80 to 90% efficiency $$ savings?

Al Roethlisberger

I currently have a 15 year old naturally ventilated Carrier(rebadged Dunkirk) hot water boiler, that is rated at 80% efficiency. It currently works fine.

I am planning to do a number of upgrades to the system next Spring including outdoor reset, some piping/pumping changes, etc... to increase efficiency, as well as likely adding DHW.

But I was looking around, and many new condensing boilers at 90%+ efficiency are fairly affordable.

My question is how much could one expect to save by changing out to one of these new boilers from a relatively new 80% efficient boiler? Is it as easy as figuring up about 10% of one's yearly energy(gas) costs and deducting it as a rough/gross figure? Or are there other factors as well to consider?

I basically curious about the ROI(return on investment) and how long the new boiler might take to pay for itself, hence curious about the savings or other conveniences a new boiler may offer.

Thanks
Al

John Ruhnke

49%

Al,

Ten schools had there conventional boiler changed out and replaced with mod/con boilers and a form of water temperature set back. They saved an average of 49%.

This is from an ASHRAE paper titled "Boiler System Efficiency". by Thomas Durkin P.E.

You will save way more then 10%. If done right, you should save around 49%.

JR

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Brad White_9

Far better

The published percentages tell a fraction of the story. Condensing gains are, to me, a bonus. The biggest savings is in modulation, the ability of the boiler to track the actual load.

With modulation, the boiler has the option to down-fire, whereas your on-off boiler has one choice: Off. On-off operation entails a warm-up and cool-down cycle with inherent inefficiencies and periods of unstable combustion at each start-up. Modulation allows you to use the amount of boiler you need.

Actual experience has demonstrated reduced therm use in the range of 30,40 even 50 percent. Many variables, not all of which will apply to you, but you will do better than the catalogued differences, which are laboratory-derived steady-state combustion efficiency differences.

John Ruhnke

Condensing and lower water temps...........

Brad,

The savings mostly come from the condensing mode and lower water temps. Yes some savings come from the modulationg burner. Just because Mark did one test on his house and says different doesn't mean that everyone here on the wall should start to change opinions. One study by Mark is not a reason to change the rules of science. All the other studies and rules I have read say that you definatley need to run that boiler in condensing mode and get the temps way down under condensing mode to see big savings.

A condensing boiler will save you a lot more money (At least twice as much dropping from 130 to 100 degrees vrs dropping an atmospheric boiler from 180 to 150 degrees.

JR

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Al Roethlisberger

Thanks everyone, this is very helpful feedback.


It will certainly influence my decision for next year.

With some of the models including outdoor reset and DHW priority, etc buit-in... this definitely makes me lean toward a new boiler.

BTW, I assume that with these direct vent systems, there is no way to re-utilize the chimney as intake/exhaust, versus having to punch a hole in the side of the house?? Just thought I'd check if there was anything creative, as I hate to cut my old masonry.

Thanks
Al

Brad White_9

John.....???

John, what study and which Mark?

I do not subscribe to any particular study save my own and those of ASHRAE and the BNL... I am no sheep you know! Besides, you are not only preaching to the choir but to a lay minister. My opinions do not change "just because"; rather I use logic coupled with experience and common sense, understanding the limitiations of applicable variables. Never change your understanding of science on just one study without looking deeper into things.

The condensing is great and low water temperatures even well below dewpoint are awesome, no doubt.

But my own experience with my Monitor MZ (a condensing but non-modulating boiler) has illustrated the benefit by the lack of modulation. All other things being equal, my gas consumption stayed the same or slightly up on a "per degree-day" basis compared to a Burnham 204 with an even greater input.

The cycles were croaking efficiency, 30 seconds on, 20 seconds off during mild weather. I mean, we all know it is a system, but even a 2-stage burner would have been welcome. 4:1 or 5:1 modulation plus condensing would have been a mitzvah.

Only after I installed a buffer tank and ODR (largely to mitigate cycling), did I begin to recover the former efficiency I had enjoyed.

All of the manufacuturer's data I have makes it clear that low fire modulation is where the biggest savings lie. (Edit: JP touched on this below.) All of that surface area absorbing a reduced mass flow of combustion products and avoiding a shut-down while promoting condensing.

ALH_4

Efficiency

Most of the huge reductions in fuel costs for boiler replacements come from the old boilers having been grossly oversized or just being of inferior design. Notice that even outside of traditionally accepted condensing range of 130F and less, that the efficiency can be quite high with a condensing boiler at partial load. Outdoor reset controlled modulation is the primary feature that promotes condensation on the heat exchanger. You cannot make the most of condensation without modulation unless you have a relatively fixed load. What I like about modulating boilers is the continuously variable input that matches the continuously varying heat loss.

Uni R_2

Factor in everything...

All boilers waste their fuel. They produce heat that just ends up getting leaked out of the house at some point. We don't mind this waste because it provides comfort in a safe way. We do try and keep the costs down, but without comfort and safety any fuel efficiency issues really are secondary. When safety and comfort are overridden by efficiency, there's a good chance that the story has a sad ending.

In terms of safety and comfort, a high efficiency boiler is more comfortable. It can modulate it's heat output to match your home's heatloss. Most are as quiet as a good fridge. Some may also argue that a modcon is also safer because the boiler won't fire if the flue is plugged. Somewhere between safety and comfort is the piece of mind that comes from it being reliable. On that, some would still make a point for a simple bang bang boiler if you live in a remote location.

The savings area is not as simple as an extra 10% in savings. You will be using outside combustion air instead of sucking moist air in through your walls. Moisture reduces the effectiveness of fibreglass insulation - and that is very hard to quantify. You won't be able to warm you bare hands over an chimney in the middle of the winter, and you can warm your hands on most atmospheric boilers even if it has been idle for hours. It's a different system. One that is most efficient on the days where your atmospheric boiler is least efficient. Apples and oranges...

It's hard to put out a number. The 49% John mentions had a small number of schools that ranged from 33% to 64% in savings. They also had building envelope improvements at the same time so it is tough to say how much can be attributed to the structural improvements and how much to the boilers.

Fact is you'll save a lot more than 10%. There will be savings from the boiler as well as savings from how the boiler intakes air and exhausts its fumes outside of the house. The modulation will help your comfort factor.

A new boiler probably won't pay itself off with savings in a fast enough time period, but the savings really should be secondary. First you want comfort and safety in any heating system. Those are constants. The savings are highly variable and essentially start off with what you pay for heating now. If your current heating bill is under $1K a year, using a simple payback will be far longer than if your heating bill is over $2K a year.

The savings are definitely much much higher than 10%, just don't forget to also consider whether or not a new boiler would be a better boiler in terms of keeping you comfortable and safe.

I'm a homeowner and I just made this move. Hard to compare the savings in my case because I had oil before and 2 large tanks which was far harder to monitor usage than just going outside and looking at the gas meter each day. And yes, you can use your chimney as long as it doesn't have anything else venting out of it.

Uni R_2

Brad

The irony of your oversized MZ is the fact that the larger model would have saved you more fuel. That has to be a first!
I think the secondary kicks in when the delta T is over 11 or something like that...

jp_2

lower temp=mod

john,

the way the boiler lowers its temp is through modulation, so the two are the same, not different as you are eluding to.

your statement:
The savings mostly come from the condensing mode and lower water temps. Yes some savings come from the modulationg burner.

Brad White_9

Not by much though, Uni

The 2-stage is 70 or 72 MBH each, my heat loss is around 48 MBH and the one-stage is 94.5 MBH.

A wee bit better but I understand your point.

Thanks!

John Ruhnke

RE:

Uni R,

You said "They also had building envelope improvements at the same time so it is tough to say how much can be attributed to the structural improvements and how much to the boilers."

The study claims that they had small envelope improvements that reduced fuel consumption, that was offset by lighting improvements that increased fuel consumption. He is saying that the heatloss remained the same because one offset the other.

49% is about right.

JR


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John Ruhnke

RE:

JP,

I guess it depends how you look at it. The modulating feature saves a lot of money by reducing cycles and matching the input to the load. While matching the load if it reduces water temps it will save even more money from that too. It is not ODR that saves money. It is the reduced water temps that ODR create. If you can reduce the water temps in another way other then ODR then you will see simular savings.

JR

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dana_3

chimney

need to clarify further. you can use the chimney, if no- other appliance is using the same flue. if the existing flue, is capable of accomodating two pvc pipes (2or3 inch)to the roof

John Ruhnke

No you can't........

Dana,

The condensate in condensing boilers is very acidic. You can't use the chimney. It would destroy the chimney. Unless you insert something in the chimney that can handle the acidic flue gas. It would have to be sealed 100% tight to prevent flue gas spilage. The flue gas is under positive pressure.

JR

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Uni R_2

Chimney Chase

You are correct, however the discussion is about using the unused chimney as a chase for running the air intake and exhaust vent pipes. Many boilers include venting options for doing this.

John Ketterman

We replaced a 30-year-old cast-iron boiler with a mod-con Munchkin about half the size of the former boiler. We're saving about 20%.

In our previous home we replaced an old forced-air furnace with a condensing 2-stage; we saved about 30%.

Maybe there is someone out there who saves 49% (which means the usage is halved) but it is not normal. It's more like the fine print in those weight loss commercials: "results not typical".

John Ketterman

> the way the boiler lowers its temp is

> through modulation, so the two are the same, not

> different as you are eluding to.


Huh? The two are completely separate. A nonmodulating boiler can have its aquastat set to any low or high temperature.

Mike T., Swampeast MO

Then you can call me atypical as that's about what I've achieved with a mod-con and well-adjusted heating curve.

This compared to a system that already had TRVs, constant circulation, reset and warm-weather shutdown. Simple cast iron boiler from the 70s.

Presuming the original poster's system has none of these, I'd suspect that a mod-con directly driving his gravity conversion (TRVs added) would achieve very similar fuel savings.

John Ruhnke

49% was the average............

R Kalia,

49% was the average. It was not a marketing study. It was a scientific study.

You need to have the right control strategy too. If you saved 20% that is on the low end. The system in the schools was an engineered system. Which means everything was designed to work well together. Piping, boiler cycles, water temps and wiring matter.

JR

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Uni R_2

And an average 68% savings over steam...

http://www.roienergy.ca/pdf/Boiler_Efficiency_ASHRAE_JULY_2006.pdf

Personally, I wouldn't consider this a scientific study since it was far from controlled. But the fact is, there is far more difference in savings than what pure AFUE numbers suggest, especially in low temperature hydronic systems.

John Ketterman

Math question. If your usage is 10,000 BTU/degree-day and it used to be 15,000, would you consider that a 50% change?

Uni R_2

I'd say 33%

That's half the problem with all of this... or one third! LOL

Brad White_9

Almost, not quite

I think the point being made was that modulation following the load tracks better and can maintain lower average water temperatures for longer periods. This particularly where the return water temperature is being used as a reset variable to lessen the firing rate. Outdoor reset does this also but as we all know, automatically. Modulation is temperature control after all.

John Ruhnke

Cut in half

The study that I am quoting is cutting the fuel bills in half. If you use 44,000 btu's before then you will use 22,000 btu's afterwards. 49% is cutting the fuel bills in half not by 1/3.

JR

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ALH_4

math

I had some issues with the math in a paper on overall efficiency posted by JR on The Wall a while back. I assume this is the same one, and I think J.Cricket hit the nail on the head.

Mike T., Swampeast MO

That's the way I've figured as well. I began with the BEFORE to determine percentage.

Al Roethlisberger

Great feedback!


Great discussion, even at the low end, this seems like a potential win.

It is indeed correct that my current system, beyond having been converted to a sealed circulator system, is bone stock. Upright cast-iron radiators, no TRVs, no outdoor reset, etc.

Thanks!
Al

jp_2

well R.kalia?

what would the modulation do if not to vary the temp on a constant flow boiler? the entire system is driven soley by temperature difference.

correct, but an aquastat is just going to switch the boiler on/off/on/off....not vary burner to keep water temp at a set point.

John Ruhnke

This is the paper I am quoting from..........

Uni R,

You found the paper I am quoting from.

http://www.roienergy.ca/pdf/Boiler_Efficiency_ASHRAE_JULY_2006.pdf

JR

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John Ketterman

OK. As you pointed out, they didn't just put in a condensing boiler, they also did other things. I note particularly the energy-management systems. These studies are for schools, which are occupied for a much smaller fraction of the time than homes. It is quite possible that their old systems were inefficiently managed for the 2/3 of the time the building is empty.

I also think it is curious that they list therms used, but not the degree-days. I suspect (but cannot prove) that the "after" year was warmer.

dana_3

chimney chase

you didn't read the complete posting

John Ketterman

The temperature is set by the aquastat, and then the modulation level is determined by what is needed to reach and hold the temperature.

This may sound like a chicken-and-egg problem. What is cause and what is effect, if the two are related?

But the fact is, the modulation rate varies based on several factors other than water temperature. On startup, the boiler will go to full fire to heat the water faster. As it approaches the setpoint, the heating rate will drop. As TRVs open and close, the modulation rate will vary to hold a given water temperature. The modulation range is always finite, so sometimes the boiler will go on and off to hold a very low water temperature.

So it is NOT correct to say that modulation level and water temperature are the same thing.

jp_2

confusing.....

kalia, I do not wish to go round and round on this, but you are saying the same thing, you say the aquastat set temp but the mod part varies around that set point....

for one, you misundersatnd what i said to begin with.
modulation controls water temp!

John Ketterman

> kalia, I do not wish to go round and round on

> this, but you are saying the same thing, you say

> the aquastat set temp but the mod part varies

> around that set point....

>

> for one, you

> misundersatnd what i said to begin

> with. modulation controls water temp!

Gordy

I disagree

Brad is spot on John, you gain way more from modulation than in the condensing part of the equation. To me condensing is a gain on the fuel burning efficiency, modulation is a savings in the amount of fuel consumed by the structure its used to heat.

A far greater savings from the modulation of a boiler with 90% efficiency plus modulation to a boiler in the 80% range on and off will be seen. Condensation is a by product of low return temps, and the icing on the cake.

Gordy

GW

I can't resist

Please tell me it's not all about ROI, how fast does your car pay you back? Your computer? bla bla. If you want a cheap, just hire the cheapest contractor you can find!

OK, I'm done.

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Brad White

I am with JP on this one

Sorry RK... An aquastat whether the standard fixed-setpoint device by actual name or a sensor reporting back a variable setpoint, is but a tool, a sentinel. The modulation of the burner is the means to reach that.

Modulation is the principle form of temperature control especially in fixed-flow boilers but in variable flow boilers as well.

Modulation can respond to too low a discharge setpoint, (ramp up) too high a return water temperature (ramp down) and yes, when below the modulating range, she will cycle off. That is a function of sizing and the limitation of modulation range.

But the point remains, it is serving water temperature either coming or going. Not "several factors", at least no others that I can think of.

John Ruhnke

It was adjusted for degree days........

R. Kaila,

The study was adjusted for degree days. It says so right on page 55 of the article.

JR

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Steamhead (in transit)

Those savings figures are totally meaningless

unless you know what condition the old steam systems were in. Given that these systems were in schools, it is reasonable to assume until proven otherwise that they were in terrible shape from neglect and poor maintenance. I've said this before and I'll say it again, comparing a steam system that has been abused and knuckleheaded to a new hot-water system is not, has never been and will never be a fair comparison.

It would be very interesting to see how poorly the new condensing boilers work after a few years of the neglect they will suffer in these buildings.

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Joannie_12

Modulation

We have modulting units that have fixed temperature settings. The firing rate is modulated. The temperature setting is not. When I think "modulation" I don't think temperature change. I think firing rate modulation. When I think "reset" I think water temperature setpoint change.

Joannie

Laars Heating Systems