The furnace manufacturer says
The furnace manufacturer says my furnace will deliver the advertised efficiency when you, the designer/installer, provide it with a proper distribution system. The furnace has a rating plate that clearly states the maximum external static pressure the furnace or air handler can work against and still deliver the goods. Once you exceed the parameter, which is 0.5″ w.c. total external static pressure, then bad things start happening. It’s like a car manufacturer says his car works properly on quality paved roads. Take it off road or haul heavy loads beyond its rated capacity, then, once again, bad things start happening. Heating contractors have to make sure the road (duct system) is as good as it can be.

Wouldn’t mold grow in the ducts if the humidity reaches 86%?

To Blake’s point, it’s important to look at motor efficiency as opposed to motor watts. For example, if you replace a PSC that’s operating at 60% of design airflow with an ECM, a good contractor will take the opportunity to fix the airflow, so blower watts may still go up. If the duct system is undersized or a leaky duct system is sealed without enlarging it, an ECM may get pushed further up its power curve than you’d like. But on a CFM-per-watt basis, it’s still more efficient than a PSC.

Steve asks why furnace ratings don’t appropriately reflect blower energy… Blower energy has always been the unwanted stepchild of equipment rating procedures. For example, SEER and HSPF ratings for air conditioners are not necessarily based on tests with the matched blower. Most ratings are modeled (simulated) from baseline tests. in that case, the blower energy component (for SEER & HSPF ratings) only varies depending on whether the blower is PSC or ECM. Also, baseline tests upon which these simulated ratings are based do not have to be conducted at a particular airflow (although the tested airflow must now be disclosed on the ratings).

This situation evolved from a time when blower energy was much less consequential to system energy consumption. Clearly that’s no longer true, as source equipment efficiency improvements have far outpaced blower efficiency. AHRI rating procedures are specified by the DOE and thus are subject to public comment. There’s naturally going to be resistance (from manufacturers) to any change that would require more actual testing and/or make their products look worse.

@Mike, all good points. I’ve definitely seen bogus data in the tables based on field measurement. But excepting outright errors, I find that blower watts is usually within 10 or 15 percent. And as I’m sure you know, and to Treje’s point, it’s important to understand that blower table (watts) doesn’t purport to include power consumed by thermostats, transformers, relays or solenoids (or the inducer fan).

In any case, as a 3rd party designer and specifier, I’m often limited by what the contractor sells, in which case I may not have the luxury of empirical data. My point was simply that manufacturer blower tables are much better than the (ARHI) electrical consumption data included in RightSuite.

Thank you.

These are good baby steps in the initial learning curve about system selection. One of the problems though is that manufacturers lie and make up stuff.

The game changer is when technicians gather and analyze actual system performance data from their installations ad discover what systems actually perform best. And when they find they can’t deliver as much efficiency into the home as they want (like aiming for an A in distribution system efficiency and getting a C), things are learned like the furnace penalty of higher temperature air and resulting larger Btuh losses within the distribution system than lower delta systems (Heat pumps, low flow hydronics for example), or the penalty of lower CFM airflow and corresponding lower fan Watts (higher distribution losses due to longer transit time to destination).

Good designers comb literature and modeling programs. Great ones measure and base their decisions on actual equipment performance.