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Flue temps and condensation

If you are concerned about flue temp for reasons concerning efficiency, don't. The only information you need to know is the return temperature to the boiler. Any return temperature above 140F means no condensation and therefore no gained efficiency over the typical 87-89%. The lower you can get your return temperature, the better. This is why radiant systems attain such high efficiencies (94-96%). Instantaneous hot water cable units can achieve the 98%. Any condensation taking place in the flue pipe doesn't contribute to your efficiency to speak of.


  • Mark_46
    Mark_46 Member Posts: 312
    What's your opinion?

    Looking for some examples of how some go about measuring flue temps on a direct vent boiler using PVC pipe - of course.

    I presume a tap must be established in the flue pipe. If so at what point in the pipe? As close to the boiler as possible? What type of tap? Brand? Size? What instrument and probe do you use to measure the temp? Is a Type K thermocouple thermometer for use with a multimeter acceptable?

    Secondly, how much condensation do you expect to see in the flue pipe? I currently have a poorly glued joint in the pipe which of course must still be corrected. As a result, some condensation is also escaping (i know, i know, flue gases potentially escaping too and the associated hazards) But it was my understanding that nearly 100% of the condensation should drain out the boiler's drain tube. Apparently this is not the case. More importantly, that residual condensation I would think is likely to 'puddle' in the pipe. Normal? Again, my original question...how much residual condensation should you expect to see in the flue?
  • coalcracker
    coalcracker Member Posts: 51

    Flue temperatures vary the farther they get from the boiler. The minimum temperature to prevent condensation is about 350 degrees in a typical chimney. You can take it from there.
  • Mark_46
    Mark_46 Member Posts: 312

    coalcracker, sorry I left out of my first post that this is a mod/con boiler.
  • Roland_4
    Roland_4 Member Posts: 84
    Flue temp.

    Check this site. They make probe-type dial thermometers which you can mount permanently in the flue pipe. They are all stainless steel and nicely made.

  • JBW_2
    JBW_2 Member Posts: 67

    hey mark,

    what kind of boiler do you have??

    Josh W
  • ALH_4
    ALH_4 Member Posts: 1,790

    I agree, but with some qualifications. The 140°F return temp rule of thumb applies to conventional boilers for preventing flue gas condensation, but with mod/con boilers it's a little more fuzzy as shown in the attached photo taken from Viessmann literature. In terms of operating temperature, efficiency is probably more related to the average of the supply and return temperatures, but also is related to burner modulation. At low modulation, efficiency can be above 97%, even at 167°F supply / 140°F return. In my opinion, this is what makes mod/cons applicable to high temperature systems as well as low temperature systems.

    I would get that leaky exhaust pipe joint repaired ASAP.
  • Mark_46
    Mark_46 Member Posts: 312

    Munchkin 80M
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    To measure flue gas temperature with reasonable accuracy your sensor (and yes simple thermistors are fine as long as they have the proper range), you want to measure the gas temperature as close to the HX (or condensation recovery unit) as possible. Do not let the thermistor contact the flue wall as you will not get an accurate temperature reading. If this is a permanent installation, the opening in the flue MUST be very well sealed with something invulnerable to fairly high temperature. I suspect nothing short of a purpose-made system would ever be considered legal.

    By very definition, anytime condensation is occurring on the HX (or recovery unit) of a mod-con, the flue gasses leave at their dewpoint. As the gas travels through the flue it cools and condenses. That's why a constant pitch BACK to the mod-con must be maintained in the flue.

    Horizontally vented mod-cons make some seriously impressive ice sculptures in sub-freezing weather. In certain wind conditions even a completely legal horizontal termination can coat large areas of walls/eaves with a thick layer of frost. Numerous photos of both situations have been posted here. Vertical flues rarely (if ever) have this problem as the condensate will either be dispersed in the atmosphere or drain back to the boiler.

    The amount of condensate in the flue itself has an incredible number of variables. Just a few: length of the flue; temp of the air surrounding the flue and its outdoor termination; amount of fuel consumed by the boiler; type of fuel used; supply/return temperature conditions in the HX or heat recovery unit. While the quantity won't be high in nearly any residential system, it can be assumed to be nearly constantly occurring.

  • Mark_46
    Mark_46 Member Posts: 312


    Thanks, thats what I was looking for...just to see that some amount of condensation in the flue is normal. As I said, I thought that close to 100% of the condensate should drain out the boiler drain tube.
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