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Fan speed vs. Setback temp vs. DeltaT

I've just replaced my 23-year-old Utica Boiler and 10-year-old crappy FHA furnace with a Baxi Luna HT modulating/condensing boiler. The FHA furnace was replaced with an air handler connected to the boiler. I did not pay the extra $$ for a variable speed air handler, although this air handler supports the manual selection of 3 speeds. It is currently configured for low speed.

I live in upstate NY, in a 150-year-old, 3200 sq. ft. farmhouse. I've been in this house for 10 years. The first thing I did to improve the efficiency of the house was to insulate the attic (it had none) and install programmable thermostats on all 6 zones (originally 5 boiler zones and one FHA zone; now the FHA zone turns on the air handler). This made a big difference, and has kept the house affordable to heat. Not cheap to heat, but affordable. I'm hopeful that the Baxi will improve things further. I still need to do some air sealing to improve the envelope.

I get pretty radical with my programmable thermostats, letting the house get down to 50* when nobody is home (and do the same for the non-bedroom areas of the house at night).

I have found that the air handler, currently set on low fan speed, takes a lot longer to recover from my setback temps than the FHA furnace did. My choices are to:

1. Increase the fan speed

2. Decrease how far I set back the temp in the hot air zone

3. Program the thermostat to recover from the set back temp starting at an earlier time

4. Some combination of the above

How would you recommend I decide which way to go?

Thanks, Steve


  • Brad White_9
    Brad White_9 Member Posts: 2,440
    All Options Listed Have Merit

    None of what you suggest are mutually exclusive, Steve.

    1. Increasing fan speed increases capacity by increasing mass flow. But there is a back-slide in that you will move more air but at a lower temperature, so the net increase is not that much, 10%-15% or so. Check your fan/coil capacity charts to be sure. Still it will work out to be in the plus column though and turnover will be faster.

    2. Setbacks are always an issue with ModCons it seems. And you are finding that the Air Handler Zone is dictating how low a temperature of supply water you need. They tend to do that.

    Have to ask if the coil capacity was selected for the default 180 degree water? Or did you opt for a 4-row coil which would bring the HWS temperature requirements more into line with the other emitters? I usually go for a 140* EWT selection to start.

    3. Optimum Start Time from Setback- If you 'stats are the "smart response" type this should be automatically happening if "recovery mode" is set up correctly. Do a CBA (cost benefit analysis) to see if longer times translates to fuel savings. May be close to even in my experience. The longer I am away, the deeper the setback is my rule.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Don't forget that the old furnace was self-contained. Now, it has a hot water coil connected to the boiler. If other zones are recovering at the same time that means less energy available to the air handler.

    Deep, daily setback of room temperature is in perfect opposition to the philosophy of condensing/modulating boilers where the highest efficiency is achieved by maintaining space temperature using the lowest possible supply temperature.
  • clammy
    clammy Member Posts: 3,092
    mikes right

    I have alao found that it cost more when you use deep set backs and have advised most of my custermers that it is a un wise practice espically with properly sized equipment (mod cond) and either way what temp you lose during set back is what your putting back in at a later time unless it's a prolong abstance from your home but deep set backs require your heating system to run longer to put it back in .Stick to tightening your building enevople peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Steve Goldstein
    Steve Goldstein Member Posts: 35
    This is an old debate

    I've seen this topic debated on the forum before. While I agree that the boiler runs most efficiently at lower water temps, many people on this forum continue to argue that you can't forget the DeltaT factor (inside house temp vs. outdoor temp). The lower the DeltaT, the fewer calls for heat there will be.

    Let's face it--there's one thing more efficient than a high efficiency boiler, and that's any boiler that's currently OFF. The lower the DeltaT, the more OFF time you have.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Condensing/modulating boilers add yet another variable to that old debate: as they modulate down, their efficiency increases slightly. Modulating burners strive to be "less on" (not off completely) as the deltaT decreases...

    Produce a heat call requiring a significant increase of room temp and the boiler will greatly increase the firing rate. Depending on how the system is controlled, deep daily setback of a highly zoned system could easily result in little or no modulation regardless of weather conditions.
This discussion has been closed.