Condensing boiler vs conventional boiler

Kevin_10

in a radiant floor system in a concrete slab. My system was built with a conventional boiler and an indirect hot water heater without an outdoor reset. Considering an upgrade and correction of my current system. Should I get a condensing boiler for my application or keep the conventional? Can I use an outdoor reset efficiently with a conventional boiler and an indirect heater? Any suggestions.

Rocky

Conventional boiler w/ reset

Kevin,
I am afraid I am not much help in the arena of condensing boilers as we have exactly "0" here in Fairbanks, Alaska. However, we use outdoor reset on every boiler install we do here and all of our boilers are conventional, non-condensing boilers. The majority are connected to indirect water heaters. We use the snot out of the tekmar 260. It gives us boiler reset and DHW priority. If you have all low temp radiant, I would include a tekmar 356 variable speed injection control to mix your radiant system, or, you could just go with the tekmar 363. It does boiler reset,radiant system reset via either variable speed injection or mixing valve,and DHW. Can also do high temp zones via a boiler demand signal if you have any hi temp zones. If your radiant system uses water temperatures below 127* (the point at which condensing boilers condense) then perhaps a condensing boiler may be your answer. Good luck. I'm sure other more well-informed "Wallies" will respond.
Good luck
Rocky

Matt Connolly

Condensing is the only way to go...

Especially with radiant. The conventional unit in there is probably there because it was the easiest to do at the time, given the products and knowledge available. Since you don't have to mix down for a condensing boiler, you can save about 50% on fuel costs vs. a conventional unit. For dhw recovery, make sure the boiler can switch and ramp up to 180 for that job then go back to the proper temp (based on outdoor reset) for the heat. The Monitor MZ series of condensing boilers has these features built in. More info can be had by going to www.mzboiler.com. I've put in about 20 of them over the years and have had terrific results.

Good Luck!

Matt

Steve Ebels

It never made any sense to me........

To take a standard boiler, fire it to 160-180 degrees and then mix the water temp back down to achieve the temp you should have had in the first place. Wasted energy.

For a low temp application like a slab a condensing boiler is the only way to go. They'll run all day at 110* or less and love it. Monitor, Viessmann, Munchkin are all worth looking into. Buderus is rumored to be introducing a stainless steel fire tube boiler this summer that sounds very interesting. I'm not convinced on the aluminum heat exchanger models that are out there. How long does an aluminum car radiator last?

Yes! Reset by all means! A modulating burner is a good thing also!

UniR

50%

Matt,
Ahem... save 50% on fuel costs by not having have to use a mixing valve to lower the supply temps? Is that heat that is being mixed off taking a shortcut out of the house somehow?

Does that mean that someone would actually have to spend more on energy for in-floor radiant than rads or HWBB if they have a conventional boiler because they would have to mix down?

If a new condensing boiler could save "about 50%" in energy costs over a new conventional boiler (and that's the question here) for in-floor, all things being equal (outdoor reset, fuel type), then I'm going to lose all faith in physics. Sorry... I just don't believe that claim. I could easily see 10% but 20% would be absolute tops. And this 10 to 20% comes at a higher install cost and possibly a much shorter life than a cast iron boiler. Plastic venting is a really nice plus though... :-)

There are obviously efficiencies to be gained with modulation (when is the Monitor going to modulate?) and low water temps but would you guarantee Kevin in writing that his savings will be that high if he bases his decision on your advice?

Just playing the devil's advocate here...

Michael B

it appears

that this thread is beginning to get ugly. In an attempt to get back "on subject" my humble advice would be to add outdoor reset to the existing conventional system. If the boiler is in relativly good shape why throw the baby out with the bath water so to speak. If however you are jones'n for a condensing boiler I would seriously look at what is on the market now vs. what is going to be coming out in the market place "this summer". We have had great luck with the Viessmann Vitodens (fully condensing, modulating, 316 stainless steel, integrated computer control, dhw capabilties, etc, etc, etc. Yes it is more money Yes it is worth more. As to the 10-20-50% savings issue. I have seen dramatic drops in fuel bills over "standard" non-condensing boilers. I think we would all agree that there will be a savings as to how much depends on a number of factors.

The one last thing I would like to address is "more time installing". Take a look at how easy it is to pipe this boiler. I would hold this up against any non-condensing boiler in terms of overall piping and wiring. Is it different the first time...yes. But hey what isn't?

Mike T., Swampeast MO

50%

Sounds fantastic but is truly not unheard and reasonably verified.

I realize that "by the numbers (AFUE)" this is impossible, BUT there are many inefficiencies not addressed by AFUE.

As I understand it, the AFUE number could be roughly described as "the peak efficiency achieved by a perfectly sized boiler maintaining space temperature at outdoor design conditions via HIGH temperature supply AND return." Change these conditions for a conventional boiler and the efficiency drops--sometimes drastically.

It is a system with oversized boiler, high mass/volume, digital control and low supply temperature requirement (too common by the way) where a condensing boiler will truly shine as a replacement. The more a system resembles this model, the greater the potential for energy savings.

Lowering the supply/return temperatures of a conventional boiler seems to have the greatest efficiency penalty--teamed with more frequent cycles in an oversized appliance and digital control, inefficiencies seem to magnify one another. Again this is my understanding, but in a condensing boiler, much of this inefficiency is recovered through the "afterburner" section. While combusion efficiency [may] drop with supply/return temps in a condensing boiler it is very slight by comparison. When you consider that there is less transmission and jacket loss at lower temperatures, a condensing boiler may well seem to increase its efficiency with lower temperatures.

The truly perfect boiler would: 1) have an exhaust gas temperature exactly equal to the return temperature 2) supply the EXACT number of BTUs required by the space at any given indoor/outdoor temperature set. Believe it or not, but some condensing boilers are kissing this ideal.

BTU production though is only part of the efficiency equation--those BTUs must be transferred as perfectly as possible to the space being heated. Call me crazy with my views of radiation, but I believe this occurs when the emission device is delivering pure radiation just sufficient to MAINTAIN THE DESIRED TEMPERATURE OF THE SOLID OBJECTS IN THE SPACE. Again, some systems kiss at this ideal--the keys are large radiating surfaces and MODULATING CONTROL in individual spaces.

In a truly static world, modulating control of the emission device would be unnecessary. We do though live in a highly dynamic world: outside temperature changes; clear cold nights suck radiation from the structure; the sun adds it radiation, wind changes direction and intensity; and most importantly the definition of "comfort" is HIGHLY subjective. Just as the modulating flame and reset (outdoor or indoor) in some boilers strive to keep it operating at peak efficiency, modulating flow devices deal with the inherent dynamics of the space allowing any emission device to operate at its peak of radiation. I stand by that last phrase absolutely--it is "proved" mathematically, objectively and subjectively.

To me, the REAL debate over condensing vs. non-condensing is essentially a question of reliability and longevity. Condensing boilers will only result in increased lifetime efficiency if they overcome their increased energy of production through their lifetime. This is a question that only time will answer. Meanwhile, it only seems prudent to select a condensing boiler of finest material, highest standards of manufacture and least complicated design. Some (maybe even all) condensing boilers may prove to be much less efficient over their service life than implied by their efficiency of operation. Some designs are most certain to prove less reliable than others. Even the best are certain to evolve as problem areas are discovered--such is the nature of nearly everything humans and nature have ever produced.

Boilerpro_3

A always, Mike, well put

however those 50% savings numbers seem very unrealistic. When replacing many larger commerical boilers (oversized) with two or more stage fired conventional 80% boilers with full system temp reset and partial boiler reset and sometimes indoor feedback controls, I see a minimum savings of 40% and some as high as 75%. Your comments about there being many variables are right on. I bet much of those 50% saings are simply coming through higher system efficiency due to the onboard reset controls on most condensing boilers. Better system control equals lower heat loss rates from a structure which means fuel savings. I did have one installation where the owners could not afford a complete new heating plant, so I installed one large 80% Burnham 208A boiler, and then later added a large water content Hydrotherm AM100 condensing boiler as a fixed lead boiler. Fuel bills dropped some, however the large boiler was short cycling due to control limitations and it being much larger than the typical day heating load.

Boilerpro

Mike T., Swampeast MO

Real-world dynamics

One of the few true statements to come out of Madison Avenue is, "Your actual [savings] may vary, [ratings] are for comparison only."

A big problem with many ratings is that they are conducted in an absolutely controlled environment--and typically under static conditions. While I understand the need to eliminate as many variables as possible in an attempt to make direct comparison, such tends to enhance the performance of devices that are essentially static in operation (like many/most conventional boilers and control systems) and penalize those systems designed to operate dynamically.

I'm not at all saying that deception or collusion is involved in boiler rating--just that much better, more realistic models should be able to be produced. A good example: Recall the early MPG ratings for cars and how they were unrealistically high. Current MPG ratings seem MUCH more accurate because [I believe] they are a dynamic composite.

Personally I think that most people are MUCH better at comparing dynamic conditions with each other than dynamic with static. Why? Our brain is absolutely bombarded with constantly changing input from an enormous variety of sources--both internal and external--yet our attention cannot be truly divided. It merely seems to be divided by virtue of "on demand" switching. When our environment ceases to change, say a sensory deprivation chamber, our brain literally creates its own dynamics and we hallucinate.

The dynamics involved in maintaining an essentially constant blood-oxygen level in our body are beyond belief, yet it's just one tiny element of what our brain accomplishes. Our brain IS WIRED DYNAMICALLY--it is designed to comprehend dynamics--not stasis.

For instance, I (and I'm certain many others) can accurately mark a board and "plunk" it against the miter gauge of a table saw in perfect position. Not every time, but often enough to be WAY beyond cooincidence. Consider the dynamics involved; lighting, position of miter gauge; width and thickness of the wood, position of the blade, position of your body, etc., etc., etc.

Of course dynamic comparisons are, by definition, subjective and science doesn't like subjectivity. Personally I think it would be an easier task to derive quantum physical formulas (essentially predicting the probability of a sub-atomic particle occupying a given space at a given instant) than a formula to accurately predict the overall efficiency of a given boiler in a given real-world structure. YET, I believe, our brain is capable of doing so quite accurately if we only pay attention to our senses and experience. Yes, it's the "art thing" and you'll NEVER make a mathematical formula for art.

I gotta write a freakin' book some day, maybe along the lines of, "Dynamic Hydronics--Understanding Heat in All of its Motions". Daunting, I know, and certain to be disregarded by academia. Much easier though than the book I'd REALLY like to write: a novel with the dialogue of Margaret Mitchell (Gone With the Wind); the research and uncanny prediction of Jules Verne (20,000 Leagues Under the Sea); the philosophic profundity of Isaac Asimov (I, Robot) the easy reading of Franklin Dixon (Hardy Boys) and the characterization of William Shakespeare (The Merchant of Venice).

T.A. Williams

Venting Exhaust Gases

Don't forget that most of these condensing boilers require special venting to exhaust the flue gases. Most models require a venting material of the special stainless steel called AL29-4C. This high grade of stainless steel will stand up to the corrosive effects of the acids in the condensation. Most other materials rot out after a period of time.

TomW

Boilerpro_3

Never seen a condensing boiler that uses stainless

only plastic. What boilers are you referring too?

Boilerpro

UniR

Choir Practice

I think we are all preaching to our own choir here. My point was simply that Matt was effectively making it sound like you could get 50% savings by going condensing and we'd all agree that it would be pretty easy to see those savings **IF** the conventional boiler were poorly setup, was over-sized and didn't have any outdoor reset logic.

However the point I was trying to make was that if both systems are properly tuned, sized, and both systems use outdoor reset, then the condensing boiler would not end up using 50% less fuel. I didn't want the original poster to think that condensing is "that much more" efficient than a properly set up conventional boiler.

Let me say we all sing the condensing boiler praise song well and I'd really like to hear the inside skinny from Jeff Cook about the condensing oil boiler in the works. I think that ultimately ceramics will make condensing the only choice in the future... seems like an excellent application for it.

Glenn Harrison

Weil McLain VHE series comes to mind.

as it has a secondary condensing (to a point) heat exchanger. W.M. Gold series also. Otherwise, technically any boiler vented in S.S. will condense, but only in the S.S. flue. Maybe that's what he was refering to.

Boilerpro_3

Those VHE's and Golds really aren't suppose to condense

except for a very short period on start up,I thought. True condensors are 90% and up, IMHO.

Boilerpro

Steve Ebels

I agree BP

A true condensing boiler condenses in the heat exchanger, not in the flue, under "normal" operating conditions. If condensation occurs in the flue the latent heat given up is still lost and not transfered to the heating fluid. Condensing will always result in efficiencies of 88%+ for natural gas, LP gas and oil. (Well, with a good burner on a good boiler you can run 89% and not suffer excessive condensation in the HX) I have a Vitrond with a Riello delivering 88.5 to 88.7 everytime I've checked and it shows just a trace of condensation. Fabulous piece of equipment.

Glenn Harrison

I agree with ya

they are not continuous condensing boilers, but were set up to condense due to cold start up and get a few extra percentage points out of the afue rating. They draw a fine line in the definition of "condensing", but like you said a true condensing unit is above 90%

Unknown

VHE and Gold and others are

classified Category IV for venting which means they will typically condense in the flue. There are a lot of boilers and furnaces which condense in the flue (even some that are not supposed to). Most of them now have to be vented with AL-29C (totally non-ferrous by classification) or comparable flue pipe.

To be classified a pure condensing boiler or furnace the flue gases have to condense in the boiler or furnace. This is typically accomplished by retaining the flue gases (water vapor) removing the latent heat from the water vapor which causes it to cool hence condense. The heat removed from the flue gases is absorbed back into the system in any one of many ways. Because they have such low flue gas temperatures they can use PVC (schedule 40 unless specified otherwise) safely without concern for damage.

Actual condensing mode starts typically at 87.5% and on up to the mid and high 90% range.

The volume of condensate you get from a dedicated condensing package is a lot more than some that are classified as condensing only because they condense in the flue.

I think the confusion sometimes is that the requirement for venting and classifying a boiler or furnace Category IV makes folks think it is a condensing boiler or furnace.

Furnaces have been condensing here in the USA a lot longer than boilers. They use a secondary heat exchange which is actually a condensing coil most of the time to accomplish this.

A lot of boilers simply circulated the return water back in a coil and passed the hot flue gases across the coil. Sort of a heat reclaimer. This did not typically become a condensing situation.