Sq ft steam - under versus oversizing

JohnNY

....will not provide enough heat.

Oversize in moderation when faced with a decision.

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steve_173

Is there any rule of thumb when sizing a boiler to the sq ft of steam? Should one always go to the next biggest boiler or only if you are more than a certain amount beyond the smaller boilers capabilities? Does this depend on the number of very cold days expected? anything else?

steve_173

I agree, but what is moderation in this case? Boilers come in sections so another section can be quite a large swing in sq ft steam. This oversizing is wasteful and I have had argued to me might lead to short cycling. Yes, some can be de-rated (gas in my case) with some effort of changing the orifice.

How much of the 33% pickup can one tap into, i.e., EDR required should equal or exceed I=B=R sq ft steam. Now in reality the boiler makes about 33% more. I can't imagine the pipes using that entire 33% so there must be a small buffer.

Larry in Elkins Park_4

The math!

I just replaced a boiler, and because of the boiler we got we went down from 225K gross to 207K. I was concerned, so I did some research and was able to get the EDR for most of my radiators, and an approximation of the 3 convectors I could not do exactly. And added 1.333 and came up with about 171,000 BTU after conversion--with my boiler netting 171K.
I then went and measured pipes, and did the math on them--length x diameter x pi ==square feet. Beleive it or not, this came out to about the same amount ofsquare feet as the 1.33 x radiators added above. Although I can not see all the pipes, I have a good handle on how the hidden pipes must run. This also did not count the header. So I figure I am oversized by only a little, certainly less than the old boiler. Looks like the 1.33 is a good approximation.

steve_173

I am replacing an old coal converted to oil boiler. It was firing a 2.25 nozzle (~315 Mbtu) and then was lowered to a 1.75 (~245 MBtu). In order to get the sq ft of steam needed, I actually need a larger input than the old boiler. I think this is because the water capacity of a new boiler is a quarter or less than that of the old boiler. The old boiler took some time to boil, but when it did a lot of steam was available.

Larry in Elkins Park_4

Correction, I can't type!

That should read, I came up with **155,000 BTU** based on all the radiator EDR plus the 1.33, and again by radiator EDR plus square footage of piping, with the boiler 171K BTU net. If I use 1.5 as a pick-up, as some have suggested for an old system, the load is 175K BTU. While my measurements are not 100% accurate, I think with soem 66 square feet EDR to spare, I am okay.
This is a gas system, and always was--after it was converted from township-supplied steam when built in 1900.

steve_173

Hmm, if I use a 1.5 pickup factor then the sq ft steam and input BTU become somewhat more aligned. I suppose that might indicate that my 1919 extended home needs a 50% factor.

kevin coppinger_4

it depends...

what do you have for radiation and what boiler(s) are you looking at? Look into more than one model/ make. Also remember that the rads were based on the original windows and insulation...have you made insulation and window changes? That might mean you have some buffer room...even if the radiators don't heat all the way through..ie. you now have smaller rads...kpc

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steve_173

I am looking at any and all gas boilers. The more efficient the better.

House is original, no insulation. Only storm windows were added. My understanding is one needs the sq ft of steam even if that results in too much heat regardless of added insulation.

One can't tell each radiator to heat partially. The ones further down the line will suffer.

gerry gill

heres a question for you..

is the system in question, one pipe steam or two pipe vapor?

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kevin coppinger_4

again...

what are the numbers? What is the EDR?

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steve_173

single pipe steam

gerry gill

i would round up

on one pipe steam, and install stage fire capability on the boiler..

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steve_173

I have 6 convectors of at at least (can't say for sure if some are the same as each other) 4 different sizes. There are 12 radiators plus one that needs to be added back to a bathroom, but for which I need to acquire a used or new radiator.

I get an EDR of between 733 and 756 depending on how I figure the convectors so say about 745 to be safe. I have not yet selected a boiler. Burnham comes as 725 or 812 (deratable up to 10% with a custom orifice). Peerless is 746 or 873 (yes the 746 is very close, but not sure I want the Peerless). Smith seems to be 654 or 775 (I have to double check if these are the gas ratings).

steve_173

What do you mean by "stage fire capability?"

Thanks.

gerry gill

two stage firing.

since the boiler would be rounded up to the next size, it will build up pressure especially on a long burn..when the pressure builds up it will down fire to a lower gas usage..automatically..then upon pressure fall, it will ramp itself back up.

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kevin coppinger_4

that's not....

that bad of an over fire. I just replace a boiler today... oil fired, W-M, 5 section 455 edr...I put in a Smith 3 section fired on lo w/ and edr of 268....the connected load was 202.

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steve_173

Oversizing is an even bigger concern on a smaller boiler as percentage-wise the extra section produces a lot more sq ft steam.

steve_173

How does this differ from a modulating burner? I thought this was only an option with hot water. Is this an option on any gas steam boiler or only certain boilers/burners?

How does that compare to going with a Smith G8?

gerry gill

there is no modulating

gas burner for steam, on an atmosheric boiler that i'm aware of..it just provides two firing levels, one high, one low.

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steve_173

Do all gas boilers offer this automatic high/low firing option?

nicholas bonham-carter

two stage firing

i also would like to know more about this hi-lo arrangement for my peerless 1-pipe steam[1,050,000 btu]
is it controlled by one vaporstat,or must one have a second one?
i immagine you could set it up to get up to minimum pressure [6 oz.] at hi-fire then go to low fire to more slowly take it up to 12 oz before the burner cycles off.
does this require a second gas valve?
this arrangement might mimic the old coal fire--hot to start with then firing lower to maintain it in the "sweet spot".nbc

gerry gill

it does indeed

require a second gas valve and another vaporstat..and you are correct that it is the desire to mimic the coal boilers operation..i think you've got it figured out now..like we say, if you want to control the pressure, then you better get control of the fire.

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Daniel_3

Most manufacturers build the pick-up into their steam ratings; be careful and ask first.

steve_173

I have only seen a 1.33 or 1.333 factor. I haven't seen any (but I'm only a novice) that didn't include the factor in the I=B=R rating.

Can one get gross sq ft of steam by doing net sq ft times the pickup factor? So if it is 100 sq ft I=B=R is it then 133 gross with a 33% piping an 150 with a 50% piping. One could then work backwards to pick a larger boiler with a higher I=B=R to account for the larger amount of piping in a older home.