Condensate Volume of Mod-Con

Glen

elevation, dewpoint(dependent on the H2 content of the fuel and CO2 in the flue gases), relative humidity, which will all affect gross flue gas temps. With your return temps, without compensation of above variables - I have another chart from Viessmann that would suggest about 1.4 oz per KWh. You can do the math.

Jim_64

Here's a bizarre set of numbers produced by a Vitodens 6-24 directly (no LLH or P/S) driving a fully TRVd, converted gravity system with 1,049 sq.ft. EDR of standing iron rads and a few small radiant floors--everything under constant circulation with a Manual-J loss of 100 mbh @ 8F.

Temps given here are rounded to the nearest degree Fahrenheit. For the hour of concern, none of the temps changed more than plus/minus a couple 10ths of a degree.

Flue (at terminal--outdoors) - 93F

Flue (approx 1" from heat exchanger) - 95F

Combustion air (test fitting of co-axial flue) - 72F

Supply (surface of black iron approx 3' from boiler) - 98F

Boiler Temp (reported by boiler) - 104F

Fuel (natural gas) input: 26,500 btu/hr

Return - 87F

Outdoors - 22F

Indoors - 66F

Condensate produced - 1.64# per hour

-------------------------------------------------

Comments:

1) Note the significant amount of preheating of the combustion air. From 22F to 72F in only about 5' of horizontal flue! Boiler had shut down for an hour or so about 4 hours before this test period. While the inlet temp is not datalogged, I again observed it frequently and it took more than 2 hours of continual firing for the inlet temp to maximize. I cannot absolutely discount recirculation of exhaust, but by all measurements and observations any significant degree of recirculation seems highly unlikely.

2) Theoretical condensate volume was 2.53#; actual was 1.64#. This produces a "condensate value" (Viessmann term - theoretical divided by actual condensate) of 0.652 This [appears] to be at the very high end of what is technically possible and by any measure leaves extraordinarily little energy loss due to unrecovered condensate. Only 0.89# of condensate was not recovered and with only 21F difference in temp between combustion air and flue air at the terminal (outdoors) we're talking about around 0.1 percent.

3) According to the efficiency equations on page 4 of the Viessmann white paper titled Condensing Technology my flue gas loss cannot be more than 1% and is likely around 0.5% No--I cannot measure CO2 percentage of the flue gas, but with approx 5% CO2 for an atmospheric burner showing only 0.97% flue gas loss... Note: I used the terminal (outdoor) flue temp for this calculation as well as the measured inlet temp. This would [seem] to be the only "fair" way to calculate given the amount of combustion air pre-heating I am seeing in cold weather. If not, how do you compensate for the pre-heating? After all, if it were either drawing indoor air for combustion or used a separate line for combustion air, don't you have to consider the combustion air temp essentially identical to the outdoors? (If the combustion air comes from indoors it becomes infiltration, correct?)

4) I am fully aware of assumptions being made regarding the actual heating value of my fuel as well as the limitations of my measurement devices and lack of a true CO2 reading, but I've calculated similar in other circumstances. Regardless of conditions, and regardless of how I attempt to calculate, I'm getting 99%+ efficiency! In quite cold weather! In the real world! (At least with the boiler truly modulating e.g. firing constantly for at least a few hours...)

---------------------------------------

My heartfelt suggestion:

ANYTIME you see a mainly unmolested system that originally operated under gravity and uses standing iron radiators:

1) Recommend insulation as much as practical and weatherize as much as possible, but no reason to go to extremes.

2) Install TRVs on ALL RADIATORS!

3) Remove any radiator covers! You'll NEVER get this kind of efficiency when the boiler is forced to fire to a higher temp to turn the radiators into convectors!!!!!

4) Calculate your Manual-J heat loss and pick a Vitodens 200 model whose input most closely matches. Seriously--you can even go 10%-15% lower--I did!

5) If a Vitodens 200 6-24 or even an 8-32 you can almost certainly install the boiler without a low-loss header or without any form of primary-secondary. You will use ONLY the built in variable-speed circulator that is under the control of the boiler itself.

6) If loss utterly requires a larger Vitodens 200, then pester Viessmann over and over to let us know what VARIABLE SPEED "standard" KM-BUS CIRCULATOR TO USE!!! Their own literature regarding variable speed circulators says that such are "standard" but none of the suggested circulators for the larger Vitodens is variable speed. What's up?

7) Instruct the homeowner well on how to adjust the reset curve. Let them know that THEY ARE IN CONTROL OF EFFICIENCY! Keep the TRV settings as constant as possible with unused spaces set back as deeply as practical. Keep the reset curve as tight as possible and press the "party" button if for some reason you need to raise heat fairly rapidly.

8) ALWAYS use Viessmann coaxial venting! It's expensive, but even 6' of straight vent (including terminal) [appears] to be sufficient.

9) Let the homeowner enjoy the comfort and fuel savings of a system whose energy source operates at efficiency levels with no meaningful room for improvement!

Doug_7

Preheating Combustion Air in Concentric Flue

"Note the significant amount of preheating of the combustion air. From 22F to 72F in only about 5' of horizontal."

Concentric flues do conserve energy and increase efficiency - but by how much ??

All this is happening outside the boiler - the flue is condensing some more going through the concentric flue, giving up some heat and the air supply is being preheated - which helps boost boiler efficiency.

Does anyone have a figure on this efficiency increase ?

What could the efficiency increase be if the concentric flue was a properly designed heat exchanger ?

jp_2

supply temp

mike, its inaccurate to give supply temp at 3 ft. you need to give supply temp reported from boiler, thats the true delta T of the heating plant.

Cliff Brady

ME Did this with his Munchkin 50

Vertically for 20 some feet, IIRC from a long ago post. I searched but could not find it. What say you ME?

Would a section of stainless steel tubing for the inner exhaust vent be feasible as a tertiary HX?

C

Mike T., Swampeast MO

I did give the temp reported by the boiler. A agree with you regarding the delta T of the heating plant, but for the system as a whole, I that the supply temp about 3' away from the boiler is far more representative. I find that same supply temp throughout the supply side of the entire system. The only place the "boiler temp" happens [seems] to be in a few ounces of water inside the boiler itself.