Bad Zoning Froze Your Coil. Here's What to Do

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HVAC zones are a great feature that can improve comfort. But they're only effective if they're properly designed and installed.

When they're not, they cause problems.

Perhaps the best kind of zoning involves multiple HVAC systems. Maybe you've got one system for upstairs and one system for downstairs. In theory, these systems were individually designed to meet the heating and cooling needs of just one area of your home. If this is the kind of zoning system you have, you're probably in great shape.

Another great option is to use variable speed HVAC equipment with communicating zoning boards. These modern systems do a great job delivering just the right amount of conditioned air to different areas of your home.

But a lot of homes have multiple zones for just one standard HVAC system.

This kind of system can work, but it's problematic. A while back, we published an article about good zoning, bad zoning, and bypass dampers, which goes into a lot of depth about these systems.

The long and short of it is that single HVAC systems split into zones nearly always require dampers that cut off air to one zone while another zone is being heated or cooled. What's bad about that is the fact that one or more zones will receive too much air any time another zone isn't receiving any air.

To address this problem, HVAC contractors usually install bypass dampers. These dampers redirect the excess air back to the system's return, relieving static pressure.

Except that when you do this, you risk creating a whole other problem.

The excess air can freeze the coil.

And a frozen AC coil is really really bad. For one thing, the AC will eventually quit working. You're gonna be hot! And as the ice thaws, it can cause water damage to your home or destroy HVAC components.

Thankfully, there are ways to prevent this from happening! If you have a zoning system with a bypass damper and you're having problems with ice forming on the AC coil, here are some techniques our team uses to solve this problem and save your zones.

1. Install a freeze stat

A freeze stat is a device that measures the temperature of your copper refrigerant line. When the temperature drops below a certain threshold, the freeze stat shuts off the outdoor unit temporarily. The system doesn't totally turn off — the blower will still blow air — but the compressor will stop running.

After the temperature inside the line goes back up, the freeze stat will allow the outdoor unit to resume operation. Basically, the freeze stat prevents the cool air coming through the bypass damper from "overdoing it" and freezing up the coil.

2. Install a discharge air temperature sensor

This works similar to the freeze stat except that it measures the temperature inside your duct system, not the copper line.

We install the sensor inside the system's supply plenum. When things get too cold, it turns the outdoor unit off. After the temperature inside your duct system warms up again, the outdoor unit can start up again.

3. Send bypass air to the home

This really only works if the bypass damper is inside a mechanical room or otherwise inside the home's living space. Instead of sending the bypass air back into the return, we can set it up to release air into the home instead.

If you've got an unconditioned basement, we can sometimes dump the bypass air there — a good option if you've got a humid basement and want it to be more comfortable!

4. Discharge bypass air farther from the indoor unit

Most of the time, a bypass damper won't freeze your coil if it sends air to a part of the return that's far enough away from the unit itself. When it's possible to discharge the air at least 15 feet from the blower, this is a workable solution that's unlikely to result in a frozen coil.

5. Oversize the zones or combine zones

In some cases, an HVAC contractor can install larger ducts for one or more of your zones. As long as the ducts aren't so big that the AC can't cool the space, this approach can relieve the pressure inside each zone and minimize the amount of air the bypass damper has to relieve.

When possible, you might also consider combining zones. For example, maybe you have a very small HVAC zone in your home: a garage-level office, say. If you're willing to combine that zone with your home's living room/kitchen zone, doing so might sufficiently relieve pressure in the system and prevent the coil from freezing up.

6. Get a better zoning situation

All of these bypass damper strategies, while totally workable, are essentially band-aid fixes for less-than-ideal zoning arrangements. A better solution is to use staged equipment whenever you're ready to replace your HVAC system.

Two-stage and variable speed systems, when designed and installed properly, can be great platforms for zoning. And you shouldn't need a bypass damper at all!

The best solution depends on your home's zoning situation

That's the key takeaway here. When you have this problem, one of the above solutions is likely to be better or more workable than the others. To determine the right course of action, our team will visit your home to see what's going on with your bypass damper.

If you're having trouble with your AC and have multiple zones in a single system, give us a call today! We'll send an experienced pro to your home ASAP to diagnose the problem and find the best solution.