An architect, insulation installers, door and window salespersons/installers, and a mechanical contractor are in a classroom together, and the instructor asks: “Should you upgrade the building envelope or replace the heating and air conditioning equipment? You can only do one or the other first due to budget constraints.”

Only the mechanical contractor says HVAC equipment first and everyone mocks that answer. The energy audit instructor says, “Of course you’d say that — you sell HVAC equipment. Care to explain why you think you’re right?”

There’s only one problem: The mechanical contractor already crunched the numbers and had documented proof he is right. That contractor was me, and this really did happen.

So how do you diplomatically tell a room full of professionals they and the instructor are wrong? “Carefully, wary carefully,” as Elmer Fudd would say.

For one thing, the all-day class had just started and there was going to be a test at the end. Tick off the instructor and that test score might suffer! So, I told them that if they were referring to on/off HVAC equipment, they’re only partially right and here’s why.

Upgrade the envelope and wouldn’t you all agree the existing more-likely-than-not already oversized heating/cooling equipment will be grossly oversized? Yes, they all agreed that would be correct. And we can all agree their heating/cooling bills will be lower — assuming all conditions remain the same, right? Yes, indeed. I then asked if they were aware of what happens to grossly oversized heating/cooling equipment?

“No, but I’m betting you will tell us,” the just-out-of-college energy audit instructor says.

 

The documented proof

Here’s what happens to furnaces and boilers:

  • The burners will be on/off in short cycles, which will cause flue gas condensation to occur. Flue gas condensation is acidic with an average pH of 4 to 6. Depending on the extent they are oversized, you may have a heat exchanger failure within one or two years’ time.

  • The furnace blower motor will be an older type that draws higher amps than a new ECM (Electrically Commutated Motor). As most folks know, the lion’s share of energy consumption is when starting an older style motor. With an upgraded envelope, you’re now compounding the parasitic energy losses on both the fossil fuel and electrical usage. 

  • Those chimney-vented appliances probably have mild steel flue piping. Properly, or nearly so, sized heating appliances are designed to have exiting flue gasses above the dew (condensation) point. Acidic flue gas condensation will rapidly rot out the metal flue pipe and fittings.

  • There will be sustained flue gas condensation in the chimney. In many older homes, the chimneys do not have a terracotta liner and are brick-and-mortar. Many of them utilized soft, unglazed bricks.

The acidic condensation will rot out the bricks and mortar, the surfaces will shed off and fall into the base of the chimney, potentially blocking the vent pipe(s).

Carbon monoxide issues are now exacerbated. Who will accept responsibility for the debilitating injuries or death(s)? Of course, they said the mechanical contractor, but I reminded them I’d been left out of the upgrade loop.

  • Boilers have, in most cases, automatic water feeders. Are you incorporating leak detection equipment for the water-feed line or floor sensors to alert the homeowners if a leak occurs, or will they simply have an ugly surprise one day for a flooded area, mold and expensive repairs?

  • Equipment “rated” to have operating efficiencies around 82% that operate under short-cycling conditions will have operating efficiencies substantially below their factory rating.

Let’s look at heat pumps and central air conditioning:

  • Old style heat pumps use old style motor-driven compressors and fans (both the condenser and air handler blower). Here again, oversized equipment will short cycle, increasing power consumption due to the frequent starts.

  • Oversized air conditioning will not properly dehumidify the air. You’ll get complaints about your customers feeling clammy or printers grabbing multiple sheets of paper on each printing order. You had better be adding UV sterilization to bathe the evaporator coil surfaces to prevent mold growth!

  • Ductwork, both metal and ductboard, will grow mold under damp/wet conditions. You think it’s expensive to replace the equipment when upgrading if the customer is keeping their old on/off HVAC units? You might want to put a lawyer on retainer!

On the other hand, the homeowner(s) could install new HVAC equipment that utilizes outdoor reset to modulate comfort-energy output that is right-sized for most of the heating/cooling seasons. Given that this will reduce their energy consumption by, on average, 30% to 50%, if they’re wise, they will bank the difference and save up to make improvements to the envelope.

  • Heat pumps and central air conditioning options today are virtually unlimited for utilizing variable-speed inverter-driven equipment — both unitary central ducted units and room-by-room mini-splits.

Inverter-driven motors use a fraction of the energy those older induction motors used. Efficiencies of higher-end inverter mini-splits can rival geothermal performances. For buildings with a diversity of load, VRF inverter systems can provide simultaneous heating and cooling by diverting refrigerant within the system. Refrigerant is miserly metered to provide just the right number of Btu, fan speeds modulate and the equipment sips just the energy required to meet the building’s heat loss/gain at that moment in time.

  • Modulating condensing boilers and combi-boilers can utilize outdoor reset to constantly alter their Btu output and water temperature — constantly adjusting to the home’s needs for comfort.

  • Furnaces can be multi-stage or step-modulation to continually adjust their output based on the home’s actual heat loss at a given moment in time.

 

Swaying the skeptics

Still skeptical? Indeed! I detailed a real-world example.

The homeowners had an antique boiler, which a heat-loss calculation illustrated was oversized. They planned to upgrade the windows and doors as they could afford to in this old mansion. Single pane, metal sash windows with no storm glass! Drafty old doors.

We installed a modulating condensing boiler, divided the three floors into multiple zones, installed ECM circulators and 1-watt zone valves to target both the fossil fuel and electrical consumption sides. The immediate result (after a winter’s usage while comparing heating degree days) was a 35% reduction in natural gas consumption. As they upgraded the windows and doors over the next three years, their fuel bills continued to be lower despite the cost for natural gas rising.

You see, the boiler was resizing itself with each envelope upgrade and able to operate using lower water temperatures, thereby increasing its operating efficiency.

In other homes, and especially in a number of churches, the owners have experienced dramatic reductions in energy consumption following the installation of inverter-driven equipment for their heating and/or cooling. As much as a 70% reduction! That leaves money in their budgets to target envelope upgrades.

And then I shut up, praying the instructor would give me a passing grade after the test! He did.