A/c freezes at night


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Old 06-10-11, 07:11 PM
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A/c freezes at night

A/C- heat pump is three years old, worked great first year. Last two years the a/c works great all day long, whatever the outside temp, but freezes up at night.

I've had two techs look at it and check charge and there doesn't seem to be a leak. Coils are clean and filters changed regularly.

How can it work great all day long and then freeze up at night? Any suggestions of where to start? Seems counter intuitive to me, but I know nothing about a/c.

Dave
 
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Old 06-11-11, 06:09 AM
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how cold is it at night? You can't or shouldn't run the unit below 65F outdoor temp in cooling. If you want to run the unit below 65F outdoor temp then you need a device called a head pressure control and a qualified technician would need to install this. If the outdoor temp is not below 65F and the evap freezes up then you have an air flow issue or refrigerant charge issue or a bad metering device.
 
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Old 06-11-11, 07:06 AM
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hvactechfw, thanks for the reply. It does not run below 65. In fact, it has been 75 or 80 when it starts freezing up at night, never below 70 all night long, and 90s during the day. It seems to run better (less freezing) when it is cooler.

As I said, it runs great all day long, no freezing. It seems the hotter it is the better. What I can't understand is why the harder it runs the less freezing, that just doesn't make sense to me.

When you say an air flow problem, are you speaking of the outside unit? I keep the coil and screens washed down and super clean.

The two techs I've had look at it seemed to have no idea why it was happening. They seemed responsible, but dumbfounded by the problem. Know any good A/C guys in Indy?

Dave
 
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Old 06-11-11, 09:47 AM
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what freezes up at night? the indoor or the outdoor unit? when it freezes up can you feel air coming from the vents and has the t-stat met it's temperature? Im thinking that the tstat is satisfied and shuts off but for some reason the a/c keeps running, causing it to freeze. tonight turn the fan to the "ON" position on the tstat and leave it all night, let us know what happens.
 
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Old 06-11-11, 01:03 PM
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No, the temp is not met. In other words, when it freezes up it has no cooling and the fan no longer is working blowing out air -- hot or cold.

I was talking with a couple of guys at the airport today who are very bright about things mechanical -- They both restore cars, build their own airplanes, plus one is a technician for Ratheon, working on helicopters, including their air conditioners. One suggested that there is a sensor that should tell the A/C to shut down when it starts freezing, which may be defective. The other suggested that since it is a heat pump the reversing valve might be defective.

Does any of that make sense?

Dave
 
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Old 06-11-11, 03:08 PM
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Not to me. Even if it had a sensor which it probably doesn't that still wouldn't explain why it is freezing up in the first place.

I'm not much of a heatpump guy but in my experience a bad reversing valve wouldn't cause a freezeup.

Any chance that the indoor motor is crapping out sometimes? Also the reasons hvactechfw gave are what I agree with.
 
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Old 06-12-11, 06:08 AM
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Turned cold last night, into the 50s, and A/C wasn't called on to work all night, so couldn't test the effect of having fan on. Looks like a week of nights in the 60s, so may be awhile before it is a problem. I'll report later.

Dave
 
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Old 08-01-11, 08:14 AM
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Problem solved! Found a guy who is an expert on cooling units in grocery stores who found the problem.

Simply, the belt tension on the fan motor was too lose, not allowing fan to turn at maximum because of slipping, which reduced air movement. I don't fully understand the physics, but somehow the reduced air flow effected the pressure, which allowed the line to freeze.

Since tightening the belt the unit works perfectly.

Dave
 
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Old 08-01-11, 08:25 AM
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No HVAC Pro....but I've never seen a belt driven air handler in a home. Most just have a fan directly attached to the motor shaft.

What you state would absolutely cause it. If the coil isn't warmed by air blowing across it (so the heat can be removed) it will freeze. Same thing happens when filters get extremely clogged or air to the fan is restricted some other way.
 
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Old 08-01-11, 08:54 AM
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GG, my last home had an American Std forced air gas furnace and it was a belt drive blower. I would switch pulleys seasonally for air and for heat.
 
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Old 08-01-11, 09:31 AM
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Ack! What kind of design is that? Having to change pulleys and or belts every season? Thats like having to change your mower blade depending on whether you are cutting the front or the rear of the yard....lol.
 
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Old 08-01-11, 11:42 AM
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I think their equipment is ancient. Like 1960's.
 
 

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