mod con techie question

Unknown

you need to connect a thermocouple tester to the intake pipe bringing in outside air. That way you are comparing with flue gas temp versus intake outside air.

This requires an additional probe which will allow this temperature to determine the net stack temperature; it will not affect other combustion gas calculations.

GW

how come?

When I was at a Jim Davis seminar years ago, he said the calculations in today's combustion analyzers are a joke, and that if the flue gas is cooler that the air or water which is being heating, you're going backwards. made good sense to me.

So, how is it you can extract heat from the flue gas and send it into the heated medium when the heated medium is already hotter than the diminished flue gas temp?

Thanks, Gary




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Derheatmeister

Or what I Am seeing is:

Supply @ 134 F.

Return @ 102 F

Outdoor @ 14 F

Flue @ 112 F

GW

OK

So, all your jobs are like this? I'd be surprised. In any case, my question was missed, how do you explain lower flue gas temperature than what the water is being heated (maybe only on my jobs)?

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bobbyg_10

If I understand you right, the point is the flue temp. your talking about is "after" the flame has given off its heat to the water. The flame temp off the burner is much hotter than the flue temp.

Unless I am missing something?

Tim_41

condensing tech

This may help you: http://www.viessmann.de/en/literature/Technical_Series.DepartmentDescription.35207.downloadlistitem.86633.FileRef.File.tmp/ts-condensing-technology.pdf

Derheatmeister

What kind of boiler..and how are you checking all that data?
as per the values that i posted before, they were JUST from a TT prestige on the Display this Winter..Which is maybe Plus,Minus Lincoln tunnel.and yes the Flame is MUCH hotter Check the viessmann information i.e. 900 C inside the Chamber.. and 40 C on the Water and this all at 45 C on the Exhaust..Which is pretty good! Can you go Lower? If so how..and How long? Sorry must run..Will check back later .Thanks Heatmeister

GW

yes

I'm discussing flue gas temp. i realize the combustion is certainly hotter that the water/air being heated (boiler/furnace). And that's where these make-believe combustion efficiency ratings come from... from the flue gas temp (compared to co2).

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Tony_23

Look at the unit

Can't speak about Viessmann, but W-M and T-T both are "down-fired" units, meaning the flame is at the supply side outlet. The flue is at the return side inlet. Return temp of 120 and flue temp of 130 is very doable with a supply temp of 140.

Hvacman

In laymans terms:

The flue gas exits the appliance at the return water entrance to the boiler... Your initial assumption is incorrect in that the flue gas temp is certainly hotter than the return water temp. edit: or should be if you want the most efficient (ie: condensing) operation. I hope this helps clear it up!

archibald tuttle

showing the condensing boiler cold return

I have a question that follows on to this. Obviously where you are looking for low stack temp you have to have low return temp.

This is what the condensing boilers enable in the first place.

But the Buderus GB 142 has a built on manifold for primary secondary style piping. Assuming an actual circulating primary loop this would be a technique that might be intended to not show condensing temps to the boiler by the primary circulating loop mixing hot feed water into the cold return. Thus Primary/Secondary wouldn't be the best piping for a condensing boiler.

I'm monitoring the performance of one that was installed without a full primary loop but picking up the feeds and returns and allowing the buderus manifold to act as a bypass of sorts.

Thus the boiler gets coldest water in the return but just not the full volume of it, dependent on the relative pressure drop across the boiler and circulating loops.

The last finesse I'm trying to reason out is how to set the pump speeds for the load loops vs. the boiler loop. The GB-142 does not currently appear to be short cycling. I am not intimately familiar with the onboard setback controller. I assume maybe it has an algorithm that has contributed to this lack of short cycling. This piping setup appears to have worked reasonably well, but my friend whose house it is asked me if there was any fine tuning to pump speeds or approach he should take. Judging by his results this winter my inclination was tell him to leave well enough along, but I can't resist a chance to fiddle around a little bit and see what happens.

My theoretical approach would be to look for a high delta T off the load loops (some under floor radiant and some enormous cast iron raditors left from gravity hot water system) so that says that I should run a low speed on the load loop. My temptation is to run a higherspeed on the boiler loop in order to minimize bypass, but I am slightly worried that this could lead to such a high relative pressure drop at the boiler inlet that you might get some short flow of the heated supply backing through the bypass manifold thus defeating the purpose of insuring only cold return water hitting the boiler.

Getting ready to try a couple of alternatives while observing closely but thought I'd ask for theoretical or previous emperical data to guide this undertaking. I was going to just try some different pump speeds under approximately equal loading (system at same temp, similar outdoor temps and wind) and see if I could use contact or infrared sensing to detect alterations in the temperature of the bypass and boiler return piping and response times for the feed side. Have you found target infrared close enough for such work or?? There aren't enough pressure gauges or tappings to allow me to simultaneously monitor the pressure at the various points of the system which would help immensely.

Thanks for any thoughts.

Brian

scott markle_2

p/s flow rates for condensing apps.

Brian,

I did not fully understand your exact piping arrangement. I have some direct experience with Gb primary secondary issues.

While lower secondary flow rates will provide higher deltas, these flows rates must be greater than primary for the boiler to "see" these lower temperatures.

The nature of a primary/secondary piping will allow mixing of flows. The direction of this mixing is determined by which flow is greater.

You can determine which flow is greater without actually measuring the flow. To do this look at the temperatures at different locations in the pipe-work. look at the return to the boiler, the supply to system and the return from system. If the Boiler return is hotter than the system return this is indicative of primary>secondary flows. In other words there is a multi-directional flow in the closely spaced T's that is bypassing the system loop and returning boiler supply directly to the boiler. In the case of a cast iron boiler with minimum return requirements this may be desirable, for a condensing boiler it is not.

In most cases the lowest setting on the gb boiler pump is sufficient. The lowest setting on the system pump that provides higher(or equal) flow in the secondary loop is where this pump should be set. Multiple zones and changing secondary flow rates complicate this, it's very likely that a small zone running alone will not require a flow rate that exceeds primary, inevitably in these situation undesirable tempering of boiler return will result.

As a general rule in condensing boiler P/S systems: Secondary>or= Primary flow rates should be observed.

bob_46

Gary Wilson's Question

Gary, it appears everybody including viessmann is evading your question. The viessmann article says that under ideal conditions flue gas temp. can "approach" return water temp.. You and some other posters are saying that you actually measure flue gas temps lower than return water temp. Thermometers respond to changes caused by the addition or removal of sensible heat, they don't tell you anything about latent heat. Moisture in flue gas is contained in the form of superheated steam at low pressure. When we cool the the flue gas/steam mixture to the dew point temp we have removed the superheat, the steam is in the condition of saturated vapor and condensation begins. Now it gets tricky. Everytime you condense a drop of liquid from the mixture you reduce the dew point temp of the mixture. The mixture then has to see a lower temp. to continue condensing. Let's say our boiler is well designed and the gas/steam mixture leaves the heat-x at return water temp. some of the steam has condensed and we have SATURATED mixture entering the flue pipe at return water temp. There is a pressure gradient in the flue system the highest pressure in the combustion chamber and the lowest at the exit to atmosphere. As the pressure drops the boiling point of the remaining water in the mixture drops below it's temp. so it has to flash into superheated steam. This takes heat and the only place it can get any is from the flue gas so it removes sensible heat from the gas and lowers it's temperature (below return water temp.) and adds this heat in the form of latent heat to the steam. My .02 bob
EDIT: DUH I reread garys question and I didn't answer it either! If the flu gas is cooler than return water temp you can't extract more heat from the flue gas.

Tony_23

bob

Reread my post and look at the numbers. Tell me where I said the flue temp is lower than the return temp.

archibald tuttle

tony...

I agree that your post did not indicate a return temp. lower than stack temp, rather it was a supply temp. lower than stack, but I think the discussion of latent heat released during condensation and where physically the condensation is taking place is critical to understanding any of the comparative temps. The notion being that if you can condense in the boiler near the flue gas exit and the return water input then a good deal of heat can be rendered to the return water without further dropping the stack temp. Maybe even if the flue gas approaches or reaches the return temp.

I'm thinking of a growing understanding of this mystery that I got when I was talking to folks who make snow on a ski mountain about how they can make snow up to 3 or 4 degrees above freezing if the air is dry because the more atomized water vapor sprayed has high evaporation that literally chills the air and turns nearby spray into snow.

Brian

bob_46

Tony

I miss-read gary's question. The numbers you posted are what I would expect to see. There have been posts on the wall that claim that they have measured flue temps below return water temp. There are two ways I can see how that could happen. One is my above explaination and I'll add that in that senario the ∆T would be very small, probably !ºF or less the other would be inaccurate thermometers or methods. bob

Tony_23

My experience shows

That the flue temp is measured within inches of the block where the return fitting is. The condensing takes place in the HX.

Open up a W-M Ultra burning LP that is out of spec and you'll see that it's condensing in the HX and the result.

Couderay

Flue temp below return water temp

Could a underfired boiler result in flue temp below return water temp?? What about out going temp?

Tony_23

No

How could it ? If it was then the water would give up heat to the flue, raising the temp of the flue. Heat goes to cold, always.

GW

flue gas

Thanks for the comments; I will be paying much closer attention in the future when working on condensing equipment to the flue temps and the water temps.

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RonWHC

What happens when

your Bacharach, UEI, Wohlers, Testo, subtracts ambient temp from flue gas temp of your mod/con for efficiency calcs? Real numbers? Or not? How low can the net temp go in the calc?

RonWHC