Comparing new vs existing system efficiency


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Old 10-19-09, 04:09 PM
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Comparing new vs existing system efficiency

Just had old faithful (oil burner) cleaned and serviced. Efficiency test showed 85%, it always comes in around 83-85%.

Looking at new boilers, such as the Munchkin with 92% efficiency.

Question - is the overall savings simply the difference between a new unit's efficiency and the existing unit, or are there other things to take into account?
 
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Old 10-19-09, 07:56 PM
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Physics and chemistry dictate that COMBUSTION EFFICIENCY at the very best can be no more than 85-87% efficient... a fact of life. Approximately 15% of the heat goes up the chimney, and it HAS to... in a 'standard' boiler.

The Munchkin is a CONDENSING boiler, and it is designed to take back some of that heat from going up the chimney... by CONDENSING the flue gases. It's designed to do that without damaging itself in the process. Flue gas condensate is acidic, and will destroy flue pipes and cast iron.

Neither of these efficiencies are to be confused with AFUE (Annual Fuel Use Efficiency) which is a measure of how well the heat is actually USED in heating the home.

So, be careful that you are comparing apples to apples...

Even an older boiler properly set up can achieve 85% COMBUSTION efficiency, but could easily have an AFUE much lower than modern systems.
 
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Old 10-21-09, 12:41 AM
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In the middle of the winter when you hit that really cold snap, the efficiencies of the two types of boilers will be at their closest point. In the milder shoulder seasons the differences in efficiencies will be greatest. This is when conventional boilers perform at their WORST and condensing boiler perform at their BEST. AFUE really doesn't take this into consideration but you should.
 

Last edited by NJT; 10-22-09 at 05:00 PM.
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Old 10-21-09, 05:17 AM
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We have to remember efficiencies change by the minute while the boilers are running. As the temperature of the water increases the efficiencies go down and as the water cools (another zone calls) the efficiencies go up. The mod/cons take a bigger hit due to the ability to operate in the condensing and non-condensing mode. When they stop condensing their efficiency drops much quicker than a cast iron product, but bear in mind they are more efficient when condensing. At 9% CO2 they are condensing at about 126. If the returning water is less than 120 the efficiency will be higher than the rated efficiency. The mod/con efficiency will increase much faster than the cast iron boiler with cooler water.
The temperature your system needs to operate at helps determine which is the best application. This is determined by the heat loss of the home and the amount of and type of connected load (radiation).
I guess to answer your question there is more to consider other than combustion efficiency. Combustion efficiency can be good but the thermal transfer of the heat to the water could be poor. This is a design issue which addresses how much heat is transferred from the flue gasses to the water. Newer boilers have much greater fireside heating surfaces as compared to water volume.
The old boiler could have been maintaining water temp and the new ones do not. How much build-up is on the iron that does not come off when cleaned. This is usually caused by flue gas condensation and solidifies when dry. When brushed out it will leave a small footprint on the iron which acts like insulation.
Near boiler piping can change the fuel consumption if not done properly now. Because it works does not mean it is correct.
Any air in the system will impact heat output even it it does not seem like there is any ion there there still could be. Air will make the pipe carry less btu's from the boiler due to air bubbles taking up space, reduces the pump head which means water slows down. Slower flow means less heat from the radiation, increased flow more heat off the radiation. Air also means more corrosion on the water side of the boiler, again, insulation.
So is there more to fuel savings than combustion efficiency.........Yes!
 
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Old 10-21-09, 04:08 PM
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Originally Posted by Who View Post
...In the milder shoulder seasons the differences in efficiencies will be greatest. Conventional boilers perform at their best and condensing boiler perform at their best.
Who, did you mean to type above:

...at their WORST and...

in place of the bold italic?
 
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Old 10-22-09, 03:46 AM
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Trooper, I would agree with Who. The shoulder season is when the mod/con can shine a little brighter due to lower water temp and condensing. The cast iron will short cycle more. The colder temperatures is when the cast iron runs longer cycles and the mod/con may not condense and will loose efficiency dependent on the application.
 
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Old 10-22-09, 04:57 PM
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Yes Troop... can you fix it for me?
 
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Old 10-22-09, 04:58 PM
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Sure can! and did... need 25 characters...
 
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Old 10-22-09, 08:56 PM
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OK.. a related question..

If "the best" burner can hit 85-87% combustion eff in theory..

At what comb-eff do they rate the AFUE at ?

My cast boiler specs say AFUE=80. My tech peaked my settings to a comb-eff of 81%. Does this imply that something is affecting the system, and Im actually running *below* AFUE-80 ? (could be fuel temp/quality..flue sizing/draft.. etc etc ) ??

Im supposing boiler manuf's set up AFUE tests in the best light possible for the units..

And.. possibly important for the OP to compare systems.. has the AFUE standards for testing changed in recent history ? (the ratings system for fridges/freezers changed some time ago, making it hard to compare old units to new units..)
 
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Old 10-22-09, 08:59 PM
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It also depends where the boiler is located. If its in the basement the heat escaping from the cast iron still warms the house, so the heat is not really 'lost'. I made up a nice spread sheet last year comparing different replacement systems. Basing on efficiency numbers and oil shooting up to $5/gal., the expensive high efficiency unit would only begin to pay off if I plan to stay in the house more than 5-6 years. It would be longer now that the price of fuel has come down.

I ended up refurbishing my 'old faithful' (40 year old Thatcher) with updated controls and a new burner, and it works like a charm. Efficiency is still only around 80% which probably represents a couple hundred dollars a year more in oil than a condensing unit.
 
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Old 12-05-09, 01:22 PM
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I was searching for other info and ran across this thread. One other 'measure' of efficiency could also be regarding an oversized older unit vs a newer properly sized.

correct me if my thinking is wrong:

even though at times my old 225K unit with 1.25 nozzle tested at lower to mid 80s efficiency, the same percent efficient new boiler would save me money since it has only a .75 nozzle (we'll ignore the fact that it's coil vs low mass for the moment). So replacing an 85 percent unit with an 85 percent sounds pretty silly but you have to know all the other factors as well

i think.
 
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Old 12-05-09, 02:56 PM
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The difference in efficiencies is often misunderstood, and many times repeated here...

When the technician tests efficiency he is only looking at COMBUSTION EFFICIENCY, which is basically meaningless if you are trying to compare units for FUEL EFFICIENCY.

All COMBUSTION EFFICIENCY really tells you is that you are mixing the right amount of fuel with the right amount of air, and that the unit is burning as cleanly as possible. (There is a little more to this, but very basically, that's the story...)

AFUE = Annual Fuel Utilization Efficiency is a better measure of how the heat that is produced is actually transported into the home. Older boilers won't be rated, so you have no way of knowing.
 
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Old 12-05-09, 07:51 PM
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So, if the combustion efficiency doesn't mean a lot. Then what part of the boiler does?
This thread should become a sticky.

The combustion reading (efficiency) only relates to the BTU of fuel that is converted to heat within the boiler. With an 80% efficiency, out of 100K BTU of oil/gas feed in, only 80K BTU went into the water. The other 20% went up the chimney.

Note that this is normal. What went up the chimney as heat is used to prevent condensation. Which is another topic of discussion. A condensing boiler, which is not part of this post is a topic for another thread. But has a higher combustion efficiency (along with higher install cost and costs to maintain).

The point being what other losses are involved.

Now, 100K BTU is dumped into the boiler, for 80% efficiency. So out of 100K BTU, 80K BTU heats the water in the boiler.

The circulator pump kicks in and starts to move that heat (80K BTUs) around the circuit to the radiators and what remains back to the boiler.

Most (hopefully) of the BTUs go to heating the radiators which puts heat into the house.

However, during this time the delivery & return pipes may be in a craw space or a basement. These pipes also release heat into those areas. Which may not be in the "heating comfort zone."

And end up being wasted heat. IOW, it is a loss. If the basement is 90* F, while the thermostat in the living quarters is set to 70* F. That is wasted heat, which is a loss.


Another area of loss is heat the escapes via the chimney while the burner is off. The boiler water may be at 140* F to 180* F at this time. There is usually a draft through the burner, through the boiler, and up the chimney during this time.

This is fuel that has been burned and does not go into heating the building. Note that during shoulder seasons this is more of a loss then during cold weather.

There are several ways to combat this loss. Outdoor reset (ODR) is the best method. Using a flame retention burner also helps. If using an atmospheric gas burner then a flue damper cutoff is required.

The idea is to reduce or eliminate the flow of air through the burner, through the boiler, then up the chimney during idle periods.

For some real insight, consider the 50 year old boiler/burner here. It was a hog, burned oil like we changed clothes.

With the installation of a flame retention burner the first item noticed was how much longer it took for the boiler to cool off. During idle times (shoulder season) the boiler held it's temperature much longer. Hmmm, didn't expect that.

The flame retention burner also ran cleaner at a higher efficiency. Although this wasn't more then 4%.

Then with the addition of ODR then overall fuel consumption dropped quite a bit.

I've posted it here someplace. However, IIRC, it was about a 30% reduction in the use of fuel.

This is with the flame retention burner, and the ODR together.

While the flame retention burner showed a 4% increase in combustion efficiency. It was the reduction in idle loss (lower draft while off) and the ODR that made the biggest difference.

Back to the basement, where the boiler is located along with the pipes. I was going to insulate them as it was always too hot in the basement. But with the ODR this is no longer so. The boiler room area temperature is much lower.

Which negates the need to insulate the pipes, and goes to show that the losses into a non-living area has decreased. Which goes toward lower overall fuel consumption.


For other installations there are items such as hot water (domestic) water. If a boiler has a DHW coil then it is doing what is called maintaining temperature. That is, the burner is firing to keep the boiler water temperature high. This is for when someone required hot water.

The standby (or idle) losses are huge. The easiest way to look at this is to never run a hot water faucet and check to see how often the burner runs. If no hot water is used, then each time the burner runs is a waste of fuel.

Al.
 
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Old 12-05-09, 08:04 PM
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If using an atmospheric gas burner then a flue damper cutoff is required.
Not previously recommended for oil because of 'problems' with the damper getting 'gunked' up... Field Controls claims to have solved those issues with some design innovations and now us oil burners have the option to use a vent damper too!

Field Controls Oil Vent Damper


image courtesy Patriot Supply
 
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Old 12-06-09, 04:50 AM
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Just a small note to add to oldboiler's comment. The newer boilers also have more fireside heating surface which extracts more heat from the flue products and improve the thermal transfer of heat from the fireside to the water side of the boiler. This is achieved in cast iron boilers with more pins or smaller flue passes with baffles. The idea is keep the flue gasses in the boiler as long as you can without producing problems. Today there are pressure fired oil boilers on the market which instead of having a negative pressure in the combustion area it is actually a positive pressure which assists in increasing the thermal transfer.
 
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Old 12-06-09, 07:07 AM
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Originally Posted by OldBoiler View Post
.

Back to the basement, where the boiler is located along with the pipes. I was going to insulate them as it was always too hot in the basement. But with the ODR this is no longer so. The boiler room area temperature is much lower.

Which negates the need to insulate the pipes, and goes to show that the losses into a non-living area has decreased. Which goes toward lower overall fuel consumption.
this is a very interesting topic to me since i'm deciding if it's better to insulate the pipes in my crawlspace vs letting them heat the space and 'maybe' keeping my water pipes from freezing and 'maybe' helping keep the humidity down. (i honestly dont know the affect the heat has on those 2 things) I'm assuming you want 'some' heat in your basement or else it would pay you to insulate the pipes? - you said it now is not necessary with ODR and i have ODR as well now.

My 3 ft deep crawlspace has a sensor which is 10ft from a crawl window, and is not airtight because i need to run satellite wires around it into the crawl, and is now reading 55 degrees with 77RH. the house is about 65 with 43%. Outside is 35 degrees with fresh dusting of snow. so i wonder if i insulated all the pipes what would happen to RH..i'm sure temp would drop.
 
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Old 12-06-09, 07:44 AM
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Originally Posted by luckydriver View Post
this is a very interesting topic to me since i'm deciding if it's better to insulate the pipes in my crawlspace vs letting them heat the space and 'maybe' keeping my water pipes from freezing and 'maybe' helping keep the humidity down. (i honestly dont know the affect the heat has on those 2 things) I'm assuming you want 'some' heat in your basement or else it would pay you to insulate the pipes? - you said it now is not necessary with ODR and i have ODR as well now.

My 3 ft deep crawlspace has a sensor which is 10ft from a crawl window, and is not airtight because i need to run satellite wires around it into the crawl, and is now reading 55 degrees with 77RH. the house is about 65 with 43%. Outside is 35 degrees with fresh dusting of snow. so i wonder if i insulated all the pipes what would happen to RH..i'm sure temp would drop.
The best bet to lower the humidity of the crawl space is to put 4-mil plastic sheet on the ground. That will greatly reduce the amount of water vapor rising out of the ground.

As far as whether it is better to have some heat or not in the crawl area. I do not know. However, as you mention there are also water pipes in that area. So it does need to be kept above freezing. If the water & boiler pipes are basically run together you may be able to insulate them in such a way that the boiler pipes keep the water pipes warm. But not hot.

Basement: when the boiler was banging off the high limit I could wear shorts and a t-shirt in the dead of Winter. It was easily 85 F in the basement. The basement does require some heat as it is used quite a bit. Washer/drier, food storage, shop, and so on are in the basement.

I had started getting prices and availability on pipe insulation. And was finding that it isn't cheap (most pipes are 2" iron, 2-pipe reverse return. So a lot of pipe involved). Then with the implementation of ODR the basement temperatures dropped. To where now it is only slightly warmer then it needs to be.

I painted the 1" iron pipes to the Den zone gloss black. Looks nice and should cut the heat loss of those pipes a little.

The plan is to paint the 2" iron pipes silver. This will reduce the radiated heat along with looking decent.

Back to losses, in the basement the largest loss is through the upper area of the walls. This is where it is exposed to the outside, and the top 3' of cold ground. So even though a lot of the heat rises up through the floor to the living area.

There are losses through the walls that are reduced by reducing the temperature in the basement.

One item I didn't mention in the previous post is jacket losses of the boiler. This is heat that the boiler gives off, just by being there and at temperature. By reducing the boiler water temperature the jacket losses are also reduced. So overall system efficiency is increased.

(Can you tell I like ODR?).

Al.
 
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Old 12-06-09, 09:01 AM
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You won't remove the moisture from the air by heating it. You have to condense it out.

Go look through the basements/crawlspaces forum. There are a many threads on what to do with crawlspaces.
 
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Old 12-06-09, 09:15 AM
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You won't remove the moisture from the air by heating it.
While that is technically true, the Relative Humidity WILL change.

Since warm has greater capacity to hold moisture, the humidity RELATIVE to temperature will definitely go down as the air is heated.

The ABSOLUTE amount of water in the air will not change though.
 
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Old 12-07-09, 05:46 AM
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As far as pipe losses a 2" iron pipe at 200f will lose about 190 btu's per ft. At 150f the loss reduces to about 104f. This will go up slightly in your crawlspace. To insulate the 2" iron pipe at 150f water the loss is reduced to 50 btu's per foot.
Let's also remember the efficiency changes with water temperature. As the water temperature goes up the efficiency goes down and of course if the water temperature goes down the efficiency goes up.
Remember AFUE is tested at 120f return and 140f supply. When you look at the charts you can see how this affects the operation. Cast iron is affected less as the thermal transfer is higher.
ODR_Charts
 
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Old 12-07-09, 03:52 PM
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I have both 1.5 and 1.25 inch black pipe right next to the boiler so no idea whats in the crawl (assume a mixture of each maybe). but i guess there's one 'sleeve' i could buy that fits both sizes? with the ODR i have a lot of 130 degree water in the shoulder seasons but now that we are down to 20 outside that shoots up to about 155 to 160.

I have to meditate upon that chart and your reasoning about higher temp being less efficient. i'm assuming it's because more fuel expended to reach that temp?
 
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Old 12-07-09, 04:24 PM
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Originally Posted by luckydriver View Post
I have to meditate upon that chart and your reasoning about higher temp being less efficient. i'm assuming it's because more fuel expended to reach that temp?
A higher water temp, in turn, causes higher temp in the stack gas. More energy goes up the flue, and is wasted.
 
 

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