I posted this in response to another thread but I thought I would start a new one as it was a little different than that thread.

I had a very scary experience yesterday. It seems like my Slant Fin Galaxy boiler continued to stay on past it's 190 degree setting. I heard these very loud "pops" coming from the expansion tank. I touched it and it was extremely hot so I immediately shut down the system. As soon as it shut down there was a tremendous explosive sound and the pressure relief valve went off sending tons of steam and some water into the boiler room setting off my fire alarm, etc. It was extremely scary and I was wondering why did shutting the system down result in such a release of steam? Wouldn't the release valve have worked prior to my shutting it down if the boiler was that hot? Anyway, my plumber immediately came over, replaced the Aquastat and put in a new release valve. Has anyone else ever experienced this? We now have the Aquastat set at 140 degrees as my plumber said with cast iron radiators, it does not need to be set that high. I'm still shaking as what if I was not home to "hear" what was going on?

 

Think of it like the flow of cooling water was shut off. The very hot iron in the boiler gave up its heat to the water and it flashed to steam. The pressure went up and th relief valve, did its job and opened up, thereby preventing a truely catastrophic explosion.


On a micro scale similar to what happened in Japan when the cooling water stopped flowing.


Peter

 

Peter: Thank you for the response. However, where did the additional water come from? Was the circulating pump keeping the boiler cool? At what point do you think the relief valve would have worked on it's own without my brilliant move of shutting down the system?

 

Scary isnt it ????


Usually close to the what the relief valve is set for. 30psi or I believe around 250F. You were probably close to one point regardless if you turned it off or not.

I guess technically only the gas should of been shut off so the burner will stop firing, but leave the circ/circs on and all zones open.

Thank god the relief valve was piped to the floor, right? You could advocate this now because you have seen it first hand. Like myself I have seen this happen. I could tell you it would literally melt your skin instantly if you were in a direct path of this discharge. Or cause death!!!!

Mike NJ

 

Mike: THANK YOU for the response!

You're 100 percent correct! Between the sound of the escaping steam and the initial bang, I thought it was going through the roof! The people who installed the boiler over 18 years ago were all ex-Con-Ed folks and they knew what they were doing! It was piped out of the top at a right angle and then another right angle down to the floor! It blew three vinyl floor tiles right off the floor! After reading about these type of things overnight, I'm lucky it wasn't worse! The valve was indeed set at 30psi (I've checked everything since it happened! ;-)) so I was hoping that its was going to go soon so as not to take the whole boiler with it! I have no idea what psi the boiler is rated for and I don't know how high the pressure was before I shut down the system but I'm assuming it was below the valve pressure setting.

 

You might want to rethink the 140 setting.

Is your system OIL or GAS fired?

If it's GAS, you DEFINITELY want to increase it. Probably not to 190, but certainly above where it is now.

If it's OIL, you PROBABLY want to increase it.

Here's why:

The flue gas products of combustion contain tremendous amounts of water vapor. If allowed to CONDENSE in the boiler and flue pipe / chimney, this vapor will combine with other byproducts of combustion to form a number of weak acids. Over time these acids will literally EAT anything they come into contact with.

If the water LEAVING the boiler is 140° by the time it circulates and comes back to the boiler, it is going to be at LEAST 20° cooler, and with cast iron radiators, probably even more than that. So the returning water is going to be 120° or LESS.

The temp at which this condensation occurs differs between gas and oil systems for various reasons... but for GAS systems it is in the neighborhood of 135° and for OIL between 115° and 120°. This is the DEW POINT of the gases.

So what happens when this cooler water comes back to the boiler is that it allows these flue gases to condense on any surface inside that happens to be cooled by the returning water, and cooler than the DEW POINT. And if these surfaces are not heated long enough to evaporate that condensation, corrosion occurs, and continues to occur every time the boiler fires up for a heat call. The end result after a long enough time (and sometimes not that long!) is destruction of the boiler, and in a shorter time, the flue pipe and chimney.

My suggestion if this is a gas system is to turn it back up to AT LEAST 160. If it's OIL, you MIGHT get away with 150.

This is ESPECIALLY SO because you have the large cast iron radiators and larger water volume in your system.

Personally, myself, I would turn it back up to 180 and leave it alone. If your house does not need that hot of water, the thermostat will shut it down before then anyway.

There's a good chance that your plumber does not realize this or understand it.

Google the term "FLUE GAS CONDENSATION" if you don't believe me or need more information.

 

Other than the aquastat replacement, did your plumber do any further investigation as to the cause of the problem?

He should also have examined the EXPANSION TANK for condition, and done some maintenance on it. Expansion tank troubles are the number one cause of relief valve spewing, although in this case it seems likely the aquastat did 'stick'.

Keep in mind that if your system seems to be functioning properly now at 140, if you turn it back up to 180 and the expansion tank is defective or water logged, you may AGAIN have an incident.

Words to the wise...

WATCH THE PRESSURE GAUGE over the next few weeks.

When the boiler is cool/cold, the pressure should be around 12-15 PSI.

When the boiler is HOT (say 180) you should see (usually) around 20, but never more than say 25 or so. The actual amount of pressure increase from cold to hot is dependent on the condition and size of the expansion tank.

 

It would be well for everyone to have a hi-hi-limit safety aquastat that cuts off fuel when the temp reaches, say, 190 deg F. The hi-hi aquastat should be independent of the aquastat used for control purposes, and it should require a manual reset after the switch opens.

My safety aquastat is a Honeywell Type L4006H, which is a strap-on model. I have it strapped on the main boiler supply pipe. It interupts the 24-V going to each of two gas valve. I have the regular gas valve, with pilot, in series with a plain solenoid gas valve. (Gas valves have been known to stick open.)

 

Mike:

Thanks again! I will raise it as you indicated. He thought that the efficient radiators that we have needed less temperature to still be effective. We have baseboard in the basement which he did say should have 190 degree water through it. I'll raise it to 160.

 

And many jurisdictions now require a hi hi limit control on all new installation for this exact reason. Something like this case it would have paid to have a redundant safety control!

 

Yes, he did examine the expansion tank first and it was fine. I have, as you can imagine, been examining the pressure gauge regularly! I will continue to watch it and see that all is well. As an aside, the gas valve/ electronic ignition is brand new having been installed late last year. He took everything apart to ensure each component was working properly. For some reason, the Aquastat (original) seemed to malfunction for some reason. THANK YOU ALL for the knowledge and recommendations!

 

In my opinion any operating control that is used to safely operate a boiler must have a low adjustable setting ,a high adjustable setting known as operating controls and a high non operating safety limit that has to be manually reset if it is activated built into one unit,so that the safety limit is always installed, anything less is unsafe.

 

I've grayed out a portion of the text in your quote that I don't think makes any sense.

Is the way I've edited your post correct? You didn't mean to say that there should be BOTH a LOW LIMIT AND A HIGH LIMIT, right?

While it is correct to call the control that performs the normal function of OPERATING a system the OPERATING CONTROL, to call a safety limit a 'non operating' control will lead to confusion. It makes it sound like you don't care if it works or not, doesn't it? For example, I have a non operating lawnmower...

I sort of agree with what you are saying in principle. I don't think it's really practical to include more functions in one control. Costs go up... etc...

 

"Built into one unit"? That worries me - there will be a single bulb for all the limits? I would prefer that the hi-hi safety aquastat be completely independent of the regular control aquastat. I think it is safer that way.

 

At first, I wondered if this event should actually be called an explosion. But, if an explosion is a sudden and uncontrolled expansion of gases, then superheated water flashing to steam would be an explosion.

Steam does some amazing things. A small steam leak from a power boiler can cut off your leg. I was taught to use a broom to locate a steam leak.

I once attended a thresherman's convention where steam from a steam-powered tractor or threshing machine was vented into a stock-watering trough filled with water and sweet corn. When the steam entered the water, the utter violence was something to behold. (By the way, I'm very nervous to be around those old steam powered tractors when they are at pressure.)

 

And rightly so. Those antique tractors and threshers can be very dangerous, as the boilers are made to have the capacity to power the machine at full work load, and when they ars just "idling",they are very overpowered, and watched carefully. Of course the safety devices should be up to parr, especially the safety steam pressure valve, much the same valve that saved this boiler from causing more severe damage, or injury. Sorry to sort of hijacking this thread.
Sid

 

I want to thank everyone who weighed in on this. I did raise the temperature to 160 and everything seems to be functioning perfectly. I put the word "explosion" in quotes because while technically it was not a true explosion, the initial sound and the sound of the steam releasing was extremely loud! My plumber is returning on Monday just to ensure that everything is going correctly. Thanks again. It's nice to know there are so many people willing to share their knowledge with others. Happy Holidays!

 

for the future a cold boiler sits at 12psi auto feed RED assembly..as should stat calls for heat and the house heats to set point max on the OFF cycle...should be 20PSI tops 25PSI depends on floors...and at that 160F range ...160F should make a stat setting on baseboard radient with no problem...you were making steam and the relief if not cycled in years might of been higher then30PSI standrd with heat and rusting on the seat so thats why he changed the 2....blow the relief mid winter and do the same on the HWH tank to flush them....

 

Smink, for the sake of clarity... can you please explain to the audience what is a

"12psi auto feed RED assembly"

Do you mean a "12 PSI pressure REDucing valve" ?

 

As I read this thread, I thought.. dang right, Im gonna install a high-high strap-on (mounted at the boiler output point) to control my TT terminal on the aquastat (currently strapped).. but maybe its better to control via the primary/oil valve etc.. or just put a relay on the boiler mains power. .. ?
The thought of a runaway boiler scares me.. our unit is on the same floor as our bedrooms.

 

yeah then the question is who leaves thier boiler feed open or closed????

I leave mine open.

I rather have a water fed boiler on runaway tripping the relief valve then a dry fired runaway...

There are pros and cons to both.

Mike NJ

 

I leave mind closed, so I can observe a leak in due time.. but you have a good point.
A low water situation wouldnt trip a thermal switch on the outlet , would it.. ?
I check in on my boiler about once a week.. could boil dry at any time..

Double-dang then.. now I need a thermal cut-off and a low-water cut-off !!

@the OP stated that the relief valve was "set to 30psi" but was it the original valve (18 yrs old ) ?

Didnt I read in here that we should be changing these out on some scheduled period ? (5 yrs ?)

 

Read this about the guy on the slab home and what happened to his boiler.

Feed Valves

Mike NJ

 

You may not agree with this solution but i have used a domestic water pump switch with the low water pressure of 9 lb as a low water cut off to shut burner off. This leaves system full of water but tell you that there is a water leak that needs to be attended to.

 

Ahhh heck, let's make it a triple play and add a low pressure switch too! Just in case the LWCO fails. I mean, there's now a backup for the high limit, why not a backup for the LWCO also?

Mike, the way you worded your post made it sound like the guy was... well... dead!

Although it was a slab home you meant, he could well have been dead, had he been home!

 

How did you do that? Water pump switches MAKE when the pressure drops. That's kinda backwards to what we want, ain't it? You want a switch that BREAKS on pressure drop, right?

 

Trooper:The switch is made by Furnas : gardall pressure switch cat.#69wec. it has6 psi. low pressure cut out fixed .Switch is on a t20psi. and off at 40 psi.

 

That's the part you didn't mention, the 6 PSI low pressure cut-out... not all well pump switches have that ya know.

Why not use the 69 WC model? That's ONLY the low pressure cutout...

http://www.hubbell-icd.com/icd/press...les/4269SA.pdf

Something to keep in mind about using a low pressure cut out:

This would NOT protect against a runaway boiler because even if the water became low, the PRESSURE would NOT because the boiler would still be firing. Even if you don't have WATER pressure, you would still have STEAM pressure.