When is a high efficiency condensing boiler not a high efficiency condensing boiler?

 
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lsayre
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Post by lsayre » Mon. Feb. 24, 2020 1:26 pm

The answer (for NG and propane, and likely also oil) is when the water returning to it is 125 degrees F. or higher. This favors in floor radiant heating big-time for efficiency.

If a condensing boiler is rated at 95% efficient, that 95% efficiency rating is only true for about 90-100 degree (or lower) returning water. As the boilers return water temperature goes up its efficiency goes down nigh-on straight-line linearly until when at 125 degrees on the return it is only about 86% or 87% efficient (at best). It then stays at 86%-87% efficient as return water temperature climbs from there.

I'd wager that your average HVAC contractor will likely not be open to up-front telling you this little tidbit.


 
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Post by Lightning » Mon. Feb. 24, 2020 2:41 pm

What is different about a condensing furnace as opposed to a normal furnace?

 
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Post by lsayre » Mon. Feb. 24, 2020 2:47 pm

Lightning wrote:
Mon. Feb. 24, 2020 2:41 pm
What is different about a condensing furnace as opposed to a normal furnace?
Apparently they condense hot water (steam) and other condensable (SP?) stuff that would otherwise go out the stack and make their BTU's usable as heat. A non-condensing furnace/boiler would just gum up on the condensables (if permitted to happen), rapidly corrode, and fail.

On first guess a condensing furnace is likely to work much better than a condensing boiler (which effectively will not condense unless installed in conjunction with in floor radiant).
Last edited by lsayre on Mon. Feb. 24, 2020 8:16 pm, edited 1 time in total.

 
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Post by NoSmoke » Mon. Feb. 24, 2020 3:57 pm

That is really not the right question to ask.

If a condensing boiler is in a situation where it has high return heat then it is not super efficient granted, but it is still better than a non-condensing boiler.

Better is the key word.

It may not be MORE efficient then a non-condensing boiler, but it will equally not be LESS efficient either. In other words, a condensing boiler can operate as a non-condensing boiler with longevity, but the opposite is not true. A non-condensing boiler cannot operate as a condensing boiler for very long.

The "better" part comes in because the boiler is made out of non-corroding materials (like stainless steel). So the owner will still have a far superior boiler no matter what capacity it is operating.

This leads to the real question that should be asked: is the price worth it? That is a return on investment question, and has many answers.

 
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Post by Lightning » Mon. Feb. 24, 2020 4:20 pm

Oh. I was just trying to understand what exactly a condensing furnace/boiler does differently than one that doesn't condense.. so as I understand, it condenses gases in the exhaust and applies that heat to be used instead of going out the stack? So, a condensing natural gas boiler would use the H2O by product of combustion and make it condensate in an attempt to squeeze a few more BTUs out of the fuel? Or do I have that all wrong...

 
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Post by lsayre » Mon. Feb. 24, 2020 4:23 pm

Lightning wrote:
Mon. Feb. 24, 2020 4:20 pm
Oh. I was just trying to understand what exactly a condensing furnace/boiler does differently than one that doesn't condense.. so as I understand, it condenses gases in the exhaust and applies that heat to be used instead of going out the stack? So, a condensing natural gas boiler would use the H2O by product of combustion and make it condensate in an attempt to squeeze a few more BTUs out of the fuel? Or do I have that all wrong...
I think you have it. The condensed moisture is very acidic. And apparently even more so for oil.

 
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Post by Lightning » Mon. Feb. 24, 2020 4:31 pm

Yeah, I would imagine any trace sulfur compounds in the fuel would end up in the condensate as sulfuric acid..


 
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Post by Lightning » Mon. Feb. 24, 2020 4:35 pm

Once water reaches 212 degrees it takes a huge push of energy to make the change of matter into steam. That energy is recouped when steam changes back into liquid water, so I can see where the added efficiency is coming from.. pretty neat idea.

 
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Post by LeoinRI » Mon. Feb. 24, 2020 7:20 pm

My Buderus GB142 natural gas condensing boiler is made of aluminum. Condensate flows through a cannister filled with marble chips to neutralize the acid. The boiler has a micro-ampere current circuit to combat electrolysis.

 
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Post by NoSmoke » Tue. Feb. 25, 2020 5:39 am

I have a condensing stainless steel Propane Boiler hooked up to my radiant floor heat. It can easily modulate the 76-100 degree water I need for my radiant floor heating system, but it is not set up to do that.

Instead I use a metering valve operated by a PLC with various sensors to get those 76-100 degree temps. I did that because I wanted to knock down high-heat from add-on:

Wood/Coal boilers
Solar
Compost Heat, etc

If I did not go with a metering valve, then I would have had to use cold water knockdown, and to me that is just a silly concept: heat water up, just to cool it off.

The 80,000 btu boiler is the black box in the upper-right of the photo as it is a hang-on-the-wall unit.
DSCN3866.JPG
.JPG | 494.8KB | DSCN3866.JPG

 
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Post by lsayre » Tue. Feb. 25, 2020 5:46 am

Very nice set-up there NoSmoke!

 
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Post by Rob R. » Tue. Feb. 25, 2020 6:12 am

NoSmoke wrote:
Mon. Feb. 24, 2020 3:57 pm
It may not be MORE efficient then a non-condensing boiler, but it will equally not be LESS efficient either. In other words, a condensing boiler can operate as a non-condensing boiler with longevity, but the opposite is not true. A non-condensing boiler cannot operate as a condensing boiler for very long.
I think the wall hung high efficiency units have a few other advantages. The boilers are very low mass, so after the heat call is satisfied there is not a bunch of heat stored in the boiler (which often end up going up the chimney or being shed to the basement) like there would be with a cast iron unit. The high efficiency units can also modulate the output to try and match the load. This also reduces the amount of standby losses.

The downsides of the high efficiency units are the increased cost of the boiler and installation. The boilers themselves typically cost more, and many of them have to be piped primary/secondary. It can also be harder to get service and/or spare parts for some of these units.

Larry - if you are planning to install a high efficiency gas boiler, I suggest contacting your local supply houses and see what they stock/support.

 
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Post by Holdencoal » Tue. Feb. 25, 2020 6:44 am

To combat high temp returns taco and other manufactures have come up with the variable circulator pumps, which work off the delta T.

 
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Post by NoSmoke » Tue. Feb. 25, 2020 7:43 am

Holdencoal wrote:
Tue. Feb. 25, 2020 6:44 am
To combat high temp returns taco and other manufactures have come up with the variable circulator pumps, which work off the delta T.
They are called metering valves, and is pictured in my photo. It is a very misleading name because the "metering valve" is the circulating pump in the middle. It is controlled by the PLC which is the small box located under the boiler, but to the left of the zone valve relay box, also under the boiler without its cover on.

This system does have the primary (what I call the control loop) and the secondary loop going to my floor. The metering valve (variable speed circulating pump) sits between the two loops, and is what meters the amount of hot water coming off the primary (control) loop.

It's output...how much it pulls from the primary loop...is dictated by outside ambient temperature, slab temperature, return water from the floor temperature, and desired temperature.

 
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Post by NoSmoke » Tue. Feb. 25, 2020 7:46 am

lsayre wrote:
Tue. Feb. 25, 2020 5:46 am
Very nice set-up there NoSmoke!
Maybe, but I am humble enough to admit that it took the supplier explaining to me three times why this $1800 option was much better than using a $500 propane hot water heater I had intended to use.

13 years later, I am glad I bought the boiler.


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