Boiler knocking

[speedoo51]

Utica SFH4150WT oil fired base board..New Honeywell aquastat L7224U..High Limit set 180° with 10° differential...When a zone calls for heat furnace turns on and runs till the 180 limit..At about 175 it starts to make a knocking noise rising in sound and speed and goes on for about 30 seconds after high limit shut off then ceases..This started last year prior to new aquastat.. Noticed the boiler water temp will rise to about 190 in a few minutes and drop as zone uses the water eventually to the 170 start up.. I searched here and gleaned what I could..Went and checked expansion tank for heat and is hot only in top 1/4..Boiler pressure will go from 15 psi to 20 at max temp and then drop down as boiler cools..Oil Service says getting too hot and is boiling [like in an area]? Say to reduce temperature? I did reduce high limit to 175 and seemed to help some, as far as how loud the knocks are..Sorry for long post, tried to give as much info as I could..Thanks, Seb..

 

[dogdog]

give it a good flush as an annual maintenance or run the boiler at lower temp?

think there is a post here about skimming it because of oil in the system?

If you have a hydronic probably needed to clean those dirty water trap in the piping as well

might be problematic for some system with air trapped in the pipes if you drain them but you'll probably find a trick that works out for you next season......

just google skimming boiler maybe?

I flush mine (steam system) every year before and after the heating season.....

 

[dogdog]

that and also compare the actual temp of the water.... maybe you have some dirty sensor, your 175F is actual more than 212F... on my steam boiler the Pressuretrol tube have clog few times and the pressure never get regulated till the backup manual pressuretrol tripped. When inspecting the problem found the slug have clogged that tube, and many other sensors (LWCO, water guage, pressure guage etc).... That was the year I skipped maintenance. YMMV.

 

[speedoo51]

that and also compare the actual temp of the water.... maybe you have some dirty sensor, your 175F is actual more than 212F... on my steam boiler the Pressuretrol tube have clog few times and the pressure never get regulated till the backup manual pressuretrol tripped. When inspecting the problem found the slug have clogged that tube, and many other sensors (LWCO, water guage, pressure guage etc).... That was the year I skipped maintenance. YMMV.

I have checked with an infrared temp gauge [at top of boiler near out pipe] and matches what the digital read out on the aquastat says..I noticed the temp probe didn't have any type of heat transfer compound, don't know that it should...Thanks, seb..

 

[roscoe2000]

Knocking or water hammer is when hot water hit a cold section of piping with the most likely culprit is air in the system. You will need to go each baseboard unit and bleed any entrapped air out of the system. Starting with the lower units first and make your way up to the highest unit.

 

[BD1]

Yea, sounds like air to me too. What type of radiation do you have ? Old cast iron radiators with a coin vent ? Low ,modern cast iron about 8'' tall with coin vent ? Copper fin tube baseboard ? The boiler temperature should match the type of radiation, usually about 165 for cast iron and 200 for copper fin tube.
Some cases they'll run a higher water temperature to compensate for not enough linear footage of heating element.
If it is old style radiators , feel the radiator, it should be hot all the way up to the top. If not, it has air and need to be bled .

 

[speedoo51]

"Copper fin tube baseboard" I made the bleeders in the zones by putting a "T" in the line(s) with a vertical 4" long tube and manual bleeder at end [top]..The tube catches all the air, works very well..

 

[stingray1966]

Just had this with my hot water baseboard
Tied to blead system in the end it was the circulator pump burned out

 

[zmaxmotorsports]

give it a good flush as an annual maintenance or run the boiler at lower temp?

think there is a post here about skimming it because of oil in the system?

If you have a hydronic probably needed to clean those dirty water trap in the piping as well

might be problematic for some system with air trapped in the pipes if you drain them but you'll probably find a trick that works out for you next season......

just google skimming boiler maybe?

I flush mine (steam system) every year before and after the heating season.....

 

[speedoo51]

Just had this with my hot water baseboard
Tied to blead system in the end it was the circulator pump burned out

I am beginning to suspect the circulator pump not up to par and water flow is too slow but don't know how to check flow, pump is probably older than 30 years, maybe impeller eroded/worn?..I've posted on heatinghelp.com and I'll post if find a check to do, fellows there being helpful you guys are here..Thanks, Seb..

 

[speedoo51]

I checked expansion tank for air pressure and it was at 15 psi..No evidence of water at the valve..Any way to check circulating pump for flow with out removing it? If I drop the high limit to 170° is that still hot enough? Seb..