This video demonstrates how to use the Testo 410i and 605i to verify airflow and delivered capacity on a Mitsubishi ductless heat pump.
Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.
In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.
I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.
Good friend and contributor to HVAC School Neil Comparetto made this video showing the way in which he creates access ports for static pressure and gas combustion analysis. As techs we find ourselves in the tough position of needing to drill access holes to take measurements but the drilling and sealing of the holes can sometimes create real and perceived issues with the equipment. Many techs use high temp RTV Silicone, Rectorseal Duct Seal compound or even tape. In some cases these sealants may be appropriate but Neil shows how he uses plugs to make a good permanent access point. Always make sure to leave any work is a well sealed and workmanlike condition.
You can find many of these items at Trutech tools HERE
In this 60-second tech tip video by Brad Hicks with HVAC in SC. he shows us how and why to remove the weep port plugs on a condensing fan motor. I know from experience that motors can fail prematurely when this practice isn’t followed. Remember that motor orientation dictates which are removed. It (generally) the ports facing down that need to be removed and the ones face up stay in place.
When you first start checking your supply air with a thermo-hygrometer you may notice that the relative humidity is REALLY HIGH. Often the RH in a supply duct will be between 85% and 96% relative humidity on a system that is functioning as designed. The reason for this is fairly simple.
In order for dehumidification to occur the air must reach dew point, otherwise known as 100% relative humidity
Jim Bergmann explains it this way. Think of a sponge being like air and when it is fully expanded it is like the air in the return. When the sponge is fully saturated and can accept no more water it is at 100% RH and when it is completely dry it is at 0% RH. Let’s imagine that the sponge is 50% saturated and full size in the return. When that sponge (air) goes over the evaporator coil it is compressed, because colder air can hold less moisture. Once that air is compressed (cooled) enough it will begin to give up moisture. This point at which it starts to give up moisture is called dew point or 100% relative humidity. Once that air leaves the coil it still remains in approximately the same temperature state (compressed sponge) as it was when it went over the coil. This means that unless heat is added or removed from that air, it will remain at 100% relative humidity.
So why is it less that 100% RH in the supply?
There are several reasons why the air in the supply will be slightly below 100% in the supply. First is contact factor or bypass factor which are both terms used to demonstrate the efficiency of a coil at “contacting” the air. The greater the surface area of the coil and the longer the contact time of the air on the coil the more efficiently heat will be transferred from the air to the coil.
Because no coil is 100% efficient, there will always be some air molecules that leave the coil warmer than others, this causes the airstream to be warmer overall and decreases the RH of the air stream. You will notice when a system has a higher coil air velocity (speed) it will have a higher bypass factor (lower supply humidity). When you run lower coil air velocity the bypass factor will drop and the supply RH will increase.
There is also some heat added by the blower motor and possibly even the cabinet or supply ductwork. This added dry bulb heat results in a warmer airstream and thus some additional moisture capacity. Imagine a slight expansion of the sponge due to heat from the duct walls and the blower motor.
Once that supply air exits the duct and mixes with the room air it is allowed to “expand” again and the relative humidity drops below what it was initially. This is why supply air has a high RH in cooling mode.
Here is a video we did on the topic –
Recovery is the removal of refrigerant from a system to either store and send in for recycling or to reintroduce back into the same system.
Here are some top tips –
We cover all of this and more in this video –