Banging boiler has me stumped

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Old 10-30-09, 07:20 AM
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Banging boiler has me stumped

Purchased a home with an old hot water boiler (50 years?) that I cannot get to stop banging when it nears maximum operating temperature (about 180 degrees). It is a one-pipe system.

I have a weil-mclain G-7 natural gas boiler with two zones. Zone 1 is cast iron baseboard (the original) and Zone 2 is copper-finned tube. Home is single storey. Each zone has its own circulator pump on the return side of boiler. The fresh water supply (3/4") has a new backflow valve and feeds boiler ata point just above above the circulator pumps. Water supply has a combination B&G reducing valve / relief valve (both replaced this year due to sediment and wear).

Zone 1 and 2 are fed by a tee at the supply side of boiler.

Zone 1 has a "Thrush" flow control valve above the supply tee. A Fill-Trol expansion tank is connected by 1/2 inch pipe on a take-off from the center of Thrush valve. Valve appears to function properly. Zone 1 has an Auto-vent #67 at the return side above the pump and water supply. The circulator pump for Zone 1 has a control at the tee (set to 110 degrees).

Zone 2 has a B&G flo-control valve on the supply tee. Zone 2 has an Auto-vent #67 above the flo-control valve. The circulator pump for zone 2 is controlled by call for heat from a separate room thermostat.

An Aquastat control is surface mounted just above flow control valve on Zone 1. It is set to 160 degrees. The high limit control is in well on the boiler and set to 180 degrees.
Boiler fires when temperature drops below the aquastat setting and cuts off when the high limit is met. Have adjusted so that temperature in boiler does not exceed 190 (due to heat rise after firing).

I drained the system in order to repair a leak in a supply to a radiator and to replace the defective pressure-reducing / relief valve assembly. Also replaced the backflow valve on water supply. All pumps and valves appear to operate normally.

I refilled the system and bled all air from radiators. Added "Boiler-Colloid" with the fill and ran system for 2 weeks, then flushed and filled again (with added Boiler-Colloid). Got some same pieces of hard black debris in water discharged when system was flushed. I flushed, filled, ran, and flushed/filled again till specks were mostly eliminated. System has boiler colloid in water.

The expansion tank has been removed, drained and charged to 12 psi -- per specs. The tank is hotter on bottom and cooler on top when the system is operating.

The exterior of the boiler fins were scrapped and cleaned with wire brushes as summer maintenance.

There is no air detectable in the system. Boiler water is "clean". But, when the boiler fires, it makes short, sharp clanking noises with a corresponding jump in the needle on the pressure guage (from say 14 to 18 psi). This symptom persists despite adjustments of high/low limits. Expansion tank appears to be working normally. I do not hear water "boiling", but boiler still "kettles".

I am stumped. Do I need a new boiler or is there a cure.
 
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Old 10-30-09, 09:10 AM
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what's the water pressure in the boiler?
maybe it is really boiling bcs. pressure too low, or not enough water in the boiler
 
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Old 10-30-09, 02:17 PM
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The pressure is between 14 and 16 psi. I have filled boiler by lifting handle on pressure regulator till the relief valve gives and have bled all air at nearest radiator (and down the line as necessary). Is there a way to determine if boiler is full?
 
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Old 10-30-09, 02:36 PM
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15 psi is correct, and if no air, the next question, is :
is the circulator pump sending enough water thru the boiler?

Slow water flow would mean the water stays too long and gets too hot.
 
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Old 10-30-09, 03:54 PM
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could you post some pictures of your boiler,to show how the piping is set up? I had a very similiar problem with my boiler, which is a weil-mcclain CGA4, and it actually happened that the pump was pumping the wrong way, among other minor things that needed to be switched around, such as the gravity feed valve, and the zone valves.
 
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Old 10-31-09, 04:29 AM
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The circulator pumps are working fine, they flow towards the boiler return, thereby opening the flow control valves on the supply side when pumping, The overall system setup hasn't been changed in over 15 years according to former owner. This is a new problem that followed having to drain and refill in order to replace a water supply regulator and repair a radiator leak. The banging occurs after the circulator pump stops (the pump for each zone runs on room thermostat call for heat). The banging occurs after pump stops while the boiler is firing to recover its maximum temperature.
 
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Old 10-31-09, 06:55 AM
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Thanks for the advice. I might have figured it out. I used a highly accurate digital thermometer to takes readings at aquastat and in the well of the high limit switch in the boiler (after it stopped firing and the temperature gauge stabilzed). It appears that the boiler's temperature/pressure guage is not properly calibrated, reading significantly lower temps than actual, plus the temperature markings on the aquastat and high limit switch are "approximations" (as I presumed they might be). Made adjustments to the limits and got the boiler temp down to 190 max. after firing, and almost all banging is gone. Will try lowering by increments to 180 degrees. It must have been boiling the water after all. Maybe I'll install a new temperature guage.

By the way, just about every useful bit of advice about boilers I've gained from this site -- there ain't much else out there except generic explanations of how things work, or are supposed to, anyway.
 
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Old 10-31-09, 08:12 AM
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The banging occurs after pump stops while the boiler is firing to recover its maximum temperature.
I'm a little confuzzled over this statement...

You get a heat call, it satisfies, and the circ stops but the boiler continues to fire up to HIGH LIMIT? That's weird...

Boiler fires when temperature drops below the aquastat setting and cuts off when the high limit is met. Have adjusted so that temperature in boiler does not exceed 190 (due to heat rise after firing).
Is this saying that the boiler is staying hot 24/7 during the heating season? Why?

I bet yer fuel bills are gonna be killer.
 
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Old 10-31-09, 09:09 AM
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Originally Posted by NoseyJ View Post
The banging occurs after pump stops while the boiler is firing to recover its maximum temperature.
I to am a bit confused about this comment. I am no boiler expert but have worked on and installed a couple of them. One was an old unit, 45+ years old and another was brand new when installed 3 years ago. On both those units the circulator pump NEVER shut-off until the t-stat temp was achieved. As I understood the boiler tech I talked to, some boilers can actually be ruined if the circ pump stops but the boiler continues to heat the water.

This makes me wonder if the circ pump is too large for the system and it moves water faster than the boiler can heat it. Again I have never seen a boiler that couldn't heat the water faster than the pump could circulate it but I am far from an expert on the matter.
 
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Old 11-01-09, 05:57 PM
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NoseyJ Thanks for the advice. I might have figured it out. I used a highly accurate digital thermometer to takes readings at aquastat and in the well of the high limit switch in the boiler (after it stopped firing and the temperature gauge stabilzed). It appears that the boiler's temperature/pressure guage is not properly calibrated, reading significantly lower temps than actual, plus the temperature markings on the aquastat and high limit switch are "approximations" (as I presumed they might be). Made adjustments to the limits and got the boiler temp down to 190 max. after firing, and almost all banging is gone. Will try lowering by increments to 180 degrees. It must have been boiling the water after all. Maybe I'll install a new temperature guage.
If it is of any help to you, I have my high limit set to roughly 170F. It seems to work very well, set at that temp.
 
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