A/C guys- I got a problem for you to solve
 

The facts: Typical split system home a/c unit, about 30 years old. Still has the old mercury t-stat. Works OK but I hold my breath every time the seasons change from heat to a/c.

Today's problem: My wife leaves the house at 7:30 am and I get back at 11:00am, I open the door and feels like it is over 100 degrees in the house as if the heat has been on. Poor dog is looking at me with her tongue hanging out. The unit is running but the air is HOT. The stat is set in "auto-cool" position. I turn it to "off" and start looking for the problem. I find nothing. I wait a hour and turn it on. COLD AIR! It has now been running for about 4 hours and has cooled the house down to a normal setting.

So WTF? Should I make the $1000+ phone call and get the service guy out?

sealed system is fine....sounds like a simpler control issue. Might be a bad thermostat control

That's what I was thinking too. Old t-stat may have somehow flipped to the heat mode?

30 years old? Seriously? Spend some money and get something efficient. My 7 year old unit was 12 SEER new and probably is 8 or so in the condition it is in. The latest and greatest stuff is 21-24 SEER. The better equipment has turndown, 2 stages of cooling so that it runs like a smaller unit when you don't need as much cooling.

A really good T-Stat is like $50 at Home Depot and a 10 minute installation.

I don't think you are going to know what happened until you monitor the T-stat and see that circumstance again. Why not buy simple digital heat/cool T-stat with no timer settings? That old analog mercury unit has got to be on it's last legs.

Hope it's not an internal problem, that could be more expensive and somewhat dangerous. You might also try to find a small independent service guy and have you high limit switch checked out amongst other things.

I would love to rip it out and install the latest and greatest. BUT, I'm out of work right now.....:(

Try this...
Sneak up on it and see if you can catch it while it is performing this mysterious heating mode.
If you come in and you think that your house has moved next door to the depths of H E L L, fist go look at the condensing unit and see if 1. The compressor is running. If the condensing fan is running and the compressor is not, the chances are good that it is off on an internal overload or inherent protection device.
This may or may not be a bad thing. It may be tight cycling and the compressor may not be starting against pressure very well. A simple delay on break timer may take care of this problem.
It may be the start capacitor and relay may not be functioning correctly.
If the compressor is running along with the condensing fan motor, see if the unit is in the heat pump mode. Feel the line set going to the evaporator and see if one of them (large one) is hot. May be a control problem.
When it is running and cooling check the temperature of the bigger line back to the compressor. It should be cool/cold.
Start with this and let us know.

If it was heating the house the condenser outside would be blowing cold air would it not?

I some how missed the fact that the unit was 30 years old. I'm with billy on this one, I just design refrigerators.

billy, send him a bill for 1000 bucks

i would say first shot would be a new thermostat. that's easiest and cheapest option. only costs $20

What size is your unit? I found a company in your state which will install the 23 SEER IQ-Drive for $6500 (2.5 ton unit). I can't imagine the 4 ton unit would be more than $10K.

Probably gas furnace and separate A/C with evaporator in the furnace unit.

Somehow the heat kicked on. We don't know if it cycled off the high limit switch because he wasn't home. I'm guessing it did.

2.5 ton. Thanks but I won't be getting a new unit until the hopie-changie kick in.

I'm in central Florida. No furnaces 'round these parts.

Another possibility could be:

A low gas condition resulted in the evaporator coil icing up, which blocked the airflow and meant the waste heat from the air-circulation fan got dumped into the house all day.
If this was the case, turning off the system for a hour or so could allow the ice to melt and air flow through the coil to resume, resulting in a cooling system that works again...until the next hot humid day.

Just a guess.
Good luck
Les

Get a good digital programmable t-stat. You will wonder why you did not do it years ago. I made the switch last year and love it.

realize it's been 95 plus here in florida....wouldn't need the heat to be on to make it blow hot air...once the house got heat soaked it could've simply been recirculating hot air......
my last house when the a/c went out (2bd/2 story) and in 3 hours it was 90+ degrees.......
just saying......
maybe the line just froze up?

Good advice from all, thanks. I'm guessing low gas maybe an issue too. I'm looking online at T-stats. I'll do that first.

No heat? Wasn't it like freezing at this year's Daytona 500? Or is it a heat pump? Now those will go backwards and heat. If low on the gas, would it heat?