[Chuck]

This is going right down the road of the 38 page debates of multi-stage furnace efficiency in first stage!

Two things:

1) the heat exchanger is not a flat plate that you can simply measure the distance between another flat flame source and it. This argument would be valid for an atmospheric burner where the flame source is along the bottom of the unit firing up in a flat plane. Look at the combustion chamber of these boilers, and consider that the flame position has everything to do with the chamber design and the induced draft from the inducer motor.

2) When the burner is inputting and outputting less fuel and heat, the heat exchanger size has remained the same. So, if there is half the heat to transfer with the same surface area, the heat exchanger could be half as thermally "efficient" and still transfer the same percentage of heat created.

LOL Every time you downfire something the fire gets smaller. Whether it is a atmospheric or power burner.

What you need to do is calculate the GPM through the Knight boiler in the OP. You will need the head loss of the primary loop piping and the head loss of the boiler. Then with the appropriate pump curve you can see what the gpm is. Then with an accurate thermometer(or use the display interface on the boiler) you need to get the temp diff at 100% fire and at low fire.(Knight boilers read in % of fire so you will know what percentage of full input btu it is at) Then use this formula to get the BTU output: Delta T x GPM x 500(if not using an antifreeze) This will give you the true output which you can then use to calculate the efficiency.

I have done it many times on boilers and furnaces, modulating, multi stage, and fixed fire, and always see lower efficiency at the lower firing rates.

So until you go out and start measuring systems as I described above, your theory has no credibility. Until then I will bow out of this discussion.

 

[larobj63]

LOL Every time you downfire something the fire gets smaller. Whether it is a atmospheric or power burner.

What you need to do is calculate the GPM through the Knight boiler in the OP. You will need the head loss of the primary loop piping and the head loss of the boiler. Then with the appropriate pump curve you can see what the gpm is. Then with an accurate thermometer(or use the display interface on the boiler) you need to get the temp diff at 100% fire and at low fire.(Knight boilers read in % of fire so you will know what percentage of full input btu it is at) Then use this formula to get the BTU output: Delta T x GPM x 500(if not using an antifreeze) This will give you the true output which you can then use to calculate the efficiency.

I have done it many times on boilers and furnaces, modulating, multi stage, and fixed fire, and always see lower efficiency at the lower firing rates.

So until you go out and start measuring systems as I described above, your theory has no credibility. Until then I will bow out of this discussion.

Hmmm. Seems like you are doing a little more math than you need to. Does the gpm change at high fire vs low fire? Nope. So why are you bothering to calculate it? You can pick an arbitrary flow, or better yet, remove it from the equation. Does the system fluid change? Nope. Simply compare the delta t to the percentage fire/input to derive system efficiency. Or actually, you're not really deriving efficiency, you're simply comparing the fire rate to the delta t and seeing if they change at different rates or in exact direct proportion. Flow and water or brine properties are constant.

I hope you can see I'm not a dummy, and I'm not trying to insult you. Let's draw in our fangs. :grin2: You have to understand that I have some skepticism because you aren't exactly demonstrating a mastery of algebra and you do dodge the fact that combustion efficiency and thermal efficiency are directly related when ALL ELSE IS CONSTANT. If you are burning at exactly the same combustion efficiency over a given firing range, the thermal efficiency of the system can only change if you exhaust more or less heat out the flue relative to the heat you create, meaning the flue temp raises or the combustion air volume increases disproportionately as the firing rate decreases.


I do not know about the Knight specifically, but I do know that Aerco is definitely claiming higher thermal efficiency at reduced firing rates. Don't take it out on me, it's their engineers making the claim. :grin2:

So, you have my attention. I have not field tested equipment like you say you have. What are you finding? Feel free to share real results I am all ears. :cheers:

 

[larobj63]

LOL Every time you downfire something the fire gets smaller. Whether it is a atmospheric or power burner.

What you need to do is calculate the GPM through the Knight boiler in the OP. You will need the head loss of the primary loop piping and the head loss of the boiler. Then with the appropriate pump curve you can see what the gpm is. Then with an accurate thermometer(or use the display interface on the boiler) you need to get the temp diff at 100% fire and at low fire.(Knight boilers read in % of fire so you will know what percentage of full input btu it is at) Then use this formula to get the BTU output: Delta T x GPM x 500(if not using an antifreeze) This will give you the true output which you can then use to calculate the efficiency.

I have done it many times on boilers and furnaces, modulating, multi stage, and fixed fire, and always see lower efficiency at the lower firing rates.

So until you go out and start measuring systems as I described above, your theory has no credibility. Until then I will bow out of this discussion.

Let's also consider that a reduced firing rate with low return water temp will yield a higher thermal efficiency than full fire with 160+ return water. Like I said, the biggest worry for this and many residential designs is the domestic hot water recovery, which requires a hotter water supply temp, and a probably slightly or non-condensing condition. During heat calls, which can pretty much always be a condensing return water temperature if the terminal units are large enough, the thermal efficiency is increased by virtue of the fact that the boiler is condensing. If the boiler were sized spot on for the houses heat loss at full fire and low supply and return water temps (condensing condition), you might be short on domestic hot water. Increase the delta t (in order to force a condensing condition) by reducing the flow and you run into hot rooms vs cooler rooms because the fin tube temp varies too much, as previously discussed. Hmmm...

 

[larobj63]

LOL Every time you downfire something the fire gets smaller. Whether it is a atmospheric or power burner.

What you need to do is calculate the GPM through the Knight boiler in the OP. You will need the head loss of the primary loop piping and the head loss of the boiler. Then with the appropriate pump curve you can see what the gpm is. Then with an accurate thermometer(or use the display interface on the boiler) you need to get the temp diff at 100% fire and at low fire.(Knight boilers read in % of fire so you will know what percentage of full input btu it is at) Then use this formula to get the BTU output: Delta T x GPM x 500(if not using an antifreeze) This will give you the true output which you can then use to calculate the efficiency.

I have done it many times on boilers and furnaces, modulating, multi stage, and fixed fire, and always see lower efficiency at the lower firing rates.

So until you go out and start measuring systems as I described above, your theory has no credibility. Until then I will bow out of this discussion.

lol - And finally - this article is perfect for our conversation. Basically, you are right about thermal efficiency vs firing rate. But please read the whole article, it explains the advantages of modulating burners very well (not that you ever said they weren't good).

http://www.raypak.com/module.htm

:cheers:

 

[Chuck]

lol - And finally - this article is perfect for our conversation. Basically, you are right about thermal efficiency vs firing rate. But please read the whole article, it explains the advantages of modulating burners very well (not that you ever said they weren't good).

http://www.raypak.com/module.htm

:cheers:

Good article. And with better algebra.

You are correct, I never said modulating condensing boilers were not a good idea. We install them regularly with outdoor reset and customers save anywhere from 40% - 60% on evergy usage compared to the old cast iron on/off boilers. These savings come mostly from the low return water temps most of the year rather than the benefits of modulation.

I just don't like it when I hear: "It modulates so it doesn't matter if its oversized" Oversizing a mod boiler means it might never see higher than 50% fire except for a few hours per winter when it might see 70%. So it's spending most of its time at the lower thermal efficiencies. That article from Raypak is assuming a perfect world where both the mod and the other boilers were sized for 100% at design load. This almost never happens in a residential setting.

As far as standby losses, it is a much larger issue in commercial where the heat lost off the boiler is truly lost to a well ventilated mechanical room. Most residential installations the boiler is in conditioned area so the heat is not really lost, it goes into the space that needs to be heated anyway.

Picture a single stage condensing boiler on outdoor reset with a buffer tank (or a high mass condensing boiler) that allowed a 15-20 minute cycle even in low load conditions. This boiler would always run at peak thermal efficiency, with water temps that were only high enough to meet the load on any given day. While I have never seen this setup I believe it would be the most efficient possible. John Siegenthaler has an article somewhere about it, maybe I can dig it up.

 

[durussel78]

Knight

:cheers:Nice work!

 

[larobj63]

Good article. And with better algebra.

You are correct, I never said modulating condensing boilers were not a good idea. We install them regularly with outdoor reset and customers save anywhere from 40% - 60% on evergy usage compared to the old cast iron on/off boilers. These savings come mostly from the low return water temps most of the year rather than the benefits of modulation.

I just don't like it when I hear: "It modulates so it doesn't matter if its oversized" Oversizing a mod boiler means it might never see higher than 50% fire except for a few hours per winter when it might see 70%. So it's spending most of its time at the lower thermal efficiencies. That article from Raypak is assuming a perfect world where both the mod and the other boilers were sized for 100% at design load. This almost never happens in a residential setting.

As far as standby losses, it is a much larger issue in commercial where the heat lost off the boiler is truly lost to a well ventilated mechanical room. Most residential installations the boiler is in conditioned area so the heat is not really lost, it goes into the space that needs to be heated anyway.

Picture a single stage condensing boiler on outdoor reset with a buffer tank (or a high mass condensing boiler) that allowed a 15-20 minute cycle even in low load conditions. This boiler would always run at peak thermal efficiency, with water temps that were only high enough to meet the load on any given day. While I have never seen this setup I believe it would be the most efficient possible. John Siegenthaler has an article somewhere about it, maybe I can dig it up.

I agree. I like the idea a single stager with a buffer tank, but in practice I will continue to push mod cons in the meantime... That stored heat could work against you if it isn't in the conditioned space. But an interesting idea none the less.

 

[larobj63]

:cheers:Nice work!

Thanks!

 

[larobj63]

Good article. And with better algebra.

You are correct, I never said modulating condensing boilers were not a good idea. We install them regularly with outdoor reset and customers save anywhere from 40% - 60% on evergy usage compared to the old cast iron on/off boilers. These savings come mostly from the low return water temps most of the year rather than the benefits of modulation.

I just don't like it when I hear: "It modulates so it doesn't matter if its oversized" Oversizing a mod boiler means it might never see higher than 50% fire except for a few hours per winter when it might see 70%. So it's spending most of its time at the lower thermal efficiencies. That article from Raypak is assuming a perfect world where both the mod and the other boilers were sized for 100% at design load. This almost never happens in a residential setting.

As far as standby losses, it is a much larger issue in commercial where the heat lost off the boiler is truly lost to a well ventilated mechanical room. Most residential installations the boiler is in conditioned area so the heat is not really lost, it goes into the space that needs to be heated anyway.

Picture a single stage condensing boiler on outdoor reset with a buffer tank (or a high mass condensing boiler) that allowed a 15-20 minute cycle even in low load conditions. This boiler would always run at peak thermal efficiency, with water temps that were only high enough to meet the load on any given day. While I have never seen this setup I believe it would be the most efficient possible. John Siegenthaler has an article somewhere about it, maybe I can dig it up.

Chuck - in light of a few recent conversations I've had, including a visit to a boiler manufacturer's plant, I wanted to share something with regards to our discussion of thermal efficiency versus firing rate.

It seems that the general rule is that thermal efficiency does increase as the firing rate decreases. One of the variables we hadn't discussed yet is the reason - the TIME it takes for the flame and combustion gases to pas thru the heat exchanger. At lower firing rates, even if slightly over-aired to maintain (nearly) perfect combustion, the velocity thru the heat exchanger is SLOWER, allowing more heat to be transferred to the heat exchanger.

Lochinvar, Aerco and Fulton boilers are all claiming higher thermal efficiency at lower fire rates. If you daisy chain the boilers together and let them decide the most efficient way to run, multiple boilers will fire at a modulated rate instead of one at full of higher fire. This list only includes the boiler manufacturers that I have confirmed the higher efficiency at lower fire to be true - I'm sure it is true for almost all condensing boilers, and dare I say furnaces. :angel:

Great discussion, it actually pushed me to explore it more and ask some of the right people in the business. I hope you can share my enthusiasm for seeking the truth..