Home A/C Questions...

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ScottB

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Feb 18, 2006
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Hey guys,

I just got central heat & A/C put in my 100-year old house.
Here in Pasadena, we generally hover in the high 80's to low 100's in the summer.
My question is this: it gets to about 74/75 and then the cold air shuts off and the fan just blows. I will come back to cold after a while, but I'm confused about this. I called the company, and we bought a very high-end system, and one that's overkill for our square footage. He told me that the system can't keep blowing cold air as the unit will freeze-up and cause issues. Now I know a system has to cycle, but it seems to me that it should be shutting off, not continue to blow "ambient" temp air?
In my car - and I'm flat-out admitting I know very little about A/C, but in my car, when the system cycles, it's still cold.
Am I getting bum info, or are home systems different?

Thanks,
Scott
 
Lord, I don't know. It's of the latest design, I only know the "seer" number and the brand...
I'll have to look that up and post back.

Scott
 
The compressor cycling and the blower are two separate functions. Just like hot air heat. You're gonna reach a min (max cold) setpoint on the evaporator ..but your thermostat may still be on the high side.

It's not always wise to get more cooling then you need. If you get too big a unit ..it's just like getting too big a boiler. The cycling is so spread out that you overshoot your setpoint ..waste energy and don't remove enough humidity from the air.
 
Your company is full of crap. It will not freeze up if it runs too long- assuming it is working properly. I have a Carrier Infinity system in my house and on hot days it will run almost all day on low speed. It keeps the humidity between 42% and 46%.

Also, your thermostat may be set to run the fan all of the time.

When you say the system is overkill, what do you mean? Is it actually larger than necessary? Oversizing an A/C system is not a good thing.
 
The system is slightly larger because we are going to add-on in the attic. Only slightly larger than would be "normal"

Setup is:
FAN: Auto
Heat/Cold: Cold
Thermostat: 72°

As to the insulation, that's a fine question, and maybe 1/2 of the house is not well insulated.

All that said, why does my car blow cold air until I can't stand it and this until only blows cold but not until I have to turn it down?

Thanks for all your help so far...

Scott
 
quote:
Originally posted by Gary Allan:
It's not always wise to get more cooling then you need. If you get too big a unit ..it's just like getting too big a boiler. The cycling is so spread out that you overshoot your setpoint ..waste energy and don't remove enough humidity from the air.
Click to expand...
Gary is right on the money. Bigger is not better in the line of A/C. Better to have a smaller unit that runs longer and dehumidifies much better.

Do some basic troubleshooting. During times that you are approcching where yhou think the unit will stop working, mmeasure the temperature rise across the outdoor unit and the indoor unit. You can do this by getting one of those digital cheapie thermomometers with an outdoor senser. Put the sensor on the top of the outdoor unit and get the max temp and then put it at the bottom to get the inlet air temp. You should have around 25 degrees of temp rise.

Do the same for the indoor. Pick an air outlet close to the air handler and measure the temp outlet and then measure the return air temp. Again it should be about 25 degrees.

Now if you can get the temp of the larger freon pipe out of the indoor unit by using the temp sensor used above and insulating it so it touches the cold coper pipe you should be able to get a temp of it. It should be maybe 42 to 45 degrees. (about 10 degrees lower than your cold air temp.)

My guess is that you may have too low air flow (dirty filter or possibly a bad phase coil on your air handler fan). You also might be going out on low pressure - possibly have a low freon charge.
 
It is not normal for a home A/C to have the blower run continuously. The furnace blower is controlled by a relay. Usually 2 relays for different speeds for heating or cooling. The relay could be sticking, or the unit wired wrong. If your installer won't fix the problem, I would complain to the manufacturer about their local representative.
 
quote:
Originally posted by ScottB:
All that said, why does my car blow cold air until I can't stand it and this until only blows cold but not until I have to turn it down?
Click to expand...
Your car's AC doesn't have a thermostat to tell it when to stop blowing cold air. The temperature adjustment knob on a car AC isn't a thermostat control, only an air temperature control. To make the air warmer, it passes increasing amounts of cold air (that you just paid to cool down) over the heater core.

I have no idea what kind of central air unit you have but here is how most of them made in the last 15 years work, assuming that the fan is set to auto. The compressor is in the outdoor unit. If the fan in the outdoor unit is running, then so is the compressor.

1)Temperature exceeds setting on thermostat.

2)Thermostat turns on the fan and the compressor.

3)Temperature drops to setting on thermostat.

4)Compressor turns off but fan continues to run for a minute or so, then turns off. (This is done to improve efficiency, since the unit will continue to blow cold air for at least a minute after the compressor is turned off).

If you leave the fan set to "on" the unit will cycle the compressor on and off as needed to maintain the set temperature but the fan will always continue to run.

This is apparently not good for humidity control, as condensate on the evaporator coils (that's the part that gets cold inside your indoor unit) will be evaporated off if the fan continues to run (as opposed to dripping down the drain which is what would happen to it without the fan running).

For that reason, it is recommended that the thermostat be set to "auto".

If your unit always has the fan running even when the switch is set to "auto" then I suspect an installation error. Does the fan turn off when you set the switch to "off"? If not then there is definitely a problem.
 
Freezing/ice issues will occur if you have poor airflow across the evaporator/cooling coils. Changing your air filter(s) regularly is the key. Flat fiberglass (usually blue) filters last about a month, pleated filters will last about 2-3 months, unless it is an oversized deep pleated filter, like an Aprilaire, which can last up to a year.

I have a high end cooling/heating system. With a new house (lots of construction dust) and Spring pollen, I turn my fan "on" whenever we are home. My AC will blow ambient air when the temp drops below 1 degree of the thermostat's current setting, otherwise it's always blowing conditioned air. If I have the fan "off" and set only to "cooling", it never blows ambient air.
 
I'm in the A/C business. I believe California requires a heat load calculation be performed to properly size the equipment. An oversized system may not control the humidity level correctly. The 3M filters are too restrictive for most installations. 16 SEER units require 2 stage thermostats and proper setup programming. It's good you went with an R410a system. I hope the installation company sized the ductwork and linesets correctly. What model # air handler did they install.
Aloha
 
Ours is 14 seer.
If those filters are too restrictive, how can they sell them?

Thanks for the info - this is all new to me and more confusing than I thought it would be
smile.gif
 
OK, I think this is wrong:

Thermostat set to 71°
It blows air, but it's not cold.
The house is at 75°
Set to: 71°
Fan: Auto
Heat/Cold: Cold

So, based on what you guys said, it should be blowing cold or nothing?

Scott
 
Are you saying that this system NEVER shuts off? It never reaches the setpoint that you dial in on the thermostat??

That is, when you set for 72F ..it shuts off @ 75F

Is it digital or one of those dial type?

If you set for 65F ..does it get to 72F?

If so, then it's not a system problem ..it's a thermostat problem.
 
It doesn't appear the house can get below about 74° when the outside is in the 90°'s+
AS was alluded to, insulation of the house (or lack thereof) contributes to that point, but if it gets to the set-point it will shutoff.
Now, per what I've interpreted above, I think that if the temps can't be reached, and the air is coming out ambient, and not cool, then it should shutoff?

It's a digital thermostat.

Scott
 
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