Weil Mcclain CG gas boiler knocking noises when fired up
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deeMatrix
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I should thank you as well Rob for all of your guidance and advice. I will reach out to Rich as one of the designer/installers we consider.
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deeMatrix
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Dana,
Looking back at your earlier response, I was off on my number using base 60F and the roughly 9% differential between the two makes more sense. The numbers are:
40,872 BTU/hour heat load calculation at 65F
44,744 BTU/hour heat load calculation at 60F
I also have roughly 194 feet of fin tubed baseboard throughout the 5 zones, as well as two under sink vanity Twin-Flo III kickspace heater in kitchen and master bathroom. The house is 25 years old made of 2x4 construction, double pane Anderson windows, at least R30 attic insulation in 1/2 the attic and R38+ in the the other half. Basement living space walls are insulated as well. So then based on upon your information base 65 would be correct to use?
Breakdown of house is as follows:
House is ~3150 sqft. of living space with 5 zones split between 1st floor, 2nd floor and basement, all with copper-finned baseboard as follows:
1st floor ~1400 sqft. living space divided into 2 zones:
Zone 1) Family room, Kitchen/breakfast area foyer and powder room ~1025 sqft. (54' baseboard and under sink vanity Twin-Flo III kickspace heater
Zone 2) Dining room/Living room ~375 sqft. (41' baseboard)
Second floor ~1050 sqft. living space divided into 2 zones:
Zone 3) 3 bedrooms, hall, main bath, laundry closet ~650' sqft. (42' baseboard)
Zone 4) Master bedroom, master bath and closet ~400 sqft. (26' baseboard and bathroom under sink vanity Twin-Flo III kickspace heater)
Zone 5) Heated Basement ~700 sqft. living space (31' baseboard)
Using the ASHRAE 1.4x multiplier and base 65F a CG-3 boiler (saw one listed at 58,000 DOE output) would be about right. The one I saw and referenced earlier was 51,000 DOE output- different series???).
I'm not sure if the ~194' of radiation baseboard and two Twin-Flo III kickspace heaters up the requirements at all to go up to the CG-4 which I have seen listed at 71,000 DOE output and another one at 77,000 DOE output? Or does the level of micro zone radiation still allow the use of a newer condensing boiler?
Any final thoughts on this before I solicit some design/installation proposals?
Thanks again,
deeMatrix
Can you combine some of the zones to run in parallel?
For instance, the upstairs is two zones presently- if you combine them would that present a problem with room temperatures?
Same idea for the first floor and basement.....
Physically combining the zones at the boiler is quite easy to do BTW.
As stand alone zones, you're just on the boarderline of being able to use the lowest fire rate (8K BTU's) of a 10:1 turndown mod-con.
The 31' basement zone will short cycle as soon as you near condensing return water temps.
If you can combine into two zones overall, you'll be good to go for no short cycling.
Some things to consider before moving to a mod-con..
(See fin-tube radiator output and Condensing Boiler Efficiency Curve charts for reference)
Keep in mind that your current radiation was sized to your rooms/house with their output based on 180F SWT (Supply Water Temperature).
So,for example- if you have a room that actually requires 5K BTU's to maintain temp on a cold day- the builder would have installed 10' of radiator (see fin-tube radiator output chart for fin-tube output at 170F x 10ft).
Mod-cons run at considerably lower SWT, as a matter of fact- they only start condensing with RWT (Return Water Temps) below 130F (see Condensing Boiler Efficiency Curve chart), but to even reach above 90% efficiency you need RWT's at 120F or below. So you're aiming for 130F SWT to get RWT's in condensing range for mod-con efficiency. To calculate radiator output- common practice is to use SWT-10F, that gives you AWT (Average Water Temp).
If you look at the fin-tube radiator output chart, 120F AWT only gives you 210BTU's per ft of baseboard. So that 10' of baseboard is now only producing 2,100BTU's vs 5,000BTU's it produced with 170F ATW. It's pretty clear that the room will be cold with less than half of the BTU's needed.
But......
Luckily, most homes were over provisioned with radiation, so you can sometimes get away with using the original amount of radiators even at lower supply temps.
You can sometimes install additional radiation in rooms that are cold with the lower supply temps.
You can swap out your high-temp baseboard (as/if needed) for modern low-temp baseboard that almost doubles std fin-tube output at condensing temps.
You can sometimes change flow direction so that the coldest rooms get the hottest supply water first.
You can run the mod-con's at higher temps via the ODR (Outdoor Reset) curve... you just don't get 90%+ efficiency at that point.
In the end, being creative and being willing to add/modify existing radiation will go a long way towards making a mod-con work in a home designed for 180F cast iron boilers.
For instance, the upstairs is two zones presently- if you combine them would that present a problem with room temperatures?
Same idea for the first floor and basement.....
Physically combining the zones at the boiler is quite easy to do BTW.
As stand alone zones, you're just on the boarderline of being able to use the lowest fire rate (8K BTU's) of a 10:1 turndown mod-con.
The 31' basement zone will short cycle as soon as you near condensing return water temps.
If you can combine into two zones overall, you'll be good to go for no short cycling.
Some things to consider before moving to a mod-con..
(See fin-tube radiator output and Condensing Boiler Efficiency Curve charts for reference)
Keep in mind that your current radiation was sized to your rooms/house with their output based on 180F SWT (Supply Water Temperature).
So,for example- if you have a room that actually requires 5K BTU's to maintain temp on a cold day- the builder would have installed 10' of radiator (see fin-tube radiator output chart for fin-tube output at 170F x 10ft).
Mod-cons run at considerably lower SWT, as a matter of fact- they only start condensing with RWT (Return Water Temps) below 130F (see Condensing Boiler Efficiency Curve chart), but to even reach above 90% efficiency you need RWT's at 120F or below. So you're aiming for 130F SWT to get RWT's in condensing range for mod-con efficiency. To calculate radiator output- common practice is to use SWT-10F, that gives you AWT (Average Water Temp).
If you look at the fin-tube radiator output chart, 120F AWT only gives you 210BTU's per ft of baseboard. So that 10' of baseboard is now only producing 2,100BTU's vs 5,000BTU's it produced with 170F ATW. It's pretty clear that the room will be cold with less than half of the BTU's needed.
But......
Luckily, most homes were over provisioned with radiation, so you can sometimes get away with using the original amount of radiators even at lower supply temps.
You can sometimes install additional radiation in rooms that are cold with the lower supply temps.
You can swap out your high-temp baseboard (as/if needed) for modern low-temp baseboard that almost doubles std fin-tube output at condensing temps.
You can sometimes change flow direction so that the coldest rooms get the hottest supply water first.
You can run the mod-con's at higher temps via the ODR (Outdoor Reset) curve... you just don't get 90%+ efficiency at that point.
In the end, being creative and being willing to add/modify existing radiation will go a long way towards making a mod-con work in a home designed for 180F cast iron boilers.
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A heat load of 41-45K for a 3100' house is credible. And witn ~200' of baseboard it means it can deliver the full BTU with ~120F average water temp (AWT), which is in the condensing zone, making it an ideal fit for a condensing boiler. In fact you may opt to run it at a fixed temperature rather than using outdoor reset, for reasons outlined below.
The shortest zone is 26' of baseboard, which will emit ~5000 BTU/hr @ 120F AWT (nicely into the condensing zone, with 110-115F return water), so it really matters how much the toe-kick heater is putting out, or if it can be adjusted to even turn on at water temps that low (some can, others not.)
The very SMALLEST of the wall-hung combis out there have a min-fire output of about twice that, but not enough domestic hot water output to cover more than one shower at a time.
Not all cycling is short-cycling. With low mass boilers burns as short as 3 minutes are neither an efficiency or maintenance problem as long as it isn't 10 burns per hour. The previously mentioned UFT-080W had a minimum firing rate of about 7500 BTU/hr out, and at 125F out, 115F back (120F AWT) the 26' of basboard is emitting 2/3 of that, with only 2500 BTU/hr ( 42 BTU/minute) of "extra" driving the cycling. The boiler has about 3 gallons, and there is probably on the order of another gallon in the zone & distribution plumbing for about 30-35lbs of water. At 42 BTU/minute and 30lbs of water the temperature is slewing 1.4 degrees per minute, so even a 5F swing around the boiler's set point would deliver a 3.5 minute minimum burn, a 7F swing would deliver on the order of five minutes. This isn't a short cycling disaster, and the duty cycle is high enough that the odds of overlapping calls from other zones is also high. So even with just the 26' it won't cycle rapidly even if just the 26' (with or without toe-kick operating) zone is calling for heat, as long as the boiler is set up to deliver water no cooler than about 120F (for an AWT of 115F or so) which is fine.
The output of low-rise fin-tube baseboard becomes very non-linear with temperature and harder to predict at AWTs less than 115-120F, and at temps much cooler than that the toe-kick heater would be putting out a tepid wind-chill on bare feet rather than a "warm summer breeze" effect.
The NTI Trinity TX51 would work here too, with a min-fire input of 7,100 BTU and a condensing output (x 0.95=) ~6700 BTU/hr. At high-fire it'll still deliver 52-54,000 BTU/hr in near-condensing or low-condensing mode, so you'll have at least a ~1.2x or higher oversize factor, which is fine. But the UFT-80W or Navien's NHB-80 would give you more margin for Polar Vortex coolth, delivering ~70,000 BTU/hr even at water temps well above the condensing zone.
With a wall-hung combi (or bigger mod-con boiler) with a min-fire output in the 10-11,000 BTU/hr range you'd have 7500-8000 BTU/hr of extra, which would cut the burn times down to a minute or less, which WOULD be a short-cycling problem.
If going with cast iron, the CGa-3 puts out 59,000 BTU/hr (DOE) which would be about right. The CGa-4's DOE output is 88,000 BTU/hr which is twice your design condition heat load. With 59,000 BTU/hr going into ~200' of baseboard you're looking at 300 BTU/foot, so when all zones are calling for heat it'll be edging pretty close to the condensing zone, with a 140F-ish AWT, which good for efficiency, but potentially damaging to the boiler if it spends a lot of time actually condensing inside the boiler. Protecting it from condensing would require designing and implementing a bypass branch to mix boiler output water with the return water, to keep the entering water temps sufficiently above condensing. Cast iron boilers will short cycle on zone calls too (your existing boiler probably short cycles a bit- are the burns always longer than 5 minutes?), so it's important to set up the boiler controls to take maximum advantage of the thermal mass in the boiler when only one or two stubby zones are calling for heat. With your micro-zones it would be worth installing a retrofit heat purge economizer control, such as the Intellicon HW+ to limit the short cycling.
The Burnham ES2-3 is about the same size as the CGa-3 but is set up for on-board smart heat-purging controls to managing cycling, and is internally protected for low entering water temperatures (down to 110F return water). For those reasons it would be a better bet than the CGa-3, since it's a no-brainer install, no design skills necessary.
The Burnham ESC-3 has a DOE output of 52,000 BTU/hr which at ~1.2x oversizing is also about right, and like the ES2 is also internally protected from cool return water temperatures. Like a mod-con, the ESC series are "direct vented", meaning that the combustion air is ducted from the outdoors, and the exhaust is power vented. That would allow you to block up the port to the chimney, which lowers the actual heat load, since the parasitic drafting of the chimney when the boiler isn't firing goes away. But it's probably going to be more expensive to install than the smaller mod-cons, since the exhaust venting has to be stainless steel instead of plastic.
The shortest zone is 26' of baseboard, which will emit ~5000 BTU/hr @ 120F AWT (nicely into the condensing zone, with 110-115F return water), so it really matters how much the toe-kick heater is putting out, or if it can be adjusted to even turn on at water temps that low (some can, others not.)
The very SMALLEST of the wall-hung combis out there have a min-fire output of about twice that, but not enough domestic hot water output to cover more than one shower at a time.
Not all cycling is short-cycling. With low mass boilers burns as short as 3 minutes are neither an efficiency or maintenance problem as long as it isn't 10 burns per hour. The previously mentioned UFT-080W had a minimum firing rate of about 7500 BTU/hr out, and at 125F out, 115F back (120F AWT) the 26' of basboard is emitting 2/3 of that, with only 2500 BTU/hr ( 42 BTU/minute) of "extra" driving the cycling. The boiler has about 3 gallons, and there is probably on the order of another gallon in the zone & distribution plumbing for about 30-35lbs of water. At 42 BTU/minute and 30lbs of water the temperature is slewing 1.4 degrees per minute, so even a 5F swing around the boiler's set point would deliver a 3.5 minute minimum burn, a 7F swing would deliver on the order of five minutes. This isn't a short cycling disaster, and the duty cycle is high enough that the odds of overlapping calls from other zones is also high. So even with just the 26' it won't cycle rapidly even if just the 26' (with or without toe-kick operating) zone is calling for heat, as long as the boiler is set up to deliver water no cooler than about 120F (for an AWT of 115F or so) which is fine.
The output of low-rise fin-tube baseboard becomes very non-linear with temperature and harder to predict at AWTs less than 115-120F, and at temps much cooler than that the toe-kick heater would be putting out a tepid wind-chill on bare feet rather than a "warm summer breeze" effect.
The NTI Trinity TX51 would work here too, with a min-fire input of 7,100 BTU and a condensing output (x 0.95=) ~6700 BTU/hr. At high-fire it'll still deliver 52-54,000 BTU/hr in near-condensing or low-condensing mode, so you'll have at least a ~1.2x or higher oversize factor, which is fine. But the UFT-80W or Navien's NHB-80 would give you more margin for Polar Vortex coolth, delivering ~70,000 BTU/hr even at water temps well above the condensing zone.
With a wall-hung combi (or bigger mod-con boiler) with a min-fire output in the 10-11,000 BTU/hr range you'd have 7500-8000 BTU/hr of extra, which would cut the burn times down to a minute or less, which WOULD be a short-cycling problem.
If going with cast iron, the CGa-3 puts out 59,000 BTU/hr (DOE) which would be about right. The CGa-4's DOE output is 88,000 BTU/hr which is twice your design condition heat load. With 59,000 BTU/hr going into ~200' of baseboard you're looking at 300 BTU/foot, so when all zones are calling for heat it'll be edging pretty close to the condensing zone, with a 140F-ish AWT, which good for efficiency, but potentially damaging to the boiler if it spends a lot of time actually condensing inside the boiler. Protecting it from condensing would require designing and implementing a bypass branch to mix boiler output water with the return water, to keep the entering water temps sufficiently above condensing. Cast iron boilers will short cycle on zone calls too (your existing boiler probably short cycles a bit- are the burns always longer than 5 minutes?), so it's important to set up the boiler controls to take maximum advantage of the thermal mass in the boiler when only one or two stubby zones are calling for heat. With your micro-zones it would be worth installing a retrofit heat purge economizer control, such as the Intellicon HW+ to limit the short cycling.
The Burnham ES2-3 is about the same size as the CGa-3 but is set up for on-board smart heat-purging controls to managing cycling, and is internally protected for low entering water temperatures (down to 110F return water). For those reasons it would be a better bet than the CGa-3, since it's a no-brainer install, no design skills necessary.
The Burnham ESC-3 has a DOE output of 52,000 BTU/hr which at ~1.2x oversizing is also about right, and like the ES2 is also internally protected from cool return water temperatures. Like a mod-con, the ESC series are "direct vented", meaning that the combustion air is ducted from the outdoors, and the exhaust is power vented. That would allow you to block up the port to the chimney, which lowers the actual heat load, since the parasitic drafting of the chimney when the boiler isn't firing goes away. But it's probably going to be more expensive to install than the smaller mod-cons, since the exhaust venting has to be stainless steel instead of plastic.
One thing have noticed on my mod-con is that the current control software is still somewhat "lacking" in it's ability to modulate down fast enough (if it's running at a mid range to high output rate) to prevent bouncing off the high limit and shutting down combustion when a large zone shuts down leaving just a short zone running.
Once the SWT drops then drops down below the differential setpoint (because the short zone is still calling for heat) the boiler goes through the whole pre-purge and startup procedure again in response to the call for heat.
If it happens enough times it may be putting excess wear and tear on the boiler with many unnecessary cycles.
If you can combine some of the short zones into longer zones, you will mitigate the above situation.
Just a suggestion at any rate.....
Once the SWT drops then drops down below the differential setpoint (because the short zone is still calling for heat) the boiler goes through the whole pre-purge and startup procedure again in response to the call for heat.
If it happens enough times it may be putting excess wear and tear on the boiler with many unnecessary cycles.
If you can combine some of the short zones into longer zones, you will mitigate the above situation.
Just a suggestion at any rate.....
deeMatrix
New Member
Thanks again Rob and Dana. More food for thought. This sugar is complicated!
If it were simple even hacks could get it right, and you wouldn't currently have a 117K DOE output boiler serving a design heat load that probably isn't over 42K (including standby and distribution losses.) That is a 2.8x oversize factor- fully twice the ASHRAE recommended max.)
If you have the time and curiosity, time a few burns on the beastie-boiler and count the burns per hour. (This is easier to do with data logger.) Ideally the minimum burn time for a cast iron boiler would be 10 minutes or longer, but 5 minute burn times isn't a disaster as long as it's only a handful of burns per hour.
The down-modulation response time inducing overshoots followed by re-start NY_Rob is experiencing isn't likely to be inducing excessive wear on the boiler. I suspect even with those events his system is still averaging fewer than 5 burns/hour, possibly even fewer than 3. Is it wear & tear, as compared to continuous modulation? Sure, but the impact on efficiency and maintenance costs is still down "in the noise" unless it's happening more than a handful of times per hour as opposed to a dozen times per day. After tweaking in the reset curves obsessively to within 1F of perfection people tend to get spoiled, and like to see it modulating continuously with a single burn lasting from early November through early April with NO flue purges and re-starts.
If you have the time and curiosity, time a few burns on the beastie-boiler and count the burns per hour. (This is easier to do with data logger.) Ideally the minimum burn time for a cast iron boiler would be 10 minutes or longer, but 5 minute burn times isn't a disaster as long as it's only a handful of burns per hour.
The down-modulation response time inducing overshoots followed by re-start NY_Rob is experiencing isn't likely to be inducing excessive wear on the boiler. I suspect even with those events his system is still averaging fewer than 5 burns/hour, possibly even fewer than 3. Is it wear & tear, as compared to continuous modulation? Sure, but the impact on efficiency and maintenance costs is still down "in the noise" unless it's happening more than a handful of times per hour as opposed to a dozen times per day. After tweaking in the reset curves obsessively to within 1F of perfection people tend to get spoiled, and like to see it modulating continuously with a single burn lasting from early November through early April with NO flue purges and re-starts.
As usual, you're almost spot on with your figures!
In my case it's more of an annoyance/pet peeve then anything else as I can predict when it's going to happen but the boiler can't.
It can easily be circumvented via controller board software, but sadly it somehow slipped passed the Korean engineers during the QC reviews.
Lochinvar has way more sophisticated software than my HTP UFT, wonder if they have similar issues?
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