Dealing with a Problem Home, A ”Basket Case” Case Study

When we approach problems, we have to determine whether the HVAC equipment or the house is the source of those problems. Ideally, HVAC contractors would be able to act as a consultant that refers customers to people who can solve the problem.

The case study in this video took place in Miami and dealt with a house that was too warm; it didn’t maintain the set point and had a high indoor RH%. To begin diagnosing the problem, the technician had to have a productive conversation with the customer and understand what the diagnostic tasks really accomplish.

In a case like this, a visual inspection would let us see if ducts were leaking, but there were no obvious duct leaks in this case. The design review would allow us to determine if the equipment sizing is correct via a load calculation. (In this case, the sizing was almost perfect.) System airflow and TESP measurements can let us know if the equipment is performing properly (it was normal). House pressurization gives us an idea of the HVAC system’s impact on the house pressure; you use the HVAC equipment rather than a blower door to manipulate the pressure (the difference was 2 Pa, which indicated a supply duct leakage problem).

Room pressurization tests weren’t applicable, so the technician had to go straight to the envelope (blower door) test. He found out that the house was leaky but not unusually leaky for a house of its age in Miami. (Plus, the Manual J accounted for the leakage but NOT the house’s depressurization by the system.)

TEC’s See Stack simulation tool relies on indoor temperature, outdoor temperature, home height, and known leakage to estimate how much air leaks in and out (in CFM) and the house pressure at the highest and lowest points of a home. It also accounts for CFM exhausted from the home due to the depressurization leakage unaccounted for by Manual J. In this case, instead of 102 CFM (predicted by Manual J), the ducts actually had 384 CFM of leakage. The house sucked in hot air when it was depressurized, meaning that there was a larger load than what the equipment was sized for; the house was losing capacity AND increasing the load (especially the latent load) all at once.

Overall, the house was covering only about 41% of the latent load and 74% of the sensible load. Manual J didn’t account for the extra load from depressurization, so the “perfect” sizing was actually very far off. Most of the loads tended to stay consistent, but the infiltration load was variable and had the potential to cause severe comfort problems inside the home.

A larger system would NOT have done the trick because it wouldn’t have done anything to correct the leakage (depressurization would get even worse, sucking more hot air in from outside). Putting in a 5-ton unit would have helped the clients’ comfort only a little bit. Instead, the recommended solution was to seal or fix the ductwork.