How do you prevent thermal shock to a boiler?

Heaterton

How do you know when the boiler is cool enough to add cold city water?

If the boiler is hot, can you crack the valve to slowly feed water in?

Just curious.. thanks... any tips are appreciated.

Zman

If you have a hot water boiler, you should be OK adding water slowly if the circulator is running.

Heaterton

Zman said:

If you have a hot water boiler, you should be OK adding water slowly if the circulator is running.

Ok thank you. What about when the boiler was recently drained to service something, and it's still hot? Same applies there perhaps?

Steamhead

Heaterton said:

Zman said:

If you have a hot water boiler, you should be OK adding water slowly if the circulator is running.

Ok thank you. What about when the boiler was recently drained to service something, and it's still hot? Same applies there perhaps?

NO! Let it cool down to the point where you can touch the pipes connecting to the boiler. Then feed it slowly.

Is this a steam or hot-water system?

Heaterton

Steamhead said:

Heaterton said:

Zman said:

If you have a hot water boiler, you should be OK adding water slowly if the circulator is running.

Ok thank you. What about when the boiler was recently drained to service something, and it's still hot? Same applies there perhaps?

NO! Let it cool down to the point where you can touch the pipes connecting to the boiler. Then feed it slowly.

Is this a steam or hot-water system?

Just in general. I'm learning the trade. This can theoretically take hours. What does a Tech do while waiting for it to cool? Perhaps do other calls then return if close by?

(I'm starting as an apprentice soon once I graduate from my HVAC classes. Also just got the "Pumping Away" book delivered tonight.)

Steamhead

Plan your work accordingly. Do the part where you have to drain the boiler first. Then go and do something else on that job, and come back to the boiler.

Or, have the customer shut the boiler down a couple hours before you come.

Heaterton

Steamhead said:

Plan your work accordingly. Do the part where you have to drain the boiler first. Then go and do something else on that job, and come back to the boiler.

Or, have the customer shut the boiler down a couple hours before you come.

Thank you Steamhead.

EBEBRATT-Ed

With a hot water boiler if it's hot when you drain it AND you have to drain the system or zone and the work doesn't take a long time (just changing a gauge or something) you should wait to fill the boiler until you can hold your hand on it.

If the system has valves and you can isolate the system or zone and keep it full of hot water while you drain the boiler then when the work is done let the hot zone water into the boiler and feed cold water slowly it will mix with the hot which will warm up the cold water. Run the circulator

It also depends on what you call hot. 130, 140 degrees is probably not an issue if you feed the boiler really slow with cold water. 190-200 degrees is another story

Heaterton

EBEBRATT-Ed said:

With a hot water boiler if it's hot when you drain it AND you have to drain the system or zone and the work doesn't take a long time (just changing a gauge or something) you should wait to fill the boiler until you can hold your hand on it.

If the system has valves and you can isolate the system or zone and keep it full of hot water while you drain the boiler then when the work is done let the hot zone water into the boiler and feed cold water slowly it will mix with the hot which will warm up the cold water. Run the circulator

It also depends on what you call hot. 130, 140 degrees is probably not an issue if you feed the boiler really slow with cold water. 190-200 degrees is another story

This makes sense. Thank you, I will remember this.

pecmsg

1st
Why are you draining it?
a cast iron boiler even at 180* empty will cool down fairly quickly so 30 - 60 minutes?
If at 140* when shut down I wouldn’t have a problem slowly adding make up water. 

Heaterton

pecmsg said:

1st
Why are you draining it?
a cast iron boiler even at 180* empty will cool down fairly quickly so 30 - 60 minutes?
If at 140* when shut down I wouldn’t have a problem slowly adding make up water. 

Not draining anything - I'm just asking in general.

I do not want to crack a boiler and I'm curious in how the experts combat this.

From the helpful information in this thread I can say the most important factor here is simply time - I must plan ahead.

If on call, hopefully I can isolate the zones and then flood the boiler with hot water from those zones and then slowly add cold water into the system to bring it up to temp.

HVACNUT

Most times, if your replacing parts like a tridicator gauge, air eliminator, extrol tank, relief valve, circulator..., you don't even need to drain the boiler. Just isolate whatever you can, prep your new part(s), drop the pressure to zero, (just drain enough water until the water stops flowing,  and close the drain valve before air gets sucked up the hose) and let her rip.
On most residential boilers, your not gonna lose more than 3 or 4 gallons so the cold make up water shouldn't be a concern. 

delta T

Agree with @HVACNUT, I rarely have to drain a whole boiler down. Ususally it is possible to isolate and only drain a part of the system that I need to work on.

EBEBRATT-Ed

Work fast but use common sense don't get burned

Heaterton

Thanks guys for the tips, very helpful - working quickly but using common sense...