Too much modulation?

hot_rod

A good read on high turndown burners and potential problems associated with high turndown. I'd imagine this would apply to smaller residential equipment also?

Thanks to David Connors with Patterson- Kelley

https://www.hpacmag.com/features/boiler-modulation-better/

SeymourCates

An interesting article and technically correct. However, the authors have not done their homework regarding the current crop of 10:1 mod-cons. They typically run with not more than 30% excess air at full modulation, thereby mitigating nearly all of the claimed inefficiency.

They are completely accurate regarding the inefficiency created by stopping and starting a mod-con repeatedly which will occur on a regular basis in most installations with 5:1 mod-cons.

hot_rod

SeymourCates said:

An interesting article and technically correct. However, the authors have not done their homework regarding the current crop of 10:1 mod-cons. They typically run with not more than 30% excess air at full modulation, thereby mitigating nearly all of the claimed inefficiency.

They are completely accurate regarding the inefficiency created by stopping and starting a mod-con repeatedly which will occur on a regular basis in most installations with 5:1 mod-cons.

Laminar conditions and hot spots in the metal? I certainly see a lot of failed heat exchangers in my travels, many just a few years old. I think he is spot on with the causes of the failures we are seeing in the field.

I see and get data from the field like this all the time. This one last week. I travel to over 100 wholesalers a year, I see what is in the returns section

An e-mail last week:
" Had to change a HX on an 8 month old fire tube boiler. Fill water was DI with inhibitor added. System ran at 127, never above 50% modulation since installed"

It seems like fire tube design is more prone to early failures according to what I see and here. I know they tend to trap air up high, so that metal over-heating is a viable observation if attention is not paid to that condition.

The cause of the HX failure rarely gets relayed back to the installer and HO, if in fact it is ever diagonised by the manufacturer?

SeymourCates

A couple of questions:

Were these failures the purvey of 10:1 mod-cons, exclusively? Or is it possible that the 5:1 mod-cons also suffered similar failures due to improper installation (insufficient waterside flow)?

Most mod-cons still require P/S for the specific reasons cited in the article. The manufacturer is greatly concerned about localized overheat at low flow rates.

The discussion regarding laminar flow on the fireside is interesting. If this was truly the situation, the manufacturer of said equipment would suffer 100% failure rates as the mod-con typically spends more than 30% of its time operating at minimum fire or close to it. You really have to make the assumption that the fireside has been tested at minimum fire for extended periods of time to ensure that the aforementioned possibility is not going to compromise the installation.

Mark Eatherton

Interesting article, but I think they are killing gnats with a sledge hammer with their points. They are making a LOT of assumptions (lower water flow velocities?). It almost reads like a 'we can't get our burners to do well below 20%, therefore anyone who can is pushing the limits of modulation'... Interestingly enough, none of their boilers go less than a 5:1 turn down.

To me it looks like an article that was dictated by the marketing company, and fulfilled by the engineering department. And before all the haters jump on me, I've installed PK boilers before, and found them to be just as troublesome as any other large modulating boiler, pick a brand. It seems to me that higher turn downs have been around long enough that if in fact there was a problem, the industry would be well aware of it by now and would be backing away from high turn down ratios.

In fact, I met the chief engineer (at the time) for VIessmann boilers, and asked him what their turndown ability was with the Matrix burner. He said that they were able to do a 100:1 turndown (1% increments) and that the only problem they encountered was flame signal stability. No mention of hot burners, or boiling or significantly changing dew points.

I think one has to read it with a grain of salt....

I also remember sitting in on a hydronics institute meeting where they were discussing the allowable turn down ratios allowed for gas burner boilers. At the time, 2:1 was the minimum recommended standard (atmospheric, in shot burners) and there were a couple of staunch opponents to allowing anything greater than a 2:1 turn down. It was held back for numerous years until the good ol' boys club decided to allow greater turn downs to occur. And to their point, an atmospheric burner can NOT be turned down too low, or the burner will experience flash back and flame aspiration in the primary mixing ports of the burner. None of the good ol' boys club had a premix variable speed burner available, therefore they didn't want to approve it.

Times change, and so does technology. I know some people who could write a book on the fallacies of using aluminum in direct radiant exchanges as well...

ME

hot_rod

SeymourCates said:

A couple of questions:

Were these failures the purvey of 10:1 mod-cons, exclusively? Or is it possible that the 5:1 mod-cons also suffered similar failures due to improper installation (insufficient waterside flow)?

I doubt any manufacturer analysis every failed boiler, I know one wholesaler that had over 400 failed last year, I suspect some are water quality related, some weld issues, an some thermal stress related. I wonder if any were actually analyzed for failure,cheaper and easier for manufacturers to field scrap and honor or refuse warranty.

Most mod-cons still require P/S for the specific reasons cited in the article. The manufacturer is greatly concerned about localized overheat at low flow rates.
Actually one of the selling features of fire tube design is direct pipe, no P/S or separators required, if circulated properly. Cast iron boiler run without any flow, don't try that on any of the mod cons I know.

The discussion regarding laminar flow on the fireside is interesting. If this was truly the situation, the manufacturer of said equipment would suffer 100% failure rates as the mod-con typically spends more than 30% of its time operating at minimum fire or close to it. You really have to make the assumption that the fireside has been tested at minimum fire for extended periods of time to ensure that the aforementioned possibility is not going to compromise the installation.

I think you assume too much WE are the R&D department for many of the new high efficiency residential designs.

SeymourCates

You might be correct.

However, 400 failures of unknown origin cannot really condemn any specific manufacturer unless they were all from the same manufacturer. If from various manufacturers, the number of failures versus the number of boilers sold is still a small percentage.

I'm usually quite skeptical myself but I have a difficult time believing that various manufacturers are offering a 10 year HX warranty knowing full well they will never achieve it.

Regarding the "selling feature" of the firetube design:

Take a look at the installation manuals and you will find the direct piped system is posted at the very end of the installation diagrams with all kinds of caveats. Now, why would that be if the manufacturer was not greatly concerned about flow rates?

Even the latest crop of 1 GPM firetube designs show P/S piping as the preferred choice.

hot_rod

Mark Eatherton said:

Interesting article, but I think they are killing gnats with a sledge hammer with their points. They are making a LOT of assumptions (lower water flow velocities?). It almost reads like a 'we can't get our burners to do well below 20%, therefore anyone who can is pushing the limits of modulation'... Interestingly enough, none of their boilers go less than a 5:1 turn down.

To me it looks like an article that was dictated by the marketing company, and fulfilled by the engineering department. And before all the haters jump on me, I've installed PK boilers before, and found them to be just as troublesome as any other large modulating boiler, pick a brand. It seems to me that higher turn downs have been around long enough that if in fact there was a problem, the industry would be well aware of it by now and would be backing away from high turn down ratios.

In fact, I met the chief engineer (at the time) for VIessmann boilers, and asked him what their turndown ability was with the Matrix burner. He said that they were able to do a 100:1 turndown (1% increments) and that the only problem they encountered was flame signal stability. No mention of hot burners, or boiling or significantly changing dew points.

I think one has to read it with a grain of salt....

I also remember sitting in on a hydronics institute meeting where they were discussing the allowable turn down ratios allowed for gas burner boilers. At the time, 2:1 was the minimum recommended standard (atmospheric, in shot burners) and there were a couple of staunch opponents to allowing anything greater than a 2:1 turn down. It was held back for numerous years until the good ol' boys club decided to allow greater turn downs to occur. And to their point, an atmospheric burner can NOT be turned down too low, or the burner will experience flash back and flame aspiration in the primary mixing ports of the burner. None of the good ol' boys club had a premix variable speed burner available, therefore they didn't want to approve it.

Times change, and so does technology. I know some people who could write a book on the fallacies of using aluminum in direct radiant exchanges as well...

ME

The article does reference data from Johnson boiler research. So at least two marketing departments dictating engineering put out this misleading info?

I wonder why all the current Viessmann resi boilers are limited to 6.5-1 turndown? The 100, 200, and 222. I recall a conversation with them predicting failure issues with higher turndown rates.
The higher turndown Euro boilers I have seen use multiple gas valves and burners.

I think the higher turndowns we see are actually a damper in the inducer air flow, not actually lowering burner output below 6-7:1?

RayWohlfarth

I enjoyed the article and think like the others it should be a case by case basis. Oversizing can do tremendous damage to a system. A couple things I have found with some higher turndowns is the burners sometimes require more air at low fire to allow better turbulence to mix the fuel so that lowers the dewpoint temp. In some instances the velocity is excessive at low fire becaue the combustion chamber is designed for the furnace pressure at high fire. Another factor is the Jacket loss of the boiler into the boiler room. If it is 2% of the boiler rating at full boiler capacity and we have a 10-1 turndown, our jacket loss jumps to 20% of the input. A couple of independent sources:
EPA says boiler efficiency drops significantly when operated below 50% firing rate and is most efficient when fired between 50-80% Honeywell says a Low High Low burner uses 10-15% less fuel than a modulating burner. I am not sure who you believe
Hot Rod, I agree I have been the R&D arm for many boiler companies over the years. Thanks for stimulating the neurons today. The picture below is from a job with an oversized boiler that loafed at low fire all winter.

Tinman

The higher water volume in the Vitocrossal 300 makes it a pretty darn good choice for a 5:1 boiler.

Eastman

The Johnston technical brief that this article references is focused solely on a stack loss comparison between 4:1 and 10:1 modulation rates. I care less about boiler thermal and combustion efficiency than total system efficiency. Also, the 10:1 condition ran with 70% excess air.

Gordy

case, and point when the 10:1 KHN line gets down to the smallest boiler in their lineup it’s KHN 56 Peters out at 6.6:1.

I think the direct pipe method should be full of caveats with any critical flow rate dependent boiler since the variables (installers ability to calculate proper pump selection,and systems variable requirements) go up astronomically in achieving proper flow in all conditions.

I often wonder why the big V doesn’t follow suit in the 10:1 market.

Eastman

@Gordy

I kinda suspect that it's more difficult for water tube boilers to achieve higher turndown ratios. Also, wouldn't gravity negatively effect gas flow across the heat exchanger in horizontal arrangements?

Solid_Fuel_Man

I've often wondered why we don't see some arrangement of a dual burner for high turndown arrangements. Anyone with a combustion meter is well aware of the "need" for higher excess air the lower the fireing rate is to keep the physical burner cool as @RayWohlfarth showed. I just setup a burner with 40% exc.air at low fire, what the spec called for. A smaller burner would have less exc.air to blow the heat up the flue and raise the dewpoint.

The whole argument proves that in larger markets multiple boilers is more efficient, maybe even an asymmetrical setup where the lead boiler is smaller. Rather than 2 boilers at 10:1. Who knows?

SuperJ

The commercial Vitocrossal is a 3 boilers in one. Ie. 3 burners each with 5:1 turndown.

Solid_Fuel_Man

If only it went so darn expensive. It is a great boiler and wonderfully engineered. I looked into one and at 2.5times the cost of my fire tube of choice it ain't gonna fly very high.

jumper

>>Times change, and so does technology. I know some people who could write a book on the fallacies of using aluminum in direct radiant exchanges as well...<<

So do codes. Now allowed to treat exhaust to meet air quality standards. So future might be energy efficient combustion and then deal with NOX at the back end?

njtommy

I think the Lochinvar crest 2.5 million btu boiler is something like 20.1 tdr. That’s pretty impressive. I’m sure it’s challenging to set up a burner to run at 17-30% excess air.

Look at the Fulton pulse boilers. You could run them at 1% to 100%. We’ve had a few that where absolutely nightmares. To keep running for long periods of time.