Oversized furnace?

Discussion in 'HVAC Heating & Cooling' started by pitteach, Jul 7, 2009.

  1. pitteach

    pitteach New Member

    Messages:
    27
    Location:
    Hamburg, NY
    I have an opportunity to purchase a brand new Heil 100k 92% 2 stage natural gas furnace for an unbeatable price.
    The house is in Buffalo, NY, 40 yrs old, 2 story, 1800sq ft plus a finished basement, new LOe 3 windows, 8-9 ft ceilings, marginal insulation in walls and attic.
    I know that a single stage 100k furnace would be too much heat. My thought is that this could work with the furnace operating at 60% for the majority of the time. I am looking for thoughts and ideas as I am trying to make this work. The plus is that I can install this for far less than the smaller 80k furnace, what I am looking for are the possible minuses that I might encounter.

    Thanks.
  2. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    Nothing usually beats actually matching the burner to the actual load. (And yours could easily be under 50KBTU/hour on design-day, if not right now, after you've spent short money on fixing the most eggregious air leaks and attic insulation.)

    Low mass systems like hot air furnaces don't suffer quite so badly as higher mass systems when oversized, but your unbeatable deal may not look so great when you look at it's de-rated efficiency due to oversizing, in a 5 year cost analysis.

    OTOH, if it's a sealed combustion model (taking outdoor air into the burner, not room air) it may in fact do slightly better if only 2x oversized:

    http://simulationresearch.lbl.gov/dirpubs/42175.pdf

    (See figure 6, the Induced Draft BONN 85 curve.)

    If it's not sealed combustion, it'll be operating somewhere below it's rated thermal efficiency whenever it's cycling on/off. It's derating will be something similar to those other curves (but at least the 1.0 factor line is ~92-95%, not 80%, so it can be 3x oversized and do about as well or better than a well-matched 78% AFUE model.)

    Since you won't be modulating, stepping up/down much, it'll likely see more ignition cycles than if perfectly matched & running fairly continuously in mid-winter, which will wear it out a bit quicker. If there's a way to add some hysteresis into the controls it'll last longer and run more efficiently delivering the partial-load heat with longer but fewer cycles.

    I'm assuming you've already figured out where/how you'll be venting the exhaust and disposal of the condensate? If not, take a big step back. You can't just hook this thing up to a masonry chimney where the old one was and pass inspection. (The flue condensate of high-efficiency burners would destroy the chimney.) If the chimney has the right liner and isn't too long you might be able to use it though. See if you can't find the installation manual for the unit- it'll probably have most of the necessary info for getting it right, but it's not usually a DIY project for the uninitiated.

    The price of the unit itself is only one factor in the installation cost...
  3. pitteach

    pitteach New Member

    Messages:
    27
    Location:
    Hamburg, NY
    Oversized furnace

    Thanks for the reply Dana. This is in fact a sealed combustion model with separate intake and exhaust lines. The previous "chimney" was a chase through the center of the house with a double wall pipe. I am replacing the hwt with a RUUD power vent system to eliminate the metal piping which seems to have charred the chase structure over the years. I am contemplating whether to run vent and intake lines back through the old chase and out the roof or out the side of the house. Both have pros and cons. As for the condensate, I will be using sump pump to remove the water.

    I have installed many of these types of furnaces, but they have always been single stage units. This particular furnace runs 12 minutes @ 65k btu and then calls for high heat @ 92k after that if needed. If I were to put in a single stage 80k btu, it would run at a constant 73k. My thought is that the low heat on the 2 stage would be enough to heat the home without calling for high heat on those days that are not as cold, and on the exceptionally cold days, the high heat will be used. I believe that the low heat scenario would make the furnace cycle longer, but less often?
  4. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    In that case, go for it!

    If there's a way to defeat the high output burn, lock it into the low-output mode or reprogram the 12 minute lo-burn to an hour, that will probably be more than meet your actual design day heat load, and ensure that it's in condensing mode most of the time. Recovery times from deep overnight thermostat setbacks would be slightly longer, but the low-fire steady state thermal efficiency is probably ~95%, and the high-output steady state thermal efficiency is probably no better than ~87%, in which case any savings from overnight setbacks go up the flue in lower combustion efficiency if most of the recovery heat gets delivered at the high-output burn.

    Have you done a Manual-J or similar heat loss calc (or careful fuel use against heating-degree-days) on the place? Even 80K/73K seems a bit high (but probably not by more than 2x). But the 2-stage unit will almost certainly run at higher seasonal efficiency. You may want to limit overnight setbacks as part of the efficiency strategy. (Play with it a bit and you'll find the happy medium.)
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