Weil Mcclain CG gas boiler knocking noises when fired up

[deeMatrix]

We have a 25 year old Weil Mcclain CG Series 11 gas boiler that makes annoying knocking noises when fired up that resonates through pipes and heard on first floor above. It is not overly loud but consistent and not normal. Noise coming from boiler itself as I can hear percolating type sounds and some hissing when next to it and feel the corresponding vibrations on boiler itself. The noise starts soon after fires up and continues throughout burn cycle and then diminishes once boiler reaches set point and goes away completely after a few seconds while circulator continues to pump to calling zone(s).

The system has five (5) Honeywell zone control valves with a singular B&G Series 100 circulator pump. A new Amtrol expansion tank was installed last season and set to 13PSI. All 5 zones get hot with no air noise in any of the baseboard units throughout house. System pressure is around 13PSI cold and increases to around 18PSI when hot. Aquastat set to 180F.

Noise started last year or perhaps end of season before and initially thought maybe it was mineral buildup in boiler itself so added Rhomar HyrdoSolv 9100 for a few months to boiler last year and then drained / filled with Rhomar 922 corrosion inhibitor at end of last heating season.

Not much of difference in noise, if any, this heating season. Not sure if it is just too much mineral buildup in boiler itself and Rhomar 9100 wasn't strong enough to remove. Looking to see if there is something else that can be done other than replacing boiler at this point as perhaps there is something else going on? Any advice/input would be greatly appreciated.

Thanks in advance,
deeMatrix

 

[deeMatrix]

We have a 25 year old Weil Mcclain CG Series 11 gas boiler that makes annoying knocking noises when fired up that resonates through pipes and heard on first floor above. It is not overly loud but consistent and not normal. Noise coming from boiler itself as I can hear percolating type sounds and some hissing when next to it and feel the corresponding vibrations on boiler itself. The noise starts soon after fires up and continues throughout burn cycle and then diminishes once boiler reaches set point and goes away completely after a few seconds while circulator continues to pump to calling zone(s).

The system has five (5) Honeywell zone control valves with a singular B&G Series 100 circulator pump. A new Amtrol expansion tank was installed last season and set to 13PSI. All 5 zones get hot with no air noise in any of the baseboard units throughout house. System pressure is around 13PSI cold and increases to around 18PSI when hot. Aquastat set to 180F.

Noise started last year or perhaps end of season before and initially thought maybe it was mineral buildup in boiler itself so added Rhomar HyrdoSolv 9100 for a few months to boiler last year and then drained / filled with Rhomar 922 corrosion inhibitor at end of last heating season.

Not much of difference in noise, if any, this heating season. Not sure if it is just too much mineral buildup in boiler itself and Rhomar 9100 wasn't strong enough to remove. Looking to see if there is something else that can be done other than replacing boiler at this point as perhaps there is something else going on? Any advice/input would be greatly appreciated.

Thanks in advance,
deeMatrix

Below is a Soundcloud link to audio file of boiler noise. Knocking comes pronounced at 2 minute mark through to end of track and very noticeable on floor above. Thoughts?

Thanks,
deeMatrix

https://soundcloud.com/user-851536758/boiler-fire-noise

 

[Sponsor]

 

[NY_Rob]

By your description and the audio file, sounds like a classic case of Boiler Kettling.

In many cases it's due to limescale and mineral deposits at the bottom of the heat exchanger.

There was a recent thread where the same topic came up (starting at post# 6 ) :
https://terrylove.com/forums/index.php?threads/boiler-over-pressurizing.72671/#post-536154

 

[Dana]

I the system pressure also ~13psi when measured by a separate pressure gauge?

Does the kettling sound abate a bit as the system pressure rises during a burn? If yes, you you have enough headroom to raise the system pressure to 20psi (cold/tepid) without blowing open a 30 psi pressure relief valve, and that may quell the symptom.

The boiler water treatments are always a bit of a craps-shoot. I have no experience with Rhomar HyrdoSolv, but Fernox sometimes works.

Any 25 year old cast iron boiler is pretty much at end of service life. Between the corrosion on both the fire side and corrosion + liming on water side of the heat exchanger they are usually no longer able to hit their nameplate labeled steady state combustion efficiency (= DOE BTU-out/BTU-in), so even if you can get by with it for another year or five, it's time to start contemplating the replacement and have that figured out before a potential leak or other catastrophic failure so that the decision making doesn't have to happen in a panic. That always starts with a heat load calculation.

While ACCA Manual-J load calculation method is the gold standard, most HVAC pros who use it have a far less than golden record of using it correctly (garbage in == garbage out) and will tend to skew far to the high side. Many HVAC pros don't even bother, and just replace with something the same size, or size it to balance with the output of the radiation at 180F. Those methods tend to oversize the boiler by 2-3x on systems as old as yours. That level of oversizing has consequences for both comfort and efficiency, especially with cast iron boilers. ASHRAE recommends no more than a 1.4x oversizing factor for the load at the 99th percentile outdoor temperature bin, which is a large enough factor to use overnight setbacks without unduly long recovery times, and is usually enough to cover the peak load during Polar Vortex type cold snaps with overnight lows 15-25F below the typical 99% outside design temperature for your location.

Since you have a heating history on the place you can use fuel use and the nameplate boiler efficiency to MEASURE the heat load, using the boiler as the measuring instrument. Even though it's steady state efficiency has fallen off a bit, and it's probably 2-3x oversized, as long as you use winter time fuel use, ignoring the shoulder seasons it will usually be far more accurate than a sloppy Manual-J, and pretty close to a properly executed Manual-J. Fuel-use load calculation methods are covered in some detail in this bit o' bloggery.

If you think you'd be considering a high efficiency condensing boiler, it's worth doing this bit of napkin-math too.

 

[deeMatrix]

I the system pressure also ~13psi when measured by a separate pressure gauge?

Does the kettling sound abate a bit as the system pressure rises during a burn? If yes, you you have enough headroom to raise the system pressure to 20psi (cold/tepid) without blowing open a 30 psi pressure relief valve, and that may quell the symptom.

The boiler water treatments are always a bit of a craps-shoot. I have no experience with Rhomar HyrdoSolv, but Fernox sometimes works.

Any 25 year old cast iron boiler is pretty much at end of service life. Between the corrosion on both the fire side and corrosion + liming on water side of the heat exchanger they are usually no longer able to hit their nameplate labeled steady state combustion efficiency (= DOE BTU-out/BTU-in), so even if you can get by with it for another year or five, it's time to start contemplating the replacement and have that figured out before a potential leak or other catastrophic failure so that the decision making doesn't have to happen in a panic. That always starts with a heat load calculation.

While ACCA Manual-J load calculation method is the gold standard, most HVAC pros who use it have a far less than golden record of using it correctly (garbage in == garbage out) and will tend to skew far to the high side. Many HVAC pros don't even bother, and just replace with something the same size, or size it to balance with the output of the radiation at 180F. Those methods tend to oversize the boiler by 2-3x on systems as old as yours. That level of oversizing has consequences for both comfort and efficiency, especially with cast iron boilers. ASHRAE recommends no more than a 1.4x oversizing factor for the load at the 99th percentile outdoor temperature bin, which is a large enough factor to use overnight setbacks without unduly long recovery times, and is usually enough to cover the peak load during Polar Vortex type cold snaps with overnight lows 15-25F below the typical 99% outside design temperature for your location.

Since you have a heating history on the place you can use fuel use and the nameplate boiler efficiency to MEASURE the heat load, using the boiler as the measuring instrument. Even though it's steady state efficiency has fallen off a bit, and it's probably 2-3x oversized, as long as you use winter time fuel use, ignoring the shoulder seasons it will usually be far more accurate than a sloppy Manual-J, and pretty close to a properly executed Manual-J. Fuel-use load calculation methods are covered in some detail in this bit o' bloggery.

If you think you'd be considering a high efficiency condensing boiler, it's worth doing this bit of napkin-math too.

Thanks for assistance. As for questions, kettling sound doesn't really abate as the system pressure rises during a burn. Once burners go off and circulator continues to run you can still hear some knocking for a bit and then slowly dissipates. I haven't tested the system pressure outside of the actual boiler gauge itself and will have to get an external one to do that.

As for boiler sizing, the existing one (see attached pic is CG-5 Series 11) and we had a Man-J calc done last year when assessing whether time to change it out with a high efficiency one and the numbers came in around 80,000 BTUH and the company recommended a Weil Mcclain ECO 110 at 110,000 BTU which also included hot water for house.
Thanks again for your perspectives on this.

 

[Dana]

A properly performed Manual-J done by a disinterested third party (not an HVAC contractor) that comes in at 80K @ NJ type outside design temperatures implies either a fairly large house (well north of 4500 square feet), or a barely insulated, air-leaky house. It's always worth running the fuel-use numbers as a sanity check on that Manual-J, and that's especially true if the load calculations were performance by an HVAC contractor. If the fuel use load calculation comes within 10% of the Manual-J it was probably done correctly.

If it's 20% or more UNDER the Manual-J, trust the fuel use number, not the Manual-J. But if you have hard copies of the Manual-J it might be possible to figure out where they went astray, or had a thumb on the scale.

It's (almost) never appropriate to upsize a boiler when adding an indirect for hot water, since the indirect can be run as a "priority" zone, with reasonably short recovery times with just about any boiler, short enough that the house won't lose even half a degree as it recovers. (A standalone 50 gallon tank typically has ~30-35000 BTU/hr of burner output.)

The minimum fire output of the W-M ECO-110 is about 21,000 BTU/hr at condensing temperatures, and with five zones it's possible or even likely that one or more of the zones doesn't have enough radiation to emit 20,000 BTU/hr at condensing temperatures. The ECO-80 would be the "right" boiler for 19 out of 20 houses in the northeast, but it still delivers over 13,000 BTU/hr at condensing temperatures, which is still high enough that doing the napkin-math the zone radiation would be warranted.

There are 80K-in boilers out there with sub 8K output at minimum fire that might be more appropriate, and 100K boilers with sub-10K output, if that 80K load number turns out to be real. Don't just leave it to "the pros"- this forum is full of threads dealing with professionally installed short cycling modulating condensing boilers, where even the most rudimentary calculations would have avoided the problem. Since the heating system still works, you have time to run those numbers.

 

[deeMatrix]

I the system pressure also ~13psi when measured by a separate pressure gauge?

Does the kettling sound abate a bit as the system pressure rises during a burn? If yes, you you have enough headroom to raise the system pressure to 20psi (cold/tepid) without blowing open a 30 psi pressure relief valve, and that may quell the symptom.

The boiler water treatments are always a bit of a craps-shoot. I have no experience with Rhomar HyrdoSolv, but Fernox sometimes works.

Any 25 year old cast iron boiler is pretty much at end of service life. Between the corrosion on both the fire side and corrosion + liming on water side of the heat exchanger they are usually no longer able to hit their nameplate labeled steady state combustion efficiency (= DOE BTU-out/BTU-in), so even if you can get by with it for another year or five, it's time to start contemplating the replacement and have that figured out before a potential leak or other catastrophic failure so that the decision making doesn't have to happen in a panic. That always starts with a heat load calculation.

While ACCA Manual-J load calculation method is the gold standard, most HVAC pros who use it have a far less than golden record of using it correctly (garbage in == garbage out) and will tend to skew far to the high side. Many HVAC pros don't even bother, and just replace with something the same size, or size it to balance with the output of the radiation at 180F. Those methods tend to oversize the boiler by 2-3x on systems as old as yours. That level of oversizing has consequences for both comfort and efficiency, especially with cast iron boilers. ASHRAE recommends no more than a 1.4x oversizing factor for the load at the 99th percentile outdoor temperature bin, which is a large enough factor to use overnight setbacks without unduly long recovery times, and is usually enough to cover the peak load during Polar Vortex type cold snaps with overnight lows 15-25F below the typical 99% outside design temperature for your location.

Since you have a heating history on the place you can use fuel use and the nameplate boiler efficiency to MEASURE the heat load, using the boiler as the measuring instrument. Even though it's steady state efficiency has fallen off a bit, and it's probably 2-3x oversized, as long as you use winter time fuel use, ignoring the shoulder seasons it will usually be far more accurate than a sloppy Manual-J, and pretty close to a properly executed Manual-J. Fuel-use load calculation methods are covered in some detail in this bit o' bloggery.

If you think you'd be considering a high efficiency condensing boiler, it's worth doing this bit of napkin-math too.

Dana,
I have confirmed the boiler aquastat is about 1PSI higher than the external one I attached to the boiler drain valve. To see if any noise reduction, I raised the cold/tepid system pressure to around 18PSI and the system now gets to around 22-23PSI when burners go off at around 180F. No noticeable difference in noise reduction and sometimes think may be worse with some rapid knocks at times.

I also see the aquastat pressure indicator shaking a little so not sure what that is? Should I go back down to around 13PSI cold/tepid at this point?

Thanks in advance,
deeMatrix

 

[NY_Rob]

^ the pressure needle bouncing around is symptomatic and is due to flash boiling "hot-spots"creating momentary pressure changes.

You can try lowering your high limit temp from 180F vs. 170F, that helped quiet my system a bit before I replaced it.

 

[Dana]

NY_Rob has it right, both on the recommended course of action and the explanation. Most house can be heated with ~150F water or lower, since the systems as-designed tended to be more than 2x oversized for the actual load, far more margin than is needed (or makes any sense.) If you have enough radation to not short-cycle the CG-5 at 180F output, it'll probably still deliver at least 3-5 minute burns with 150F water, and longer burns at 160-170F.

Terminology: "Aquastat" refers to a control device that opens/closes switch contacts in response to water temperature, not a temperature or pressure indicator.

 

[NY_Rob]

Keep in mind that lowering supply water temps, changing pressure, etc... is just buying a small amount of time before you're forced to replace the old boiler.
Start doing your research now into boilers and installers before it's all hands on deck crisis mode.

 

[Dana]

What NY_Rob said, now is the time to plan the replacement, when you have the time to do the math, and get it right, rather than waiting for conditions to worsen. You may be able to get one, even two more heating seasons out of it, but probably not four. Leaving it up to the HVAC pros will almost always result in sub-optimal oversizing factors, even RIDICULOUS oversizing factors (which is probably where you're at right now).

So, over then next few gas bills run a fuel-use heat load calculation.

Then, measure your radiation zone by zone on each of the five zones, to see if your system is a good candidate for a modulating condensing boiler, and pick one with a low enough output at minimum fire that it won't short cycle at condensing temperatures.

With that information already spelled out and filed away you will be in a position to assess proposals, or even direct the contractors regarding the necessary specifications so that nobody is wasting their time. If you can schedule the replacement at time other than the middle of the heating season it'll be a lot easier, and you can shop it around for better prices or better proposals.

 

[deeMatrix]

Thanks Dana and Rob for your input. Yes I need to learn the terminology. I will start to plan a replacement as I too would prefer to have this be a planned rather than reactive installation.

 

[NY_Rob]

Go to https://heatinghelp.com/find-a-contractor/ and input your zipcode.
Most of the Pros there will at least do a proper heatloss analysis vs. sizing a new boiler by looking at the capacity plate on your existing boiler. If you have enough radiation you could also talk to the contractor about a moc-con instead of conventional cast iron boiler.

 

[deeMatrix]

What NY_Rob said, now is the time to plan the replacement, when you have the time to do the math, and get it right, rather than waiting for conditions to worsen. You may be able to get one, even two more heating seasons out of it, but probably not four. Leaving it up to the HVAC pros will almost always result in sub-optimal oversizing factors, even RIDICULOUS oversizing factors (which is probably where you're at right now).

So, over then next few gas bills run a fuel-use heat load calculation.

Then, measure your radiation zone by zone on each of the five zones, to see if your system is a good candidate for a modulating condensing boiler, and pick one with a low enough output at minimum fire that it won't short cycle at condensing temperatures.

With that information already spelled out and filed away you will be in a position to assess proposals, or even direct the contractors regarding the necessary specifications so that nobody is wasting their time. If you can schedule the replacement at time other than the middle of the heating season it'll be a lot easier, and you can shop it around for better prices or better proposals.

Dana, Great article on fuel-use heat load calculation you provided.

I used last mid-January 2017 to mid-February 2017 gas bill and did the fuel-use heat load calculation as outlined in link above with the ASHRAE 1.4x sizing factor and didn't make any error adjustments to numbers but arrived at:

57,221 BTU/hour @ 65F minimum load
62,642 BTU/hour @ 60F maximum load

So my existing CG5 Series 11 has a DOE of 117,000 so at least 2x oversized? If these calculated min/max numbers are close and assume we wanted to stick with a Weil McLain cast iron replacement boiler then the CG4 at DOE of 71,000 would be right size versus CG3 which is only 51,000?

Thanks again in advance,
deeMatrix

 

[Dana]

Are those the raw load numbers, or is that a sizing factor that includes the ASHRAE 1.4x multiplier?

If the raw load numbers came out 40,870 BTU/hr using base 65F and 49 500 BTU/hr using base 60F, with cast-iron you'll want something with a DOE output in the 50s, not the 70s, and the CGa-3 would be a better fit, at about 1.25x oversize factor for the base 65F calculated load, and still has the higher load number covered.

The CGa-4 would be nearly 2x oversized for the load calculated at base 65F, which is a realistic base temperature for an insulated 2x4 framed house with clear glass double panes (or storms over single-panes). But if yours is a 2x6 framed house with R38 or more in the attic, with Low-E windows and an insulated foundation base-60F is more realistic, making the CGa-3 a bit marginal, and the CGa-4 would be a better fit.

Better yet would be to find a condensing boiler with 60-80K of input and a good turn-down ratio, with a minimum modulated output under 20,000 BTU/hr. There are some pretty good fire tube condensing boilers out there, that are sometimes even cheaper & easier to install than cast iron. A CGa-3 or 4 runs $1.7-$2KUSD for the boiler, but so is an HTP UFT-080W (or it's Westinghouse branded version, the WBRUNG080w.) Like most fire-tube condensing boilers it's heat exchanger has a pumping head comparable to small cast iron boilers, and can be pumped direct in almost all systems, making it an easy drop-in replacement, and since it can throttle back to ~7600 BTU/hr-out it can handle some fairly stubby zone radiation without short cycling. There are other similar condensing boilers, if local support is lacking in your area.

 

[deeMatrix]

Thanks Dana for follow-up, which type of boiler is easier from an annual maintenance and ongoing repair perspective? My CG-5 lasted 25 years and required only periodic minor repairs and maintenance. I read an article last year that the newer higher efficiency boilers are more complicated, require more maintenance and a lot of technicians don't have the knowledge to adequately support them so maybe better to just stick with cast iron?

 

[Dana]

I've heard that theory too, mostly from folks who don't (0r can't) read the manual!

Twenty years ago there was a case to be made that modulating condensing boilers weren't all they were cracked up to be, but the majority of issues had to do with oversized boilers and installers that hadn't a clue about how to set them up, or really much of anything about hydronic heating design beyond a few rules of thumb. Many condensing water tube condensing boilers need to be plumbed primary/secondary and specifying the pumps required som math. Condensing boilers with aluminum heat exchangers were more sensitive to system water chemistry and had higher than usual failure rates, etc.

But fire-tube stainless steel heat exchangers have changed all that- they're much less sensitive to the system design or contaminants, and can go the distance. The most important factor is sizing it correctly for both the load and radiation, and dialing in the outdoor reset temperature curve to where burns are measured in hours, not minutes. Once you have tweaked in the parameters to the outdoor reset curve for max comfort & efficiency there's little to do other than enjoy the higher comfort and take the savings to bank.

Most techs won't take more than a WAG at where the reset curve should set, but it doesn't take a rocket science degree to fine-tune the program yourself. Some are simpler than others, but none are difficult.

With enough radiation and a well adjusted curve the savings are usually larger than the comparative AFUE efficiency numbers might imply, especially when comparing it to a 2-3x oversized boiler (typical.) Even if it craps out in 15 years (most don't), the savings will have more than made up for a potentially shorter lifecycle. NJ will even spot you $300 up front as an incentive (a far cry from MA, which has a rebate subsidy 10 times bigger than that!)

Having the napkin math already in your pocket when soliciting proposals gives you the tools for winnowing the true hacks from the cream of the crop. A contractor who can follow the math listens to what you want is also more likely to be willing and able to explain the nuances of why they picked one boiler over another for your system, and what you may need down the road in terms of maintenance & service, etc. The sad fact is that the average boiler installer is pretty lousy, but you don't usually have to pay extra to get a good one. You have to be able to figure out how to spot them though, and that's what this exercise is about.

 

[deeMatrix]

I've heard that theory too, mostly from folks who don't (0r can't) read the manual!

Twenty years ago there was a case to be made that modulating condensing boilers weren't all they were cracked up to be, but the majority of issues had to do with oversized boilers and installers that hadn't a clue about how to set them up, or really much of anything about hydronic heating design beyond a few rules of thumb. Many condensing water tube condensing boilers need to be plumbed primary/secondary and specifying the pumps required som math. Condensing boilers with aluminum heat exchangers were more sensitive to system water chemistry and had higher than usual failure rates, etc.

But fire-tube stainless steel heat exchangers have changed all that- they're much less sensitive to the system design or contaminants, and can go the distance. The most important factor is sizing it correctly for both the load and radiation, and dialing in the outdoor reset temperature curve to where burns are measured in hours, not minutes. Once you have tweaked in the parameters to the outdoor reset curve for max comfort & efficiency there's little to do other than enjoy the higher comfort and take the savings to bank.

Most techs won't take more than a WAG at where the reset curve should set, but it doesn't take a rocket science degree to fine-tune the program yourself. Some are simpler than others, but none are difficult.

With enough radiation and a well adjusted curve the savings are usually larger than the comparative AFUE efficiency numbers might imply, especially when comparing it to a 2-3x oversized boiler (typical.) Even if it craps out in 15 years (most don't), the savings will have more than made up for a potentially shorter lifecycle. NJ will even spot you $300 up front as an incentive (a far cry from MA, which has a rebate subsidy 10 times bigger than that!)

Having the napkin math already in your pocket when soliciting proposals gives you the tools for winnowing the true hacks from the cream of the crop. A contractor who can follow the math listens to what you want is also more likely to be willing and able to explain the nuances of why they picked one boiler over another for your system, and what you may need down the road in terms of maintenance & service, etc. The sad fact is that the average boiler installer is pretty lousy, but you don't usually have to pay extra to get a good one. You have to be able to figure out how to spot them though, and that's what this exercise is about.

Your sad fact is what concerns me most as the average boiler installer I've come across is pretty lousy so I have to figure out a good one. In looking at the NJ link it brings up the combi unit rebate of $700. Last year both firms that came in recommended this versus a separate hot water heater. I think it was required for the overall home energy efficiency savings rebate. Any thoughts on providing hot water to house out of same system versus separate water heater? The $700 versus $300 rebate makes me ask the question as well as my AO Smith now being about 15 years old.

 

[Dana]

Wall hung combi boilers tend to work best in houses with larger than average heat loads, and lower than average domestic hot water loads. There are few tank-type combis that can work though, and the inherent thermal mass of tank combi's make them essentially short-cycle proof, even on micro-zones. Within HTP's lineup (I swear I don't work for them!) the all stainless Phoenix Light Duty or the smallest Versa would work fine in most homes, but they have wall hung versions too.

But run the size numbers on your radiation first, since that determines the minimum-modulated output the combi boiler would need to meet. Some of Navien's wall hung combis have decently low minimum-fire output, but as with any tankless water heater it becomes a matter of how much hot water you need to serve up at one time.

How many bathrooms you have, and are there are any oversized bathtubs to fill?

When it's time to solicit proposals, I don't know how cheap this guy is (in Toms River, NJ) and I've never met him, but from his web-comments he seems to be a competent hydronic designer.

 

[deeMatrix]

Wall hung combi boilers tend to work best in houses with larger than average heat loads, and lower than average domestic hot water loads. There are few tank-type combis that can work though, and the inherent thermal mass of tank combi's make them essentially short-cycle proof, even on micro-zones. Within HTP's lineup (I swear I don't work for them!) the all stainless Phoenix Light Duty or the smallest Versa would work fine in most homes, but they have wall hung versions too.

But run the size numbers on your radiation first, since that determines the minimum-modulated output the combi boiler would need to meet. Some of Navien's wall hung combis have decently low minimum-fire output, but as with any tankless water heater it becomes a matter of how much hot water you need to serve up at one time.

How many bathrooms you have, and are there are any oversized bathtubs to fill?

When it's time to solicit proposals, I don't know how cheap this guy is (in Toms River, NJ) and I've never met him, but from his web-comments he seems to be a competent hydronic designer.

Thanks Dana for all of your sage knowledge and advice. It is truly appreciated by all.
deeMatrix