two pipe air vent boiler replacement

mikeInMA

Ran across a 1900's mansion with a two pipe air vent system and the original boiler still there. It has finally died and needs replacement. My question is how do you address the drastic reduction in water volume in a new boiler in a house with miles of piping. This boiler we believe fires at 680,000 BTU input. The connected load is just under 300 SQ feet of radiation. Also the first floor is severely under-radiated. I am wondering if they counted on the heat of the boiler radiating through the floor to compensate. I need to replace this thing and i want to be certain that i can or cannot use a return pump and tank. Also do I size this for the radiation available or do I need to be larger for lack of water volume?

Steamhead

Sure it didn't have indirect rads for the first floor? Look for steam lines going into sheet-metal enclosures in the basement.....................

KC_Jones

mikeInMA said:

Ran across a 1900's mansion with a two pipe air vent system and the original boiler still there. It has finally died and needs replacement. My question is how do you address the drastic reduction in water volume in a new boiler in a house with miles of piping. This boiler we believe fires at 680,000 BTU input. The connected load is just under 300 SQ feet of radiation. Also the first floor is severely under-radiated. I am wondering if they counted on the heat of the boiler radiating through the floor to compensate. I need to replace this thing and i want to be certain that i can or cannot use a return pump and tank. Also do I size this for the radiation available or do I need to be larger for lack of water volume?

Why are you talking about pumps if all you are worried about is water volume? See the attached from Weil Mclain boiler manual. Reservoir pipe to add water volume and not add a bunch of moving parts to a simple system.

Jamie Hall

If you are concerned about water volume, add a reservoir tank as @KC_Jones suggests. No moving parts. No fussy controls. No nightmares.

And as @Steamhead suggests, there may well have been indirect radiation on the first floor. It was very very common back in the day -- particularly in mansions and the like, as it meant there was no visible sign of heat. How fancy! There is even a good chance that some or all of it may still be there... and need to be taken into account in your sizing. It's not always obvious...

EBEBRATT-Ed

600,000 btu on the old boiler with only 300 square feet of raidiation? 300 x 240=72,000

Something is amiss

Jamie Hall

EBEBRATT-Ed said:

600,000 btu on the old boiler with only 300 square feet of raidiation? 300 x 240=72,000

Something is amiss

One of the reasons I'm wondering about indirect -- either still there and missed, or taken out at some point.

mikeInMA

the grilles are still in the floor but the coils are gone.

Jamie Hall

You do have a problem. Well, two problems.

First, and I have no way of knowing how important this is, is the combination of two of your comments: the first floor is under radiated, and someone removed the coils from the indirects. You are going to need to work with the client on this. Do you restore the indirects? Can be done, not really a big deal. Do you add new radiation? That can be done too, again, not really a big deal. Or are they happy with the way it is?

In any event, address that problem first.

Second, how big a boiler? And what to do about miles of piping? How big a boiler is easy -- like any other steam job, match the boiler to the installed radiation (and that is why you do the first problem first, since it may alter the amount of installed radiation). That's really not optional. The second aspect of that -- what to do about the miles of piping -- is also easy enough: you may well need a reservoir tank for that new boiler. This isn't difficult. It's simply a tank with a nice big surface area located at the water line of the boiler. It's equalized to the steam header and fed by your wet returns.

The theory is not always immediately obvious, because we tend to focus on the total water volume of the boiler (and, with a reservoir tank, the tank), but that's not correct. What we are really interested in his what volume of water can we convert into steam, send out into the system, and wait to come back all without changing the water level in the boiler too much. So it is the surface area at the water line that we want to make bigger -- not the total volume (although that doesn't hurt).

Example. Suppose that we have a very large system, and that we figure that the total volume of the system is around 1500 cubic feet which we are going to fill with steam. That's 1 cubic foot of water converted to scheme, approximately. Now when we boil that 1 cubic foot away, we only want the water level to drop 2 inches in the boiler. So... we need a surface area of about 800 square inches. A tank with a 30 inch diameter would be ample. Or about 28 inches square. Easier to fit in. But 1500 cubic feet of volume is a very very large system indeed! And even at that volume, there may well be enough surface area in the boiler to manage it.