Pressuretrol and low pressure gauge

bergensteamguy

@KC_Jones, just making sure we are not mixing up our wires... You are stating it should run less than half pound of pressure? Just wanted to be sure you were not thinking 6 pounds of pressure.. I agree! that would be a lot of pressure. I do understand steam slows down with more pressure. However, I did not think for a residential boiler that half pound of pressure was a lot.

Mad Dog_2

No worries the brass nipples are very strong 💪...just don't hang Dumbells off it!!!  I do it just like that all the time. The Black pipe IS 
MORE BETTERER but overkill   I did it because was MY boiler, made it in to the Burnham Calender and HVAC TV and Dano did a show on my brand new Vapor system with antique rads.  Mad Dog 🐕 

ethicalpaul

However, I did not think for a residential boiler that half pound of pressure was a lot.

It's a lot more than anyone needs, but it's a lot less than many see.

KC_Jones

bergensteamguy said:

@KC_Jones, just making sure we are not mixing up our wires... You are stating it should run less than half pound of pressure? Just wanted to be sure you were not thinking 6 pounds of pressure.. I agree! that would be a lot of pressure. I do understand steam slows down with more pressure. However, I did not think for a residential boiler that half pound of pressure was a lot.

Pressure is a function of boiler size to system size with venting mixed in. The pressutrol is a safety to prevent pressure from getting too high, it does not "make pressure" and it technically should not be viewed as a control. With a properly sized boiler, and proper venting, you should never hit the pressutrrol setting and run at very low pressure, less than what you are seeing.

@ChrisJ system exemplifies this by running at a few inches of water.

Roughly speaking, 1/2 lbs pressure is probably 5-10x what you need to heat the building effectively.

bergensteamguy

@KC_Jones the boiler is sized correctly that I am sure of. I went around and calculated the EDR for each radiator myself.. I would have to then assume perhaps i do not have enough venting on my mains. However, I did recently install 3 new vents. I tapped into the end of my main a 1/2 inch hole and piped two Gorton #2 vents there. I also piped in a Gorton #D in a separate 1/8 hole I tapped in my other main. My mains split into two. one is a 50ft 2inch run and the other around 5 ft 2inch pipe and reduces to a 4 ft at 1 1/2 inch. I am just wondering if I should have tapped a bigger hole on both and put on a reducer. Two questions.. one, I am assuming improper venting would cause higher than normal pressure? Two, would making the tapped holes bigger help, even though the size of the vent doesnt change. i.e. Gorton #2 is half inch threading. I can post pictures if needed.

Jamie Hall

Improper venting causing higher than desired pressure? Well... sort of.

Let's consider what happens when the boiler starts firing Until the water boils, nothing -- of course. But now when the water boils then what? Well, steam starts moving out into the mains. If there is no venting, or woefully inadequate venting, the pressure will rise -- slightly. But most of what happens is that the steam gets out into the mains -- and promptly condenses. As the mains heat, the steam is moving farther in the mains, and the pressure at the boiler will increase enough to make the steam move -- perhaps a few ounces in most two pipe systems. Meanwhile the air is escaping from the mains, either through main vents or crossover traps. When the steam does finally reach the main vents or crossover traps, they will close and now the steam is moving into the radiators, where it condenses.

What happens next depends on the relative size of the boiler vs. the radiation. If they are well matched, the pressure will not rise at all, or may rise very slowly. If the boiler is significantly too big, the pressure will rise much more rapidly and it may be necessary to tell it to shut off for a bit to let the radiation catch up, and in this instance the vapourstat is a control, not a safety.

But let's step back a bit. What happens if the main venting is too small? Instead of the pressure holding more or less constant at no more than a few ounces, it will continue to rise to try and force the last of the air out of the vents.

So... there is a very simple way to determine whether you main venting is adequate, assuming you have a low pressure gauge. Watch what happens as the system starts to make heat. If the pressure rises slightly, and then almost completely levels off, your main venting is adequate. If it never really levels off but continues to rise, your main venting needs to be increased.

This is tied in to what to set your vapourstat at for your specific system: watch the pressure again. When it starts to rise again (if it does) after a longish interval (may be quite long if the system is well matched, if it ever does), that's when you want your control vapourstat to shut off the burner to let the radiation catch up. Let's say that your plateau pressure is around 3 ounces. When the pressure starts to rise again, at perhaps 6 ounces (about twice the plateau pressure), the vapouostat shuts things off.

Note that the control vapourstat is not the same device as the safety vapourstat or pressuretrol. Different functions, two different pieces of equipment. The safety device is there to protect the bits and pieces -- and should, for residential systems, never be over a 2 psi cutout, but needn't be much less.

bergensteamguy

@Jamie Hall , this is very insightful. Thank you