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HVAC System Duct System Problem

I was recently called out to a home to look at the second floor heat pump system. The homeowner was asking about upsizing the unit as the floor never cooled well.

The current unit was a 2.5 Ton Trane split system with the air handler located in the attic. Insulated ductwork, pretty standard installation.  While I was there, I looked at the downstairs unit too. It was also a 2.5 ton split system with the air handler in the basement. Total house about 3,400 square feet.

In inspecting the ductwork, I was able to count 7 separate 6” supply runs to the second floor. In contrast, the first floor unit had 11 separate 6” runs.

I also took the time to perform a load calculation on the home. We’ll cover load calculations in depth at a later time, but for now, it is enough to know that the load was calling for a 2.0 – 2.5 ton unit in the second floor – and the same in the first floor

Let’s convert all of this to BTUs. 1-Ton = 12,000 BTUh. That is British Thermal Units per Hour or enough heat to raise 1lb of water by 1 degree Fahrenheit.

At a certain temperature rise you can expect a duct to carry a consistent heat energy at a specific CFM or Cubit Feet per Minute. Think of CFM at air quantity. Given that heat pumps have a very consistent delta T (or temperature change), most HVAC contractors focus on the CFM rating.

So, here is the easy math: 1 ton = 12,000 BTU = 400 CFM.  This client’s 2.5 ton unit (30,000 BTU) needs 400 x 2.5 = 1000 CFM.


Now let’s look at those 6” runs. We would need to size the truck work too, but we are going to focus on the distribution runs here.

  • With a static pressure of 0.05 – 0.07 per 100ft (Static pressure is how hard the fan is pushing) which is what residential equipment is designed for, we get 80-95 CFM per 6” line.

If each 6” run moved 85 CFM and there are 7 runs, then we have airflow consistent with 595 CFM.  See the problem? Even at just a 2.0 ton load, we would need 800 CFM. Important rule here.  It doesn’t matter how big is your equipment. I could put a 5.0 ton unit in for this house and the home is still not getting more than 595 CFM. It’s like trying to squeeze an almond down a drinking straw. It just doesn’t fit.

The downstairs unit had 11 runs. 11 x 85 =935 CFM – much closer to the 1000 CFM we are looking for and more than enough to satisfy the load for that part of the house.

To fix this problem, I employed a two-prong solution: First, after identifying that one of the second floor bedrooms was particularly short (and the customer confirmed it was hottest there), we decided to add two additional runs in that room.

9 runs x 85 CFM = 765 CFM. Still not ideal, but a lot closer and consistent with the load for the home. And if you employ a slightly higher static pressure (and thus higher CFM per run), we are fine.

Second, we deployed a communicating variable speed heat pump system that will operate between 25%-100% of its capacity, constantly ramping up and down to meet the demand of the zone. The communicating thermostat will maximize the output of the duct system, constantly adjusting the fan pressure. And with much longer operating cycles, the system will continuously push out conditioned air and thus better balance the temperature across the zone yielding better comfort.

End result, more often than not, the problem lies not in the size of the equipment, but in the size of the ductwork.

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