Sizing replacement boiler - how much over SQ FT estimate?

lizg

With assist from generous forum members, I've researched SQ FT for the 4 types of radiators in our 1885 Victorian (Helena, MT). My estimate is 768 total SQ FT, for a net I-B-R of 184,250 and heating capacity of 245,000. Our current boiler is a 45-50 yr old HydroTherm, 350,000 input/Net I-B-R 210,000 & 875 SQFT. (Boiler photos and SQFT worksheet attached).

The boiler has worked well in the past, keeping us quite comfortable on cold days in most rooms and esp. on 2nd floor--but we haven't been very observant about how often it cycles on and off. We have noticed less comfort in the shoulder seasons, something we're told is to be expected with steam.

Our Burnham replacement choices are STMX299 (771 SQFT) and STMX350 (892 SQFT). One plumber suggests 299 (is this enough overage? or would it run better if more closely matched to SQFT?); the other plumber strongly recommends the 350 (the safer choice since we've already experienced this amount of BTU). Does the change in technology over the past 50 years make a difference in how closely we should size it to SQ FT?

I'd much appreciate insights on which way to go, or any other things to consider. Many thanks!

Steamhead

Is the boiler leaking, or otherwise unusable?

The lack of comfort on milder days might be a main-venting problem. Are there any vents at the ends of your steam mains?

Big Ed_4

Insulate the supply pipes , that header is used to deliver dry steam for heating.....

bburd

I and many others here will advise you to choose the smaller of the two boilers you are considering. The net square feet figure includes a 30% allowance for "piping and pickup factor" which most of us think is excessive for a system with insulated steam piping and good venting. Bigger is not better in a steam system. And construction professionals do not guess… we calculate.

EBEBRATT-Ed

Agree with @bburb. no need to go larger. more important than the boiler is that the installer read and understand and follow the boiler piping detailed in the manual as a minimum. and clean the waterside of the new boiler and take care of any venting issues

mattmia2

The boiler piping isn't right but it might work ok with that being an older boiler and the very long riser.

Replacing the boiler won't fix the system problems that are causing uneven heat on short cycles. You need to figure that out before you replace the boiler or you will have the same problems. it could just be maintenance if little has been done in a few decades like replacing failed vents.

Mad Dog_2

Sizing is Art & Science times experience.  O have oversized and I have undersized...Trust me undersized is worse.  A good boiler guy can adjust your firing rates and dial it down a bit if need be. Without seeing your system, I'd Pull the trigger on the 299..thats right in there.  Make sure your main venting is good.  Mad Dog 🐕 

gerry gill

Unless you have tons of un-insulated pipes or mass quantities of risers running up exterior walls that aren't insulated just size to the sq. ft. rating. If you size to the sq. ft. rating, there is already a third extra boiler added in. 10 % for heating the piping and roughly 23% for coming out of a set back condition on the thermostat. Known as pick up factor.

ChrisJ

lizg said:

With assist from generous forum members, I've researched SQ FT for the 4 types of radiators in our 1885 Victorian (Helena, MT). My estimate is 768 total SQ FT, for a net I-B-R of 184,250 and heating capacity of 245,000. Our current boiler is a 45-50 yr old HydroTherm, 350,000 input/Net I-B-R 210,000 & 875 SQFT. (Boiler photos and SQFT worksheet attached).

The boiler has worked well in the past, keeping us quite comfortable on cold days in most rooms and esp. on 2nd floor--but we haven't been very observant about how often it cycles on and off. We have noticed less comfort in the shoulder seasons, something we're told is to be expected with steam.

Our Burnham replacement choices are STMX299 (771 SQFT) and STMX350 (892 SQFT). One plumber suggests 299 (is this enough overage? or would it run better if more closely matched to SQFT?); the other plumber strongly recommends the 350 (the safer choice since we've already experienced this amount of BTU). Does the change in technology over the past 50 years make a difference in how closely we should size it to SQ FT?

I'd much appreciate insights on which way to go, or any other things to consider. Many thanks!

I'd go with the guy recommending the 299. The 350 is grossly oversized.
If it was me personally doing it in my own house, I'd go with a STMX250 honestly. If all of the piping is insulated well and everything is done properly the 250 would be amazing.

The 350 would be a nightmare.

KC_Jones

Not that you asked, but make sure they at least pipe it to minimum spec from the manufacturers manual. Your current one is not as near as I can tell from the pics you posted.

ethicalpaul

Don't use the guy suggesting the larger boiler anymore, he failed the easiest test in steam.

KC_Jones

lizg said:

With assist from generous forum members, I've researched SQ FT for the 4 types of radiators in our 1885 Victorian (Helena, MT). My estimate is 768 total SQ FT, for a net I-B-R of 184,250 and heating capacity of 245,000. Our current boiler is a 45-50 yr old HydroTherm, 350,000 input/Net I-B-R 210,000 & 875 SQFT. (Boiler photos and SQFT worksheet attached).

The boiler has worked well in the past, keeping us quite comfortable on cold days in most rooms and esp. on 2nd floor--but we haven't been very observant about how often it cycles on and off. We have noticed less comfort in the shoulder seasons, something we're told is to be expected with steam.

Our Burnham replacement choices are STMX299 (771 SQFT) and STMX350 (892 SQFT). One plumber suggests 299 (is this enough overage? or would it run better if more closely matched to SQFT?); the other plumber strongly recommends the 350 (the safer choice since we've already experienced this amount of BTU). Does the change in technology over the past 50 years make a difference in how closely we should size it to SQ FT?

I'd much appreciate insights on which way to go, or any other things to consider. Many thanks!

I missed this the first time. Whoever is telling you that is wrong. I would speculate between an oversized boiler, poor piping, and poor venting you currently have a less than ideal system. Get someone in who will size it correctly, pipe at or above manufacturers spec and get the venting sorted out, and you will have a nice running system.

Also as stated above, do not oversize. The bigger boiler literally does absolutely nothing for your comfort, and can actually reduce it in some circumstances.

lizg

Hi all, Thank you for all your responses and for being so generous with your knowledge and experience. You've made us feel much more confident in our choice of boiler size. I'm a bit stunned to learn that the piping and venting were not done right when the boiler was installed (in 1982), and to learn that cooler indoor temps in spring/fall aren't true of steam systems in general.

To answer a few questions:
--The current boiler is indeed leaking and none of the plumbers consulted thought it could or should be repaired.

--There are no vents anywhere on the mains.

--Thanks for the comment about insulating the header. We'll talk with our plumber about that and about insulating the supply pipes that currently are not insulated--less than 1/4 of supply piping in basement. (The four 11-12 foot risers, running through first floor to second, are all on the interior of the downstairs rooms.)

--We'll talk with the installer about meeting the manufacturer's specifications on piping (although I have no idea what that means or how to make certain it happens). Would all the supply pipes near the boiler need to be replaced with different size pipes? I've attached some IMAGE files of the pipes, with circumferences noted in green tape. (I can see that I need to read Ch 4 and 8 of The Lost Art.)

--Cleaning the waterside of the new boiler: I'm not sure what that means. But wonder if it is related to a story about "rogue steam" that a neighbor related recently. Four years ago, first run of his new Burnham Independence 380K and there was no heat to second floor. Installer (experienced with steam) and neighbor puzzled for a while, neighbor (who understands control mechanisms) noticed something about the level in water gauge. Final diagnosis: "rogue steam" from chemical interaction between boiler interior and water that changed the way the steam happened, so the first bunch of (rogue) steam satisfied the system and the thermostat shut down the boiler before radiators on second floor could heat. Solution: Drained the boiler completely, and continued to do so every 2 months during the first winter. Problem solved. A year later same thing happed to his next-door neighbor's brand-new Burnham boiler. Installer (different from my friend's installer) changed the piping, no better result, and finally agreed to try what my friend recommended. Problem solved. Does any of this make sense to you and does it have anything to do with "cleaning the waterside of the new boiler"?

Thanks again for your support and help on this journey into steamland.

Steamhead

lizg said:

--There are no vents anywhere on the mains.

There should be. Main vents allow the steam to distribute quickly through the mains before it rises to any radiator. This not only evens out the heat, but reduces the boiler's run-time. The gas company probably won't appreciate the resulting fuel savings, but who cares about them?

Measure the length and diameter of each steam main and we'll tell you what you need.

EBEBRATT-Ed

@lizg

Download the boiler manual for the boiler you plan to use. Get it read and study it before the boiler and installers start the job with special attention to the piping diagram. Make sure the installer understands that he is expected to follow the boil piping as shown in the manual as a minimum spec, Also read the section on skimming and cleaning

lizg

Steamhead - Make sense, thanks. Is there a way to translate circumference of pipe (are they cast iron?) to diameter, since I can only measure circumference? The circumferences are 6.0", 7.5", and 9.0.

KC_Jones

6.0 = 1.5” pipe 
7.5 = 2” pipe 
9 = 2.5” pipe

lizg

Thanks, KC _Jones!

ChrisJ

lizg said:

Steamhead - Make sense, thanks. Is there a way to translate circumference of pipe (are they cast iron?) to diameter, since I can only measure circumference? The circumferences are 6.0", 7.5", and 9.0.

That's 1.9, 2.38 and 2.86 OD

1 1/2,  2" and 2 1/2" pipe.

lizg

Thank you, ChrisJ

lizg

EBEBRATT-Ed said:

@lizg

Download the boiler manual for the boiler you plan to use. Get it read and study it before the boiler and installers start the job with special attention to the piping diagram. Make sure the installer understands that he is expected to follow the boil piping as shown in the manual as a minimum spec, Also read the section on skimming and cleaning

Thank you for the great advice! Will do.

ChrisJ

I didn't notice kc's response previously.

Sorry about that.

New England SteamWorks

Assuming your EDR calculations are correct, and all your basement piping is insulated, we'd actually install the STMX250. Plenty big enough, given the built in pickup factor.