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    Lochinvar Knight and Condensing temp vs efficiency (5 Posts)

  • Solarstar Solarstar @ 12:00 PM
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    Lochinvar Knight and Condensing temp vs efficiency

    Is it in full condensing mode with 140 deg return temp ,with a 150 degree supply going out.. , regardless of flow rate thru it. Where do i find the actual Eff. curve for these temps.???   ( 2 stacked 400K BTU units)

    Oh : the outdoor reset has picked this temp for 18 deg F outdoor temp. and I would have assumed a bigger delta T????         Thanks Paul
    This post was edited by an admin on January 22, 2010 12:02 PM.
  • stakrellis stakrellis @ 10:08 PM
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    Firing rate determines the delta T

    The actual firing rate of the boiler is determining the Delta T since the flow rate is fixed by the pump.

    What is your Delta T at full fire?

    Take a look at your Flue temp as well the lower the better for condensing.
  • eluv8 eluv8 @ 10:29 AM
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    Lochinvar Efficiency Curve

    With a search on the internet you might find an old Lochinvar brochure. They used to post a curve with their literature.(I think it was fudged a little for marketing)  I have not seen any currently circulated literature that defines their efficiency in any detail. Typically the Giannoni heat exchangers with good combustion are between 92-96% Efficiency with 90 - 120 returns and with around 60-70 returns can get above 98%.

    Typically if your returns are above 130 degrees you will see little to no condensing in the boiler. Excess air, outside air temp, water content in the gas and combustion air, quality of burn and location of combustion in the chamber, surface area, all effect  the point the boiler will begin to condense and amount in which the boiler will condense and can vary significantly, higher turndowns typically increase excess air thereby lowering the temperature in which the boiler can condense. I have seen boilers with 115 degree return water not condense, and I have seen a condensing boiler with 135 return stream some pretty good condensate.
  • Mark Eatherton Mark Eatherton @ 7:33 AM
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    I think too much energy is spent on attempting to keep the appliance in the condensing mode....

    See the attached chart. Unless you are living in a home with WarmBoard emitters of equivalent, and bare stone floors you will not be in the full condensing mode the majority of the time. Other readers of this board (Jimbo) keep trying to lower their return temperature in order to theoretically keep their appliance in the condensing mode. It's just not worth it. A BTU is a BTU, and if you are losing it to an unconditioned space, you are keeping that space warm, but you are still paying for it and it's not giving you that good of a return on the investment.

    My thought is that the majority of the increase in seasonal efficiency from the application of this appliance is from controlling the excess air that would normally carry off paid for BTU's in the combustion process. Note the 10% excess air on the chart. That is critical. Most atmospheric appliances run in the 50% to 100% excess air category because for the most part, it (excess air) is uncontrolled and dictated by stack height, operating temperatures and other conditions that we have little to no affect over.

    I think it is the MOD that saves energy, and not the CON.

    Now, if the appliance CAN be maintained in the CON mode, then the fuel savings will be compounded, and the AFUE will approach the theoretical maximum, but these types of systems have to be designed from scratch, and in retrofit conditions that is tough to do.

    Now snowmelt, that is where these beauties shine. But that is considered a HUGE waste of energy by environmentalists, and will probably be one of the first energy hogs to be cut out of the energy future if the tree huggers have their way about it. Water source heat pump systems (rare) would also be an excellent application for maximum potential savings (75 degree F max operating temps).

    So, in short, the energy savings comes from the MOD, not the CON. Unless your emitters are GROSSLY oversized, you will not be in the condensing mode for most of the time, but the minimum fuel savings I have seen with the application of these appliances is still 30%, and that is nothing to sneeze at. The maximum I have seen was in the 50% category, but that was a 12 story apartment complex that had about 3 pounds of washers on the barometric damper, and was 12 stories tall.

    You are ideally matching the load to the capacity of a highly controlled heat source with little to no short cycle inefficiency and little to no excess air in the combustion process. When i gets cold outside, the appliance turns on and stays on, exactly matching capacity to demand, until the demand goes away.

    By the way, the boiler does still condense, even at the higher temperatures. Just not as much as it will at significantly lower temperatures. But in the upper operating regions, I don't think it is that important.

    http://www.csemag.com/photo/167/167345-boiler_efficiency_vs_temp_2.jpg

    Oh, and don't forget the need for annual service, or all this energy savings potential is for naught. An unserviced, or improperly serviced modcon will WASTE fuel, not save it.

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Steve Ebels Steve Ebels @ 5:10 PM
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    Condensing factors...........

    You just hit the nail on the head describing all the factors that keep a boiler in condensing mode and excess air is a biggy.  I saw a neat presentation on that very subject at a Viessmann training meeting dealing with the new Vitodens and how they have optimized combustion based on all the factors you noted. The performance of the burner in that unit is what every fuel burning appliance should be targeting. 
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