN-5 that is rated for system that precisely.   

358 EDR x 240 BTU/EDR =  85,920 BTU     This is what the rads can give off, and therefore, the heat that must be provided to them.  This is the NET BTU.   Keep in mind that a steam system must be sized with an extra allowance of 33 or 34% for piping losses and pickup.  So, the output of the boiler must be bigger, right? 

85,920 NET BTU x 1.34 Output BTU/1 Net BTU = 115,132.8 BTU

For clarification, the gross output is what it says it is... the total gross output that comes out of the boiler.  The difference between the Input and the Gross Output is lost up the chimney.   The Net Steam ratings has the pickup factor in it and can be used to size the boiler to a system if you are using the BTU rating of the radiation attached to the system.   The Sq Ft Steam rating, also had the pickup factor in it, and can be used to size a boiler to a system using only the sq ft EDR rating of the connected radiation.



Hope this helps....

jv100

Let me repeat that back to you in my own words...

I go to the specs of a boiler; in this case, let's use Burnham IN-5.  Here's what I find:



1. Input (MBH): 140.  This is the amount of energy I put into the boiler.



2. DOE Heating Capacity (MBH): 115.  This is what is available to me to use b/c this steam boiler runs at 82.5% efficiency (140 MBH x 0.825 = 115).  The difference has gone up the chimney.  also known as "Gross Output"



3. I=B=R Ratings (listed as Net I=B=R on Weil-McLain specs):



     a. Steam (MBH): 86.  This is how much capacity I have available for the radiators, having factored in a loss of 33% for warming up my pipes, etc  (115 MBH /1.33 = 86).  aka "Net Output"



     b. Steam (sq ft): 358.  This is how much radiator EDR I can supply, having factored in a loss of 33% for warming my pipes, etc (115,000 BTU divided by 240 BTU/sq ft = 479 sq ft, then divide by 1.33 to get 360 = 358 with rounding errors). aka "EDR of the boiler"



3a and 3b are essentially the same thing, just using different units.



Have I got that right?? 

jv100

Dave in QCA, one more rad measuring question...

When measuring height, I've been measuring from the floor; i.e. including the feet.  That's how it is shown in the diagram in the article you linked for me.  I just want to double check that I'm doing it correctly.  The rad in my garage doesn't have feet (it is hanging on the wall) so I just measured the length of the end section.

Thank you again!

JV

Rod

"Boost Factor"

Hi-  Yes-  on measuring the radiator height,   it usually includes the feet of the radiator.



On your calculations you seem to basically have it though you might want to look at it in a slightly different way.



“1. Input (MBH): 140.  This is the amount of energy I put into the boiler.



2. DOE Heating Capacity (MBH): 115.  This is what is available to me to use b/c this steam boiler runs at 82.5% efficiency (140 MBH x 0.825 = 115).  The difference has gone up the chimney.  also known as "Gross Output"



Think of it more as “out put divided by input times 100 equals percent efficiency” ie: 115 divided by 140 equals .82142 times 100 equals 82.14 percent efficiency



3. I=B=R Ratings (listed as Net I=B=R on Weil-McLain specs):



     a. Steam (MBH): 86.  This is how much capacity I have available for the radiators, having factored in a loss of 33% for warming up my pipes, etc  (115 MBH /1.33 = 86).  aka "Net Output"



     b. Steam (sq ft): 358.  This is how much radiator EDR I can supply, having factored in a loss of 33% for warming my pipes, etc (115,000 BTU divided by 240 BTU/sq ft = 479 sq ft, then divide by 1.33 to get 360 = 358 with rounding errors). aka "EDR of the boiler"



3a and 3b are essentially the same thing, just using different units.



   You basically are correct here though I think referring to the pickup factor as “a loss” confuses things. Think of the pickup factor more as a “boost” factor and the IBR ratings as the maximum amount of radiator EDR that the boiler will supported when the “Boost factor” is included.

    If the boiler's capacity (DOE Heating Capacity) was just enough to support the radiator's total EDR, even without the “pickup factor” included, they would still get hot just that it would take a much, much longer time as the piping would have to heated first. (This is where the benefit of insulating your steam pipes really makes a huge difference.)  After the “Boost” (pickup factor) does its job of heating the piping, it  becomes a surplus source of addition heat for things like indirect HW or HW heating zones which is why I don’t think you should consider it as a “loss”.

- Rod

Dave in QCA

Rod's Answers are Precisely Correct

Rod answered your questions before I got back to my desk.  His answers are 100% correct and in fact, better stated than I would have been able to do.  Boost Factor is a perfect term!

jv100

The fog is beginning to lift!

Thanks Rod, and Dave in QCA, for the great explanations.  Slowly but surely, I feel like I'm starting to grasp these ratings!



Rod, thanks also for the answer about the radiator measurements.



Two final questions:



1. Does an EDR of 303.25 sq ft sound reasonable for my house?



2. If my EDR of 303.25 sq ft is correct, do I just choose a boiler with the closest size up from my EDR?  I assume that on a bitter cold morning (by NYC metro standards), we would set the thermostat to call for steam early to warm the system, so that we would have that "boost factor" capacity available for the indirect to support our hot showers when we get up. 



Thanks again for taking the time to reply!  You Wallies are awesome!!

Rod

EDR and setbacks

Dave- Thanks! Coming from you, who always writes the most eloquent responses, it is indeed a complement!



jv100-  Your calculated ECR should be correct. The only other things you might factor in is what changes have occurred since the "Dead Men"  first installed the system. If a lot of improvements like double pane windows,insulation etc. were added and you then found a boiler slightly under your EDR you could them probably go with it otherwise go with the closet one that is on the higher side (though again not too high!)

On the early mornings- I don't know whether you have run across this yet but the majority of opinion on the Wall seems to favor small thermostat setbacks (5 degrees or under) on steam systems as it is felt you use less fuel maintaining a set temperature than you use making up a deep setback. You will also need a thermostat on which the cycles per hour can be set. On a steam system this is one cycle per hour.

- Rod

jv100

I've read that about the setbacks...

...and am trying to imagine living with it.  I'm so used to setting the programmable thermostat to go to 65 degF in the morning (and sometimes 68 if I'm feeling wimpy), down to 55 while we are gone during the day, back up to 65 for the evening, then down to 55 at night.  I'm sure the cats would be glad if I left it at 65 during the day, but at night I like a cool bedroom.  Hey, I'll leave the windows open, just like my steam system was designed for!  Seriously, I'll search the Wall for discussions of how people have lived with the setback issue, but I'd love to hear options that you have come up with.



Thanks for the mentioning the cycle issue for the thermostat.  I'm currently making up a list of questions for my 3 bidders, and I'll add that to the list.



Thanks again!

JV

Rod

Use TRVs

I like a cool bedroom too so I use TRVs in  the bedrooms set to the low 50s. I also use them to shut down (set to low 40s)  individual rooms (guest rooms etc.)  we rarely use in winter as that saves on fuel.

- Rod

jv100

Excellent ideas!

Thanks again for all the great info, Rod.

JV