Steam sizing

McSteamer22

I’m replacing a steam boiler soon, current set up is an older boiler with an output of 533sq ft.
The house requires 420sq ft, so we are installing a megasteam 513,
(know it’s close to to a 396 but i can’t go under)
Question is about how many sq ft that can travel through a 2” pipe. I’m dealing with a 2” header with two 2” take offs, total sq ft again is 420sq ft. So I’m not sure if i should leave the 2” header or upsize it to 3”

Thanks for input

Dave0176

Don’t go with the 513 because you have to add another 33% to that number thats already built into the boiler for the pickup factor bringing your EDR total to 682 sq ft. Do yourself a favor and go with the 396. Run both of the wimpy 2” outlets into a 3” header.

hvacfreak2

I'm heating 431EDR with a MST396 ( this will be the 3rd season ). I guess I could say that I would like to have more power from a cold start ( wouldn't we all I guess ) , but once that's over with sizing is where it needs to be. I still have not insulated my header and some piping , still playing with main venting , etc. so I think I will improve that a little when everything is tight.

EBEBRATT-Ed

@hvacfreak2 is the installation you are referring to in your own home?? If it is that's fine and you can spend the time to tweak things.

As a contractor you have to get in and out and be able to walk away from a system that works.

Undersizing a boiler may not allow for that

MilanD

Wait - I thought boiler EDR rating on the sticker (sq ft of steam) always shows actual output to connected radiation and is already 30% less than actual btu output, accounting for 33% pick up.

Jamie Hall

MilanD said:

Wait - I thought boiler EDR rating on the sticker (sq ft of steam) always shows actual output to connected radiation and is already 30% less than actual btu output, accounting for 33% pick up.

That's my understanding, too.

MilanD

> @Jamie Hall said:
> Wait - I thought boiler EDR rating on the sticker (sq ft of steam) always shows actual output to connected radiation and is already 30% less than actual btu output, accounting for 33% pick up.
>
> That's my understanding, too.

In that case, op new boiler still gives 20% or so of pick up factor. Should be plenty, esp. if main and header are insulated.

hvacfreak2

The Megasteam EDR ratings are net with 1.33 pick up included.

MilanD

> @hvacfreak2 said:
> The Megasteam EDR ratings are net with 1.33 pick up included.

Huh... What do you know - it sure does!! 513 is still good size. Gives 20% or so pickup. Should be enough.

KC_Jones

In the OP's case the 513 gives a 62% pick up factor the 396 gives 25% pickup factor.

I seriously doubt being "short" by ~8% is going to cause it to not perform.

EDIT: see my next post, didn't look at the actual rating before posting this.

MilanD

Apparently, Megasteam EDR rating includes the pick up factor. Thus, if connected radiators total EDR is 420, 513 EDR Megasteam gives 22% pickup to 420 EDR connected.

396 will be too small, or rather, it will be sized to load and give no pick up factor. No?

KC_Jones

hmm they are doing something weird there.

396 rated 127k gross multiply by 86% = 109k/240 per sqft = 455 not 396 and still doesn't account for the pickup factor.

interesting. It isn't a traditional pickup, but it's something they are adding/subtracting there.

MilanD

> @KC_Jones said:
> hmm they are doing something weird there.
>
> 396 rated 127k gross multiply by 86% = 109k/240 per sqft = 455 not 396 and still doesn't account for the pickup factor.
>
> interesting. It isn't a traditional pickup, but it's something they are adding/subtracting there.

Per brochure, 396 net output is 95k. 95k/240 = 395.83 edr gross. (396/133)*100 is 298 EDR of radiation.

KC_Jones

The gross input is 127,000 btu
The efficiency is 86%
127,000 x .86 = 109,220 btu
109,220/1.33 (traditional pickup)=82,120 btu
82,120/240= 342 sq ft
They rate it at 396 which would be 14% pick up factor
Again they are doing something different from tradition with the ratings on that boiler.

Can't ignore the 127k input everything comes from that.

And they have this note under the ratings:
"Net AHRI Ratings are based upon pick-up allowance of 1.333. Consult manufacturer with unusual piping and pickup requirements
before selecting boiler."

So starting from 127k, 86% efficiency and their stated 1.33 pick up, I'd love to know how they get to 396.

Fred

That's the way all the brochures read. They quote "Net Output" as it relates to Sq. Ft. of steam available to satisfy connected radiation. The Piping and Pick-up factor is in addition to that. What is the gross BTU output? That will tell you that about 33% has not been accounted for in the "Net" number. A 396 should be fine.

MilanD

Look at the Megasteem brochure.
For 396 model: 127k input. 95 output. (By this efficiency is calculated at 75%?) EDR 396.

95k output / 240 EDR = 395.83 EDR.

What am I missing?

Looks to me they do rating like this:
127k input divide by 1.33 = net btu / 240 EDR = EDR net. This is sort of in between what my understanding of EDR calc is. That leaves less for pickup than 33%. More like 10-12%. Maybe that's ok too?

I was under the impression you calculate EDR like this:

gross input times efficiency = net output
Net output divide by 1.33 (for pickup factor), divide by 240 = load EDR (combined rad EDR).

Fred

Per footnote #2 and #3, the efficiency factor has already been factored into the DOE of 127, so the calculation is 127 divided by 1.33 = 95.4. They are not telling you the gross input, which is 127 divided by .86 (stated efficiency) = 147.7

MilanD

> @Fred said:
> Per footnote #2 and #3, the efficiency factor has already been factored into the DOE of 127, so the calculation is 127 divided by 1.33 = 95.4. They are not telling you the gross input, which is 127 divided by .86 (stated efficiency) = 147.7

Aaahh! Thanks Fred. I figured they'd want to tell you what your input is, but I guess it doesn't matter. I'm one of those guys that needs to read the manual too!! Haha!

In that case op should go with 396. 512 will be waaay too big.

(Autocorrect is making me have to edit my posts. Sorry guys.)

KC_Jones

I'm an idiot. The 127 is after efficiency, its firing at 1.05 oil is 140k per gallon. That means gross input of 147,000x .86 for stated efficiency =~ 127,000 then apply 1.33 pick up gets to to down to 95,000.

So basically back to my original statement the 396 has 25% the 513 has 62% pick up.

MilanD

@KC_Jones, I was right there with you...

Fred

Brain freeze, guys. Wait till you get my age.