Your thoughts please...

Mark Eatherton

I am doing a bunch of mechanical room energy audits/physical plant reviews, and am seeing a whole lot of copper fin tube boilers (pick a brand) with multiple (as many as 4) stages. These appliances, in some cases, have 4 gas valves, but only two combustion zone pressurization fans. When only one burner is lit, the combustion efficiency is bad (excess air, cooling flue gas, high CO etc). That causes me concern, because most of these boilers spend most of their time on low burn, which doesn't look efficient on the combustion side.



The other thing that causes me concern is the basic design. You have a 2 pass copper fin tube heat exchanger, and regardless of which burner section (1 vs 4, or 4 vs 1) you set up as the lead burner, as the fluid passes through the combustion zone, it immediately flows through an unheated zone, giving up heat to the flue that was paid for.



Ideally, the burner compartments should be divided between the front and the back of the boiler, not left to right, or right to left. This way, the burners could be staged such that no unheated surfaces would see internal heat, nor lose heat to an unfired section.



As usual, in most cases, the appliances are grossly oversized, and this does cause issues with short cycling, and that short cycling is causing the staging controls to compound the problem of system inefficiency. Programming the controls to allow the full burner tray to come on during a call for heat increases the combustion efficiency, but compounds the short cycling inefficiency. (driving a tack with a sledge hammer)



Other than the obvious (replacement with a good mod con boiler) what can a person do with these beasts to make them more efficient (short of complete re-design from scratch on the manufacturers side)?



Tuning them is a given, but it seems to be a bandaid on a bleeding artery type of fix.



Thoughts and comments welcome. (including manufacturers)



TIA



ME

The Steam Whisperer (Formerly Boilerpro)

I agree with your assessment...

that is just plain and simple, piss poor design.  This leads back to a topic I have continued to harp on, the need for some realistic efficiency standard when large boilers are used for space heating.  All you see are thermal efficiencies and the like, which only apply at full load conditions.  They don't take into account problems like you are describing and the general issues of standby losses when large boilers are used for space heating.   For instance, No one in thier right mind could possibly believe that an atmospheric boiler  could be within a point or two of efficiency when compared to a power burner design, especially one with a modulating burner.    Apparently, for years there has been a group fighting to change boiler efficiency standards listed at AHRI to reflect these issues, but continues to get nowhere.  And it is costing the steam/ hot water heat industry potentially its future.   The costs are starting to really pile up now, if the efficiency rebate programs in other areas are similiar to those here in Illinois.  You get only a $400.00 or so rebate for upgrading a roughtly 2 million plus btu/hr gas input boiler to a power burner boiler over an atamospheric....roughly the same as upgrading a 80% furnace to a 95% furnace.  Guess what most multi unit building owners and condos are thinking...get rid of all the boilers and put in forced air.

Zman

I agree

I am getting ready to do some some work on some very similar sounding systems.

I have yet to do the combustion tests but have observed significant short cycling. My plan is to increase the differential on the main loop and tune the boilers. If that does not control it, I am thinking about a buffer tank. Great subject!

Jack

This is exactly the case

On the "tankless water heater as boiler" I've seen. I have been called out on some jobs that had HUGE gas bills with the tankless burning 3 out of 16 burners burners ALL the time. in trying to get the owners to understand I'd tell them to put their car in low and drive at 1500 rpm all the time. Do it for a couple weeks and call me and let me know how it works for you. As I wander around the internet to different sites I am amazed at how many people absolutely insist on doing this.

Steamhead

Is there any way

to run the whole rack of burners in a standard lo-hi-lo configuration? 

NYplumber

.25 to .5

Along the lines of what I was thinking.



Minor rewiring of the gas valves, that both valves engage when their respected fan runs. Essentially making it run on a 2:1 turn down, not a 4:1.



For short cycling, if a large buffer isn't an option, how about incorporating a tekmar or similar control to delay the firing.

Charlie from wmass

simply a poor design

Frank mentions the only way to make it a little better but these boilers are made to make low bidders make a profit. They are short lived poorly designed products. The only way to make them work a bit better would be to make them ramp up and down as a full burner set with controlled air flow. That's what most condensing boilers do. The problem is if the copper fin boilers see condensation they die even faster then usual. Using staged boilers would be better then cramming all the water in and out one set of fins and would reduce the heat loss as the heated water passed through the unheated sections.

DavidGeorge

Product Manager, Lochinvar Corporation

Mark and crew-

Thank you for your inquiry.  You made some very good observations and the design does lend itself over to these ideas.

Allow me to introduce myself.  My name is David A. George and I am a Product Manager for Lochinvar Corporation.  Long time listener; first time caller.  Let me begin by apologizing for the length of my response.

In 1993, Lochinvar developed the very design you describe with its Copper-fin II boiler and water heater series.  The design is horizontal heat exchanger over a row of multiple burners.  It is a stage fired design featuring one or two blowers.  These are non-condensing boilers designed for high temperature boiler systems.  They operate at 85% thermal efficiency.

Your first concern was about bad combustion.  Lochinvar designs for these combustion values at high and low fire on the Copper-fin II.  ~8.0% CO² / >20 ppm NOx / >100 ppm CO / ~320°F Flue Temperature.  Note: our target of 100 ppm of CO is ¼ th the national standard of 400 ppm and the low NOx rating meets all low NOx standards around the country. 

As you know, the true measure of boiler operation is thermal efficiency.  You were concerned that unconsumed air flowing through a set of unfired burners would reduce the heat transfer.  I am pleased to report that is not the case.  The unconsumed air has little effect in stripping heat away from the heat exchanger.  In fact, we see equal or greater heat transfer when one burner group is firing.

The key is increased heat transfer surface area.  Consider the boiler firing at 100% Btu/hr input.  All the burners are firing into a matching surface area over the total heat exchanger from one end of the combustion chamber to the other.  When only one burner group is firing, it has a larger surface area and can transfer more heat.  You can see this dynamic on mod-con efficiency graphs where the highest efficiency is delivered at the lowest Btu/hr input.  

As for oversized systems and short cycling, we could not agree more.  Many of the short cycling issues we see in low mass equipment are the result of the product being used as if they are large mass boilers with a large volume of water.  Low mass equipment is designed to save energy and dollars when properly applied.  These are not “your granddaddy’s boiler”.

Short cycling will shorten the lifespan of all boilers and we ask everyone to do their part to stop this problem.  From our side, we recently added SMART SYSTEM controls to the Copper-fin II which features a Short Cycling protection program.

As you know, we also offer newer, more advanced, modulating, condensing boilers.  So why do we continue to offer the copper-finned tube boiler?  The reason is simple.  There are thousands of high temperature, non-condensing boiler systems that were established long before low temperature, condensing boilers were so commonly available.

As we all know, a high temperature system will keep the mod-con, condensing boiler from delivering the up to 98% thermal efficiency for which it was designed.  The system will operate in a non-condensing mode with non-condensing efficiencies.

However, we can confirm there is an opportunity to condense in the “shoulder months” when outdoor reset or night setback reduces the operating temperatures.  So there is a cost/benefit analysis opportunity here for the owner.

Last note, in addition to the 4-stage Copper-fin II, Lochinvar also produces the 2-stage Copper-fin.  Several of the follow-up comments were interested in that idea, which we offer with the Copper-fin boilers and water heaters. What’s more, the 4-stage Copper-fin II can be programmed to fire in 2-stages by giving the stages 1 and 2 the same setpoint and giving stages 3 and 4 the same set point.

These boilers are the product of extensive design and testing at all the different stages in every combination. Each is designed to meet a specific part of the market similar to the mod-con.  We hope you will continue to use Lochinvar products.

Thank you.

David A. George, Product Manager, Lochinvar Corporation   

 

 

Mark Eatherton

Thank you David...

I always appreciate it when a manufacturer comes to The Wall.



So, if I understand you correctly, If I have a 4 stage boiler that spends most of its time running on a single stage, that the air flowing through the non fired sections of the copper heat exchanger are not that significant as it pertains to heat loss up the stack?



I had one today that was doing DHW, and I had the opportunity to test it under 3 different conditions. They and the C/A results are as follows;



Boiler size; CFN1261PM

Altitude: 5,280 ' A.S.L.



1 stage;

O2 = 17.3

CO = 9 ppm

Efficiency = ---

CO2 = ---

T stack = 156 degrees F

T air = 66.6

Excess air = ---

CO (air free) = ---



With 2 stages running,



02 = 12.0%

CO = 28 ppm

Efficiency = 84%

CO2 = 5 %

T stack = 221 F

T air = 69.7

Excess air = 119.4%

CO (air free) = 66 ppm



With all 4 stages running,



O2 = 7.6 %

CO = 57 ppm

Efficiency = 83.6%

CO2 = 7.5%

T stack = 294 F

T air = 70.4

Excess air = 51.1%

CO (Air free) = 90 ppm.



I should note that this boiler is connected to a 120' tall stack, and that there were no barometric dampers on any of the other (4 total) boilers, and draft at the appliance breaching was -.08" W.C. I have recommended the installation of barometric draft control dampers in my report.



What concerns me, is the information gotten when the appliance was running on the first stage. I had no flow meters on either the gas or the water, so I really couldn't tell you what the net thermal efficiency of the appliance was at that point, and am hoping that you can address this and let me know if there is a major issue, or if it is normal to not be able to see what the combustion analysis results were on low burn. These results echo my other findings on low burn. The boiler was running the DHW (direct), and entering water temperatures were probably in the range of 100 degrees F. I intentionally cooled down the system so I could run the boiler through its paces and record the results.



My concern is, that this boiler spends MOST of its time during the day running on just the first stage. What has your labs found in the way of thermal efficiency at low burn rates. I honestly can't tell you what it is, because there was not enough CO2 in the flue gas stream to allow my C/A to determine what was going on.



Again, I (we I'm sure) appreciate your willingness to come and speak to our concerns.



Thank you for being here for us.



ME

furnacefigher15

Mark

I have run into this very same situation many times.



The bigger issue I have is that I've been running across burners that are badly deteriorated on the stages that see more off time.



Without physical dis-assembly, the burners are not visible, the only portion that can be seen is the venturi of the burners.



There have been multiple occasions where I have been the third or forth service company called to look at the boiler. No one else bothered looking at the burner, but rather just started replacing controls when ignition problems started to occur.



The trouble seems more prevalent in systems using direct outside air for combustion. I have concluded that there must be condensation occurring in the off sections causing the trouble with the burners.

Charlie from wmass

My Grand daddy's boilers last for 100 years

Yep they are terrible, only 80%- 86% efficient when installed properly and maintained properly. But that is neither here nor there.

Is the combustion issue discussed here brand specific? Do all fin-tube heaters use the same firing pattern?  How does the change fire rating change the venting requirements? Vertical venting can be adversely effected by low flue temps. how is this address with the large draft hoods? Would they be better served by using adjustable barometric dampers so the draft could adjust as the fire rate changes?

icesailor

Intake Air:

If you are in an area with a lot of cold humidity, you can really have a problem. Cols air will hold a lot of moisture and when temperature and dew-points get together, problems occur. Some boiler manufacturers are suggesting to not take fresh air from outside if you are near the ocean or areas where the intake air has a lot of moisture.

I saw a 5 YO direct vent horizontal furnace, properly installed, with the intake/exhaust facing the West and the PVC exhaust was absolutely black, it was running so black. I told the owner to get hold of the guy who installed it.

I don't know what the answer is.

DavidGeorge

Thank you for the report

Mark, wow, this is an excellent and detailed report.  If we got regular feedback like this, our Tech Service department could conquer the world.

Your idea to add a barometric damper is a logical conclusion that we support.  We'd like to see the draft drop from 0.08" negative to about half that or more.

To the questions from Charlie from wmass, yes sir, different manufacturers arrange multiple stage boilers in different firing patterns.  We have designed our product for simple construction but optimum operation.

Lochinvar, like many manufacturers, makes a variety of products to be installed in widely different conditions for Alaska to Florida.  You guys are the local experts in your area and we benefit when you chose the best product for your customers.

Thank you.

David A. George, Lochinvar

Mark Eatherton

Additional information...

David, Thank you and Lochinvar for responding. As promised, I did get an opportunity to check another 4 stage unit, and was able to start on the 4th stage, and back my way down to the single stage, recording the combustion analysis data for each stage along the way.



Stage 4;



O2 = 7.4%

CO = 67 ppm

Efficiency = 83.7

CO2 = 7.6%

T stack = 293 F

T Air = 71.3

Ex Air = 49%

CO (AF) = 104 ppm



Stage 3



O2 = 7.2%

CO = 62 ppm

Effic. = 83.5

CO2 = 7.7 %

T stack = 303

T Air = 71.2

Ex Air = 47.2

CO(AF) = 95 ppm



Stage 2



O2 = 11.1

CO = 57 ppm

Effic. = 81.6%

CO2= 5.6 %

T Stack = 293 F

T Air = 71.4

Ex Air = 100.1

CO(AF) = 122 ppm



Stage 1



O2 = 14.8%

CO = 46 ppm

Effic. = 78.3 %

CO2 = 3.4 %

T stack = 267 F

T Air = 71.7

Ex Air = 217.7%

CO(AF) = 160 ppm



Draft was monitored during all stages and stayed steady at -.02" W.C. (properly adjusted barometric dampers.)



These boilers were doing space heating and I had cooled the loop down to force it into a full burn scenario. Water temperatures were between 160 and 180 degrees F during the testing.



I am not allowed to make adjustments to the appliances during these evaluations, and I suspect that there were "issues" based on the amount of CO (Air Free) in the flue gas samples. These results, unfortunately, are quite typical of what I see in the field on a regular basis. Preventative maintenance is virtually non existent (Housing Authority) and I would LOVE to go back and use these appliances as a test example to show what kind of increase is possible with a little fine tuning. They operate under the premise of "If its HOT, don't mess with it..."



I appreciate your involvement and contributions, and think that Lochinvar is a stand up company for allowing contributions like yours.



What kind of numbers SHOULD a person see under "ideal" conditions?



Thanks for commenting.



ME