There was a time when furnaces had as much flexibility as boilers with respect to Btu/h input and output. Same for the air delivery side! Hard to believe, right?

Fan/limit switch. Fan/limit switches had a helical probe that was inserted between the heat exchanger sections. As the heat exchanger warmed up, the helical metal strip contained inside the slotted tube would twist to turn a dial inside the control box inside the furnace cabinet. That dial had adjustable limit stops: fan off (low temperature); fan on (higher temperature); and burner off safety overheat limit, but the fan would continue to run if the call for heating or until the fan/limit switch dial twisted back to break the lower temperature fan off setting. We could adjust both the fan-off and fan-on pointers on the face of the dial with its temperature lines. Set the lower fan off pointer a bit too low and the fan would never shut off, which led to customer complaints! The thinking was to have the fan continue running after the call for heating ended and, thereby, extract the last remaining Btu from the now cooling heat exchanger. Conversely, we could also raise the upper fan on setting if we needed to add more Btu/h input. Here’s how:

Adjustable gas valves and oil burner nozzles. If a customer complained about air delivery temperature and while observing the dial on the fan/limit switch, you found the furnace settled in at 140° F, you could adjust the gas valve to increase gas supply and air shutters to maintain proper combustion and observe the temperature increase of the airstream while watching the fan/limit dial to ensure you weren’t creating an overheat condition whereby the fan/limit was bouncing off of the safety high limit, which would cycle the burner off/on as air temperature rose/fell. With oil burners, the process was a bit trickier because combustion had to be accurately dialed in if you down- or up-sized the nozzle to decrease or increase Btu/h input. Combustion testing back in those days was accomplished using dumbbell-like chemical testers we pumped flue gasses through to check CO and O2. You also needed to check over-fire draft and flue draft along with a smoke test and stack temperature.

With the huge push for decarbonization and electrification of practically everything, you are now faced with more options than ever before in the history of our PHVAC trades!

Blower assemblies. Here too, we could widely adjust airflow by installing an adjustable pulley on the motor (smaller of the two pulleys). By resetting its width, the belt would ride higher (faster speed) or lower (slower speed) inside the pulley. The motor position was adjustable too, for placing the proper tension on the belt. If you increased the Btu/h input to accommodate a customer’s complaint about air delivery temperature and you determined, via the fan/limit dial, the heat exchanger was close to overheating, you could speed up the fan, deliver a bit more air and bring the temperature down on the fan/limit switch dial.

That was then/this is now

Today, with a few exceptions, we no longer have single-speed blower assemblies. We have progressed from control modules where there are set speeds for heating and a higher speed for air conditioning modes that can be changed by swapping out the spade connectors. ECM blowers can now automatically adjust to deliver the design cfm (cubic feet of air per minute) you dictate from programming a compatible electronic thermostat. Direct-drive motor/blower assemblies no longer have pulleys and belts, so no fiddling with tweaking air delivery to be had there. Fan/limit switches no longer offer adjustments and the helical pug-tails gave way to sensors. The fan on/off is now a timed event and no longer adjustable.

Gas valves are often no longer adjustable. Oil burners can still be tweaked by altering the nozzle input and they are rated in gph (gallons per hour) at specific oil-pump pressures, which vary depending on the age of the oil burner — see the I&O manual! Or, today, go online via your smartphone to determine how far you can go.

Flexibility to the max

Modcon (modulating/condensing) boilers have been offering extreme flexibility with respect to Btu/h output and water temperature ranges, they can not only tolerate but excel at extracting almost all of the heat energy from the flue exhaust when operating at lower temperatures while in full condensing mode. Hydronic heating offers exceptional forgiveness, too. We have all seen the occasional modcon installation where the installers never connected an outdoor sensor, ignored programming a reset curve and you discover the modcon was treated like an old cast iron 1950s relic left to bounce off of its upper safety temperature limit. It’s often detected by a glance at the discolored and deformed PVC exhaust piping!

Indirect water heaters and combi units, oh my! The modern well-insulated “normal” size home no longer requires a massive Btu/h input, which creates somewhat of a quandary for sizing an indirect water heater: upsize the tank with a lower Btu/h output boiler; or use a smaller gallon indirect and oversize the boiler? To a certain degree, you can fudge with upsizing a modcon because, after all, they do modulate while striving to be right-sized for the heating side of comfort conditioning. I always upsized the indirect tank because a right-sized modcon, or older style cast iron boiler, will always have an overall better operating efficiency under those conditions.

There’s a new game in town

Heat pump this, that, and everything! You’ve been exposed to plug-and-play heat pump water heaters for many years now. Designed to drop into the standard 30-amp electric water circuit, they became an ideal retrofit addition to your arsenal of sales tactics that greatly benefit your customers and fatten your bottom line. With the huge push for decarbonization and electrification of practically everything, you are now faced with more options than ever before in the history of our PHVAC trades!

Inverter mini-splits: Long since mainstream, but sales continue to increase year-over-year. No longer just a supplemental resolution for temperature imbalances within a home or business, they are often deployed in either whole-home retrofits or from the get-go in new construction. Let’s call them the Old Reliable.

Air-to-water inverter heat pumps: While not exactly “new technology,” these modern systems exploded onto the scene at the recent AHR Expo in Chicago. Everywhere you turned, manufacturers had new models on display. As a contractor, this should be one clue so loud and clear to you about where the market is headed, you had best be aware of what these are and how they operate. More importantly, how you need to alter your mindset with respect to how you design and install the balance of components that comprise the rest of the system, be it hydronic heating/cooling; forced air; chilled/heated water mini-split-like zoned units; and let’s not forget, these can handle domestic hot water as well.

The times, they are a-changing. Are you going to ride this train and reap the associated profits or be an old stick-in-the-mud while sales pass you bye-bye baby, goodbye.