Hi Velocity AC freezing problem

[bparizadehhvac]

Hello,

I'm a home owner in the Toronto area, hoping to get some help from hi velocity experts as I've been through several service calls and no luck. I am having issues with freezing lines on my AC. There are two units and they both freeze after different run times. They were installed 5 years ago. No issues keeping temp for 3 years. Noticed last year the flow from registers reduced and temp was not going down and freezing on the lines outside. On one of the units, I had the outside AC and evaporator replaced. It did not fix the freezing problem. Currently, my upstairs unit freezes after 30 minutes of running and my main/basement unit runs for 1.5 hrs before it starts freezing. My air handler is HE-B101. There is a 4 ton unit outside and the evaporator is on the supply side of the air handler. I have approximately 30 registers for each unit and all are open.

I've been doing some reading and it seems you need to have the evaporator on the return side of the air handler for hi velocity systems. The people who installed my system put it on the supply side, and the air handler companies say that will never work, but the thing is it worked for 3 years. I just don't get what could have caused the issues that still persist.

Before going down the costly road of changing out evaporators, Im hoping someone can shed some light and help me get to resolution.

Happy to provide more information as well.

Thank you.

[Sergroum]

Its a matter of static pressure.  Icing up with high velocity is pretty common. Regardless of where the evap. Coil is. 

Do you have a freezestat?  Thats usually how thats controlled. Multiple reasons, dehumidification is one of those.

[bparizadehhvac]

I believe there is a freezestat on the suction line coming out of the evap. It wasn't connected to the air handler board until recently, and I've confirmed it kicks in, but it seems to kick in long after solid freezing on the lines outside and freezing on both lines coming out of the evap. It would be ideal if the system takes a break soon after freezing on the outside lines as the flow starts to reduce and the air temperture is not very cold anyways.

One observation. Today is the hottest day so far, and the main floor system has been running for almost 4 hours without freezing (previous was about 1.5 hrs in the last week). Is this a result of static pressure you mentioned in some way? I looked up the concept, but I don't fully understand how it applies to my situation.

Either way, outside of a freezestat, are you suggesting the system operates within normal bounds? should I be doing anything else to improve freezing? I can't quantify it but it does feel like the system isn't as efficient as a few years ago. I currently am scared to push it past 78.