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Bobbalou Member Posts: 4
I am considering updating my early 1990's HVAC equip with a hydronic air handler with a AC coil and a Laars Mascot 2 boiler. If the entering water temp is 180 degrees with a 20 degree delta T, and returning water is 160 degrees, am I correct to not expect the boiler to operate in condensing mode? I am trying to understand the the relationship of widening the delta T spread to obtain a lower return temperature on this application. Thanks in advance for advise as I am a begining hydronic convert.
Bobba Lou


  • Tim McElwain
    Tim McElwain Member Posts: 4,608
    You will need to get

    the return temperature below 140 degrees in order to even begin to condense. Depending on the location of the air handler in relation to the boiler you may be able to go with a lower output temperature to the air handler.

    Another option is to use outdoor reset and maybe install some baseboard somewhere to drop some heat on the return side of your air handler.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,537
    Oversize the coil

    then you can run lower temps,don't use or use the least amount of fan delay you can. Done many hydroair jobs with condensing boilers and the surprise is how much they condense!11
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • tom_49
    tom_49 Member Posts: 269

    why run 180o to coil? we have tons of em out there running 145-150 supply water temps. oversize coil. run fan as low as possible.
  • Bobbalou
    Bobbalou Member Posts: 4
    Hydronic Air Handler questions

    Thanks all of you for the good advice. I was hoping to be able to oversize the air handler as the heat loss calcs came out to 95000 BTU for my residence. The unit I am looking at is a 125000 BTU heating/ 5 Ton cooling. It appears from the spec sheet that the fan speed is adjustable. Would a variable speed unit be a better choice?
    Bobba Lou
  • Brad White
    Brad White Member Posts: 2,398
    Variable is

    almost always a better choice. The difference between the actual load (volume, capacity) and the best calculations, can vary enough to allow saving some energy if your actual load is less.

    If you can get an ECM motor, they are rebate-eligible and will also provide constant torque adjustment. Basically constant airflow once set, even as filters load up.

    Let's set up an example, your 5-ton system might move 2,000 cfm on cooling (could be more, could be less, but just say).

    On heating, you have that coil selected at 125 MBH yet you only need 95 MBH.  Your comfort might dictate that  you want less but warmer air, so now you can do that. Subtle and subjective.

    But back to energy and capacity- Say your 2,000 cfm system really works best at 1600 cfm, a 20% reduction. Your 2,000 cfm system working at 0.75" total static pressure and 70% efficiency, might draw 0.337 brake HP (a 1/3 HP motor full-out).  Call it 1 kW per HP (337 Watts) and say your electric rate is 18 cents per kWH.

    Your fan full out will cost you 6 cents per hour to run. Over a years time running half the time, that is $263 just to move the air.

    When you reduce your air volume by 20% your horsepower/kW drops by the cube of the difference. So your 337 Watts will drop to 51.2%, call it 51% or $134 per year. You save about $129 per year if you run 50% of annual hours.

    Such a deal.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Bobbalou
    Bobbalou Member Posts: 4
    Variable is

    Thanks for this insight. I had not considered the electric current savings in the plan.

    I will order the variable unit.
    Bobba Lou
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