# real worl efficiancy with condensing boilers?



## 907plumber (Mar 13, 2010)

So my former boss was trying to tell me that if you install a condensing boiler on a typical baseboard system that you will not see the claimed efficiency. the summary from him was that the return water is not cold enough.

Can any one elaborate on this?


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## RealLivePlumber (Jun 22, 2008)

I can elaborate. He does not know what he is talking about.


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## Ishmael (Dec 9, 2009)

I don't know how to explain the technical aspects of it, but the Weil-McLain Ultra UG-299 claims a combustion efficiency of 96.4%, but they also state that _"...when used in low-temperature applications such as radiant heating, it can achieve efficiencies up to 98%._" So, theoretically, if you had two identical houses right next to each other - one with radiant and the other with baseboard - I guess the house with radiant would have lower gas bills by about 99 cents over the course of a year?


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## RealLivePlumber (Jun 22, 2008)

Yeah. Course what you made up in savings by having that "cold return water", would be quickly offset by repalcement boilers.:yes:


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## ZL700 (Dec 8, 2009)

He is correct.

Condensing of boilers stop at about 140 degrees plus. Condensing boilers are called just that since they produce condensate under the correct conditions. Condensate is produced as a byproduct of combustion which is virtually the moisture in combustion air and fuel (gas).

There is as much as 1000 btus to be gained from 8.3 pound of condensate water (1 gallon).

So when you install a high efficiency boiler and run it on a baseboard system, with the industry standard of 180 degree supply water temp, and if the return water is 160 degrees because the baseboard was designed at a 20 degree delta t, the boiler never condenses, because the flue temp is elevated for its not operating in a hi-efficiency mode. 

However there is some operating efficiency to be gained by replacing a large mass hi water volume with a smaller footprint, lower water content boiler since it has less system water to heat up even when operated at 180 degrees.

That is why radiant systems are designed at lower temps and do so well, the other option is to add more baseboard allowing the operating water temp to be lowered, and still meet the heating requirements on the coldest day.

More often the newer boilers incorporate outdoor reset that allows the boiler to operate at cooler temps on milder days and only going to the higher temps when needed on the coldest days, then a condensing boiler may be in condensing mode 40 to 70% of the heating season, thereby offering energy savings without heating system changes beyond the boiler replacement. 


N


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## OldSchool (Jan 30, 2010)

907plumber said:


> So my former boss was trying to tell me that if you install a condensing boiler on a typical baseboard system that you will not see the claimed efficiency. the summary from him was that the return water is not cold enough.
> 
> Can any one elaborate on this?


Your former boss is right...

At higher water temps the boiler would not condensate.... making it a non condensating boiler...

You will still be in the 90% range but not at the 98% like the rating says...

ZL700 explained it well...

But one must also consider that most condensating boilers are also modulating and that alone is a big cost savings in fuel....

I would always recommend putting in a modulating condensating boiler in every application possible.


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## 1703 (Jul 21, 2009)

ZL700-

Trying to learn here- you say 1K btu per pound of condensate, then put 1 gallon in parenthesis.

Could you explain a little more as a gallon of h2o weighs more than a pound?


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## ZL700 (Dec 8, 2009)

Colgar said:


> ZL700-
> 
> Trying to learn here- you say 1K btu per pound of condensate, then put 1 gallon in parenthesis.
> 
> Could you explain a little more as a gallon of h2o weighs more than a pound?


Oops, typing on a smartphone
8.33 pounds/1 gallon can have up to 1000 Btus claimed from condensate


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## 1703 (Jul 21, 2009)

ZL700 said:


> Oops, typing on a smartphone
> 8.33 pounds/1 gallon can have up to 1000 Btus claimed from condensate


Phew. I was afraid I was gonna hafta bust out the "No, YOU don't know chit.". :thumbup:


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## 907plumber (Mar 13, 2010)

Thanks Guys. I was working with another j-man, we were installing a triangle tube boiler and I was telling him that these people werent going to get the claimed efficiency. He argued they were and that triangle tube designed it to work with baseboard. I just shut my mouth, till I could get my facts straight.

Still gonna be hard to tell him that the reason they wont get the efficiency is cause it wont condensate. Cause I dont see what condensation has to do with efficiency


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## Scott K (Oct 12, 2008)

When combustin occurs in a boiler, the burner takes CH4 (Methane - Natural Gas, roughly) and O2 Oxygen (and Nitrogen, but Nitrogen is inert)
and ignites them with heat (you need 3 things to achieve combustion - fuel, oxygen, and heat). The By-products of perfect combustion are 3 things - CO2 (Carbon Dioxide) and H20 (water) as well as heat. Since the products are hot (have you tried putting your hand in front of a flame) the water is in a vapour state (steam). 

Now lets forget about this combustion stuff for a second and look at water. When you apply heat to water in its fluid state, say you have a pound of water. A BTU is the amount of heat to raise the temperature of 1 pound of water 1 degre fahrenheit. So say you applied heat to water. AS the heat is applied, the temperature of water is steadly increased. This increase in temperature is temperature you can sense and is called "sensible heat." Sensible heat changes the temperature of the water.

But when water gets to the point of turning into steam (212 F), it doesn't just flash to steam. You have to pump a whole crapload of BTU's into water to get it to convert to steam. This is called LATENT heat - a change in state, not a change in temperature. 

These are the 2 types of heat. Sensible - and Latent. ONe is a change in temperature you can sense (sensible), the other is a change of state from solid to liquid, or liquid to vapour (steam).

Now for every pound of water you raise 1 degree F, as I said above, it requries 1 BTUH. But to convert 1 pound of water to steam (at the same temperature - 212 F), it requries 970.4 BTU. JUST TO CHANGE THE STATE (no temperature change at all). So the same thing happens in reverse. Instead of having to apply a crapload of heat to change the state, when that water condenses back into a liquid from steam, it also GIVES OFF 970.4 BTU per pound. It is giving off its latent heat before it can turn into a liquid and start losing temperature (sensible heat). It works both ways. 

So back to the burner here. WAter (stream - vapour) is a product of combustion. So you have this burner and it's giving off it's heat and products of combustion (CO2 and H20). Now around the burner is a bunch of stainless steel water tubes that water is circulating through (this would be called heat exchanger) . Now if the water inside these tubes is cold enough (typically 135 degres or less), the steam in the products of combustion from the burning process will condense on the heat exchanger walls and give off its latent heat to the water circulating through the tubes. This is the route of where a condensing boiler gets it's extra efficiency. The COLDER the water in the tubes, the MORE of the vapour in the products of combustion will condense and give off its latent heat, the more efficent the condensing boiler is. So if the entire heat exchanger is below 135 degrees - say you're only pushing out 120 degrees max to the radiant floors which means the return water might be 110 or 100 after giving off some of its heat to the surface, you're going to see the entire heat exchanger condense.

Now if you're running water above 135 degrees, the water vapour in the products of combustin will not condense and subsequently all that water vapour will get carried up the flue, or will condense in the flue which won't give off this extra latent heat to the heat exchanger.

Are you with me yet?


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## OldSchool (Jan 30, 2010)

Scott K said:


> When combustin occurs in a boiler, the burner takes CH4 (Methane - Natural Gas, roughly) and O2 Oxygen (and Nitrogen, but Nitrogen is inert)
> and ignites them with heat (you need 3 things to achieve combustion - fuel, oxygen, and heat). The By-products of perfect combustion are 3 things - CO2 (Carbon Dioxide) and H20 (water) as well as heat. Since the products are hot (have you tried putting your hand in front of a flame) the water is in a vapour state (steam).
> 
> Now lets forget about this combustion stuff for a second and look at water. When you apply heat to water in its fluid state, say you have a pound of water. A BTU is the amount of heat to raise the temperature of 1 pound of water 1 degre fahrenheit. So say you applied heat to water. AS the heat is applied, the temperature of water is steadly increased. This increase in temperature is temperature you can sense and is called "sensible heat." Sensible heat changes the temperature of the water.
> ...


And there you have it in a Nut Shell.... nice explaination :thumbup:


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## plumber666 (Sep 19, 2010)

That was awesome, Scott. :thumbup1:


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## 907plumber (Mar 13, 2010)

Thanks Scott, I had to read it a few times but I think I understand it now. So where does the H2o from combustion come from? The humidity in the air inside the home?

Can you tell me how a modulating boiler works?

Thanks for your time!


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## Scott K (Oct 12, 2008)

907plumber said:


> Thanks Scott, I had to read it a few times but I think I understand it now. So where does the H2o from combustion come from? The humidity in the air inside the home?
> 
> Can you tell me how a modulating boiler works?
> 
> Thanks for your time!


The "humidity" is a byproduct of combustion. When you combine Natural Gas (CH4) and Oxygen (O2) in the right quantities i.e. within the flammable limits (i.e. 10 parts air to 1 part methane is the standard stoichiometric/perfect combustion ratio) and you apply heat (through a spark or a pilot - aka the ignition source) it combusts and the products of this combustion are Carbon Dioxide and Water. 

The formula looks like this in basic form:
1 CH4 (1 part Natural Gas) plus 2 parts Oxygen (O2) plus 8 parts Nitrogen (N2) (10 parts Air - remember the Nitrogen however is inert so it doesn't interfere with the combustion), plus an ignitiion source (heat - either a spark or a pilot) produces:

1 CO2 (1 part carbon dioxide) plus 2 parts H20 (2 parts water) plus 8 parts Nitrogen - and most importantly, heat (which is emitted by the flame). Due to the heat of the combustion process, the water in the products of combustion coming out of the flame is in the vapour (steam) state. 

If you put a cold surface near a burner that is giving off these products of combustion (again, carbon dioxide and water) (in this case, a cold surface is about 135 degrees Fahrenheit or less) the vapour in the products of combustion will condense on this cold surface. 

Now remember above I mentinoed that everytime water changes from water to steam you have to apply 970.4 BTU's per pound to convert it from water at 212 F to steam at 212 F This is called "latent heat." You apply latent heat to change the state of water, but NOT the temperature. 
And at the same time, it works both ways. Everytime water vapour/steam condenses back into a liquid it also gives off 970.4 BTU's per pound. 

So think back to this cold surface I mentioned above. If you have a cold surface near a burner. In this case, a cold surface would be some stainless steel tubes containing water, and that surface is 135 degrees F or less, the water vapour in the products of combustion will condense onto the surface of these tubes. "Condensing" is the water changing its state from steam to water and remember what I said above about what water does when it changes states? It gives off its latent heat (970.4) to the water inside the tubes by condensing on the tubes. 

In a conventional boiler, which typically has a cast iron heat exchanger (which is the material containing the water on the hydronic side), you do NOT want the water to condense on the surface because cast iron can not handle condensation, so you need to send some heat up the exhaust vent to keep the water in the vapour state until it is exhausted from the building. You also must ensure the water in the "tubes" does not fall below 140 degrees or this vapour will condense and eat the heat exchanger.

In a condensing boiler, you want the opposite to happen. Because the heat exchanger (the tubes containing the water) is made out of stainless steel, it can handle the water being condensed on the surface of the tubes. Everytime the water condenses on the tubes, you gain the extra heat from the latent heat of stream turning back to water. 

Now what does modulating mean? 

A basic cast iron conventional boiler typically has 1 firing rate. It turns on, it turns off. It only fires at 1 BTUH input. So you'll look at the rating plate and it'll say "96,000 BTUH" input. This means this boiler uses 96,000 BTUH everytime it fires. If you fired it for one hor straight, it would consume 96,000 BTUH's worth of fuel. It may also have an output rating as well, which factors in efficiency of the appliance. For a 80% efficient appliance, the output is the useable heat that is not wasted (the "wasted" heat would be heat you send up the flue to ensure the products of combustion are kept hot so the water vapour from the combustion process is less likely to condense on the heat exchanger walls or in the venting). So 80 percent times 96,000 BTUH equals roughly 77,000 BTUH. So you'll get roughly 77,000 BTUH transferring to the appliances water doing work you can feel by increasing the temperature of the water in the tubes, and the other 19,000 BTUH is used to keep the products of combustion hot so the water vapour is less likely to condense which could cause all sorts of problems on metals that are not designed to have water condensed on them such as the B-vent, chimney, and boilers heat exchanger walls. 

Now the problem with an appliance that only has one on/off firing rate is that sometimes you do not need 77,000 BTUH's of heat. Sometimes you only need 25,000 BTUH. Sometimes you may only need 65,000 BTUH. So how does an on/off appliance with one firing rate deal with this? Well it comes on for a bit, heats up the water to a certain point and then shuts off. Depending how much heat is required, it may do this many times in an hour. Everytime it does this, it puts wear and tear on ignition components and parts. Also, since an appliance takes a certain amount of time to get up to its rated efficiency, it causes the appliance to not run as efficiently. It can also cause flue gas condensation as at a certain point when the appliance starts up after having been off for a while, the water in the heat exchanger is cold and may take a bit of time to get up above 140 degrees. In extreme cases this can cause the boiler to live a much shorter life, plus it wastes fuel. Everytime you have to get the boiler up to operating temperature, it wastes fuel, and the boiler also burns in a much less clean state with more emissins until it gets up to temperature. Also consider things such as thermal stress due to expansion and contraction of the boiler constantly starting and stopping. The heat exchanger is 120 degrees one minute, and 350 degrees a few minutes later. Overtime this puts stress on the heat exchanger, mating surfaces, and any gaskets or connections, especially if differing metals are involved.

The best case for a heat exchanger and for efficiency of the burner is to try and keep the burner running as long as possible. A modulating burner is one way to do this. By varying the input rate, if you properly set it up, you can ensure the burner runs for a much longer time at a reduce rate instead of having it come on, off, on, off, etc (short cycling). I've always said that I try and set up my mod-con's so that they come on in November, and don't turn off until about March. This is the hallmark of a modulating condensing boiler that is properly sized for the load is intended to heat. 

So when you combine the efficiency of condensing appliances, with a modulating burner, you have a recipe for excellent efficiency assuming you picked a "mod-con" that will work with the buildings heating requirements.


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## Plumbdog (Jan 27, 2009)

Scott K said:


> The "humidity" is a byproduct of combustion. When you combine Natural Gas (CH4) and Oxygen (O2) in the right quantities i.e. within the flammable limits (i.e. 10 parts air to 1 part methane is the standard stoichiometric/perfect combustion ratio) and you apply heat (through a spark or a pilot - aka the ignition source) it combusts and the products of this combustion are Carbon Dioxide and Water.
> 
> The formula looks like this in basic form:
> 1 CH4 (1 part Natural Gas) plus 2 parts Oxygen (O2) plus 8 parts Nitrogen (N2) (10 parts Air - remember the Nitrogen however is inert so it doesn't interfere with the combustion), plus an ignitiion source (heat - either a spark or a pilot) produces:
> ...


Thanks for a great post, I have refered back to it several times (just trying to all the info through my thick skull). If you don't mind could you explain what exactly you would do to make sure that the boiler was set up correctly to "run from Nov-March" as you say.

The reason I ask is I'll be installing a Mod con soon with a radiant slab on the first floor and with either low temp base board or over sized rads on the second floor in order to keep water temp low. I just want to make sure that the system performs as it should.

Thanks
Dog


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## Scott K (Oct 12, 2008)

Well you can start by trying to run the boiler at the lowest water temperatures possible. This DOES require a bit of trial and error, and may result in one or two call backs, but a certain amount of experience can help you tweak things, especially if you have a boiler controller that is a little more interactive than say just a simple knob that controls a heating curve (which may work too, however, not all the time). Having the boiler up and running on just outdoor reset alone during temporary heating for example, while you work to finish the system will allow you to tweak the system a bit this way. 

The other thing is anti short cycle controls. What does the boiler have in its controller? Most boilers have anti cycle logic built in that will allow the boiler to exceed the setpoint a bit before cycling off. How about a controller that allows you to slowly ramp up the system upon a call for heat so it runs as long as possible, that not ony a design water temperature setting, but a max temperature setting. Or a controller that has the option to tell you how far it is allowed to the specified temperature before it cycles off. This may buy the boiler some time for other zone valves or actuators to open and dump some cooler water back to the return of the boiler bringing those temps down and allow the boiler on minimum firing rate to do some work. 

Sizing your pumps correctly is critical. Say you have a pump that is way oversized pushing more water than it should. If a zone has been sitting there dormant for a while to the point the water has cooled and then it kicks on and that pump cranks the water out to the boiler, the boiler may ramp up quite a bit to heat all this water coming to it, but the slab can't absorb the BTUH's right away and some of that heated water will head right back to the boiler causing it to potentially ramp down quickly to minimum fire and then potentially off. This is where balancing and proper pump flow & selection in tune with your properly sized boiler, is critical. Consider variable speed ECM pumps like the Alpha or Stratos which have various settings to help you tune the flows, for the heating system. 

If you can have a dedicated indirect pump that is the sole pump that comes on when the indirect requires heat, and then primary secondary your boiler and system loops, you can tune your boiler pump to the flows it will need to inject into the system loops. (say if your system requires 5 GPM on design day, have your boiler pump pushing 3-5 GPM's). This will ensure the boiler pushes nice Delta T's through the heat exchanger. 

Perhaps I am taking for granted outdoor reset? This is a MUST. 

The other thing you can do is ensure you are utilizing the smallest boiler possible that will heat the space. Don't think about max fire rate, think min fire rate. Of course the max fire rate has to be a bit above the buildings heat loss, but size the boiler around the house. You can always buffer the reduced output of the boiler with a larger indirect hot water tank (say up one size). 

With the design of the system - try to push the homeowner into not having a bunch of small zones if possible, but combining a few of them. This will save the money, but also allow the boiler to run longer with more flow through the boiler. 

In some systems, if you have a bunch of 1-2 loop super small zones, and the boiler minimum firing rate isn't that small, it may be time to consider a buffer tank as well. 

I'm sure I will think of more things. 

The bottom line is this - don't just install a system because it will make you money. Look at the bigger picture - you of course want to heat the house on the coldest day of the year. I get a kick out of walking into a house on an extremely cold day and seeing how the boiler is performing. But the hardest part of a good, well installed installation is setting it up to run the other 90% of the year when the system is no where near it's design temperature. This is the hallmark of a conscientious installer and designer, and someone who is in tune with the relationship everyting in your system has. You are there to make a difference. If everyone of your boilers uses 10% less fuel, and your pumps use 10-20% less electricity because you are in tune with what you're installing and understand the design, after 10 installations you just made a big difference in not wasting fuel or the worlds electricity.


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## tk03 (Mar 4, 2011)

Scott K,
I know with cast iron boilers when you reduce the input you loose thermal efficiency due to not filling flue passes with flue products.
If we produce less products of combustion how can we not loose thermal transfer?


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## SlickRick (Sep 3, 2009)

tk03 said:


> Scott K,
> I know with cast iron boilers when you reduce the input you loose thermal efficiency due to not filling flue passes with flue products.
> If we produce less products of combustion how can we not loose thermal transfer?


An intro is requested from all new members. In case you missed it, here is the link. http://www.plumbingzone.com/f3/.

The PZ is for Plumbing Professionals ( those engaged in the plumbing profession)

Post an intro and tell our members where you are from, yrs in the trade, and your area(s) of expertise in the plumbing field.

This info helps members who are waiting to welcome you to the best plumbing site there is.

We look forward to your valuable input.


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## U666A (Dec 11, 2010)

SlickRick said:


> An intro is requested from all new members. In case you missed it, here is the link. http://www.plumbingzone.com/f3/.
> 
> The PZ is for Plumbing Professionals ( those engaged in the plumbing profession)
> 
> ...


And he made a spelling mistake... Get him! 
Lol


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## rebeccaasmit (Mar 9, 2011)

Thanks for posting this , in between your conversation you also resolve many issues of mine.


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