Condensing boilers in High Temp Applications

CDM2

I am curious as to why anyone would install a condensing boiler in a high temperature application such as copper fin tube or cast iron radiator jobs? It's my undertanding that a boiler will not condense with return water temperatures of 130 degrees and above. The WM Ultra literature states the 95+ efficiency is achieved with 110 degree supply, and 90 degree return temps. If a return temp of 130 degrees yields 85+% efficiency, it seems to me there is no economical incentive to spend money for the condensing boiler. Sure, the modulating burner is a great feature, and with outdoor reset mild outdoor temps will call for lower supply/ return temps. I can also see where you could estimte the amount of run time during the heating season with outdoor reset to determine how much runtime could be achieved at condensing temperatures.

I am not a contractor. I am a Bryant Territory Manager in Central Illinois. I know many contractors that are selling Ultra's, Munchkins, Peerless (Munchkin), etc because they are "High Efficiency" boilers. I spend alot of time trying to point out to them what seems to be in my mind, when a condensing boiler should be applied. Am I missing something here? I do not want to give out bad advice, but it is like hitting a brick wall at times when I have these differing views of condensing vs conventional boilers.

Thanks for your input. I value the knowledge of all the contractors here on The Wall and I learn from you all daily.

Regards,

Dean Meece

Unknown

I worked on a Bryant tonight.

That gas fired boiler is the biggest piece of crap I've seen sold by the gas company for a while now. To answer your question about condensing boiler technology,,, It works. They condense even in high temp applications especially with OA reset.

Worth every penny if you plan on being here for the next few years. BTW ignitors are only available for this Bryant (POS) boiler by the gas company.

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Ted_9

Also the key is to use outdoor reset with that condensing boiler. It can work well with baseboard and great with cast iron. So most of the heating season, you will use lower supply temps thus condensing even more. They work, we have many out there like this.

PATRIOT HEATING & COOLING, INC.

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Constantin

Dean, consider the system as a whole...

... Assuming the person who installs the equipment does a heat loss calculation, the equipment will be sized to meet the heat loss on days that occur 2.5% of the year. For now, let's say 50% of the year, the boiler isn't running.

That means that 47.5% of the year, the boiler will be oversized WRT the heating load. You can approach this problem from two directions. You can cycle the boiler on/off and pump blasts of hot water through the system (causing noise, wear and tear, etc.) or you can modulate the firing rate so that the heat being pushed into the system equals approximately the heat loss from the house.

Running a on-off boiler by bringing water to 180°F during much of the year makes no sense, even with low-efficiency emitters. The boiler water only needs to be that hot on design days. In fact, the better heating professionals will nowadays install enough baseboard to meet heat loss at 140°F water temperature. Baseboard is dirt cheap, you might as well use it to allow the system to heat the home at lower water temperatures.

Condensing, modulating boilers can best take advantage of constant circulation and low water temperatures. But it can be done with CI boilers too, they just need a 4-way valve or injection loops to ensure that they don't start condensing (the Vitola being a notable exception). Constant circulation, consistent water temps not only lead to a noiseless system, it also lowers the heating bills because the lower the water temp circulating around the system, the lower the BOP losses before the water gets to the emitters.

Coming back to the 47.5% of the year that the maximum burn rate of a boiler is too large for the home... a traditional CI boiler won't modulate and will hence use a lot more fuel duing the shoulder seasons whenever the return water temps could be very low, even on a BB system. These shoulder seasons predominate in our heating needs (i.e. most of the degree days occur when it's not that cold out.) During the shoulder season, a condensing boiler will almost always condense and hence save the customer money even if it cannot condense on design days because of undersized heat emitters within the home.

Take a look at the distribution of heating degree days within the year. You will see that most of them are a) not design days and b) will allow a condensing system to run in condensing mode most of the time, even with high-temp emitters like BB.

Now, let's take the whole thing up a notch and assume that the homeowners also want to be energy efficient about their water heater. Considering the incoming vs. outgoing water temperatures on most IDWH, I would like to understand how a customer that needs a IDWH tank would not find the boiler condensing 100% of the time. That's a huge energy savings over the 60% AFUE gas water heaters that most people seem to be using. It's an even bigger savings over a 80% AFUE high-mass CI boiler that has to come up to temp first before hitting those AFUE numbers or (worse) a high-mass CI system that is kept boiling hot all summer to serve a tankless coil.

I agree that condensing systems aren't for everyone. I have a cast iron vitola in my basement that is fed via oil. However, if the US fuel oil wasn't so contaminated with sulfur and other sludge, I would have considered a condensing boiler for my application as well. With any hope, we will see modulating Herrmann oil burners in the sub 1 GPH range at some point before we all switch to bio-fuels.

Mark Eatherton1

2 words...

THERMAL EFFICIENCY.


These little condensors kick butt even when they are out of thier condensing range. Your dinosaur of a boiler can't match the THERMAL efficiency of these modulating devices. AFUE is hog wash. It should stand for A Fuel Used Efficiently, but instead stands for A Foolish Use of Efficiency.

If you, as a manufacturer of boilers doesn't want to be left out in the cold, you'd better start looking at this fully modulating technology and learn how to apply it to your methodology of heat transfer. While you're at it, you might want to review your heat transfer methodology too...

When I can displace one of two 2.5 mill fire breathing atmospherically vented horizontal steel tube boiler with 600,000 btuH of condensing technology and reduce the consumers gas bill by 40 to 70%, theres something SERIOUSLY wrong with the technology at hand...

Time for ALL conventional boiler manufacturers to wake up and smell the coffee. Look across the pond to see what the trend is going to be. They are slaying their dragons.

ME

Steve Rockwell_2

It's a balmy 38*

Here in Grand Rapids today. My Ultra, feeding "high temp" copper fin baseboard and standing iron, is cruising along at 126* supply, return about 110. My 78 year old house is warm and toasty throughout.

Can you say "condensing?"

Copper baseboard. It's not just for breakfast anymore!

Jay_14

High Temp Heat Emitters

Condensing boilers achieve higher efficiency by absorbing the latent heat in the flue gas. The requires the heating system to operate below the condensation point which is roughly 130F.

Copper fin tube and cast iron radiators are generally thought of as high temperature emitters. However, the amount of heat emitted is somewhat proportional to the amount of surface area the copper fin tube or CI Rads have. Therefore a lower water temperature can used when the heat emitter is "oversized".

Now keep in mind that many older homes have been insulated. This results in the emitter being "oversized" in order to heat the building.

It is in these cases that a condensing boiler, when used with outdoor reset, can result in higher efficiencies. Modulating boilers further increase efficiencies by reducing the number of cycles on the boiler.

That said, I feel that there is a large misrepresentation in the industry. Manufacturers often downplay the return temperatures required to achieve these higher efficiencies while some installers (not all!) just believe the efficiency sticker on the side of the boiler without regard to system operation.

Jay

DaveGateway

A well designed heat exchanger

can bring higher temp returns down to condensing levels. Dunkirk quantum claims condensation @ 160°

Tony_8

The best reply here

Once again, ME nails it.

And who said CI rads are always high temp ? I don't call 140 high temp, and that's what my and my customers' rads usually run at on the coldest of days, -20F.

The devil is in the details. The thermal efficiency of the aluminum block of the Ultra is the key, as is the other low mass materials used by other mfg's. Heck, I've had plain copper tube boilers with an AFUE of 82% outperform CI boilers rated the same. Consistently. Condensing and modulating just sweeten the pot.

chuck shaw

not to mention

most condensing boilers are very low mass, holding less than a gallon of water for a 150,000 + BTU boiler. How many gallons of water in a conventional boiler? How much more energy is required to raise that mass of water and cast iron up to a temperature where it can deliver the requested temp?

Chuck

Greg Swob

Gary Wallace- I have a Bryant boiler to repipe after Christmas which was installed by another firm last year. Made by Dunkirk - what do you not like about the one you worked on? Wondering what may be in store for me to look out for? I believe this one is a BW-3: as the co-worker who looked at it said it was draft induced and non-standing pilot. I've worked on BW-1's before- pretty basic.

I am having a lot of trouble getting a replacement piece of insulation (rigid fibered material) which goes horizontally just above the burners and roll out shield. It's not shown in detail in HVAC Partners and I'm losing a lot in translation with tech and parts people.

Thanks- Greg

Mark Hunt

38% reduction in total therms used


I have a customer that proved this.

Munchkin 140m, outdoor reset, baseboard convectors.

You folks that cling to the "high temp" mantra need to understand one thing. If it's 50 degrees outside, I don't need 180 degrees inside.

It works, but if your a competitor of mine, it doesn't.

Mark H

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Constantin

Good Point!

I wholeheartedly agree that a low-mass system will respond to requested changes much quicker than a high-mass system. This is point is particularly relevant for high-mass systems that are used to heat water for an IDWH in the summertime.

However, the equation goes a bit beyond water temperature and mass. For example, our Vitola has 31 gallons (!!!) of water in it - the definition of high-mass. Yet, the Vitola also has 3" of insulation all around, a very high AFUE for a non-condensing boiler (when used with oil), etc.

What sets the Vitola apart is the construction of the HX, which makes it perfect for non-condensing usage in a radiant floor system. No other CI boiler can tolerate the low return water temps that a Vitola can. The only reason we have a 4-way valve on our radiant manifold system is to allow the boiler to fire for the IDWH and the radiant floor system at the same time.

The Vitola is probably the pinnacle of CI oil-fired technology. The next logical step in the evolution of oil boilers is to go to condensing, low-temp models. Once US fuels are no longer allowed to be laden with 1700PPM of sulfur and other goodies, we may see more condensing oil boilers on these shores. If Kyoto won't force their adoption, the growing scarcity of oil will!

However, if your fuel of choice is gas and you don't use a condensing, modulating boiler that modulates water temps with outdoor reset and which uses constant circulation (or better yet, gravity flow!), then I will suggest that there are more energy efficient ways to hydronically heat your home and/or potable water.

As installers and homeowners have noted here, their clients have saved between 25-40% on their fuel bills just changing from a old CI to a modern condensing system. Would the change be as dramatic if they made the jump from a 85%+AFUE system to a 95%+AFUE system? Probably not, though gas prices these days are high enough to justify all sorts of equipment improvements with a 3-year payback on the marginal cost of going with a condensing over a conventional CI boiler.

eleft_4

Hey Mark,

Would you agree the stats, 38% is comparing an apple to an orange if the boiler removed was not set up to use outdoor reset?

I wish you and yours, Merry Christmas and a Munchkin Better New Year!

al

Mark Eatherton1

Field observations of going from 85 to 95%

My partner, Tom Olds, installed a Buderus 84% er with an Ecomatic control and had it running for about 7 years. He then installed a beta testing Buderus condensing boiler parallel to his 84er and saw a 30% reduction in fuel consumption with a slightly cooler year this year.

Fluke you say?? Not so. Dennis Bellanti saw the IDENTICAL situation at his home, and I saw the same at my own home with the lil Munchkin I installed.

All three of our homes have a mix of emmitter technology varying from RFH to CI radiators with a few radiant wall/ceiling panels thrown in for good comfort.

This statement wasn't made to make their cast iron boilers look bad, it was made to show what the application of sealed combustion condensing technology will do.

Another point, Tom and Dennis' new condensing boilers are not "smart" boilers just yet. They don't modulate until they approach whatever set point they are set for (150 to 160). They don't have the equivilant of a Vision 1 control packagae applied yet. I expect theirs to show even greater savings once that technology is applied. Hows that for apples to apples...

ME

Mike T., Swampeast MO

It's not just the condensing, it's the modulation that comes with many of these boilers that's really making people wonder just what constitutes a "high temperature" system anymore...

A number (contractors and homeowners alike) here have been reporting their systems' operation with these new condensing/modulating boilers. In moderate weather, the homes are being heated perfectly at temperatures that traditional output graphs (both fin b/b and standing iron) show no output!

NOTHING in the status quo is designed around the idea of a boiler giving out just the amount of heat required at any given time, yet this is what the condensing/modulating boilers are trying to do. Even when the "modulation" is in fact a series of stages, they seem to be doing this quite well.

Comparison based on AFUE between traditional and modulating equipment seems almost meaningless. The modulating boiler is essentially re-sizing itself with changing weather. This allows the boiler and the heat emission devices to work together to maintain the efficiency of the system as a whole. AFUE calculations do not consider this.

Even with "plain" (non-modulating) condensing equipment the AFUE numbers don't seem to tell the whole story with condensing equipment generally greatly outperforming the difference shown in the numbers.

When it comes to sizing a heating system there are overstatements at every step in the process. From determining loss to forgetting about occupancy loads to sizing the actual emission devices nearly every system winds up significantly larger than necessary--even at design conditions!

In a way that's good because some people do like it hotter--some people do like to sleep with an open window in the winter time. Structures generally become leakier as they age. Unusual cold spells occur.

All of those things combine however to lower the temperature of the emission device--thus lowering the supply temperature requirement.

When you have true modulation throughout the entire system with no form of digital control things change even more. The room temperature no longer "bounces" around some air temperature setting. Each one of those "bounces" requires a bit more energy to get back to the top than the amount lost on the way to the bottom. Under complete proportional control bounces are gone--what you get instead is a gentle flow that corresponds to the solar cycle as delayed by your insulation!

Mike T., Swampeast MO

See my Current Post

titled, "1st Gas Bill With New Vitodens"

The control "goodies", outdoor reset, TRVs, constant circulation, warm-weather shutdown, etc. were all in place for years. Comfort was great but efficiency sucked.

The new Vitodens is simply and directly piped into the old system--in fact the old boiler and circulator are still in place for emergencies and experimentation. Even the reset curve is essentially unchanged!

55% reduction in fuel consumption???? Granted this is based on comparison using degree days and only 31 days of operation of the Vitodens in moderate weather, but I honestly don't expect much change as the weather gets colder. My initial prediction (posted here) was for a 53% reduction.

CDM2

Believe me, I understand the need for CI boiler manufacturers to wake up and smell the "modulation". Unfortunately (or fortunnately) Bryant purchases their boilers from Dunkirk, as does Lennox, etc. Maybe you could elaborate on your comment of "review your heat transfer methodology too.."

Thanks for your input.

Dean

CDM2

Constantin,

Thanks for your input. I always perform a heat loss on replacement boiler jobs and there have been occasions where the existing radiation will handle the load with 140 degree water at design conditions. In those cases, I always recommend a condensing boiler. There are many here in Central IL (whether fin tube radiation or cast iron) that there simply is not enough radiation to handle it at lower temps. And I ALWAYS include a tekmar 256 outdoor reset control on EVERY boiler I sell. On the cast iron boilers, I will also recommend variable speed injection controls, tekmar 356.

It makes sense to me that most of the heating season a boiler will be operating at temps that are going to be at or below the condensing point, thus a condensing boiler is the best choice.

Again, thanks for your input.

Dean

CDM2

Jay,

I agree whole heartedly. If there is enough radiation in the house, (regardless of style) to handle the load at a lower supply temperature, by all means- condensing boilers are the way to go. Unfortunately, some contractors that I have experience with ignore the available radiation and like you said, look only at the "AFUE" sticker on the side of the boiler.

Thanks for your input.

Dean

eleft_4

ME,



Of course you are talking NG.

An oil cast iron can be set up to provide the same comfort using P/S V/S inj. With Outdoor Reset.
Is the boiler cost a factor in the heating efficiency factor or just the fuel consumed.
We old guy's, you know.

Merry Christmas

al

Tony_8

A BW3

is a Dunkirk XEB with tan paint. Basic and dependable boiler just like the BW1's & 2's. That's one reason Bryant buys them and resells them.

Gary's bashing them because they're inexpensive, he loses work to them when people can't afford Viessman, he didn't have the necessary part to repair it and is frustrated/embarrassed at his inability to fix it immediately. He'll probably slam me now for my observation of his usual rants, but I can take it
Sticks and stones....

Unknown

usual rant

Tony is correct in such that they are an inexpensive boiler. However, I really haven't lost any work to them. Viessmann is not the only equipment I will sell. It's just the one I enjoy installing the most:)

I couldn't get the necessary part to repair this Bryant same day and was indeed frustrated (not embarrassed) at my inability to fix it immediately. It's winter time. When I was told that the ONLY way I could get the part I needed immediately, was for me to call the gas utility, I got quite aggrivated. I'm not the only one here having a lot of trouble getting replacement pieces. See above post.

The only problem I had with this unit is that the ignitor/pilot assembally is not easily accessable. The thing is attached to the center burner tube (far in back) and quite difficult to remove/re-install. The HX & CI burner tubes were badly corroded but, that's not the boilers fault. It's the original installer's lack of understanding of what sustained flu-gas condensation does to a boiler. Useually the inexpensive units are thrown in with inexpensive installation practices. This is not good for any any homeowner regardless of how deep or shallow their wallet was.


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Mark Eatherton1

Heat transfer methodology

Given, you're going to produce a higly efficient peice of condensing equipment. Your choice of materials is extremely limited. You can use cast iron, which is a poor conductor of heat, and not completely corrosion resistant. You could use copper, which is highly conductive but not greatly resistant to acidic corrosion. You could use aluminum, which is highly conductive and relatively resistant to corrosive tendencies, but requires a LOT of attention to fluid pH details, or you could use stainless steel, which is fairly conductive and shows excellent resistance to corrosive tendencies...

You pick...

ME

Floyd_7

Eleft......

In another post you said, show me the numbers.....
you have many numbers here.......

As for myself, here's some observations...... and some numbers......some may surprise you, some may not.....

I have seen condensers save 60% when hooked to identical loads, in low temps applications, like the pig barn job that I have used often here. Even when the condenser is running at or near high fire all the time. The payback for this boiler can be months, like about 5, even with installation, at todays propane prices.
Now in so called high temp. applications the results can be much different. In checking the gas bills for me house I have found that the payback period would be much longer, as in years.......5 or more..... but as prices keep rising the payback time will come down. Also I have to consider here than the two children that I have at home yet are going from the stage of baths in a few inches of water to twenty minute showers, and from one change of clothes a day to two and sometimes three.......however the water usage part of the equation is not as significant as I thought either.

Okay here's the real scoop...... the summer bills have been cut in half.... the condenser teamed up with the indirect, instead of a standard hot water heater was a big saver.
Heating in the transition months with the condenser has been cut by about 20%, I have gotten it a bit higher by tweaking various things and I still think I can get more out of it.

Now,...... in the coldest months, Jan, Feb. mostly for me. the savings really haven't been so dramatic. They started out at about 5% and I have been able to tweak things to get them to 10%, However the COMFORT level has risen substatially, by having the rads warm ALL the time instead of the feeling of warm rising and falling with the cycling of the heat, the house has become a much friendly place.
Remeber, my house is brick, with absolutely no insulation, so when the heat went off the cold drafts, started. By having the constant heat, I have a much more "bare"able living area. I like that....espec. when the wife and I get a few minutes home alone...:-), which doesn't happen very often.

So would I spend the extra money for a condenser in a "high" temp. application... in a minute.....I get to save some money and I get the ULTIMATE comfort that I can get out of the system that I have. It was no different when I paid more for my Suburban, because I wanted the comfort of the large vehicle, four wheel drive, leg room for may long legs, and the capacity to haul nine people or large loads even while pulling my 20' boat......

Do I recommend them to my customers.... yup.... they are the best thing out there and I always strive to give my customers the best I can give. I will install less.... but why would I ever start at the bottom and work up??????

Bottom line.... whatever you sell, you better belive in it!!! If you don't belive in condenser technology, then you better not sell it. I strongly belive in it and I can sell it most of the time......


COMFORT, baby..... I say it in one word..... that's what most people want. The condenser does it best.....


Floyd

Constantin

Why limit yourself?

My father is using a flat-plate Titanium HX for his boat engine. These plates are usually made for the dairy business, less expensive than I could believe, and should outlast his boat and probably the Smart-car engine in it too (marinized version). Naturally, working Ti is a whole different ball of wax...

Then there are some of the older WH that Larry should probably have a copy of... monel inner tanks last pretty much forever.

Never mind some of the more bizarre combos like liquid-impregnated carbon-carbon HX's that are good to 1000°C. The perfect solution for those of us that want to emulate the Russians and have molten sodium HX transfer medium. The Navy nuclear reactor is extra though.

jerry scharf_2

when I think of Ti, it's on a blackbird

SR71 that is. Their tanks leaked on the runway, but sealed up just nice when flying at mach 3. I doubt the extra effort of working with Ti is merited in a hot water boiler.

BTW, did you get the email I sent?

jerry

jerry scharf_2

being picky

Mike,

You always refer to digital as a bad thing, I think you mean binary (on/off.) My controls and sensors are all digital right up to the proportional globe valve actuator input. That turns everything into "a simple matter of programming"

jerry

CDM2

Seeing the Light

Thanks to all who have helped me truly understand applying condensing boilers. I have actually just finished plotting a boiler with design temps of 0 degrees outdoor, 170 degrees supply water at design in a bin hour chart I use with contractors. To my amazement, at just about 19 degrees outdoor temperature, my boiler will meet the load with 130 degree supply water. Definitely will condense here. And I noticed that 80% of my run time during the heating season will be at water temperatures of 130 degrees and lower.

Once again, the hydronic professionals have taught me a valuable lesson. I really appreciate you all.

Still learning,

Dean

Mark Hunt

Hi Al


I agree completely that the outdoor reset and modulating input are big factors in the reduction. But one thing that is constantly over looked in this "debate" is stack temps.

I have never seen a stack temp above 115 degrees in any of the Munchkins we have installed or on the Vitodens.

If you try that with a cast boiler, it will be a pile of rust in no time.

Of course I agree that if you have an existing cast boiler or a condensing boiler is not an option, by all means add reset to the system and the therm usage will drop.

I just don't think you would see more than 15% at best.

A VERY Merry Christmas to you and yours sir!

Mark H

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Mark Eatherton1

Cost...

is obviously a factor Al. In all of our cases, these were beta testing units, so with the exception of installation costs, they were essentially free. But even with a retail price tag attached, a 30 to 50 % reduction in fuel consumption at todays cost will show an excellent R.O.I.

Hope that answered your question.

ME

S Ebels

Modulation, Good point Mike.......

I think a huge thing that people don't take into account, or maybe even realize, is the reduction of btu's going out the stack with a modulating boiler.

The lower input that a modulating boiler can run at allows it to waste less fuel even if all other things are equal.

Take a good medium efficiency boiler like a Buderus or a Viessmann that runs at 85-86% and compare it to a Vitodens or a Munchkin, Ultra......whatever. If you look solely at the AFUE you'd think the savings would simply be the difference between the two. Usually about 5-8%. Not a huge number on the energy cost for most homes.

For the sake of simplicity let's say the heating load of the house is 85,000 btu's at design so we are going to use a 100K boiler of each type. The fixed fire boiler is stuck with wasting 15% of the available btu's. Every time it fires, 15,000 btu's go out the stack whether the outdoor temp is 50* or -10*. It's a well known fact that design output is needed less than 3% of the heating season. That means the standard boiler is wasting the 15K whether it's full output is needed or not.

The modulating burner however is able to "match" the burner output to the load which in most of the season is far less than out theoretical 85,000 btu's. This directly translates into a subtantial reduction in fuel costs because the modulating burner is "turned down" below maximum firing rate 97% of the winter. Even though its efficiency is only a few points higher than the standard boiler, the actual fuel savings are far greater.

If the modulating boiler is able to heat the structure with an input of 50K on a given day, It is only sending 10% of that input out the flue. In other words you are wasting only 5,000 btu's instead of the 15,000 the standard boiler is dumping outside. That is a substantial reduction in wasted fuel even though the efficiencies of the two units are a mere 5-8 points apart.

Once again, AFUE fails to present the whole picture. It represents what a particular appliance does under a static, constant load. I, for one, have never encountered those conditions in the field and I know that no-one else has either.

Constantin

That 's a great insight.

I'm happy that the Vitotronic 200 can control a modulating burner. Now someone just has to make one for up to 1 GPH capacities...

larry

I'm not following your logic here. I must be missing something. Please help me understand this.

If you have standard boiler A with 100,000 btuh input that is 85% efficient you will get a constant output of 85,000 btuh.

If you have modulating/condensing boiler B that is 90% efficient with 40,000 to 100,000 input you will have an output that varies from 36,000 to 90,000 btuh.

If you have a design day load of 85,000 btuh for 24 hours then both boilers will run non-stop and A will consume 2,400,000 btu and send 360,000 btu up the flue. 15% of the consumed BTUs are lost. Boiler B will consume 2,266,666 btu and send 226,666 btu up the flue. In this case 10% of the consumed BTUs are lost.

If you have another day with half the load at 42,500 btuh for 24 hours, then boiler A will run for 12 hours while B will run for 24 hours. In this case A will consume 1,200,000 and send 180,000 up the flue which is 15%. B will consume 1,133,333 and send 113,333 which is 10%.

The other metric I thought might be interesting is to look at stack losses as a percentage of total house load. In this case boiler A is about 18% and boiler B is about 11%. These percentages don't change as a function of modulation.

Of course this ignores cycle losses and other factors which makes the condensing/modulating boiler more efficient.

What other factors are at play here? I'm sure it's right in front of me, and I'm not seeing it.

Constantin

It's the cycling losses that make the difference.

If the condensing boiler can run constantly at varying firing rates, then it'll hit that steady-state nirvana almost 100% of the time. The other boiler will cycle on-off which means increased losses until the HX hits its steady state.

Plus, the condensing boiler can operate constantly even when the water temps to supply the load are very, very low. Consider shoulder seasons, for example, where the condensing boiler doesn't just modulate down but can operate at a steady-state where the regular boiler has to short cycle to inject short bursts of heat, with all the attendant standby and start-up losses.

I can also see other benefits, such as a much longer life for the ignitors and their related electronics since they get used a lot less. I'm not so sure about the fan and the circulators - how start-stop relates to life vs. constant motion at different speeds.

S Ebels

Nailed it Constantin!

The on/off cycle efficiency is what kills the AFUE rating on any boiler so designed. The constant, variable firing rate of a modulating burner actually allows the boiler to stay at "steady state" efficiency.

You also hit the nail on the head as far as component wear and tear. It's the number of on/off cycles that cause the pieces and parts to fail. Notice the cycle counter on your Vitotronic 200 and the service alert that can be programmed to notify you when a set number of cycles is reached. On a Vitodens there is no cycle counter, just hours run if I'm not mistaken. The cycles become so long as to be irrelevant.

I have to say that a condensing, modulating boiler is a good choice for almost any application. Putting one into service for an all low to medium temp system is the best scenario you could have.

The only standard on/off boiler that comes close is a Vitola, due to it's ability to handle unrestricted low water temps and high mass. Personally, I am hoping that a modulating burner is brought out for the Vitola. THAT would be the berries!!

Constantin

Funny you should mention that...

... yesterday I was drooling at the Hermann web-site over their HL 70 series. I am happy that Ms. Vitola and the logic that runs her are all set to run either a high-low or a fully-modulating burner.

Considering the price point of Viessmann and their history of innovation, it's only a matter of time before they release a model with a modulating oil burner. In the meantime, I can dream...

Mike T., Swampeast MO

It Does Get Confusing

One problem is that the chance of a design day load of say 85,000 btuh for 24 hours is so remote as to be considered only possible in a laboratory. The efficiency ratings you see are based on steady-state under very carefully controlled conditions. They are also based only on the heating appliance itself, not the interaction of the appliance with the system.

When all heating appliances interact similarly with the system this is a reasonable way to make comparisons regarding the efficiency of the appliance itself. Once the appliance is installed into a full system your efficiency will drop in many? most? all? conditions. In general the more oversized the appliance, the lower the efficiency compared to steady-state. Even if the appliance is very well sized to the design heat load, such loads don't occur often and even when they do, it's not for any long period of time. Such an appliance is generally grossly oversized.

Now comes a heating appliance that is designed to interact dynamically with the system in which it is installed. It is striving to produce that steady-state condition continually despite changing weather and changing load in the structure. Because these appliances are also capable of low temperature operation where generally wasted energy is recovered from the flue gas, they actually become more efficient under lower loads that require lower temperatures!

When a traditional boiler fires into a system under low load, to retain efficiency that system must be able to utilize that energy as fast as it is being produced. Guess what? In most cases it doesn't! Not even close! That difference between what the boiler is producing and what the system can accept goes right up the flue--regardless of any efficiency rating. This is why buffering is a common and reasonable solution to low loads with a high input. That buffer gives a place for the heat to "collect" where it is gradually consumed at the rate the system can liberate.

Even in best operating conditions the flue gasses coming out of a traditional boiler are significantly higher than the supply temperature. A condensing/modulating boiler however will generally have flue gasses somewhat less than the supply temperature (but of course higher than return temp). That's the condensing part doing its job of extracting the most possible heat from the flue gas.

In many cases, the heat exchanger of a traditional boiler is a very massive thing. Each time it begins to fire it has to heat that exchanger significantly before it can ever begin to impart its heat to the water. With a high load on the system some of that energy returns to the system once the flame stops. As the load diminishes, you get less and less of that initial heat back--instead it goes up the flue.

Your control system also affects operation. With a wall thermostat and a conventional boiler, the space temperature is NEVER steady--it's always either rising or falling. The colder it is outside the faster this cycle of rise/fall occurs. A condensing/modulating boiler with a wall thermostat is similar, BUT the boiler does its best to modulate the flame to keep these "bounces" as long as possible. With only one or two thermostats, it can do this very effectively, but with a lot of thermostats things get more chaotic and you're again faced with the prospect of buffering the load.

Now, if you team a condensing/modulating boiler with a system that regulates temperature in a modulating fashion--remember it can ONLY do this by regulating flow--the system is MUCH more like one with only a single wall thermostat even though you can individually regulate temperature in each space. The system "springs" into balance once the temperature regulating devices are maintaining temperature. The "ups and downs" are nearly gone, varying only slightly each day with the sun and occupancy.

Condensing/modulating boilers invariably include reset as an integral part of their function. They essentially strive to provide a "target" temperature based on outside temperature. BUT, the indoor load is STILL changing because outdoor temperature is only a rough indicator the the required supply temperature. What happens now? With the Vitodens driving a proportional system at least, that supply temperature stays the same--when the system has more load at that outside temperature it's the RETURN temperature that changes. More load=lower return=higher efficiency!

Jason Horner

If my memory serves me correctly, the Viessman Matrix burner (or variant) was shown to me as an oil burner 5 years ago.

ALH_3

Conductivity

Actually cast iron is generally a much better thermal conductor than stainless steel. When welding and brazing stainless steel, its relatively low thermal conductivity is very evident.

The heat exchanger wall thickness is where the gains are made. Stainless can be much thinner and can be formed in to much more space-efficient shapes than cast iron.

At least that's my take on it.

-Andrew

Constantin

Indeed...

Welding thin stainless (or Al) is an art to itself. Besides the reactivity of the components in stainless to oxygen, there are all sorts of considerations WRT the strength of the weld as a result of the various alloys coming out of solution and clumping instead of being finely dispersed.

However, I think the reason that many CI HX's are as thick as they are because:
a) there may be pressure vessel code requirements
b) manufacturers have to account for wall casting thickness tolerances
c) manufacturers have to make the stuff as thick as it is to guard against a warranty claim
d) CI was cheap

Stainless has a very low heat conductivity, thus a thinner HX is to your advantage, particularly if the grade of the stainless is high enough to tolerate just about any water condition you can throw at it. However, the thinner the HX, the greater your chances for a pinhole leak later on, whether it's at a weld, a propagating crack, etc.

High-efficiency boilers are amazing feats of engineering. Even though they have the same output, the final product is smaller, quieter, and more economical than the monsters they replace.