Furnace upgrade

Discussion in 'HVAC Heating & Cooling' started by quarry, Mar 1, 2012.

  1. quarry

    quarry New Member

    Messages:
    7
    Location:
    Upstate NY
    I have a 42 year old oil fired Williamson 5 in 1 furnace(heat,cool,humidify,elect air cleaner and 2 stage filter) . What a cadilac this thing must have been back in the day, Actually its been a great furnace and was maintained yearly. I want to upgrade to a bigtime super high efficiency condensing oil furnace ex: Adams mfg, They boasts worlds most efficient oil furnace in the mid 90s AFLU for the 140kBTU-4ton. Cant find any pricing on these things. Any one hear good, bad or indifferent about these units.

    Also does a A shaped evaporater coil actually hurt the air flow in a furnace in the heating mode. When i remove the coil inspedtion side cover on the furnace its like trying to close a door on a windy day. While being more exspensive would it be better to separate heating and air conditioning completely.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,270
    Location:
    New England
    Before you start looking at new stuff, you need to run a heat load analysis to see what you really need for both heating and cooling. Replacing what's there for the goal of efficiency can all be negated by getting a unit bigger than needed. A 4T a/c unit is pretty large, too, and may not be providing the best comfort and economy. A boiler and a/c unit that are separate will offer you ease of updating part of it. They are unlikely to have the same service life, and improvements would be MUCH easier if they were separate.
  3. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    First questions: has it been able to hold set point in the coldest winter temps and the hottest/most humid summer temps?

    Sounds awfully big...but probably a 60-65% efficiency unit so about 85-90k output. Beware of anyone wanting to put a 90+% unit in that has an output over about 95k assuming your furnace is in the range I suspect. Have a heat/cooling load calc done. Or you can look at your oil use and heating degree days for your area and back out your design load. If you have years of data this could give a pretty good value.

    Most furnaces are oversized by a huge margin. I can't think of any place I've ever lived where the furnace couldn't hold set point in the lowest temperatures, even nights 15-20 F below the supposed "design temp" for the location.

    I would expect it to be difficult to do with the blower running! The evaporator has a pressure drop associated with it, that's the only way you can move air past the fins and get some heat transfer (and air has lousy heat transfer coefficients which is why the air side has fins.) Fortunately, it doesn't take much DP to move air around.

    That said it is also possible that the A coil is dirty and more restrictive than normal.
  4. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    What's your zip code (for weather and design temp information), and how much oil do you use per year (or what is the K-value stamped on the oil bill, as it customary by most NY oil dealers)? It's pretty easy to establish an upper bound on just how big a furnace you need using that information, and right-sizing it has a positive effect on comfort & efficiency (particularly on the electrical power costs of the air-handler.)

    Replacing a 140KBTU/hr antique with a 140BTU condensing burner is ALWAYS going to be mistake- the heat loads were never that high in the first place. It's almost impossible for a normal sized house to have design condition heat load anywhere near 126KBTU/hour (90% of 140K) unless you have a lot of single pane glass, including several with some of the panes missing. If your ducts are like more 40+ year old ducts, they are leaking at least 25% of the air, and uninsulated, driving air infiltration increasing the heat load, so no matter what the fuel-use analysis tells us, the odds are that the true heat load is no more than 75% of that number, and could be 50%. The right thing to do is to pay for a room-by-room "Manual-J" heat loss calculation, but have the fuel use number in your back pocket as a backstop. If the pros tell you the heat load more than what is implied by the fuel use numbers (it happens) they either didn't do the necessary work or made a fundamental input error in the software , and you can push back on them to do it right.

    A true heat load calc is more than just a half-hour's work, and it's definitely NOT a, "Lessee, ya gots 3000 square feet times 35BTU a foot- call it a hundert-thou give or take, make it 125K just to be sher" kind of deal, which reliably oversizes it, usually by more than 2x.

    If your true design condition heat load is under 50KBTU/hr (probably is) and your design temps warmer than -10F (probably are) you have other options to consider. At current oil prices split-system air source heat pumps can heat your house at roughly half the cost of 90% efficiency oil burners. If you're in a cheaper-than average electricity market it could be less than half. What is your delivered cost per kilowatt-hour for electricity?

    And with a mini-split or multi-split you also get VERY high efficiency air conditioning built-in.
  5. quarry

    quarry New Member

    Messages:
    7
    Location:
    Upstate NY
    Zip is 12546. Tri state corner,NY,Mas & Conn. Gets pretty cold. House is ranch style 2x4 framing w/ R11batt insullation(not the greatest), T-111 siding with all anderson window and all duct work was just wraped and sealed. I have three pellet stoves on thermostats that heat during the day when im home and use the furnace heat when i am not home during the day and at night. the thermostat is set to 64 at night and the furnace cycles about every 20 to 25 min. We do have very poor air flow with the existing unit and with base board vents ,the heat works fairly well but the AC takes forever to cool the house, the new unit i pick is going to have to have a variable speed fan or at least two speeds. Yes Manual J and or heat load analisys is my next task. i will check on the cost per Kwh.

    thank you for helping me open new paths of thought and giving me greatly needed direction.
  6. quarry

    quarry New Member

    Messages:
    7
    Location:
    Upstate NY
    yes the existing old Furnace unit has a 3ton AC and a 4 ton may well be to much for a small ranch. the AC is run approx 20 days during the summer here in the north east. To way the cost of a separateA/C - Heat pump unit ( attic unit ) with all new duct, insulation and defusers against the amount of time it would be used is also somthing to think about.

    You may not be a pro but you sure know more than I. thx
  7. quarry

    quarry New Member

    Messages:
    7
    Location:
    Upstate NY
    Holding set point is marginal, on very hot and cold days. I really do need to get somone in to do the H/C load calc asap. You know two weeks ago i cleaned my file cabinet and dumped three years of Electric and oil bills showing usage data.

    As for the AC coil, is there any prefered shape coil, I have seen A , H and what looks like a Z shape.

    Thanks Much.
  8. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    The 99th percentile design temp for Kingston NY and Waterbury CT are both +2F, so your design temp is probably not lower than 0F. In a climate like that you can expect an average heating seaason coefficient of performance (COP) of 2.5 or better out of a better-class split system heat pump (mini-split or multi-). With a COP of 2.5 you get 2.5 x 3412 BTU/kwh= 8530BTU/kwh. If you are on the expensive side for electricity you might run 20cents/kwh, so for every dollar you'd get (100/20) x 8530= 42,650 BTU/$ of heat into the house.

    With a 90% condensing oil burner you'd get 0.9 x 138,000= 124,200 BTU of heat into the ducts per gallon, so if the ducts are super-tight, well insulated and perfectly balanced, at $3.50 oil you'd bet 124,200/3.50= 35,486 BTU/$.

    But in an air-leaky house with less than perfect duct design pressure differences between rooms drives higher infiltration rates due to the air handler creating pressure differences between rooms, so that needs to be discounted by at least 10%, maybe even 15% if there isn't a return in every room that has a supply duct. Call it ~32,000 BTU/$ and that may be an overestimate. So even at 20cent electricity(which is probabably an over-estimate) and $3.50 oil (probably an under estimate) heating with a mini-split would give you a 25-33% discount over heating with CONDENSING oil. The real numbers are probably even more favorable for the mini-split.

    Either way, tightening up and spot-insulating the house will always be cost effective and add comfort at your R values & construction. With T111 siding and R11 batts you can probably retrofit cellulose to ~3lbs density with ~1" holes on the outside (without removing the batts- just compressing them) with minimal repair/repaint. Doing so would tighten up the place considerably, lowering your overall heat load from infiltration, and would give you a stable R12-13 center-cavity R. With R11 batts even if perfectly installed your R-value at 0F design temp would be about R8-R9 due to convection currents within the fiber. With the air retardency of cellulose loss of performance at the temperature extremes from internal convection disappear, and air leakage at the seams drops by over 90%.

    For a quickie rough estimate of a Manual-J, what is total square feet of the house (better yet the outer perimeter length), and the number of windows & doors (and what's the U value rating of those Andersens)? Full basement, crawlspace, or slab on grade? Insulated foundation? Attic insulation (R value & type?)
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