I think you have been given a good answer, but perhaps asking a different question might have been even better. Let me ask then, is there much or any downside to having an oversized furnace? Assuming the air handler and duct system are compatible.
I'm asking about real-world choices, not something ridiculously oversized. It has been my observation that there are not that many choices about furnace BTUH capacity, and one cannot dial in an exact need as closely as with air conditioning. I am under the impression that having 160,000 BTUH when design heating load is perhaps 87,000 is not too big a problem.
I think the question gets more interesting with air conditioning.
I wonder if measuring residential AC duty cycle (on-time vs. off-time) during hot summer days would be a legitimate way of determining whether an AC were oversized. This method would by default take into account the owner's thermostat preferences, sun exposure, infiltration, duct leakage, presence or absence of radiant barrier, etc. Just about every factor would be included in this measurement without having to explicitly correct for it. This would of course work only for houses already built, whereas Manual J is the model which can be used before building.
I'm thinking that a measurement method such as this could confirm an estimate made by Manual J. In my mind at least, a measurement inherently is more reliable than an estimate.
This info could probably be obtained using a Hobo or other temperature data logger located close to any supply outlet. A duty cycle of near 100% at design conditions, would probably indicate sizing very close to what Manual J aims for. If duty cycle did not get over 75% in a hot summer day, I am thinking that would be proof of significant oversizing. Anyone care to comment on the strengths or weaknesses of this idea?
I wonder if measuring residential AC duty cycle (on-time vs. off-time) during hot summer days would be a legitimate way of determining whether an AC were oversized. This method would by default take into account the owner's thermostat preferences, sun exposure, infiltration, duct leakage, presence or absence of radiant barrier, etc.
The unit is sized properly when it can remove the amount of heat necessary to maintain indoor design conditions under outdoor design conditions. Say the design conditions for your area are 75 degrees indoor dry bulb @ 50% relative humidity, and outdoor design is 95 degrees dry bulb. If the system can maintain the design indoor comfort levels (75/50) at the design outdoor temperature (95°F), it has meet design conditions.
Under these conditions the unit will likely cycle little or not at all, as it is removing heat at about the same rate heat is entering or being generated in the structure (you might compare it to the "balance point" for heat pumps, where when in heat mode the heat pump has reached a point where it is adding heat to the house at the same rate the house is losing heat). In humid climates such as Houston, the unit will probably run continuously at those design conditions due to an added latent load. Though latent heat is not sensible, it is nonetheless heat and must be removed from the house and the system along with the sensible heat.
Going by cycle rate alone to determine system capacity would strike me as going at it bass-ackwards, since there are many variables. One day under the design conditions above you might have one person in the house sleeping all day doing nothing more than adding his body heat to the air. No doors or windows opening and closing, no laundry being done, no baking or cooking, no TV or computers running, etc.
The unit may cycle during the heat of the day.
The next day he invites everyone he knows over for a midday baseball game on TV and a pool party. That's a lot of body heat. And with the game on you'd have TV heat. One of the party goers, bored with the baseball game, starts surfing the net, so there's computer heat. Then there's the in and out traffic through a sliding door of those using the pool, along with the moisture they carry into the house on their clothing and skin. The wife's in the kitchen making chili and baking a pie, and doing laundry at the same time because she's not into baseball, computers, or swimming. She's also running the dishwasher to keep up with the demand from the partygoers.
Do you think the unit is going to cycle now? It's still running under design conditions, as far as outdoor temperatures go (95 degrees), but the indoor conditions have become too extreme for the capacity of the system.
Electricity makes refrigeration happen.
Refrigeration makes the HVAC psychrometric process happen.
HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.
Thanks very much for your comments. I agree it is kind of backing into a sizing answer. And of course you are right the people in the house must be taken into account, or the cycling measurements will be misleading.
>>on one 95 degree day it might have higher out side humidity then normal, and the unit
>>might not keep up that day, and your test would indicate that it was under sized, when its not.
Simple answer -- I would not consider a test valid if I only tested one day. Any test I imagine, would sample a fair number of the typical summer days.
While what you describe might be a very real concern for some climates, we in S. Texas are "blessed" with a really monotonous summer. Day after day of heat and humidity. HVAC-TALK recommends design humidity of 114 grains which I gather is a lot. While I am not the most able in converting between grains/cubic_ft and percent humidity, it is my impression we kinda peg the needle on humidity on an everyday basis (taking into account the night cooldown to 76-79 degrees and near the same dewpoint).