Why is my AC freezing up?

What ice on the coil actually means

The evaporator coil is cold by design. It freezes when it gets colder than it should, or when too little warm air passes over it to keep it above freezing. Everything below is one of those two.

The cost figures are Fixr's. The causes are This Old House's, except the dirty coil, which both name.

Start with the free one

This Old House is explicit:

if there's a blocked return-air filter, there's a good chance the coil will start [to freeze]

Its repair sequence begins with exactly that: clean or replace the return-air filter so it is not obstructing the flow of fresh air to the air conditioner.

Our note, marked as ours: this is the only cause on the list that costs nothing to rule out, and it is first in the professionals' own order. Do not pay anyone to find it for you. If the ice comes back after a new filter, you have learned something worth the five minutes.

The refrigerant answer, and the question to ask

A system low on refrigerant runs its coil colder than designed and ices up. Fixr prices a recharge at $100 to $600 and a leak repair at $200 to $1,500.

But Fixr also states plainly that an AC unit should never need a refrigerant refill unless a leak has been fixed or the system has been opened for a major repair.

Our conclusion, marked as ours. On a frozen coil, "it needs gas" is a diagnosis that is only half finished. The refrigerant went somewhere. A recharge will clear the ice and the ice will come back. Our AC not cooling page works through the same point for the wider symptom.

The cause nobody expects: the unit is too big

This is the part of the This Old House guide that is hard to find anywhere else, and it is counter-intuitive enough to be worth the whole page.

If the condenser is too big for the house, it can cause ice to form on the evaporator coils and shut down the system.

Its sizing rule:

Typically one ton of cooling is needed for every 500 to 600 square feet of living space.

Our arithmetic, marked as ours. One ton is 12,000 BTU, so one ton per 500 square feet is 24 BTU per square foot, and one ton per 600 square feet is 20. That is a third rule of thumb, and it is the most generous of the three we have found. Our sizing page sets it against Fixr's published table, where the implied figure runs from 18 down to about 13 BTU per square foot as the house gets larger.

Our multiplication, from their rule. Use it as a ceiling, not a target. If the unit on your wall is above the top of your row, oversizing is a live explanation for the ice, and This Old House's own expert says so: "oversizing is one of the biggest issues in air conditioning".

What fixing that actually involves

This Old House's remedy for an oversized system is not an adjustment. Its sequence runs: pump out and collect the refrigerant from the existing too-large condenser, disconnect and remove it, install a new appropriately sized condenser, then remove the old evaporator coil and replace it with a new coil that matches the tonnage rating of the new condenser, reconnect and reseal the ductwork, braze new copper connections, insulate the suction line, re-pipe the condensate drain, and recharge.

Our reading: that is a new system with the ducts left in place. Fixr puts a central air installation at $8,750 on average, in a range of $5,800 to $14,400. Our central air page has the full figures and our evaporator coil page covers the coil half on its own.

So the honest order of expense on a frozen coil is: a filter, then a cleaning, then a leak repair at $200 to $1,500, then a coil at $400 to $2,700, and only at the end a system that was wrong from the day it was fitted.

While it is frozen

Neither source gives a step-by-step for the ice itself, and we are not going to invent one. What both make clear is that the system cannot work while the coil is iced, and that running it in that state is what puts the strain on the compressor. Our AC compressor page has what that part costs, and it is the most expensive line in Fixr's table at $600 to $3,000.

Before you call

1. Change the filter and wait. It is free, it is the first thing the professionals check, and it is the most common cause.
2. Look at the outdoor unit for leaves and debris blocking the fins.
3. Find the tonnage on the data plate and check it against the table above. A unit well over the top of your row changes what the ice is telling you.
4. If you are told it needs refrigerant, ask where it went. A recharge without a leak repair is a seasonal expense, not a fix.
5. Ask for the coil and the condenser tonnage to be checked as a pair. This Old House replaces both for a reason: a mismatch is its own fault.
6. Check which refrigerant it uses before agreeing to a recharge. On an older system our Freon page has what that changes.