Modulating is it efficient?

Tim McElwain

Definitely a point for some discussion. Assumption here is that the system piping and duct work is compatible with the heating unit. A heat loss was done and system and heating unit are matched up.



When we look at say a 100,000 BTU boiler or furnace that fires on every call at full input is that efficient? Taking into consideration proper cycling rate.



If we however cause that system to modulate based on outdoor reset is it more efficient?



Now what about the combustion issue? When we fire at less than full input into a chamber designed for full input what is the trade off?



If we are firing at a lower input is excess air still the same?



What is the Stoichiometric (ideal or perfect)  Flame temperature for Natural Gas?



Is flame temperature affected by modulating?



Which has the bigger problem with the modulating concept as far as efficiency the boiler or the furnace?



Are there any charts or tables available for determining comparison of combustion analysis related to input on low fire versus high fire? Compared to Flue gas temperature on low fire and on high fire?



A lot of questions hopefully we can get some good discussion or maybe just get us thinking a little more about what is really happening with all this new equipment.

Jean-David Beyer

If we are firing at a lower input is excess air still the same?

I am not a professional, but I would love to know the answers to the

questions you posed. They seem important to me. I may well be naive

about this, but I would have assumed that boiler manufacturers would

have dealt with these issues, but the fact that you ask these questions

makes me question my assumptions.



Do you settle for manufacturer's claims, or do you want actual measurements?



On my mod|con boiler with outdoor reset, the manufacturer claims that the balance between the natural gas input and the air input is regulated. And it seems to be so in that the mixing area (before the mixture enters the combustion chamber -- heat exchanger) is a lot like an internal combustion engine carburettor. It does not need to evaporate the gas, of course, but the gas supplied depends on the air flow through a venturi, and the air flow blower is varied to do the modulation. So if it is designed right (I have no way of knowing this), it should always run at the proper ratio of gas to air.



I assume the way they do this is no secret, so any mod|com boilers can do much the same thing. I guess this assumption can be wrong.



"Is flame temperature affected by modulating?" I am not sure I understand your question. If you want the boiler to be modulating, you have to change the heat input to the thing. So you either change the flame temperature, or the amount of flame. I suspect it is hard to vary the temperature of the flame very much and still get good combustion, so I assume they just change the amount of flame. Is that not right?

Tim McElwain

I personally want to see some real

data on these efficiency claims.AFUE is not enough for me to get all excited and start turning all GREEN with envy.



A gas representative I was in the company of posed this question. Why if we are seeing all the rebates for higher efficiency do those systems when tracked for actual gas consumption based on degree day analysis not show some real changes in usage? What do you think could be the cause for this?



Does lower flame temperature affect efficiency?

eluv8

that is the right question....

I am interested to see what people have to say.



When it comes to combustion and turndown you have to make a sacrifice, how much of a sacrifice well that's debatable and seems to vary by manufacture. Another factor to consider is whether the appliance is condensing or non-condensing and its 

application.



 I would not go as far as to say that modulating appliances do not save money. I have seen to many projects in the field saving 30%-40% and sometimes more when switching from a non condensing low-hi-lo or on off appliance to a modulating condensing appliance. If that is what you are seeing I would consider looking at a different brand of modulating appliance or project design.



I have seen a chart, like many that shows the relationship between excess air, and condensing. Very enlightening.

Robert O'Brien

Tim

Do you really think a fixed input appliance is more efficient? Viessmann's Lambda Pro system targets a constant 30% excess air. I have had combustion analyzers on many mod/cons and I've never seen more than 40% EA ever,so I assume the fan RPM/gas input model manufacturers use is pretty close in the real world

Tim McElwain

I am not saying

anything is more efficient than anything else, I am asking questions to get everyones input as to real world data.



When people state here and elsewhere savings as high as 50%, I always ask 50% of what. Was it an old oil or gas system that was either not being maintained or serviced.  There has to be some point of real reference or do we just take everyones word for it?



I have seen gas systems in my many years of service that had not had anyone touch them for 10 to 15 years. That was something that was sometimes promoted by gas suppliers for years. The truth was when you tested those units they were lucky to have 60% combustion efficiency.



I have tested a number of oil systems right after they were serviced and the tag read 78 or 80% efficiency and could only get 65% using three different testers.



Here are some figures on one Mod/Con which will go unnamed:



CO2 10.5%  O2  2.3% (10% excess air) 200° net stack 86% eff 100° net stack 88.% efficient CO 100 PPM air free.



CO2 8.8%  O2 5.3% (30% excess air)  200° net stack 85% eff  100° net stack  87% efficient CO 100 PPM air free.



There is very little change from one level of excess air and temperature to the other. Would perhaps one set of readings have been on low fire and the other on high fire?

Unknown

Interesting thought Tim,,,

But if the heat is not lost up the chimney,, where is it going?

NG gives out a certain BTU perC/F or therm, if not wasted to higher stack temperatures (hence PVC use), does that not mean we are recovering more from the modulated flame?

Every M/C HX  may be too large 95% of the time,, but advanced burner technology & In/Out reset should make a huge difference.



How would one consistently alter the size of the HX to exactly match the flame at this "ideal" temperature?

bob_46

Flame Temperature

Theoretical for Nat. gas 3600 Fº no excess air. Depending on exact composition of gas and excess air 1500 Fº to 2600 Fº.

R Mannino

Comparisons

In the industry we make technical comparisons but the customer only makes one, dollars.

Constantin

Hmmmm...

As a start, I'd look at the different forms of heat transfer out there: radiant, convective, and conductive. A heat exchanger that is optimized for modulating boilers has to manage to scrub the flue gases just as effectively as when the burner is running on full-fire. I believe that is why we see a predominance of heat exchangers with very small openings for the flue gasses to pass through (i.e. a 1mm or less) in the condensing/modulating market. A radiant-dependent HX design wouldn't do as well for multiple input levels, IMO, since the flame luminosity is likely lower at low-fire than at high-fire. Not sure anyone does conductive HX, since that likely leads to CO, and other unwanted byproducts.



So, I would argue that a proper HX combined with modulation should yield higher efficiency (all things equal) in a furnace or a boiler since you're increasing the amount of surface area for a given amount of heat transfer. The charts that Viessmann has published re: Vitodens performance as a function of ΔT and firing rate seem to confirm that trend. However, the trick is for flue gases to have good contact with the HX surfaces, regardless of flue gas speed/rate.



Thus, I'd imagine that a very 'throaty' HX like the tubes typically found in the primary HXs of furnaces don't do as well at low-fire as at high fire since the flue gas flow at low firing rates may be a lot more laminar, thus insulating the hotter flues gases at the center of the tube. At higher firing rates, the turbulence caused by the inducer likely improves the HX performance. So the benefit of the HX surface area vs. required heat transfer may be negated by the lesser turbulence in the tube, leading to consistent AFUE performance across multiple firing rates or perhaps even a decline at low firing rates.  IIRC, Steve Ebels once tested a two-step furnace at both firing rates and found consistent AFUEs. 



Whether or not the burner has consistent combustion conditions at various input ratings is likely a function of design. IIRC, some current designs either meter the gas as a function of the air, while others meter the two completely independently. IMO, the independent solution is the better (tough likely more expensive) one, since it could allow you to account for changing caloric content in the natural gas, etc. in a way that a fixed ratio does not.



I noted with the many furnaces we just tore down at work that the secondary HX tends in condensing models have much narrower passages, sometimes with the addition of baffles, than in the primary heat exchangers. Perhaps not as sophisticated as some of the boiler heat exchangers, but clearly with the same aim: i.e. narrow passages scrub the flue gases more effectively than wide ones. Yet, on account of the lower modulation range of blower systems in furnaces and the lesser ability of air to transport heat (than water), I doubt that furnaces will offer the same wide range of inputs vs. boiler systems.

Jack

Other benefits

to modulation...when correctly sized, are much reduced starts and stops which in turn leads to better component reliability and seasonal efficiency. I don't intend this as an ad, but I've sold over 200,000 pieces of modulating Rinnai equipment over the last 20 years and when I first began living with the DV wall furnaces, I'd look at it operate and try to figure out, " now, why is it doing that?" My dv will start and stop on the t'stat in Sept and Oct, but by late Oct there is sufficient load to meet the 8,200 btu minimum fire. Last year the unit fired continuously from late Oct to March 24th. It simply modulated on the demand. It did not shut down in that period of time. It has done this for 12 years with no more than blowing off the fan blades and pcb area to keep dust off it. What, to me, is important is not how high a unit will fire but how low it will fire.Oversizing impacts the whole equation negatively.

What affect on efficiency does constant ignition have? The many propane companies who have installed these have said it is a dramatic reduction in usage for the customers as well as a dramatic increase in comfort. I can't offer the science you are looking for Tim, but this is my experience.

Glen

In the last ten years

I had the good fortune to commission a wide variety of retro fits; CI to Vertomat and Rondomat, Dedietrichs, Laars, IBC etc - so I have a bit of data in the archives. The quick answer to your main question is: it depends. The very best results (consumption decreases) was accompanied by equipment change and operating parameters, eg. lowering design temps wherever possible. And there was the greatest challenge - popping in the latest modulating burner (or staged burner for that matter) with ODR without an operating change sometimes did not produce the expected xx% savings. My latest data is a WM CI boiler changeout in favour of a IBC condensing unit; emitters were baseboard and we added indirect DHW. Comparing annual consumption the client saved 35% during heating season but only >5% during the summer (DHW only). The apartment residents reported a more moderate heat delivery and the twice daily - no hot water syndrome disappeared. I noted in the firing logs (built in onboard the IBC control platform) that the burner reached 85% firing rate only when there was a call for heat from the indirects. I would think that even greater savings could be realised if the residents were slowly weaned off 125 deg hot water. I have all the data archived - so it will take a bit of time to retrieve. but I would think IBC still has the data too. hope this helps the discussion.

SpeyFitter

How about System benefits of modulation?

As far as I understand it, don't a lot of mod-con gas boilers expierience higher efficiencies at their lower modulation ranges?

There are other things to factor in too. Metal fatigue from constant expansion/contraction from short cycling. They say that cars would last much longer (as an example) if they were warm all the time instead of the constant start/stop of being turned on and off everyday.

There is something special about watching a mod-con fire at a nice low firing rate during a shoulder season knowing it's getting some nice efficiencies.

And even on a high temp load I think a modulating burner makes sense. How about the thermal expansion/contraction of your piping which flexes and puts pressure on 90's and joints as they expand & contract. This with a nice variable speed pump & outdoor reset (say in an apartment building) will reduce erosion and keep the pipes at a nice warm temperature to ensure just enough heat is given off.

eluv8

combustion numbers

CO2 10.5%  O2  2.3% (10% excess air) 200° net stack 86% eff 100° net

stack 88.% efficient CO 100 PPM air free.







CO2 8.8%  O2 5.3% (30% excess air)  200° net stack 85% eff  100° net

stack  87% efficient CO 100 PPM air free.



These numbers do not surprise me when running a 180 setpoint,  it seems to me the differential is off a little and running a little rich compared to most Mod/Con boilers I am familiar with. As well as an elevated CO.



Excess air is more noticeable when running the boiler at lower temperatures according to the chart due to its affect on the boilers ability to condense.

Tim McElwain

eluv8 those figures are actually

the minimum and maximum readings allowed on a very popular MOD/CON.



I have requested from them what typical figures should be on low fire and high fire.



By the way 100 PPM for CO is not high for most MOD/CON that I have noticed in specs from manufacturers. Particularly on High fire you may see as high as 80 PPM to 125 or one manufacturer lists 135 PPM as a maximum. On Propane as high as 150 PPM. 

Tim McElwain

By the way I have invited my good friend

Jim Davis (NCI) to joins us in this discussion he may have some interesting insight to all of this. I hope he will be able to join us.

Constantin

How about that maine system I was involved with...

... replacing two 84% AFUE Buderus 124X for one Vitodens 8-32. Besides being 2x over-sized, this supply-house designed system also had endemic issues with air entrapment on the upper floors, a very hot boiler room, and near-boiler piping that made my hair stand up straight. So there were a lot of issues besides swapping a proper boiler in a proper system for a proper condensing/modulating boiler.



But the customer saved 46% on the propane bill (despite rising LP prices that year) without any other changes to use. And, unlike years before, the house now heats very evenly, without noise, etc. Point being, that a 12% difference in AFUE doesn't  tell the whole story.  It's as good as any indication that there are significant penalties associated with oversizing single-stage systems that by definition will be oversized 97% of the year, even if they were properly sized in the first place! Modulation addresses that major issue (nameplate capacity vs. required input) for much of the year.



A high-mass system may be able to ameliorate some of the issues associated with single-step firing under non-design-day conditions (i.e. limit short cycling) but a better approach is to match the input to the heat loss. That's what modulation allows across a certain range of input capacities. With time, I expect boiler manufacturers to extend the modulation range of their gear to lower and lower input capacities. However, there are diminishing returns, especially for low-mass systems (i.e. less standby loss, less mass to heat, etc.), so it might make sense to cap the lower end of inputs at 15kBTU as most manufacturers do and to design the gear to handle the resultant short cycling during late spring and early fall/winter.



This experience is why I decided to abandon Ms. Vitola (who is almost properly sized) and retrofit a Vitodens, rather than fit a gas burner to Ms. Vitola. 

burky1957

Lifecycle costs

Should we also be looking at life cycle costs?

Initial investment.

Maintenance

Replacement parts

Equipment life

I'm all in favor of energy efficiency.

But at what cost?

burky1957

Lifecycle costs

Should we also be looking at life cycle costs?

Initial investment.

Maintenance

Replacement parts

Equipment life

I'm all in favor of energy efficiency.

But at what cost?

Tim McElwain

burky1957 yes we should but

would like to try to keep on target to the questions posed at the top of the post.



Thanks for the input and by the way each one of those could be an entire posting individually.

Tim McElwain

I would like to see

some figures posted here of actual combustion testing done on equipment at time of setup, then 6 months later or a year and get some comparison as to changes in efficiency as the units get older. Remember most of these require annual cleaning and testing. Anyone have such information?

Unknown

Actually Tim,

I think ME had some data on his-own Munchkin unit he "purposely" left a few years to find-out exactly what you`re asking. Give him a shout.  ;-)  

ttekushan_3

Observations from a different perspective

Most of my observations are from Steam World (yea, I'm a Vapor Head), where the boiler temperature never changes regardless of firing rate and that modulation is base not on outdoor temperature but steam demand (as measured by gage pressure at the boiler).





Q: "When we look at say a 100,000 BTU boiler or furnace that fires on every call at full input is that efficient? Taking into consideration proper cycling rate."



Proper cycling rate is critical. Assuming we're load matched to EDR and the system is well balanced, firing the boiler at full such that it satisfies the Tstat as the boiler approaches pressure cycling, we're golden. I don't think we can get better than that. It rarely occurs but it sure is ideal. Given that combustion time, temperature and turbulence are important factors, as are the Temp diff across the heat exchanger and the heat exchanger material/thickness, we have the temp difference and turbulence working in favor of efficiency at full fire, as is the likelihood that excess air will be at its lowest.



Q: "If we however cause that system to modulate based on outdoor reset is it more efficient?"



N/A to steam unless working a vacuum subatmospheric system; I would say yes to HW if a) the HX provides proper turbulence b) air/ fuel mixture is constant, c) we are utilizing condensing technology and d) the returning water temp is low enough to allow substantial condensing.



Q: "Now what about the combustion issue? When we fire at less than full input into a chamber designed for full input what is the trade off?"



More time, less turbulence. HX design important here, i.e. water side flow rate, fireside surface area and gas passageway design. Some steam boilers with good modulating burners measure about the same efficiency regardless of firing rate. Mostly seen in some of the flexi water tube boilers like the Bryan. The old W-M MGB atmospherics (a tall, narrow boiler of low water content) claimed equal efficiency down to 20% firing. Perhaps somewhat a fictitious claim, but the unusual height and HX design might introduce enough added time for the gasses to transfer heat.



Q: "If we are firing at a lower input is excess air still the same?" Certainly not on an atmospheric with a draft hood!



Q: "Is flame temperature affected by modulating?" Educated guess here- I think the flame temperature is the same, only there's less of it. This still leaves the question of transfer characteristics thru the HX.



Q: "Which has the bigger problem with the modulating concept as far as efficiency the boiler or the furnace?" Dunno for sure but I think the boiler would have the bigger problem since the return water temp changes dramatically throughout the cycle and between cycles (and variable temp gain through the HX), whereas the furnace has a pretty predicable and low return temp.



So here's the flip side: On atmospheric steam with taller boilers (e.g. Peerless, W-M JS series, etc.), I have achieved excellent fuel savings per DD by introducing hi/lo firing even though the excess air has increased. The JS in particular develops quite a bit of excess air at a 45% firing rate. At this firing rate (which occurs when only one of two large zones is open) you can barely hear the burners, yet you can hear the steam rushing through the 6" outlet into the 8" header and through the 4" motorized globe valve. And this with a boiler steam pressure of less than 1# psig.



I can only assume that the benefit derives from the lack of short cycling (with the resulting steadiness of the steam drive and evenness of heat distribution) and the boiler design itself. It seems to me the where the excess air travels is of some importance. I recall reading some literature by Exhausto where they show one way excess air can envelop the flame, providing an insulating cushion of air between the hot combustion gasses and the HX surface. The greater the turbulence a design gives (like a tall boiler with rectangular "pins") the less likely that cushion is to develop in the worst possible place.



I use tools like this:



Combustion Efficiency and Excess Air

Tim McElwain

Reply from across the Atlantic (England)

Definitely a point for some discussion. Assumption here is that the system piping and duct work is compatible with the heating unit. A heat loss was done and system and heating unit are matched up.



When we look at say a 100,000 BTU boiler or furnace that fires on every call at full input is that efficient? Taking into consideration proper cycling rate.

- if any sort of burner fires at full rate every time, then it must waste a lot of energy overall, simply because it's using more energy MOST OF THE TIME than is actually required by the heating load.  Most boilers are over-sized anyway and need to be able to meet the maximum demand (ie. coldest predicted outside temperature) which is not the situation most of the time.



If we however cause that system to modulate based on outdoor reset is it more efficient?

- obviously!  In addition, there is the issue of type of boiler (conventional or condensing type).  If it's a high-efficiency condenser, then it's VERY unlikely that it will achieve maximum efficiency at full power.

- also VERY important to define your terms.  You have not stated exactly whether you are asking about a 'fully-modulating burner' setup or about weather compensation, or both. Burner modulation adjusts its RATE to match heat demand, with a modulation algorithm designed so that it slows down as it approaches a Flow setpoint and then maintains that Flow setpoint by burning at exactly the right rate.  Weather comp (aka

'outside temperature setback or reset') adjusts the Flow setpoint to match the heat-loss of the building and the furnace MUST be fully-modulating to achieve this.  The reality of most high-efficiency boiler designs is that they CANNOT achieve maximum efficiency at maximum temperature!  If the Flow setpoint exceeds (say) 75 degrees, it's most unlikely that the boiler will be able to condense AT ALL - the Return temperature will exceed 55 C degrees, above which condensation is physically impossible.  Most HE boiler control algorithms will prevent a delta-T of more than 20 degrees between Flow and Return, usually by throttling the burner to limit the increase in Flow temperature until the Return temperature catches up.  In a badly-designed system, you can get the situation where the Return temperature never rises enough and the boiler fails to reaches its setpoint.

 

Now what about the combustion issue? When we fire at less than full input into a chamber designed for full input what is the trade off?

- why would you design a furnace only to work at max output???



If we are firing at a lower input is excess air still the same? 

- with a zero-pressure governor, the fuel/air mix is close to stoichiometric.  Excess air is not present because the whole system is driven by the RPM of the combustion fan: increase fan revs = more airflow through the venturi = more gas pulled through governor = increased output at burner.



What is the Stoichiometric (ideal or perfect)  Flame temperature for Natural Gas?

This is well-documented all over the place but depends of calorific value (Wobbe number) of gas being used.



Is flame temperature affected by modulating?

- what's flame-temperature?  Sounds like a silly question but most burner-cap designs allow a wide range of gas/air mix pressures, so the flames can increase without much lift-off.  Temperature depends on measurement point but I imagine the temperature at the flame-tips doesn't change much.  OTOH, average combustion chamber temperature will vary a lot, according to burner modulation.  Trick of burner design is to achieve a good compromise between efficient combustion (high flame temperature) and acceptable NOx levels in the fluegas (lower temperature).



Which has the bigger problem with the modulating concept as far as efficiency the boiler or the furnace?

- don't understand the question!  (boiler, furnace = same thing?)



Are there any charts or tables available for determining comparison of combustion analysis related to input on low fire versus high fire? Compared to Flue gas temperature on low fire and on high fire?

- why should fluegas composition vary between low- and high-fire?  Ideally, it shouldn't!



A lot of questions hopefully we can get some good discussion or maybe just get us thinking a little more about what is really happening with all this new equipment

Tim McElwain

Reply from Dale Watterson

One more comment, my company is going to participate in the smart electric meter program where the appliance communicates with the electric meter to see what cost the load is based on time of day, night being cheapest usually. Anyway, I think eventually the pump/blowers will be maximized to run when the grid load is lowest. Perhaps not in our lifetimes but the young guys will see this. So, since gas or oil is a fixed time of day cost perhaps the cycling of the heating equipment will be maximized to the cost of the blowers/pumps. Always something to learn in our trade.

I numbered your bullets, hope that's OK

1. Heat loss, a very slippery topic since a lot of variability, let's say though that someone actually picks an input where the equipment runs 24X7 and keeps the house at 70 degrees at design, minus 10F here. In my 30 year career I only saw this once by the way.

2. The proper cycling rate is also interesting, I think Honeywell used to say 6 cycles per hour was correct for warm air, I think 4 cycles per hour for hot water and I always wanted 1 or 2 for steam, I think the efficiency suffers somewhat if we have full input and don't need it. I think the manufactures are saying this for warm air when the on/off condensing furnace is rated at 90 percent and the variable input at 94 percent or a little higher.

Since the warm air systems are the most common here I too always think of electrical load as well as gas and electric motors use more power at start up. I also know how little electricity is used by an ecm motor compared to a standard one, and that the ecm power requirement can be trimmed further is the heat rise can be kept up with reduced gas input because of lower load/higher outdoor temps.

3. Yes, given the problems with #1 it is more efficient although the spread varies.

4. I think the main trade off is slower response, I think the problem of combustion efficiency is fixed by a variable speed inducer motor that follows the gas input.

5. No, not in a major way if the inducer is variable speed also.

6. Good question, will need to look that up, I think 3500 degrees but will need to google it.

7. No, the flame temp. should not change, just the amount of heat. I have a tiny oxy acetylene jewelers torch that melts thin steel very nicely.

8. One problem we have seen is that there is a carbon monoxide spike when a step input valve changes input, it usually settles out with run time. I think the boiler has the bigger problem because the heat transfer is slower, sheet metal changes temp pretty fast compared to a boiler construction.

9. I think the low/high fire change over is affected by the step or variable method of input change. I am seeing more completely variable inputs, which I think are the best. I think the function of lowest possible flue temperature is really a direct result of heat transfer in the secondary heat exchanger, to me 90 degree flue temp is only 20 degrees above return air temp which mechanically is about as close as we can get.

　

Just a few thoughts Timmie, I'll be interested to see what others say, Take care, Dale

　

Steamhead

We have one customer

with an oversized W-M LGB on a large Broomell Vapor system. We hooked up the lo-hi-lo feature, with a Vaporstat to control it, and set it to drop at 3 ounces. The customer reported saving 40% on his gas consumption. Remember, this is an atmospheric with no way to control the secondary air.



40%. How much fuel could we save if we did this on every steam system out there?



All LGBs come with lo-hi-lo capability and most installers have no clue what to do with it, so they don't hook it up.

Mark Eatherton

It HAS to be more efficient...

Modulating is it efficient?

Definitely a point for some discussion. Assumption here is that the system piping and duct work is compatible with the heating unit. A heat loss was done and system and heating unit are matched up.



When we look at say a 100,000 BTU boiler or furnace that fires on every call at full input is that efficient? Taking into consideration proper cycling rate.Driving tacks with sledge hammers has never been efficient. Nor has driving railroad spikes with upholstery hammers. THe biggest problem with fixed burn rate appliances, iks that they are only at their peak efficiency potential when they are running at 100% of their capacity, and theoretically, that only happens around 2 % of the time. In reality, the ACTUAL heat loss is typically 1/2 of the theoretical, so most appliances are oversized by a factor of 2. This causes a LOT of short cycling, which kills the efficiency of the appliance./b]



If we however cause that system to modulate based on outdoor reset is it more efficient?Outdoor reset has nothing to do with appliance efficiency. It has more to do with system efficiency. The problem I se with outdoor reset is the limitations of burners to be turned down. I had an off the record with a manufacturer about their ability to go even lower on their burner, and he said they had proven in the lab they they could go MUCH lower than they were, but certain quasi-governmental organizations did not recognize turn downs lower than what was being allowed, hence 20% minimums. This statement does not hold true for all appliances. It is a matter of burner design



Now what about the combustion issue? When we fire at less than full input into a chamber designed for full input what is the trade off?In my minds eye, there really shouldn't be any "issues" provided that all parameters surrounding the extraction of energy are in place, i.e. flow rate. When dealing with heat exchange surfaces, if the surface is capable of drawing off more heat than is being placed into the box, that equates to a higher fire to water efficiency in my minds eye. I am willing to listen to opinions to the contrary tho...



If we are firing at a lower input is excess air still the same?No, excess air has been very stable in every case of a modcon that I have looked at, within reason. Remember, this is a very tightly controlled combustion process, and with the introduction of the Lambda system, it will become even more efficient in eliminating excess air and associated waste. I believe that other than the ability to modulate, this is the one area that provides significant reductions in energy consumption.



What is the Stoichiometric (ideal or perfect) Flame temperature for Natural Gas?Beats me :-) What happens to the stoichiometric conditions if we inject pure oxygen into the process of combustion?



Is flame temperature affected by modulating?Again, I'm not sure, but I do know that I don't have the instrumentation available to determine it anyway. My thoughts are that modulation probably does allow for a lower flame temperature, but so long as all the other parameters are in line, what consequences are there?



Which has the bigger problem with the modulating concept as far as efficiency the boiler or the furnace?To my limited knowledge, full range 5:1 turn downs are not yet on the market for forced error furnasties, YET. My though is that it would be harder to maintain human comfort with a modulating furnace then a modulating boiler. It's not like people are standing right on top of the hydronic heat emitters, sensing different water temperatures. With forced error system, if you are anywhere NEAR a vent termination, anything less than 100 degree discharge is going to create a condition of cold skin surface, which is equated to discomfort.



Are there any charts or tables available for determining comparison of combustion analysis related to input on low fire versus high fire? Compared to Flue gas temperature on low fire and on high fire?None that I am aware of, but I do live in a laboratory, and have a good combustion analyzer, and a nice modcon boiler. Am recovering from one heck of a head/chest cold right now, but when I get better, I will run my system through the paces, and give you four or 5 points of data to analyze between the minimum and maximum burner capacities.



A lot of questions hopefully we can get some good discussion or maybe just get us thinking a little more about what is really happening with all this new equipment.In the last 10 or so years that I have been applying exclusively modcon equipment, I have not seen LESS than a 30% reduction in fuel consumption. The majority of these savings are coming from the shoulder (Spring and Fall) heating seasons. During the peak of heating demand is when I typically see the least amount of energy savings. I am sure others will echo that same sentiment.



Another interesting question would be energy savings in NON RESIDENTIAL or non SPACE HEATING systems. In other words, if I replace an 82% efficient copper fin tube boiler with state of the art direct heating modcon appliance in a restaurant setting, what kind of energy savings would I expect to se at a minimum?



Good questions Tim. thanks for asking.



ME

Mark Eatherton

Test results

I intentionally cooled my home way down, open all doors, cranked all emmiters up, and tested at minimum RPM, and at roughly 800 RPM increments.



Ask away.



ME

Tim McElwain

Mark, let me look over those figures

and I will get back to you. Hope you are feeling better.

Tim McElwain

Looking at those

figures the one thing that stands out is at 1500 (low fire) excess air is 55.1 but the efficiency figure I come up with at that set of readings is 88%, the efficiency across the board stayed at between 87% to 89%. With that high a excess air the flame temperature would be cooled a little which may affect heat transfer but not enough to be a problem.



I agree with those figures pretty closely with two similar Mod/Con units I have tested. However they were brand new right out of the box. They were tested with room air at 70° F. Things change when air is brought directly from outdoors with no preheating before mixing with the gas. The heavier denser cold air gives higher O2/excess air in most cases. Concentric venting or preheating on these high efficiency units I have found really makes a difference. The good thing is that around 20° F and higher the figures even out. So it is only when air is down around zero that it would be a real problem as far as cooling the chamber and the gas flame.



By the way the Adiabatic Stoichiometric temperature is usually said to be between 3400° F and 3595° F. Adiabatic is a term used to describe combustion reaction in which all heat generated is retained in the products of combustion- none is lost to the flames surroundings. Stoichiometric is said to be perfect combustion with for gas 12.7% CO2 with zero excess air and 100% combustion efficiency. We are not there yet in fact far from it. It is rare by the way to find a flame temperature much above 3,250° F to 3,330° F. I believe Viessmann claims a little higher with the Vitodends 200 with Lamda Pro.



Very interesting Mark, so it would seem that from low to high fire there is not a lot of difference. It is also important to recognize that all of this is for boilers below 399,000 and furnaces below 150,000 BTU's.



So does modulation increase efficiency, I would say yes but it certainly depends on many other factors which have to be considered.



In talking to many manufacturers reps they tell me that the biggest problem with all of these units is improper installation probably caused by not reading the directions or not being properly trained.



A comparison to furnaces shows at the low end a much higher excess air so maybe a 10 to 15% drop in overall efficiency on low fire. On high fire the furnaces pretty much even out with the boilers I tested on high fire.

eluv8

Quick question for Mark

What was the Supply and Return water temperatures?



Did the return water temperature remain stable during the test? 



If this is a condensing boiler operating below 130 on the return, it would be nice to see how much of an effect getting the excess air on low fire down would have on your efficiency.



Edit: Did anyone else hit the wrong X after looking at the pictures?

The Steam Whisperer (Formerly Boilerpro)

In comparison, et el.....

While I would have to double check my records on installs, the modcons I install seem to consistently see a drop in excess air at low fire versus high fire....I have not tested in the middle.  A drop in excess air at low fire would also permit easier condensing at low fire ( condensing threshold temp would be higher).....  which I would suspect may help overall seasonal efficiencies for most higher temp systems.  I suspect Mark's unit would work better for heavy cyclical loads such as laundry water heating, because as the input increases the ability to condense also increases.

Steamhead's input on a hi,low,hi steam boiler install is of interest too.  I have discussed this same type of installation with another installer and they did not see an improvement in fuel usage, but did see a great improvement in comfort.  However,  I suspect that STeamhead's boiler was a previous over sized install, where the other install was a new, properly sized install.  As to change in excess air on the LGB from high to low,  it actually does not do so bad, only about a 2% drop in combustion efficiency.  This is much better than other brands of boilers I have tested when using low fire. Pretty much every LGB I see, I pull the high wire wire and run at low fire only since nearly all are over sized.  

Using fuel usage changes from real world experience proves to be difficult to back up Modulating benefits. There are so many different factors going on.  If you are using a tight outdoor reset curve, how the system interacts with the structure I believe has a great impact on fuel usage.   Outdoor reset appears to reduce heat loss from the structure by greatly reducing stack effect air currents through a structure and the attendant air leakage.  This is why whenever I hear someone say "heat loss is heat loss" , I cringe.   It also tends to reduce overheating and under heating throughout the structure. 

With systems that use old cast iron or newer radiant wall panel emitters,  at lower water temps, the percentage or radiant output is increased versus air convection output.  Outdoor reset will keep the emitters cooler overall, increasing the seasonal percentage of radiant output.  This in return improves comfort at lower thermostat settings and reduces stack effect air leakage, both which reduce fuel usage. 

My longest term data re modulation of input involves small commercial space heating plant replacement installations where I have been using typical 80% efficiency on/off boilers in a step fired fashion, until recently.  These step fired heating plants have never delivered less than 30% reduction in fuel usage when compared to the previous 80% to 85% firing efficiency over sized, on/off boilers they replaced.   With step fired separate boilers,  excess air is obviously much more stable, since they are always running at full input.  However, outdoor reset, indoor feedback and other improvements are typically made at the same time.  Again, how much of this savings is due to "modulation" is a question.  It may be of interest that Burnham's data claims about a 10% improvement in fuel efficiency by step firing boilers, versus a single on/off.  It varies depending on the number of steps, however, going beyond 3 equally sized step fired boilers, the efficiency gains become very small in their tests.  This makes me think that a 3 to 1 modulation is where most gains can be achieved with typical consistent heat loads.

Constantin

I guess the question is...

... can we find two Levittown or similar 'identical' homes in which to hang a Monitor MZ and another condensing boiler that also modulates? That's one way.



Another way is to hook up a BTU meter for a home and to compare a MZ-equipped home vs. one with a modulating, condensing boiler. Then compare how many BTUs go into the house vs. fuel use. See what the net difference is. Not too hard to do, a couple of thermowells, one or two flow meters, a WEL or similar data logger.



Going forward, I will be monitoring the fuel usage vs. the HDD in our home. I'm pretty convinced that we'll be saving fuel on account of the much lower standby losses associated with the Vitodens vs. a 30-odd gallon Vitola. I suppose a longer flue may help with allowing some pre-heating, but I think one grasps at straws when the return temperatures are in the mid-80's.

Mark Eatherton

Cold, cold, cold.....

It never got over 100 degrees F on the supply. My system hits a point of stability within 2 minutes.



I could have data logged the S&R, but didn't think it was necessary or relevant.



I have two mass flow meters on my system. One on the hydronic side (2.5 GPM constantly) and one on the gas side. I did clock the meter and will post that information on Tim's post above.



This is a modcon boiler (Munchkin MC80).



Don't feel bad. I kept clicking on the wrong X as well...



ME

Mark Eatherton

Tim, more information...

Interesting to see your theoretical efficiency numbers. The one number I recorded but did not report was the analyzers "Efficiency" numbers. Those numbers, that correlate with the readings taken were...



100%



100%



99.9%



99.7%



99.2%



98.8%



Per our friend Jim Davis, THIS is why we DON'T pay attention to that number,



But, I suspect my boiler is actually more thermally efficient than your calculations are showing. Tell you what I will do. Later today, I will hook the boilers computer to my PC and re-do all of these tests and show the actual fire to water efficiency and see how it compares to your theoretical numbers.



At one point, I did have the excess air lower on the bottom end, but it then made too much CO on the top end. Such is the nature of a negative pressure induction gas valve.



The other item I recorded but did not display was gas consumption. Those correlated numbers were;



11,600 btuH



21,101 btuH



28,730 btuH



43,941 btuH



49,800 btuH



62,250 btuH



Our natural gas is un-naturally derated to 830 btu/cu. foot by the utility. Guess they are worried about the pioneers hauling their own gas ranges and dryers up here from sea level in their Conestoga wagons... It comes off the well at 1,050.



As Ahnold Schwartzenegger said, "Ahl be bach"....



ME

Tim McElwain

Mark my efficiency

numbers for what they are worth are taken from a chart I use for lack of anything better. It is set up with a 1023 BTU per cubic foot at .658 specific gravity. We really do need the manufacturers to release the figures from their testing in the lab under supposedly ideal conditions so we have a point of reference to work from. Also some accurate charts and tables would help.



I have found that using my five or six different testers it (my chart) is more accurate than the actual testers readings. As you know however real things we can look at are O2, CO, and net stack as far as combustion goes.



I have also found that you really have to test a various times and conditions and then try to average out your findings.



I have not seen any MOD/CONS I have tested that are much above 90% combustion efficiency.

Mark Eatherton

I'm BAAAaaack..... It's not a tumah...

Greetings fellow flame heads.



As promised, I went back and re-evaluated the system, using the same parameters, while data logging the delta T's at the given test points. Of interest, when doing testing like this, it is nearly impossible to hit a point of  perfect equalibrium whereby the supply and return become dead nuts stable. Firstly, we are no where near design (OSA = 48 F) condition, so I am driving tacks with sledge hammers here, and the boiler keeps bouncing off the high limit setting. As my good friend Siggy would say, "It WOULD eventually hit a point of equalibirium, but where that point is going to be, and whether or not you can withstand those temepratures is a topic for another conversation on another day"...I suspect it would be out of the condensing mode when it did eventually hit equalibirium...



Ideally, in the "perfect" lab, I would have access to a very large stored volume of perfectly controlled water, and would run the boiler at a given RPM, until the supply temp stopped climbing, and then take a snap shot of that condition to assess "fire to water efficiency". Obvioulsy, I don't have that facility here. I will have it in Hydronicah, because I will have an Earth coupled heat exchanger that should have the ability to allow me to reach a point of stable equalibrium.



With all that said, I present you with the new findings.



Thanks again to Dave Davis of HTP for setting me up with this marvelous piece of equipment and the software to view it.



Enjoy!



Comments welcome.



ME

Mark Eatherton

Tim, regarding heat recovery...

I forgot to mention this, and I couldn't find my spare thermistor or I would have checked it, but my boiler has a 15' long concentric vertical counterflow vent, and there is another waste heat recovery heat exchanger that is built into the back of this boiler. http://www.giannoni.fr/Prod-ABSOLUT-CONDENS.html

I am certain that it is recovering waste heat out of the exhaust stream, but exactly how much I am not certain. In your calculations, you are probably using the Ambient air as the inlet temperatures to the appliance, when in reality, the appliance is pulling 100% outside air for combustion. What is the net effect of doing this?



The OSA was probably, heck, why guess.



at all RPM,s OSA= 45

What does this do to your efficiency calc's?



Do you need any additional information from me? This is at roughly 1 mile above S.L.



Thanks



ME

Steamhead

You're right

we didn't install that oversized LGB, we just hooked up the lo-hi-lo. It gets to 3 ounces quickly and runs on low fire for the better part of each cycle.



On a properly-sized steam boiler, the drop from high to low fire would only be just enough to shed the pickup factor. That's all you need to get a nice long burn without shutting off on pressure- even if the pressure is only a few ounces.

Tim McElwain

Mark,

I actually use a series of laboratory thermometers and strap on thermometers to measure various temperatures of both air and water along with surface temperatures of the actual boilers and furnaces to make some comparisons to jacket loss, along with piping and pickup. All not real state of the art but I have limited funds and do with what I have. It would be nice to have a setup like you have and live there to keep an eye on it.



I have three MOD/CON's that friends have in their homes that they let me play with. They are three different brands so I get a nice comparison and all three houses have similar set up and very close heat loss.



I did a lot of testing when with the gas company in conjunction with AGA back in the 90's on conventional equipment especially related to combustion air and dilution air temperatures and the use of barometrics on gas equipment with fixed draft hoods.



I have also done a lot of testing on warm air condensing units using secondary heat exchangers with conventional blowers. I have not had much opportunity to test furnaces with ECM blowers at this time. The number one problem with furnaces here in the northeast is insufficient return air by design. Many houses only have one return air opening and maybe two if you are lucky. This plays havoc with getting design efficiency out of these units. Much like putting a MOD/CON on a baseboard system and running it at 180 degrees and no ODR.



Thanks for all the good data, trying to get that from manufacturers is really tough.

Tim McElwain

Mark, how much more

efficient is the MC model versus the straight M version?