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Flue temp and return water temp. condensing?

I have basic understanding of this condensing process and efficiency.

My question is, what temp difference (on average) should I see between the return water and flue gas?

I'm currently running around a 10* return water temp drop, but my flue temp is 1* higher to about the same at the end of the cycle than return water. I usally have water dripping out of the condensation drain.

I know I need to get a lower return water temp (10* more). When I fix this, I should have a 10* difference in my flue temp?

I'm trying to educate myself.

Any info would be helpful.


  • GordanGordan Member Posts: 891
    Two parts...

    Firstly, a degree over return temp is very good. Usually this will only happen at low modulation and with decent flow through the boiler.

    Secondly, whether and how much your flue temp will drop as you take steps to lower your return temp depends on a lot of factors. Are you reducing flow through the system? Through the boiler? What kind of algorithm does the boiler use to control its burner firing rate? Since you probably won't know at least some of these answers, it may make sense to tinker with it and see. You could phrase the goal of your tinkering as "minimizing flue temp for a given burner input." Even if the flue temp does not track the return temp as you drop the latter, you still win if it drops relative to flue temp prior to tinkering.

    Of course, this would be too easy if you could simultaneously do it for all burner inputs, but something tells me that as you improve things for one firing rate, it will get "unimproved" for others...
  • Tim_HodgsonTim_Hodgson Member Posts: 59
    It is a ratio

    BTU Input (burner firing rate) : BTU output (lbs.water flow x temperature rise)

    The square inches of heat exchange surface is constant.

    Your water flow is usually constant, so a decrease in your firing rate will decrease the water temperature rise through the boiler.

     The burner, at low fire is moving less air and gas, the hot gasses flow through the boiler slower.

    This means the flue gasses are in contact with the heat exchanger for a longer period of time.

    That makes the flue temperature closer to the return water temperature

    Hope this answers your question,

  • spacolispacoli Member Posts: 61

    Gordon, Tim thanks for the input.

    I did not realized that one degree difference was that good. Most of the time it's the same or the return is one degree higher. Is there room for improvement there? If the temps are the same, is it still condensing?

    The boiler is default settings. I played with it once this heating season lowering the minimum water temp. I was trying to get longer cycles and more constant temps. My cycles were not bad ( 2 times an hour, same times running as off 15 minutes) Nor did I have terrible temp swings, I think that is due to the cast iron rads.

    I'm guessing if I do get the return water to a 20 degree difference from the supply, in turn the boiler will fire higher to make up that difference. Then I will have higher flue temps and better condensing and efficiency ?

    I know there is setting for the high fire/low fire blower speed. I don't know if this is something I should be messing with to slow the flue gas down as Tim described with the heat exchanger?

    Thanks again.
  • MagnehelicMagnehelic Member Posts: 63
    It is a function of temperature as well

    Flue gas (typical natural gas....not positive if propane is the same) begins condensing at sea level and atmospheric pressure at 133.8 degrees F. (although the dew point of natural gas elswhere I have read is actually 131.3 oF )  If your flue temp is lower than that.......condensing is taking place.........and of course.....the lower you go below that...........the MORE it is taking place......assuming that you have not yet fully condensed the total moisture in the gas.  I did a little research for some modulating scorched air furnaces we installed last year........and was told that the maximum condensate that could be produced by a 100,000 btuh burner was somewhere really close to 1.1 gallons an hour.
  • MagnehelicMagnehelic Member Posts: 63
    edited January 2012
    Here's an interesting read about it

    hopefully the pdf will show up this posts last night failed to upload the files.....
  • GordanGordan Member Posts: 891
    I think you're confusing things a bit here

    With flue temperature, lower is better because that heat is simply exhausted outside - it is wasted. Flue temperature can't go lower than return water temperature because that's what it's exchanging heat with. So, for it to be so close to return water temp is a very good thing. This is different than "delta T", which is the difference between supply and return WATER temperature.

    Water vapor resulting from combustion will condense on the heat exchanger as long as the surface of the heat exchanger that it comes into contact with is below the dewpoint. Most heat exchangers are counter-flow, so the coolest (return) water will be closest to the exhaust side of the exchanger and the hottest (supply) water will be closest to the burner side of the exchanger. There will be a temperature gradient along the surface of the exchanger between the hottest and the coolest point, and whereever that gradient dips below the dewpoint, condensing will occur. The larger that surface, the more condensing.
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