Understanding venting capacity

sobriquet

So after reading "balancing steam systems using a vent capacity chart" about 4 times, I went down to the basement to take some measurements. After doing some calculations I'm a bit confused, I must be doing something wrong. I have a 40 ft main that's 2" diameter. A 2" pipe has .023 cubic fee of air in it, so at 40', I have 0.92 cubic feet of air in that pipe. If I wanted to be able to vent that in 3 minutes, I divide 0.92 by 3 to get 0.306 cubic feet. Next I find which vent or vents can handle 0.306 cubic feet per minute in the chart. I have to be understanding this wrong, I've read and have been told I need a Gorton #2 for every 20 feet of 2" pipe and because of that I have 2 #2s on each of my 40 ft mains, which seems right to me but is some serious overkill for what I've calculated. A Gorton #2 at 2oz of pressure from the boiler can vent 1.75 cfm, which is like 5 or 6 times what I need?? I'd like to really understand this so I can accurately vent my mains and radiators.

BobC

Steam will fill pipes at VERY low pressure, my header fills with steam long before my 0-3PSI gauge needle even begins to quiver.

You want that air out of the pipes FAST, the question is how fast and how many vents (they are not cheap).

Bob

Jamie Hall

Right you are! The general rule of thumb around here is, in my not so humble opinion, definitely overkill. It is, as @BobC says, a balance between how fast do you really need those pipes to fill and how much money do you want to spend on vents...

To give you an idea of another point of view, I have roughly 180 feet of 2 1/2 inch and 3 inch steam main, which vent (there are three mains) through 3 crossover traps into the dry returns which, in turn, vent through... one Gorton #2 and one Hoffman #76. And it works just fine, thank you.

nicholas bonham-carter

Jaimie, I would imagine in your two-pipe system with working crossover traps, that you could do without any vents what so ever, with some improvement, because in the world of one pipe, you are woefully under vented.
The two pipe system is naturally more balanced, because it has traps, and no radiator vents.
In a one pipe system, we are trying to balance the resistance of escaping air from main vents, with that of the total of the radiator vents.
If you have no main vents, then the air, will escape from all the radiator vents with the closest radiators heating first, followed by the furthest radiators, and this especially in the shoulder season will cause discomfort in the building. As soon as the thermostat location gets steam, the boiler will shut down. Someone in a room further from the boiler may be cold as a result.
For even heating, you need a low resistance in the main vents, and a higher, but equal resistance in the radiators, so the mains fill before steam begins to rise up through to the radiators.
This can be seen as back-pressure on a gauge capable of measuring ounces, during the venting phase.
Therefore, think not of time, but of back-pressure, and keep adding Gortons until that value in ounces is lowest.--NBC

sobriquet

Do the mains need to be balanced between each other? I have a main running about 12 feet from the boiler, then splits into three. 2 of these lines go straight down the length of the house, about 25 feet, and end in 2 gorton #2s each (4 total), the third pipe running from these mains goes straight back to the boiler room and has to risers. This pipe is about 24 feet total, 12 feet away from the boiler and 12 feet going back towards it. Should I install a single #2 here or will a #1 suffice? The other main coming off the boiler is 1 1/2" pipe coming off about 12 feet, turning around and going straight to 2 radiators, this does not have any main venting. Does it need it? Or because of how short it is, can the radiator handle the air? If mains are vented at different rates, will that cause uneven heating? Or should I just be sure to overkill everything

ChrisJ

sobriquet said:

Do the mains need to be balanced between each other? I have a main running about 12 feet from the boiler, then splits into three. 2 of these lines go straight down the length of the house, about 25 feet, and end in 2 gorton #2s each (4 total), the third pipe running from these mains goes straight back to the boiler room and has to risers. This pipe is about 24 feet total, 12 feet away from the boiler and 12 feet going back towards it. Should I install a single #2 here or will a #1 suffice? The other main coming off the boiler is 1 1/2" pipe coming off about 12 feet, turning around and going straight to 2 radiators, this does not have any main venting. Does it need it? Or because of how short it is, can the radiator handle the air? If mains are vented at different rates, will that cause uneven heating? Or should I just be sure to overkill everything

If you get steam to the end of one main faster than the other, what do you think will happen?

Try to picture it in your head and think about it.

sobriquet

Well I'd assume that the radiators fed from that main would heat faster than the others and that id have to balance that at the individual radiator vents. But being that that's the obvious answer, I'm doubtful of it

ChrisJ

sobriquet said:

Well I'd assume that the radiators fed from that main would heat faster than the others and that id have to balance that at the individual radiator vents. But being that that's the obvious answer, I'm doubtful of it

You're pretty close, but I would try to get the mains as close as you can by tweaking their vents.

In my home, I ultimately went by how each room felt and it took a few years to fine tune everything. I want some radiators heating sooner and more than others just to keep the spaces warmer, or, some spaces have less insulation, worse windows etc. Another example is I like a warm bathroom, but want my bedrooms on the cool side. I have TRVs in my bedrooms to keep their temperatures quite a bit lower than everything else.

The objective is to keep the whole house the temperature(s) you want. If having one main faster seems to do that, then you go that route.

sobriquet

I'll try to explain better, hatterasguy. Two mains comes off the boiler, once out of the boiler room, one main splits into two, one pipe goes left, one goes right. On the left branch there is another main branch (I guess it's a main, it's only 1 1/2") tee'd in about a foot from the first split. This pipe does a 180 and heads back in the direction of the boiler room. a few feet after that, a vertical riser is connected, going to a second floor radiator, a first floor radiator is also connected a few feet after where the vertical riser is. This pipe, from where it branches off the main from the boiler, has no venting aside from the 2 radiators. And the first floor radiator connected there is always, always hot. I can rough out of sketch if need be, I have a sketch of my mains in an earlier post, but it's kind of hard to see everything clearly

sobriquet

Okay so it is just a runout. One more thing, is my boiler oversized? 2500 sq ft house in nj, 225k btu boiler. I figure with about 50 btu per square foot required, 2500*50=125000, then 125000*1.33 (the number I've seen used for boiler rating)=166250. Taking the 225000*0.82 for output=184000 btu. Close enough?

KC_Jones

Doesn't work that way on steam. You size by the radiation. Add up the EDR of all the rads and size according to the sq ft of steam. I will also say if you need 50 BTU's per sq ft you might try closing the windows to cut that down. The only way to determine btu per sq ft is to do a heat loss calculation of the building.

vaporvac

You need to compare the Sq Ft of Steam rating from your boiler front plate to the total EDR of your rads. They should be close. Don't compare the Btus. Let us know if you need help with the edr.

ChrisJ

KC_Jones said:

Doesn't work that way on steam. You size by the radiation. Add up the EDR of all the rads and size according to the sq ft of steam. I will also say if you need 50 BTU's per sq ft you might try closing the windows to cut that down. The only way to determine btu per sq ft is to do a heat loss calculation of the building.

I came up with needing around 43btu per sqft in my house @ -8F.

Yeah, I can't imagine someone needing more than me. That'd be real bad.

sobriquet

Well I think I know how to determine edr for standard tube or column type radiators, I've used this site in the past - http://www.columbiaheatingsupply.com/page_images/Sizing Cast Iron Radiator Heating Capacity Guide.pdf.

However, 6 of my radiators are baseboard style, but unlike others I've seen. They're literally 2" pipe with circular fins down the length of them, about 6'. I can post a picture if need be. So should I still use the standard 3.4 edr per linear ft?

Jamie Hall

You can always calculate it... get the area of each fin (the area of the circle less the area of the whole for the pipe and double that (two sides!!)). Count the number of fins per foot. Also get the area of the surface of the 2" pipe per foot (circumference times one foot) (all in square feet!!!!!). Add the two, and that's the EDR per foot of fin tube.

Mark N

Sobriquet the steam that is pushing the air out the vents is also being condensed back into water by the pipes. The colder the pipes the more steam they condense. Uninsulated pipes kill a lot of steam and carry much more condensate then they were designed to carry. Are your mains insulated? Also I suggest you do a search on the Main Site here at Heatinghelp for "It's All in the Venting". Give the article a read.

sobriquet

Jamie I'll try to figure out those baseboards, looks like 1.5" pipe, with the fins 4" total diameter, 33 fins per foot.

Mark, I had the asbestos removed from the mains when I bought the house, but immediately insulated with fiberglass. Everything in the basement, mains and radiator take offs, are insulated. I'll check out that article, thank you.