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condensing gas boiler

bob eck
bob eck Member Posts: 930
I have a Triangle Tube PE110 nat gas boiler in my house and I have cast iron radiators, I want to keep the boiler in condensing mode as much as possible, I will be adding 4 radiators after this heating season is over. Next year I want to be sending out 150* F max when outside temp gets down to 10*F and still keep the return water temp 130*F or less so boiler will always be in the condensing mode. can this be done with having more cast iron radiation added to the system?


  • canmar
    canmar Member Posts: 16
    low temps

    As with most hydronics, you can do most things as long as the parameters are right.  Keeping the boiler at your target temp will require you to change your setpoint.  A couple of questions though; do you have trv's on your radiators? do you have a heat loss for each room to determine the amount of btu's needed for these rooms?  Do the radiators have the capacity to handle the low temps?

    Most cast iron radiators are oversized for the rooms they are in so this becomes slightly more feasable.  I have run lower temps through cast iron radiators with great success using constant circulation and trv's but the parameters need to be right in order to maintain your comfort level.
  • Brad White
    Brad White Member Posts: 2,398
    edited January 2011
    A question of balance, of ratio

    Yes, the more radiation you have, the lower the water temperature you will need at any given time. The key is to have the radiators proportional to the heat loss as closely as possible so that they will respond about the same.

    Any radiator which is undersized, below this common baseline, will dictate that you run your system at a higher temperature to satisfy that one room. Unless you move radiators around (few actually do), the use of TRV's as canmar suggested, will level things out. Still, your one or two rooms with smaller radiators will be the tail which wags the dog.

    You start by knowing your calculated heat loss, as well as you can and for each room based on current insulation, windows and conditions.

    Then you need to assess your radiator potential output and "SF of EDR".

    I usually use 170F as an average water temperature, which traditionally corresponds to 150 BTUH per SF of EDR. (This is just a benchmark number and will be adjusted later, below.)

    Next, compare the radiation EDR to the heat loss in each room. The goal is to make them proportional as closely as you can. For example, say you have four rooms, with heat losses calculated at 5,000, 6,000,  7,200 and 8,000 .

    These rooms, say they respectively have radiators of 50 EDR, 70 EDR, 60 EDR and 75 EDR. These correspond respectively and at 170F average water temperature to 7,500, 10,500,  9,000, and 11,250 BTUH.

    For shorthand, I simply divide the calculated heat loss by EDR to come up with a minimum BTU output per SF. This in turn is matched up to the BTU per EDR temperature chart. In this way, I need, again in order, a minimum BTU per EDR output of 100, 86, 120 and 107 F. average water temperatures.

    Obviously the highest number will be your design highest HW temperature. In order to get 120 BTUH per SF out of a cast iron radiator, your average water temperature will need to be 155 degrees F. average (supplied at 165 and returned at 145 for example).

    But you can also see where you can go if you increased your radiation- The lowest number! To get 86 BTUH per SF out of your radiator for the most generously sized room, that would  require average water  of 138 degrees F. (supplied at 148 and returned at 128 for example).

    Now, you mentioned a desire to return at 130F to promote condensing. Traditionally,  returning the water at 135F or above has been given as a safe margin to prevent condensing in a cast iron boiler, where you do not want condensing. Actual flue gas dewpoints vary with CO2, degree of excess air, etc. but settle at an average of about 128 degrees F. The "135F" maintains that much margin.

    In order to reliably start and maintain condensing, your water temperature has to be consistently below that point, so 120F return water is an ideal goal.

    Do not be discouraged if you find that on the coldest day you need water temperatures hotter than you would like!

    The number of hours where these are needed are relatively few over a given season. Aggressive use of outdoor reset will keep you in condensing mode for probably 80 to 85% of the time, depending on your climate.

    EDIT: Reading back to your question, might you have meant, can you get below condensing WITHOUT adding more radiators?

    "Technically" you can, provided you maintain the same design AVERAGE water temperature in the radiator, but you are better off "admitting defeat" for those coldest hours and not play with the flow rates too much.

    Say in our example you need 155F average water and return below 130F in your example.

    Your supply water temperature to the radiators would have to be 180 F. to make this happen. (180-130) /2 = 155. So yes, you can but you need to drop the flow rate to the radiators. This creates other issues and challenges which will defeat your condensing at the boiler. The flow rates will be mis-matched enough that your boiler return will always be above 130F even if your system return is below 130F. The higher boiler flow rate needed will defeat your lower system flow rate you desire. I hope that makes sense.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • tim smith
    tim smith Member Posts: 2,743
    Re: keeping return temps down

    With the TT boiler, you can use their boost function and set up a aggressive reset on your boiler. If you set you boost to 30 minutes, that means that if it has not seen the stat satisfy, it will boost the water temp I think every 30 min. 10 degrees or 15, I forget, it is in the book. Good feature if you just want to account for the few days or so that we dip to temps not covered by the reset curve. Take a peek at the manual under boost feature.  Tim
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