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oversizing

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Greg_40
Greg_40 Member Posts: 43
in THE MAIN WALL
On forced air systems, we all are aware of the energy wasting and comfort evils of oversizing fossil fuel systems. Do similar rules of energy wasting apply to forced air electric resistance? I haven't found much research yet on this subject.

Reason I ask is it seems the energy consumption/billings don't seem out of line for this one home in particular. Writesoft shows a loss of right at 50K Btu, while system has 114KBtu resistance. Just a typical oversizing that I seem run into fossil fuel wise, but don't see many total electric HVAC here. AC is standard split system and owner is having me replace it next spring with HP. I need some better direction to advise them on the air handler aka electric furnace piece of the pie.
Thanks!

Comments

  • Brad White_164
    Brad White_164 Member Posts: 18
    Fair Question, Greg

    I would think that for electric resistance the effects of over-sizing would be minimal and would rather show up in degrees of comfort at any one time.

    For example, any fossil-fueled system has a ramp-up and cool-off period with each cycle. More over-sizing, more waste during those cycles. Converting "gross" to "net" has inherent inefficiencies which are made more dramatic by over-sizing.

    With electric resistance, your efficiency is for all practical purposes 100%.

    (No one is foooled of course, into thinking that the customer is somehow not paying for all of the inefficiencies along the way!)

    If control is on-off, say a 15.0 kW coil for a nominal 50K Btu heat loss (51.2 actually), the comfort will not be nearly as good as if you had (3) 5.0 kW stages and that not as good as if you had SCR (infinite) control.

    With a heat pump, cycle losses will show up when oversized and that to me is more of a concern for wear and tear on the compressor and how comfort might suffer when the compressor drops off with oil return delay between cycles. Not sure how efficiency would be effected with an oversized heat pump, but I would avoid oversizing the heat pump for reasons unrelated to efficiency.
  • subcooler
    subcooler Member Posts: 140
    HP

    For the cost of the heat pump vs. AC the HP is a win win scenario. Anytime the temp. is above 25*F the homeowner is going to have a unit running at 200% to 300% efficiency. If it's above 35*F it can be in the 300% to 400% range..
  • jp_2
    jp_2 Member Posts: 1,935
    over size inefficient

    my guess its just like a gas F/A system or at lest similar.

    put a pot of water on the stove, turn on high, let it warm up a bit and turn it back off, let heating coils cool, keep doing this on/off/on/off until the pot boils. all the warming up current and the heat energy during cool down doesn't get used.
    (put a small spacer bewteen pot and coil)

    any associated wiring that gets 'warm' is lost btu's. house wiring is rated at some pretty high temp's.
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