EUSAVE’s True-Up function compares the building’s modeled energy consumption to the actual heating fuel used. In other words: it compares the calculated heat loss through the building envelope components you’ve entered, to how much energy is consumed by the heating system(s) and accounts for the heat lost due to inefficiencies in the heating system(s).
Heating LOAD means calculated heat loss, while heating CONSUMPTION is the heat load divided by the efficiency of the heating system(s). Consumption is always more than load EXCEPT where efficiency is >100% as is the case with heat pumps.
In general, there will be greater heat loss in any building energy model than was actually delivered as heating energy by the heating system(s). This is because heat gains have not been accounted for. Solar insolation, along with Internal gains from lights, appliances, and human activity will lower the overall heating requirements of a building. A very broad rule of thumb (i.e. a good place to start that will rarely be correct) is that 35% of total heating requirements may be met through these heat gains. Occupant variability and behavior such as thermostat and ventilation settings, personal comfort levels, health needs, etc., also affect the true up factor.
EUSAVE starts with a default true-up factor of 65%, meaning that the heat loss of each component modeled will be multiplied by .65, thus lowering the calculated heat loss (and therefore heating energy requirements) of all component to account for the above-mentioned, and un-modeled, heat gains and behavior factors. In most cases, this adjustment will bring the model closer to actual heating energy consumption.
It’s important to remember that true up really only works when you’ve modeled the entire building. If you’re only modeling a single envelope assembly, you may want to just leave true-up set at .65 (unless you have reason to use a different value).
True up factor can vary from 40 to 90% (or more or less) depending on all of the above factors. While true-up helps to reduce the possibility of over or under-estimating improvement savings, it’s important to be as thorough as possible in your modeling to reduce errors in savings predictions. The same true-up factor is applied to both existing and proposed conditions.
A LOWER than expected true up factor indicates that the modeled building heat loss is GREATER than the actual heating fuel MMBTU consumption. The LOWER true up factor effectively REDUCES the heat load of all components proportionally until consumption matches heating fuel MMBtu’s entered on the heating page.
If the true-up factor seems unusually low, here are some things to consider (If your true-up factor seems unusually high, replace the capitalized words above with its opposite for similar occurrences):
- Too MUCH heat loss has been modeled
- Too MUCH shell assembly area entered
- Existing insulation R values are UNDER-estimated
- Air leakage is OVER-estimated
- Seasonal equipment efficiency has been OVER-estimated
- The house is kept COOLER than typical (behavior)
- When the thermostat is set LOWER than typical, the effective heat loss is REDUCED.
- Typical sources of internal gains are MINIMAL such as kitchen use, electronics, bathing, and general occupancy patterns
- HDD and fuel consumption periods are different
- Too MANY heating degrees days entered for the heating energy billing history period
- Too FEW heating fuel MMBtu’s entered