Boiler Kettling Revisited...

[KenLockett]

Hi All. I am experiencing once again a phenomena I experienced last season that I call 'kettling' which I believe is caused by the water near the fire tubes flashing and causes the so called kettling sound in the boiler and the heating zone copper piping to pulse. I tried a number of things last season to eliminate the sounds based upon suggestions from others on the forum. Ultimately my thought was that I was using wood that was too dry as I had a batch of kiln dried hardwood I was burning. I started burning some higher MC fuel and the sounds stopped. Well a year later the sounds are happening again and I am using wood with a MC of around 15% per my meter. I have posted a you tube video of the sounds and was wondering if anyone had any further input especially you guys with the Tarm gassifiers. I actually heard a couple of rumbles earlier that really made my heart race. When I shut the draft fan off the kettling stops. The you tube link is below. Any help much appreciated. The original post I believe is archived under 'Boiler Kettling' as well.

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[huffdawg]

I've had that too. I thought it was air finding its way through . It was in the first few weeks after I fired up my system . I haven't heard it since.

 

[NE WOOD BURNER]

Im no expert,but sounds like you have a sneaker in the dryer. I would check the draft fan if it where me.

 

[hobbyheater]

Hi All. I am experiencing once again a phenomena I experienced last season that I call 'kettling'

I get a sound very similar , and the source is the one way check valve opening and closing .

 

[dogwood]

Ken, this is a link to your earlier thread to make it easier for others to find. https://www.hearth.com/talk/threads/tarm-solo-plus-40-kettling.80430/#post-1038512.

 

[Fred61]

I'm no expert but that doesn't sound like kettling to me. I've heard kettling twice and it's something I don't care to hear again. Kettling would be making steam in the pipes. To me, it sounds like air. Do you have an air eliminator above the boiler? I know this is something no one wants to do but, how about opening your T/P valve for a quick shot while it is occuring to see if anything changes.

 

[KenLockett]

Why is the one way check valve opening and closing? Is it because of the flashing of the water to steam near the very hot fire tubes thus causing a pulsing of the flow through the system. Indeed it sounds like the primary source of the sound is near the 'swing' check valve at the oil boiler supply header. Purpose of this valve is to prevent ghost flow back into the oil side. Still, my bigger concern is the flashing and the pulsing in the boiler itself. As I assumed the unit is being over-fired I have increased the secondary air flow per the instruction from the Tarm manual for very dry wood. I honestly didn't think the wood was that dry.

Hobbyheater, do you just live with this all the time? Does anyone have a solution assuming it is the check valve opening and closing. I tried increasing flow on the circ pump (in addition to increasing secondary air flow and primary flow via fan damper is almost throttled completely down) and that did not help either. I'm sure throwing some wet wood in there would slow the hot burn down and reduce the flashing but why should I have to do this?

By the way, the flue temps are roughly in the 400-600 DegF range so nothing extremely hot here.

 

[KenLockett]

Ken, this is a link to your earlier thread to make it easier for others to find. https://www.hearth.com/talk/threads/tarm-solo-plus-40-kettling.80430/#post-1038512.

Thanks dogwood!

 

[KenLockett]

I'm no expert but that doesn't sound like kettling to me. I've heard kettling twice and it's something I don't care to hear again. Kettling would be making steam in the pipes. To me, it sounds like air. Do you have an air eliminator above the boiler? I know this is something no one wants to do but, how about opening your T/P valve for a quick shot while it is occuring to see if anything changes.

Fred, yes I have an air eliminator in line above the boiler as well as an automatic air bleed valve at the top of the boiler where the bleed valve. Both checked. Added the auto bleed valve last winter when problem persisted. Honestly it sounds like steam in the pipes and believe me it does get very loud and ferocious at times. Louder than in the you tube video. Like I said in last years post, as soon as I went from kiln dried to higher MC wood the sounds ceased.

 

[KenLockett]

Fred, yes I have an air eliminator in line above the boiler as well as an automatic air bleed valve at the top of the boiler where the bleed valve. Both checked. Added the auto bleed valve last winter when problem persisted. Honestly it sounds like steam in the pipes and believe me it does get very loud and ferocious at times. Louder than in the you tube video. Like I said in last years post, as soon as I went from kiln dried to higher MC wood the sounds ceased.

Fred, what is the T/P valve? I'll try anything.

 

[Bob Rohr]

Why is the one way check valve opening and closing? Is it because of the flashing of the water to steam near the very hot fire tubes thus causing a pulsing of the flow through the system. Indeed it sounds like the primary source of the sound is near the 'swing' check valve at the oil boiler supply header. Purpose of this valve is to prevent ghost flow back into the oil side. Still, my bigger concern is the flashing and the pulsing in the boiler itself. As I assumed the unit is being over-fired I have increased the secondary air flow per the instruction from the Tarm manual for very dry wood. I honestly didn't think the wood was that dry.

Hobbyheater, do you just live with this all the time? Does anyone have a solution assuming it is the check valve opening and closing. I tried increasing flow on the circ pump (in addition to increasing secondary air flow and primary flow via fan damper is almost throttled completely down) and that did not help either. I'm sure throwing some wet wood in there would slow the hot burn down and reduce the flashing but why should I have to do this?

By the way, the flue temps are roughly in the 400-600 DegF range so nothing extremely hot here.

Is the check valve sized to the actual flow rate? Sometimes an oversized check valve will develop a noise like you are describing. A swing check needs about a 1 psi delta P to open, then enough flow to keep it wide open. If you have an 1-1/4" swing check you need 12 gpm or more to swing it open. Air bubbles in the piping could also cause a check to chatter, does the piping sound air free when it is cold and hot?

 

[Clarkbug]

Ken,

What are your system pressures? Is your autofill valve open or closed?

 

[Fred61]

Fred, what is the T/P valve? I'll try anything.

Your "Blow off" valve, ie, temperature/pressure valve. The video may have muted the sound but it seems too regular to be kettling. Not enough time between pulses. For every pulse, it has to build up a pocket of steam with enough pressure to overcome your system pressure. I'm not saying that it's not kettling. I'm only throwing out suggestions to get you to look at the whole picture and not concentrate on your first diagnosis. Perhaps starting with Bob's suggestion above.

 

[kopeck]

I think the folks suggesting a check valve are right on track. I had the check installed in my Theromvar loading unit and it did something just like that. I pulled it out and replaced it with the solid plug, no problems since.

Do you have a variable speed pump? I noticed that mine go better as the pump was slowed down.

K

 

[maple1]

Should be easy to rule in or out a check valve - just get a big screwdriver & do the stethascope thing with it (business end on the valve, handle end in your ear). You could also move around the piping/system with it to check other spots too.

 

[KenLockett]

Ken,

What are your system pressures? Is your autofill valve open or closed?

Clarkbug, system pressure around 16 Psig. Autofill valve is open. I think you suggested last year that I bump up the system pressure. Did that last year to no avail. I am quite confident of the following:

(1) The sounds in the video only occur when extremely hot firing is taking place. Not necessarily as indicated by the flue temp, but simply in that I can 'hear' via hissing the extreme temps in the secondary chamber with door closed. Almost sounds like the water is trying to boil in the water reservoir. Last night I did a full load of hardwood (moisture content verified this morning to be 8-10% so very dry) and heating cycles were very rapid like 10 minutes or less. Noise continued all night and had an 8 hour burn. This morning made partial load with much less fuel (i.e. 3-4 6" diameter logs) and no pulsing and boiler up to temp with normal heating/idle cycles. My intuition tells me that maybe I overloaded the firebox based upon load with fuel that was extremely dry?? However, if I load less I will not get the longer burn, or will I?? Perhaps during idle cycles the excess wood was simply baking away??

(2) I think maybe Bob is onto something with the Swing Check Valve. I need to check but I believe that is a 1 1/2" check valve and even with both zone circ pumps running there is only 8 GPM flow to zones. WB circ pump at Med speed puts out about 12 GPM with installation head. BUT, why does it only pulse with the hotter firing?? If this was a flow issue would I not hear pulsing at all times irrespective of load size or fuel type? My intuition definitely tells me the pulsing sounds from both inside the boiler and copper piping / check valve? is related to how hot the boiler is firing. Does anyone agree with this?

 

[Fred61]

Not knowing where your pressure guage is nor where your pressure relief valve is, I'm going to just throw this out.
What if the check valve were stuck and was inhibiting the flow from the boiler thus causing just the vessel to overheat. I know you said that you changed the pump speed but did you check that the flow through the boiler actually increased? The check valve has become popular and this is the inverse of what has been discussed.

 

[KenLockett]

Not knowing where your pressure guage is nor where your pressure relief valve is, I'm going to just throw this out.
What if the check valve were stuck and was inhibiting the flow from the boiler thus causing just the vessel to overheat. I know you said that you changed the pump speed but did you check that the flow through the boiler actually increased? The check valve has become popular and this is the inverse of what has been discussed.

Not sure if flow increased, but I do know that with smaller fires this morning the noises have stopped. Furthermore, the swing check valve in question only prevents flow back into the oil boiler. Assuming there is minimal flow from the WB out to the main header the CV closes and have confirmed this via temp readings. I can buy the check valve opening and closing but I think this must be a result of some other influence. Chris with Tarm last year suggested the flashing of the water near the tubes with super hot (and efficient) burns. He suggested lower MC wood and it worked. The flashing of the water could cause the pressure dp required to open/close the CV I suppose. Ultimately, if this is a result of too dry wood why does everyone constantly tout the benefits of dry wood. I can see this on a BTU/efficiency basis but the residual result as evidenced in my video is very unnerving. Perhaps a MC band is more appropriate (i.e. 10-20%) as opposed to strictly less than 20%. Anyone agree?

 

[Clarkbug]

Not sure if flow increased, but I do know that with smaller fires this morning the noises have stopped. Furthermore, the swing check valve in question only prevents flow back into the oil boiler. Assuming there is minimal flow from the WB out to the main header the CV closes and have confirmed this via temp readings. I can buy the check valve opening and closing but I think this must be a result of some other influence. Chris with Tarm last year suggested the flashing of the water near the tubes with super hot (and efficient) burns. He suggested lower MC wood and it worked. The flashing of the water could cause the pressure dp required to open/close the CV I suppose. Ultimately, if this is a result of too dry wood why does everyone constantly tout the benefits of dry wood. I can see this on a BTU/efficiency basis but the residual result as evidenced in my video is very unnerving. Perhaps a MC band is more appropriate (i.e. 10-20%) as opposed to strictly less than 20%. Anyone agree?

I think that dry wood is a very good thing, as it has a lot of available BTUs. The issue is most likely that the range of adjustment for your boiler is limited, and it cant handle small splits with the low moisture content, and you end up with the issue you are finding now. Something like a Froiling could probably compensate for it, but perhaps not the solo. I know my boiler gets grouchy if I burn a lot of small splits of dry pine, just because I cant adjust the air enough to handle it. I have to mix it with other wood types. You could pull a turbulator or two to see if it helps, but I realize that it hinders efficiency in that manner.

Can you boost your pump speeds to move more water? Or possibly add a buffer tank to help with this?

 

[KenLockett]

I think that dry wood is a very good thing, as it has a lot of available BTUs. The issue is most likely that the range of adjustment for your boiler is limited, and it cant handle small splits with the low moisture content, and you end up with the issue you are finding now. Something like a Froiling could probably compensate for it, but perhaps not the solo. I know my boiler gets grouchy if I burn a lot of small splits of dry pine, just because I cant adjust the air enough to handle it. I have to mix it with other wood types. You could pull a turbulator or two to see if it helps, but I realize that it hinders efficiency in that manner.

Can you boost your pump speeds to move more water? Or possibly add a buffer tank to help with this?

I boosted the 15-58 circ pump speed to Hi and this made no difference. Not sure conceptually with regard to adding a buffer tank. Can you clarify Clarkbug? By the way I was using 4-6" splits last night when the pulsing was active. Not small relative to the small splits I have used in the past. I do know that adjusting the primary air and secondary air helps with regard to the intensity of the burn but was throttle back about as much as I could last evening. Unfortunately do not have anything less dry to burn at the current time. Ironic how it's usually the other way around. By the way, did you ever contact Calton Coons?

 

[Fred61]

I just went out snowshoeing for a hour and collapsed in my recliner for a few minutes, all the time thinking about your situation. I find it so hard to believe you're unit is kettling. If the pump is off, does anything change? Do you have a flow check or zone valve in the line? At what temperature does it go into idle mode? Are you positive the sound is in the piping and not in the combustion side? At 12 or so psi it has to get dam hot to boil unless there is an air bubble in there.

 

[BoilerMan]

At what boiler temp does this start to happen, also what boiler return temp? If it's as unnerving as you describe and not what we saw on the vid, but loud and spaced out more. Then you are boiling around the fire tubes, this will happen in my setup if:

1. High coal bed and very hot secondary burns
2. 190+ boiler temp
3. 180+ boiler return

If any of these changes and #3 is the one I can change quickly then the boiling stops.
I have measured a fixed 15gpm going through the boiler at all times, and 24psi at 190F boiler temp

I stop this by opening the zone valve for my large indirect via an aquastat set to 188F I set the boiler to idle at 194F to keep things from getting out of hand. It can drop the boiler return from 180 to 170 in a few seconds with a good cold slug of water mixing in the p/s loop. I mix the output from the indirect so there is no scalding potential anyway.

TS

 

[Floydian]

Hello Ken,

By the way, the flue temps are roughly in the 400-600 DegF range so nothing extremely hot here.

Is 600 really within the normal range for the Solo Plus? Seems a bit high from all my reading and research on this site about various gasifiers but I suppose higher than normal temps make sense with given your fuel.

I can't even believe I am going to suggest this, BUT.....have you considered raising the MC of your wood? Should be a whole lot easier than going in the other direction, I would think.

Man, I feel like I just violated some kind of Boiler Room code of ethics.

Noah

 

[Bob Rohr]

At what boiler temp does this start to happen, also what boiler return temp? If it's as unnerving as you describe and not what we saw on the vid, but loud and spaced out more. Then you are boiling around the fire tubes, this will happen in my setup if:

1. High coal bed and very hot secondary burns
2. 190+ boiler temp
3. 180+ boiler return

If any of these changes and #3 is the one I can change quickly then the boiling stops.
I have measured a fixed 15gpm going through the boiler at all times, and 24psi at 190F boiler temp

I stop this by opening the zone valve for my large indirect via an aquastat set to 188F I set the boiler to idle at 194F to keep things from getting out of hand. It can drop the boiler return from 180 to 170 in a few seconds with a good cold slug of water mixing in the p/s loop. I mix the output from the indirect so there is no scalding potential anyway.

TS

Water will boil, or flash in a closed loop system anytime the pressure drops below the Vapor Pressure. In hydronic terms we call this cavitation. Several places to look for cavitration would be the circ pump. The pressure at the eye of the impeller can drop below the vapor pressure even with the system pressure at 12- 16 psi. Another common place cavitation occurs is a flow restriction or a partially closed valve somewhere. A balance valve, partially open ball valve and even a thermostatic 3 way valve can coax cavitation. Cavitation can sound like kettiling as the fluid is "boiling" at the impeller or restriction.

Also as the temperature of the water goes up, density drops, further compounding the potential for cavitation.

If you are sure you have wide open piping, no flow restriction or partially pluged "devices" try increasing the system pressure to 20 psi. Ideally you would want to adjust the expansion tank pre-charge also, but for a quick test, just bump the fill valve to 20 psi and see if the noise stops.

If that works, disconnect the expansion tank from the system pressure and pump it up to 20 psi. Now remember when you increase the fill and expansion tank pressure to match, ytou reduce the capacity of the expansion vessel. re-run the sizing formula to be sure the tank has the capacity to run at higher fill pressure. If the boiler pressure rises as the system heats, keep an eye on the pressure gauge, if it nears 28 psi or so, you need more expansion capacity. Reduce the pressure and increase or add an additional expansion tank.

Kettling or perculation as it is sometimes called is a common problem in copper tube boilers on DHW systems. A small coating of minerals reduces the heat transfer and you will get the perculation sound. It's like an old water heater that is filled with sediment, whan it heats up from the burner, you hear that boiling or perculation sound.

Same thing with an old tea kettle that is limed up, that poping noise or Rice Krispy sound is perculation, or cavitation.

 

[BoilerMan]

This is a great explination of cavatation Bob! As I was working on the house (sheetrock) today I was thinking about this thread, I was thinking scale or something on the water side slowing heat transfer. I know it is almost an imposibility in a good closed loop system with no makeup water requirements, but it did come up in the thought process.

I like to keep my pressure up in the 20s in a hot system for the extra boil-protection.

TS