Residential furnace replacement sizing

Discussion in 'Environmental Discussion' started by Celtic Blue, Dec 16, 2009.

  1. Celtic Blue

    Celtic Blue New Member

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    And now for a break from the denialist troll threads for something that might actually prove useful...

    I'll be replacing my existing ~80% gas furnace in the next year and have been wondering what my actual design heating requirements are. Now, all of the guides say you to have a "Manual J" calc run. That's well and good, but it is just an estimate with a bunch of assumptions and I am more comfortable with a model when I can compare it to real world data. So it occurred to me to instead use empirical measurement of current performance to create the design case.

    What I've been doing is reading the gas meter daily and recording that along with the average daily temperature. This is providing a pretty good linear fit of ccf natural gas vs. temperature. (I'm also collecting data on shorter time spans on the coldest clear nights.) A ccf is about 100,000 Btu's (varies with composition) and this can be converted to heat load by multiplying times the efficiency. I subtract a daily water heater factor from this as well, based on correction from summer to winter water heater gas use.

    My current single stage furnace is rated at 110,000 Btu/hr input and 89,000 Btu/hr output. So if it ran 24/7 without overtemp cycling it would use 110,000/100,000 * 24 = 26.4 ccf/day. In reality from data down in the single digits it appears that at about 0 F it will run less than half the time to hold a 69 F set point. The zero F intercept on my current plot is 12.5 ccf/day.

    That 12.5 ccf/day rate would work out to a burner output rate of 12.5/24 * 89k/110k *100,000 = 42,000 Btu/hr output requirement at 0 F.

    Since my intention is to move to a 2 stage burner it looks like I have plenty of room to downsize the burner rate while staying in a given air handler range for AC and duct. Ideally, the furnace will run gently/quietly at low fire several fold more frequently, and therefore provide more uniform temperature distribution at peak efficiency most of the time, only going to high fire in recovery mode or during the coldest nights.
     
  2. jayman

    jayman Senior Member

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    The Manual J also tends to oversize/overestimate. I'd like to know:

    1. Coldest expected winter temps
    2. Windchill potential
    3. Furnace cycling

    By furnace cycling, since yours is a single stage, it is either On or Off, depending on call for heat. The blower is also On or Off. If your furnace frequently cycles, it is far too large. A furnace that runs longer is actually more efficient, its sized closer to need

    If you have a better two stage system, the furnace controller will use an outdoor temp sensor. This will become a PID control loop, comparing the indoor thermostat setpoint, outdoor ambient temp, and actual indoor temp

    The system will then stage the furnace depending on actual conditions. I have a Bryant Evolution Plus gas furnace with their Evolution control system: rated 60,000 BTU input high fire, 39,000 BTU low fire

    With the Evolution control, there is little to be gained by setback, unless you are gone for a couple of days or longer. It works very efficiently by running almost 100% of the time on Low fire

    The only time I have noticed it transition to High fire (The blower motor also speeds up quite a bit) is near -40, if there is a strong windchill

    Oh, make sure to use a one piece termination collar, preferably out the chimney you now have. In temps of -20 F and colder, the intake pipe will ice over with hoar frost, causing the furnace to lockout. This used to regularly happen to my neighbor, until he had the two-pipe system changed to a single termination collar like mine

    The single piece, the inner pipe is the exhaust, the outer pipe is the combustion air, like a sealed gas fireplace. There is just enough residual heat from the exhaust pipe to keep the combustion air intake from plugging with frost
     
  3. drees

    drees Senior Member

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    Good thread here. I also suggest the Residential forum over at hvak-talk.com which has a lot of good information.

    FWIW, I am getting a new dual-fuel system installed Friday to replace my 35 yo gas furnace (rated 100,000 BTU).

    95% gas furnace with 3-stages and a heat-pump that also does 9 HSPF heating and 12 EER and 17 SEER (according to AHRI). It's a Trane 3-ton heat-pump with a 60k BTU furnace.

    I turned on the furnace on the last day of the billing cycle last month and it jumped my normal 4-6 therms to 10. Just leaving the pilot lit doubled my gas usage (only other gas usage is a 4 yo water heater, nothing special).

    Anyway, for $50 you can buy your own Manual J calculation software for residential use (expires in 2 months) from HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations

    When I was getting quotes for my system, only 1 of the 5 guys (all were recommended) I had over really wanted to run a Manual J, and just guessed based on the existing furnace size. Even then after reviewing his numbers, had some pretty conservative temperature settings which put me at a 4 ton system with (co-incidentally all the guys who eyeballed it all said 4-tons and 80-100k BTU furnace - I'm guessing using the old 1 ton per 500sqft ballpark measurement - no wonder they don't bother).

    And then combined with the fact that the Manual J tends to oversize the system, and I'd rather undersize the system to save money/energy and my ducts aren't sized for a 4-ton system and aren't easy to get to, a 3-ton system made the most sense. If it doesn't heat/cool sufficiently, that really just means I need to get more insulation into the house since I know it's rather marginal at best now, anyway and I can deal with slightly warm or cool temps given we don't have AC now and only rarely use heat because of the gas the old furnace sucks down.

    Anyway Shawn, it sounds like you are doing it correctly and a 50k BTU furnace would do the job for you. Ideally, we'd all have smart meters that measured energy usage on a minute-by-minute basis along with ambient temps and then would be able to use that information to help size the system properly.

    I would even venture to guess that you might even be able to go to 40k BTU.

    The only real drawback to a smaller furnace is that if you do need to heat the house up quickly, it may not be able to do so.

    Also keep in mind that the less cycling a furnace does, the more efficient it is as well as having to warm up after being off takes it's toll on efficiency.
     
  4. Celtic Blue

    Celtic Blue New Member

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    I'm not surprised it overestimates. If you miss by undersizing 20% the occupants will notice and there will be hell to pay. If you oversize 20% nobody is likely to complain or even notice.

    Typical annual minimum temperature (not daily) here is -2 to -5 F from what I've gathered of the temp. data in the past 5-6 years. It can get colder, but that is rare. There are about 3-5 days per year here where the daily average is less than 10 F and about 15-20 days with the average in the teens. Dec-Feb monthly averages are right at 30 F, sometimes a little lower, sometimes a little higher.

    20-30 mph sustained winds are not uncommon and I'm on enough of a ridge that it will howl.

    I need to actually count cycles and durations on a particularly cold night. Off hand I would say it runs about 4 times an hour on a cold clear night. It is running at least half the time when it is coldest, but I need to account for the difference between the furnace warm up, the blower start, the cool down and the blower shut off.

    I had some trouble with the thermostat last winter until I realized that there was a gap in the insulation in that wall and the hole extended to the attic. Once I stuffed it with insulation the massive overshots ended. I originally suspected I had a battery problem with the thermostat, because it was obviously much warmer than what it indicated and I had seen this happen with a similar thermostat. I realized what the problem was when I touched the wall above the thermostat on a cold night and found it was very cold. The next day I went into the attic and fixed it.

    That's pretty close to the input I'm gravitating toward for a 95% unit with ECM blower. I think the next increment is around 80,000 input.

    I doubt we will have any -20 F here, although I have seen it once before about 150 miles farther south.

    Mine is likely to vent on the back wall between floors, assuming I can manage it. Otherwise I would have to add a second stack roughly 30 feet long and probably have to add an access panel in a closet for the elbow in a chase.

    I'm looking forward to sealed combustion eliminating the air infiltration of the present system. Since the builder didn't install the required vents in the utility room, I've been leaving the door open to insure proper draw (although it does well enough with the door closed...it is pulling through the joists between floors most likely.) I was going to add the vents myself, but decided I would prefer to eliminate the need for them. Furnace first, then water heater...then I will tighten infiltration points between there and the garage.
     
  5. Celtic Blue

    Celtic Blue New Member

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    Sounds like you are replacing one of the old 65% units. The pilot in them really was a waste. At least with a water heater a portion of the pilot heat offsets storage losses.

    Our current AC is 4 ton and I'll probably stick with that since there is a much wider window for summer time delta's than winter. Although normally our peak is under 100 F, our humidity averages about 60-80% during the hottest parts. Stretches of 100-105 occur every few years. The first summer in the house with this old AC the unit couldn't keep up with the upper floors in late afternoon on a couple of days. Not coincidentally that is when I got serious about balancing, tightening, sealing, and insulating. Unfortunately, it didn't get tested with this very mild summer.

    That was a major part of my reason for tracking this and right sizing it. That and comfort. The corners of the house get the extremes of the cycles. I would much prefer to pinch back more registers and push the air where it is needed most at low fire rates.
     
  6. jayman

    jayman Senior Member

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    Shawn sounds like you're on the right track, and I agree with Drees comments.

    As far as summer cooling, is it the heat, or the humidity? Up here, the summers are very brief, but usually very humid. My 2 ton Bryant 2 speed A/C runs almost all the time on the low speed.

    I can configure the A/C airflow for "efficiency" with De humidification Priority. So set, it will be "lazy" about keeping the setpoint (Can wander upwards 1-2 F) but the airflow will be slow, so maximum air contact with the evaporator coil. This really helps to dehumidify the house, and saves a ton of energy too

    I'd still recommend the one piece termination collar.
     
  7. drees

    drees Senior Member

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    Yeah, there's a 3" flame coming from the pilot all the time! Not to mention that the thing hums when plugged in - wonder how much electricity the thing pulls just sitting there...

    Be careful with pinching off registers if the other open ducts aren't big enough to flow enough air as the increase in static pressure can reduce efficiency, too.

    Hey jayman - is there anything you don't seem to have some knowledge about? :)
     

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  8. qbee42

    qbee42 My other car is a boat

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    I have a list. I can PM it to you. It's too large to post here.

    :p

    Tom
     
  9. jayman

    jayman Senior Member

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    Don't expect me to provide good advice on personal relationship problems. I once tried to console a co-worker regarding his marital issues, and within 4 months he was divorced and homeless

    :doh:

    :rolleyes:
     
  10. drees

    drees Senior Member

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    Celtic Blue - what'd you end up getting?

    I got a 60k BTU 95% furnace. Works great so far. Still oversized for my house - not surprising considering my Manual J calcs only came up with 25-30k BTU for the coldest days. Wish they made a smaller furnace that still handled my AC requirements - the 60k BTU is the smallest Trane makes (though the one I have does have multiple stages - unfortunately it's not possible to easily control what stages it uses).

    But hey - it sure recovers fast when it turns on in the morning!
     
  11. icarus

    icarus Senior Member

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    I have to say, I would never install a central warm air furnace again. For the same money I would install a number of Rinnai heaters that allow very good zone heating with very effective control, with virtually no parasitic loss due to duct leaking/poorly insulated ducts etc. And virtually no noise.
     
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