Guidance on gas steam boiler sizing when experts can't agree?

miracolix · August 15

Good morning!

We have an oil steam boiler that we'd like to replace with a gas steam boiler, so we asked three HVAC companies for quotes (one of the companies we worked with before and the other two came recommended to us; all of them have been working with steam heat for 15+ years). Still, we got wildly different recommended boiler sizes ranging from 125MBH to 300MBH (net AHRI steam ratings). Which one is right?

We measured our installed radiation as 175MBH using the Weil McLain boiler replacement guide. Our current Weil McLain boiler (see pic) has a 300MBH rating. It heats up the house very quickly (all of the radiators get hot and some are too hot to touch once the system is running).

We live in a freestanding house in Philadelphia, PA, built ~1900, 3300 sqft, solid brick, most windows have been replaced with energy-efficient windows after the old boiler was installed, but there is little insulation around the steam pipes from what we can tell.

Any guidance would be appreciated!

PXL_20230103_180819608.jpg

KC_Jones · August 15

What was the measured EDR of the radiators without converting to BTU? Also, you didn't add anything correct? Just add up the radiator EDR and use that number as is, no pick up added or anything. Just want to confirm what you did.

With that information you compare the EDR directly to the rating in the boiler catalog and pick the boiler rated closest to what you have.

If you truly do have 175 and someone is recommending 300, they literally are not doing their job. It's not disagreement, it's lazy or a hack.

Steamhead · August 15

Those 78-series boilers were built like tanks. If it were mine I'd just put a conversion burner in it. Maybe a Carlin 201Gas or 301Gas.

miracolix · August 15

@KC_Jones we measured EDR (square footage) as 729 without adding anything (we got the 175000 by multiplying the 729 by the 240 heat emission number supplied by Weil McLain)

@Steamhead thanks for the suggestion! Would you be able to recommend someone in Philly who does this sort of work? None of the people we spoke with offered this as a solution.

ethicalpaul · August 15

Compare your 729 number to the "net sq ft of steam" number that each boiler manufacturer publishes for each boiler. Don't do any extra math or conversion. The net sq ft of steam number is already overly-inflated so "round down" to the smaller boiler if you are in between.

mattmia2 · August 15

Is this one pipe or 2 pipe?

If you go the gas power burner route and if your measurements of the radiators are close, you probably want to downfire the power burner as much as Weil McClain will allow for that boiler/as much as it can be done and still get proper combustion.

STEAM DOCTOR · August 15

Conversion burners are great but only if set up properly. Unless you have somebody whom you are comfortable is competent with conversion burners, I would stay away. Much easier to mess up a conversion burner then the standard atmospheric boiler. Same reason I tell people to stay away from oil. I work with the assumption that most technicians working on these things, are somewhat incompetent. Better to have an incompetent working on atmosphere gas boiler, then an oil or conversion gas burner.

EBEBRATT-Ed · August 15

Seems like you are confusing mbh and edr. As @KC_Jones & @ethicalpaul mentioned measure all radiators and total up the edr which =square feet of steam.

Then pick a boiler that has a square feet of steam rating closest to your radiation output. No additional calculations or additions needed. Don't convert to BTUs.

The boiler rating in square feet of steam already includes a "pick up" rating of 1.33 which some think is oversized so you can downsize a little

KC_Jones · August 15

Not sure what brands are being suggested, but in the Weil Mclain line the EG-75 would fit your EDR well. Honestly I could even make arguments for the EG-65 which would still give you 13% pick up factor, but I doubt you get any contractor to go there.

Whoever is recommending the 125 seems a bit crazy to me. I'm all for sizing down as the systems can run really nice (I'm firing slightly below my EDR and it's awesome), but 125 is nuts.

Whoever is suggesting the 300 is even more insane that the 125 suggestion.

It's not so much there is disagreement among professionals, there is disagreement among people who aren't doing their job correctly. So if they aren't doing their job correctly they should be discounted.

Sizing is important, correct piping is just as important. There is a picture in every manual showing what's proper, and plenty of them still do it wrong. Having the new install properly skimmed so it will run correctly is also important. Whoever does it, need to make sure all those points will be addressed, and get it in writing.

Steamhead · August 15

https://forum.heatinghelp.com/discussion/comment/1808049#Comment_1808049

@miracolix I talked to the Carlin rep and he knows several, but would need to know where exactly in Philly you are. PM me if you don't want to post it here.

bburd · August 15

If you compare that 729 SF of calculated EDR with the existing boiler rating of 1,250 SF, you will see that you already have a significantly oversized boiler. Even if it's been downfired a bit (do you know what size nozzle is installed in the oil burner?)

Also, the current boiler is rated for 400,000 BTUH output; the input (firing rate) is higher (3.4 GPH oil x 140,000 = 476,000 or 476 MBH).

This is a simple process, but far too many contractors just don't want to learn to do it right.

PRR · August 16

all of the radiators get hot and some are too hot to touch once the system is running

It's STEAM. (And probably not vapor.) It should heat to 212 degrees. If some radiators get half-hot, that is probably air-vents, something the home-dweller may be able to replace.

We had a gas burner put in an oil boiler with troubled tankage (47' holly tree over it). We were told it would be less efficient, a generic gas burner instead of a "matched" oil burner. We were quite happy about it. True, we were on street-gas then, when gas was far cheaper than oil. (And now I know that there aren't really custom burners, just a couple angles and deflections.)

Mad Dog_2 · August 16

Once a Conversion burner is set up, I've found them trouble free. Riellos are more finicky than Carlins, but if you take the time, they work great. Mad Dog 🐕

ChrisJ · August 16

https://forum.heatinghelp.com/discussion/comment/1808063#Comment_1808063

This is a simple process, but far too many ~~contractors~~ just don't want to learn ~~to do it right~~.

I'd say with some minor changes that sentence covers a lot more issues in general, unfortunately.

ratio · August 16

Thinking is not very popular right now.

mattmia2 · August 16

https://forum.heatinghelp.com/discussion/comment/1808114#Comment_1808114

I know who i'm saying this to, so i don't want to start something, but every time i ask someone to explain something about combustion they don't, they say it has to be a professional. If no one will write it down or explain it, it is going to be hard for new people to learn or for techs to even knw that they are doing it wrong.

Jamie Hall · August 16

https://forum.heatinghelp.com/discussion/comment/1808130#Comment_1808130

In a very real sense you are quite right, @mattmia2 .And — in a sense — I agree with you. The problem is this — and I'll take flying as an example here. You can read all the books you want, and even fly a flight simulator until your computer is tired, but… let me assure you, the first time you get in the air all by your lonesome, unless you have learned the real hands on skills and feeling with competent pilot to help you, you are going to panic and you are going to bend the bird.

What I'm saying is that all the book learning in the world isn't going to give you a good result the first time you are faced with a slightly recalcitrant burner.

This is why I at least don't suggest that adjusting burners is really a good thing for someone who hasn't had at least some time watching and working with a more experienced person. Not to say that you might not be able to get close enough to stay warm at oh dark hundred on Christmas Eve, but that you most likely won't get really on — never mind be able to diagnose and fix some of the mores squirrely issues (other, of course, than that refuge of the modern age of simply firing the parts cannon at it until something sticks).

pecmsg · August 16

a combination analyzer will give you valuable information.
that information needs it be WITH proper training.
NCI Offeres classes in that.

Find a NCI Certitech and your golden!

LRCCBJ · August 17

This is why I at least don't suggest that adjusting burners is really a good thing for someone who hasn't had at least some time watching and working with a more experienced person. Not to say that you might not be able to get close enough to stay warm at oh dark hundred on Christmas Eve, but that you most likely won't get really on —

This is exactly right. Providing advice to others regarding the attempt to get "close enough" without a combustion analyzer is fraught with risk.

In fact, anyone providing any kind of advice regarding combustion on an oil fired appliance ought to own a combustion analyzer themselves and understand exactly how to use it! Otherwise, they should abstain from ANY advice.

miracolix · August 23

To give some thanks and preliminary closure on this topic: we followed the suggestion of @Steamhead to go the route of conversion burner instead of a new boiler and, with the help of @trivetman, we found a contractor in our area to do it. Whatever happens (we hope it works :), thanks to this forum, we got better info in a day than what we got from our "experts" over a much longer period of time.

Guidance on gas steam boiler sizing when experts can't agree?

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