Commercial grade registers can help a LOT with this, especially if they are located in ceilings or sidewalls. Air can be directed downwards instead of 4 ways like a standard register. Increasing CFM of the furance blower helps since air is circulated better. Correctly sizing furances so the blower is on a larger percentage of the time helps too.
If you are trying to figure heat load, the furnace should be operating at 100% when you hit design conditions. Stratification will be minimized since air will be circulating. Of course if the space has ceilings over 10' high all bets are off. The bad thing in in a space with super high ceilings is it's difficult to get the bottom warm w/o overheating the top of the space. Air infiltration has a LOT to do with the temperature difference between the lower and upper parts of the space. Any cold air leaking in will find it's way directly to the floor.
The problem is it's difficult to calculate the temperature difference between the bottom and top of the room. If the HVAC system is designed correctly and infiltration is controlled there should be minimal difference between the upper and lower levels of the room. HVAC systems that use a large volume of warm air such as heat pumps will have less temperature difference than a HVAC system such as gas heat which uses a low volume of very hot air. Commercial registers pointed to the floor in the heating season help a LOT with stratification issues. Cutting the amount of cold air leaking into a building helps a lot.
Point is there are so many factors, it's difficult to calculate how much temp difference there will be in real world conditions. Manual J typically oversizes HVAC units anyways, how many houses have you seen with heat that couldn't keep up?
This isn't an answer based upon some inside knowledge about energy software calculations, but more based upon the variables that the software must deal with, everything. In many cases, the software does its best and then those results are tested against the real world and tweaked as needed to be as close as possible. Take the base 65° we use instead of the actual 70° house temperature, tweaked I'm sure and it may include a measure for the detail you are discussing.
From average inside temperature, to combustion efficiency and the real R value of our insulation, beyond our tape measure, most of our numbers are a best guess. One of the details I had to adjust to when first entering the energy profession was to stop looking for accuracy and focus more on consistency. If our work is consistent, then we can duplicate our results and so can others. If we strive for accuracy, it is mostly unachievable and in the big picture, our results are the difference between two numbers so accuracy becomes less important.
Most of my work involves manual calculations and I do at times vary little details like this to get my calculated results to match the real world. But I would more often simply shift the entire house temperature to try and find an average for the set backs.
Run your numbers for a 70° ceiling and then an 80° ceiling and see how much difference it makes. For a whole house, increasing the temperature to 80° increases the HDD by 45%, which directly translates to energy use.
Bob and Bud,
Thanks again for making time to comment.
Getting back to total energy consumption for a residential heating system, I recall in days of yore, when attempting to match a buildings historical energy usage to the base line simulated home (which would then be modified with the addition of various system and envelope modifications) I routinely had to tweak the simulation in an effort to approach the historical record. Month to month numbers were rarely within 25%, since the software was using an aggregated, long term weather data base, so I concentrated on having the total simulated heating energy agree as nearly as possible with metered seasonal record. As I recall, adjusting air changes was a good number to fiddle with as it also affected cooling energy use. Commercial structures further complicated the process because kW demand, an important element of electrical cost, also had to match up reasonably well .
After arriving at a reasonably accurate simulated/historical energy match, I used the bench-marked home to evaluate the aforementioned modifications, weather (pun intended :-) they be operational changes, insulation, lighting mods, etc.
As you have observed Bud, consistency is the key word here and I'm looking for an answer to a question no one other than me (a retired guy with too much time on his hands) is asking. Let's drop the discussion at this point and agree that among myriad variables involved in simulation of annual building energy consumption, Delta T at the ceiling is just one of many, many factors contributing to uncertainty.
"Run your numbers for a 70° ceiling and then an 80° ceiling and see how much difference it makes"
If I still had access to software, I'd do it! Frankly, don't know how such ceiling numbers would be entered, but suspect the "occupied room temp" field has only one entry. Clearly, a room temp of 80F would result in much higher annual heating energy, but again, ceiling temp would probably be 90F.
This exercise takes me back quite a few years and I recall reading in an energy magazine of the time how researchers working on software were having difficulty matching their numbers to reality. They said they finally included the evaporative cooling effect of rain water on the home (what, no consideration of sensible cooling?) and the numbers fell right into place...right :-) Wonder if they also factored in the periodic shadow of a parked UPS truck on the south wall...
Most residential compliance software will not allow you to make adjustments for stratification (in fact, I know of none...). Much more troublesome software does allow such customization by the user at significantly more effort and probably not more accuracy. For example, in IES (Integrated Environmental Solutions) or DOE's EnergyPlus, you can horizontally divide the volume of a zone into two or more zones, and then specify temperatures for those zones. In this case, the lower zone would be set to the thermostat setting and the upper zone would be set somewhat higher. This is really the only work-around available, and as I said above, probably offers very little, if any improvement in accuracy. These software tools are also not recognized by residential certification schemes (RESNET and whatnot), and their inputs are incredibly troublesome in comparison to the typical residential compliance softwares (EGauge, REMrate, EPro, etc.). Plus, the difference is likely to be less than the variability caused by uncertainty in other elements of the model. For example, if you had 2,000 hours of 70 degree versus 80 degree temperature difference across a 1,000 ft2 ceiling, the difference in total energy use (at 100% efficiency) would only be 200 kWh. Not insignificant, but certainly not terrible, and most climates would never see that many hours at such high temp differentials...Cheers.
Thanks for your reply!
I suspected more sophisticated software might exist to take into account stratification, but as you point out, there are so many other imponderables in the process that net effect of efforts at fine-tune the model would be unlikely to raise the accuracy bar. And as Bud has observed, we're really just looking for an acceptably accurate benchmark against which to apply measures (apologies for tortured syntax, but I promised Sister Mary Recalcitrant, my English teacher in 11th grade, I would never end a sentence with a preposition :-).
I own a wireless hi/lo memory, digital thermometer, and as an exercise, will tape it to the ceiling and see what difference there is overnight. Room temps will be observed on the Honeywell.
Film at 11
If any of the above software is based on manual J it already has a huge "fudge factor" already built in. Real world conditions dictate that gas furnaces are almost always oversized, cycling even on he coldest of nights. I've never seen a properly operating gas furnace ever struggle to keep up, even the 2 burner models. Doing a run time calculation of the current system is probably the most accurate way to size the new furnace.