Viessmann & Siggy differ on efficiency at high return temp?

Retro

It's very unlikely I can use in-floor radiant in my home bec. crawl is too low clearance to work safely and ceiling is near code min. So I'm sizing alternative emitters, and that depends on return temp. Can live with ~87% but if I'm paying for a mod con why not see if we can still con with radiators or what have you.



Conventional wisdom seems to keep return temps as low as poss for efficiency (and other reasons). For example, Siegenthaler 3rd ed p78 showing curves for a "representative" mod con indicates barely condensing at 125F, even at low fire. And at high fire, maxing out at hardly better than 90% in any case.



But Viessmann eg 222-F manual p3 indicates 95%+ with return temp of 158F at a 30% load. (222-F modulates down to under 20% where I imagine it's even more eficient.) Even at 50% load it seems to be doing some condensing.



Viessmann stipulates 20F drop in emitters - is that a gotcha?



I don't see similar info for Loch or TT, wonder about that.

SWEI

Drop on emitters

depends on the emitters in question.  10-15ºF is typical on in-floor, modern panel rads can be sized at 25ºF or 30ºF



Remember, the sizing is done using the average of supply and return temps, so a supply of 145ºF and a return of 115ºF would be sized as 130ºF.  If you size panel rads this way, you will be condensing all season.

Zman

Chart

That is one confusing chart! It is not showing efficiency at a particular fixed temp. It is estimating the overall efficiency of a system with a particular design temp,outdoor reset curve and efficiency. The 1.8 or .5 defines the shape of the curve. It is not contradicting siggy's chart. It is a different chart altogether.

Carl

CMadatMe

Lamda Pro Combustion

I'll answer your question with a question? What controls the dew point? If I can control the dew point I can condense pretty much at all times, even above that magical 127-130 degree water temp line.



Maybe this will help you understand. Its a pretty good little article.



http://www.thermalsolutions.com/news/wp-content/uploads/2013/04/High-Turndown-Condensing-Boilers.pdf

CMadatMe

Deleted

Zman

Good article

Chris that is a good article.

I think the original poster was confused about the Viessmann efficiency as it pertains to the heating curves.

I am saving the article for folks considering Navien.

Carl

CMadatMe

Navien

Navien does not capture flue gas condensation to reclaim the loss latent heat. While it may condense it still doesn't reclaim lost btu/hr through the venting system. It also does not come ODR unless the installer purchases it separately. Navien, because it has a CAFUE vs an AFUE rating isn't held to the new Sept 2012 standard. That's the story to be told when competing against it. Your correct because he only posted one part of the Viessmann chart h left out the heating curve chart which is attached.

SWEI

All boilers have a set of curves like that

Viessmann is the only one I know of who publishes them widely.

Retro

trying to follow your reasoning here...

Chris and Zman I think I get what you're saying - Siggy's chart is steady state efficiency - do you think the Viessmann chart must be an estimate of seasonal efficiency? I figure, probably heat+hot water for some representative household size in a climate where design temp is -4F.



Which means, everybody, yes I definitely ought to have posted Viessmann's ODR curves. What is that diagonal scale at the lower left of those curves?

CMadatMe

Room Setpoint

That's the setpoint the boiler is using to calculate the curve. You could graph the months of the heating season with daily daytime and nightime temps and come up with your operating percentages. Might not be 100% accurate but will give you a idea of where you'd be running most of the heating season.

Retro

Dunkirk has something to say about this...

Dunkirk's Quantum Leap (discontinued I assume) advertised as follows:



"Heated condensate (from the Recovery Unit)

is sprayed down inside the Quantum Leap’s

Evaporative Re-Cooling Tower where it saturates

and heats combustion air. This raises the boiler's

dew point so that 90-plus percent efficiency can

be obtained at return water temperatures up to

160° F. The QL is the only residential boiler in

the world capable of this feat."



http://dunkirk.dev.salientlink.net/uploads/product/literature/253529d8bd958dd516417653c714da1f.pdf



While I was looking for availability (doesn't seem to be any) I also found maybe Dunkirk's name should be added to the list of makers with the smallest available mod-con boilers

http://www.dunkirk.com/product-details.asp?id=247#9

They come in 50 and 75 kBTU models and have 5:1 turndown

Stekay

Dunkirk's Quantum Leap

Yeah well, I had this boiler. Maybe that's what ate it alive from the inside out. Actually a great idea with poor execution in my humble opinion. We've learned a lot in 10 years.