unsafe aquastat settings

SteveB

I have had a few instances of customers calling technical support for their boiler manufacturers regarding the correct setting for the aquastat, especially during our recent subzero temperatures. In one case they were told to set the aquastat between 200 and 220 degrees. In the other cases, they were told to set the aquastat to 180 low and 200 high. The second case is fine IF the control is accurate. In no case was anyone told to make sure the control was accurate. Maybe they have forgotten that water boils at 212 degrees or don't realize that most aquastats do not respond accurately to boiler water temperature. I was at a house today that was set at 220 degrees, and the control overrode by 30 degrees. The customer was complaining of air in his system, steam is not air. Thankfully the pressure reducing valve worked to keep the system under pressure.
Come on people, smarten up. The little stickers you put on your boilers refering service to "qualified personnel" are there for a reason.

imatellerslie

Point taken, but

water boils at 212 F at atmospheric pressure. Raise the pressure, and the boiling point goes up. I've worked with systems where water at 500F was still liquid.

By the way, how did the pressure reducing valve control pressure? No backflow preventer? Heating system water getting into potable?

SteveB

you did miss my point

residential heating systems are not designed to work at temperatures above 200 degrees. If they were, we wouldn't be using PEX tubing on them which is rated for 180 degrees. Further the gaskets that seal the sections together and the coil plates and the circulator flanges would fail (and they do) inside of one year. My point on the pressure reducing vale (AKA feed valve? but I always thought they were on steam boilers)was that that was all that was keeping the boiler from turning into a rocket.

imatellerslie

My understanding is that the feed valve only

allows flow into the boiler. It's not designed to prevent an overpressure condition. That would be the function of the relief valve. In fact, if everything is working correctly, the pressure reducing valve can't keep pressure from rising in the boiler due to an overtemperature condition.

I did get your point, but I wanted to clarify two points: saturation temp/pressure, and the reducing valve function.

Ray Landry

If memory serves me water boiles at roughly 250 degrees @18lbs of pressure... I've found systems to become airbound between cycles, because the water temps stack due to the circulator being off...

The only thing that keeps pressure in check is the expansion tank... The PRV is only there to keep high pressure potable water from blasting into the boiler IMO

SteveB

overtemperature boilers

I had a situation this winter when an inexperienced tech turned up an aquastat to 180-200, and didn't verify the results. When I got to the job the boiler was 260 degrees and spitting steam out the hy-vents. Wouldn't the 14 gallons of water instantly become 3400 cubic feet of steam if suddenly the pressure went away? And if so would the relief handle it?

imatellerslie

I didn't mean to raise your blood pressure...

I was simply trying to point out a couple of things. I'm not in any way advocating the operation of a boiler at temperatures higher than design.

As far as your hypothesis of the water suddenly going to steam if the "pressure suddenly went away", some of the water would flash to steam. That 14 gallons of water at 260 F does not contain enough energy to vaporize completely. When the pressure dropped, vapor would form. The formation of that vapor would remove energy from the liquid water, lowering its temperature, until the temperature was below the boiling point for the prevailing pressure. I'm not sure how much would vaporize before the water got to 212 F, but probably not very much in terms of mass.

Ray Landry

To figure out when water will flash to steam, take 14 x sq root of the pressure in the system, plus 198. Ex)14 x 4.3 (roughly the sq root of 18) plus 198 is 258.2 degrees. so in a boiler with 18 lbs of pressure, you won't get steam anywhere near 212 degrees... The situation with the steam spitting out the hy- vents.. could have been a boiler that was running at low pressure? Maybe the auto feed was clogged and pressure was at say, 5 pounds. steam will form at 230 degrees then. Especially if the circ stops running and the temps stack insite the boiler.

One drop of water expands 700 times when it is turned to steam. It would take ALOT of heat to steam 14 gallons of water. IMO, water falshing to steam is prone to happen more one jobs that aren't pumping away, because of the lower system pressures more steam is prone to occuring.

Unknown

At 260 degrees and no pressure, suddenly....

ALL of it would turn to steam instantly. It is called a steam explosion. It will move walls and floors (and then roofs).

Read up on it. It's messy. It's related to, but not quite as bad as, a B.L.E.V.E.

Noel

Earthfire

Need a picture

Does anybody have a link to that waterheater explosion picture that was post here a few months ago. That was just a electric hot waterheater not a boiler that took the house apart.

imatellerslie

I'm sorry Noel, but you're

wrong. The water will not all flash to steam. There simply isn't enough energy in the water to overcome the intermolecular bonds and turn it all to steam.

imatellerslie

Let's do a \"thought experiment\"

The idea that water above the boiling point for atmospheric pressure (212F) will all flash to steam if the pressure drops to atmospheric is a common misconception. I'm not trying to be rude in pointing that misconception out; i'm just trying to eradicate it.

Imagine you have a lb of water at 260F, and its pressure is just high enough to keep it from boiling. The water would have to be at a gage pressure of 20psi (35 psi absolute pressure). Now, drop that pressure to atmospheric. This might happen because the boiler fails somehow. If the relief valve opens, it will not immediately drop the pressure to atmospheric, because the steam that begins to form will act to keep pressure in the boiler, until the temperature of the liquid water in the boiler is at 212, at which point steam stops forming.

So the boiler "explodes" or somehow, the water is exposed to atmospheric pressure instantaneously. Will all the water flash to steam? No. Here's why:

It takes about 48 BTU/lb to raise the water temperature from 212 to 260 F. It would have taken 970 BTU/lb to turn it to vapor at atmospheric pressure. So, the water at 260 F comes up 922 BTU/lb short of having enough energy to go to steam.

Take a little time and look at some steam tables. In writing this response, I looked at www.energysolutionscenter.org/TechProDemo/ MoreDetail/SteamTables.pdf

Unknown

OK

Let's do this experiment. I've been there when a drain valve broke off on a 12 PSI low water cut off on a 4 million btuh boiler.

What do you suggest the conditions in the room will be like?

OSHA's site used to have photos. I haven't checked in a while, though.

Noel

imatellerslie

I never said it isn't ugly...

and I never advocated operating any equipment at higher pressure than designed. I'm just trying to upgrade your knowledge on some fundamental principles about stuff you work on every day that you don't seem to understand. I won't say ANYTHING more, because I wasn't trying to get into an argument. Physics is. Period. You may choose to understand it, or not. It makes no difference to me, really.

Unknown

Thanks for the tune up.

I'm glad you think it's no big deal. I've seen a few steam systems in my time. Obviously, the water won't ALL turn to steam.

You make light of a dangerous situation on a site where people seek practical knowledge. I do enjoy you keeping us on our toes. Physics is, you are SO right. From your knowledge demonstrated here, you are a student. You need more time around actual steam systems. Your knowledge seems theoretical, to me. Are you here to share what you know about boilers, or get someone killed with your theories?

Been there.

8 on the sphincter scale, but there was some water left in the boiler, after.

I only would like to bring this to your attention, because as I was studying physics, my sweet little feet were beating it for the street. I couldn't see. It burned me.

But there WAS still some water in the boiler.

Noel

Unknown

Watts Regulator Company has

an interesting video caled "Danger Lurks". It has visual examples of explosions caused by super heating water vessels and pressure being devloped and then a water hammer causes pipe to break and "BOOM" a water heater lands 200 hundred to 300 yards away.

I in my short career have seen a gas water heater which had a failed relief valve (some one plugged it because it was dripping). It exploded due to temp/pressure and went from the basement through three floors out the roof and landed two houses away in the street. Not a pretty site.

I had a boiler explode due to temp and pressure and buried cast iron a half inch into the concrete wall. Temperature and pressure not a happy mix. I am not an expert but I believe that steam occupys 1700 times as much space as water. So when superheated and exposed to atmosphere it rapidly expands and has no where to go so it becomes a rocket. I do not know about all that mathmatical stuff just what I have seen in my 43 years of working on this stuff. I have many more tales of events relative to Pressure and Temperature.