CFM, a factor to considered!

Some time ago, I had the chance to speak with a consumer who chose our products for his renovations. Here's what he told me:
"My wife and I have just purchased a new heat pump. A big 4-ton central to replace our little 2-ton heat pump. It'll give us the heat we need!"

While oversizing is a problem in itself, this family will face a bigger problem when it comes to start-up:
Static pressure! 

How can the correlation between static pressure and CFM (cubic feet per minute) affect the proper functioning of the system and the comfort of this family?

Understand the correlation between static pressure and CFM.

CFM represents the volume of air we move per minute, so static pressure is the pressure this movement exerts on our ventilation ducts.

To illustrate this phenomenon, let's imagine we're blowing into a small straw. Our cheeks swell because too much air wants to pass through the straw at the same time. This pressure on our cheeks represents static pressure, and the air leaving the straw represents CFM. CFM is therefore limited by the size of the straw.

If we blow into a larger straw, there will be much less restriction. As a result, more air will escape from the straw.
The same phenomenon occurs in the ventilation system of our homes.


If the static pressure is too high, two main problems can arise, depending on the type of motor we have.

The 2 main types of motor.

First of all, there are two main families of motors. Variable-speed motors and non-variable-speed motors. 
Variable-speed motors will automatically adapt to static pressure to give a constant CFM. Yes, this is perfect for ensuring the right number of CFM, but if the static pressure is too high in the ventilation ducts, this will have the impact of creating air noise at the diffusers. 

Non-variable-speed motors won't adapt to static pressure. Static pressure therefore has an impact on motor rotation speed, creating a drop in CFM the higher the static pressure is. So you need to be more careful when making your selection.

I often compare the two types of motor, one without cruise control (non-variable) and the other with cruise control (variable). With cruise control, the car will maintain its speed even if it encounters resistance, such as an uphill gradient. Without cruise control, however, speed decreases the steeper the gradient. 

That said, for a change to an existing system, we generally recommend the use of variable motors. They adapt more easily to existing systems and ensure a stable CFM. 

How many CFMs do we need?

The number of CFMs is determined by our equipment. Most of the time, it's the heat pump that determines the CFM required. As a general rule, we say 400 CFM per ton* for heat pumps. 
* 12,000 Btu = 1 ton

Is the power of my system limited by the size of my ducts? Why is this? 

Yes, the size of the ventilation ducts can limit the capacity of the equipment you choose, because the larger the capacity, the more CFM you need.

Let's go back to the example of our family in the introduction. Their ventilation ducts were probably built around their old 2-tone heat pump. By upgrading to a 4-tone system, they go from 800 CFM to 1600 CFM. 

There's a good chance that the furnace motor won't be able to push that much CFM through the small duct without creating ventilation noise in the house.

Note: as a general rule, you don't want to reach 0.8 static pressure.
 

How can the quality and cleanliness of my filters affect my system?

It's certainly a good idea to have a more efficient filter. However, we have to make sure that it won't impact too much on the static pressure of our system, and that we stay within an acceptable range.

A more efficient filter (just like a dirty filter) creates one more restriction in the system, so the filter will increase the static pressure in your ducts. This can limit CFM or cause duct noise.
 

Smart Duct® system, a turnkey solution for new buildings!

If you're not familiar with our Smart Duct System® for new construction, I suggest you take a look!

We offer a turnkey system with a free design by our team of engineers. 

This system is designed for easy installation. Paired with our modulating furnaces and heat pumps, it ensures a uniform temperature in every room of the house. You can even add a zone system.

Installation video : 
https://www.youtube.com/watch?v=7-HiuWsmjhc 

Here are two photos demonstrating air movement in a room. The Smart Duct® System ensures even temperatures at ceiling and floor level.



In the end, our customer opted for a 2.5-ton heat pump. This is more reasonable for the size of his ducts.  Thanks to the new, extremely efficient EVO heat pumps, in both summer and winter, he's able to keep the temperature in the house stable by using only the heatpump and do so efficiently all year round.

Happy installation!


You may also be interested in these articles:
How to choose the ideal central heating equipment?
Choosing Between a Traditional and High-Efficiency HVAC System: Which is Right for Your Home?

Louis-Philippe Lebel Boulanger
HVAC Expert and Support Technician