Thermostat is wigging out...

[scruit]

I have a Honeywell thermostat which is prgrammable for weekday (wake, leave, return sleep) and weekend ( wake, sleep) cycles. The thermostat is mounted at normal height above a return. The wall does not get direct sun and there are no electrical appliance venting hot air near it. I don't know if there is a return running vertically through the wall cavity where the thermostat is mounted, but I can check... Warm return air from the upstairs rooms heatign up the wall where the thermostat is mounted may explain some of the following...


Recently I've notice the following behaviours:

1) The system will shut off during the cool cycle sometimes 3 or 4 degrees before reaching the set point. The little "Cool" display on the LCD disappears, so it's like the thermostat stops calling for cool rather than some kind of safety/thermal cutout. 3-4 minutes later it'll show "Cool" again and the AC will fire up again an continue trying to reach the setpoint.

2) When the thermostat is already at the setpoint (74deg) it'll call for cool every few minutes even though the temp has not risen. The AC will run for about 2 mins then shut off. Does this every ten minutes or so, off and on.

3) I can look at the thermostat displaying 74, and then 30 secs later it will show 76, then another 30 seconds later it will show 74 again.

I think the thermostat is either being affected by warm air from something near it, or the thermostat is just bad. It incorrectly senses the temp too low for a moment and thinks it's reached thesetpoint/stops calling for cool, or it incorrectly senses the temp too high and calls for cool.



Are these behaviours normal, or should I look at having the thermostat replaced?

[Jultzya]

How is it that all this is just now happening???????

Here are your threads, here, here, and now this one?
(all on the same day)

[henline_hvac]

I think your thermostat problem is a direct result of poor air flow across the evap. from the duct cleaning. the original hvac that charged our suppossedly (low system)just created more head pessure at the evap.helping to cool more from the poor air flow.I think you now have an overchaged system and the high pressure cut off is cycling the the condenser.

[Jultzya]

Originally posted by henline_hvac
I think your thermostat problem is a direct result of poor air flow across the evap. from the duct cleaning. the original hvac that charged our suppossedly (low system)just created more head pessure at the evap.helping to cool more from the poor air flow.I think you now have an overchaged system and the high pressure cut off is cycling the the condenser.

Did you miss this?

Originally posted by scruit
3) I can look at the thermostat displaying 74, and then 30 secs later it will show 76, then another 30 seconds later it will show 74 again.

Your diagnosis is not going to cause this issue!

And who says whether his system even has a high pressure cut-off?

[henline_hvac]

maybe you should take a stab at the problem

[Jultzya]

Originally posted by henline_hvac
maybe you should take a stab at the problem

Can't, I'm waiting for the "Rest of the Story" to reveal the real problems... seems he has several issues with this system.

The way things are going, he needs to start at the beginning and tell EVERYTHING, just like it has happened.

(we can't help DIY, due to site rules )

[scruit]

I'm not asking for DIY advice - just need to know if the thermostat behaviour is normal or not. If it's not then I'll have a HVAC tech come out and replace it.


As for 3 posts in one day... This has all been happening over about a month.

1) HVAC not cooling, so I call a tech who reams me for a $200 blow-and-go, but at least the system starts cooling better

2) 2 weeks later I call Sears out to clean the ducts, and after the leave I notice poor cooling again. After trying different things (like replacing the filter again, monitoring inside vs outside temps etc) I finally check the evap an realise it's full of crap. I clean it and the cooling performance returns

3) But now I've spent hours over the last two weeks monitoring the thermostat, so that's how I noticed the strange behaviour.


Question: When a thermostat calls for cool it displays teh little "Cool" icon on the screen... Now, if the A/C is unable to supply cool due to a hi-lo pressure safety cutoff or thermal cutoff etc, then will the thermostat realise this and stop calling for cool, and will this turn off the "Cool" icon on the lcd? Or will the thermostat continue to to call for cool and continue to display "Cool"?

Again, I'm not asking for DIY - just asking if this is normal or not. I don't wanna pay a call-out charge just to hear; "That's normal" If it's not normal then I'll have a tech come out and check it over.

[filterchanger]

Its not normal, the t-stat will still call for cool if any safety opens in the condensor.

[TheDuke]

.Here is something u can try with the thermostat to reset it to factory settings then see if it works OK.
First turn off the emergency switch on the wall for the furnace, blower.
Step 1.
Now take the batteries out of the thermostat.
step 2
Now put the batteries in the thermostat backwards.
step 3
Now take the batteries out of the thermostat.
Now nothing will show on the display.
step 4
Now put the batteries back in the thermostat correctly.

step 5
turn on emergency switch

OK now it is reset to factory settings, turn it on give it 30 minutes see if it working OK.

One more thing pop thermostat out of wall mount before u do above with emergency switch off, put insulation in hole in wall at wiring coming out block draft.

[amigadude]

This is pseudo-normal for a Honeywell.

At best, I'd suggest trying TheDuke's reset suggestion, and/or change the batteries. My <limited> experience with Honeywell programmables is that they have a mind of their own, at least sometimes. I've programmed computers for a living, so I almost understand their seemingly backwards logic.

Here is my best guess at what it is trying to do, based on your three observations.

1) Based on previous experience, the Tstat attempts to guess at how much cooling will continue after it stops calling for cool. (The idea works much better during heating season, but who knows...)

2) I'd guess that there is a small change in temperature, that does not register on the display. Like maybe its really 74.5, but it shows 74.

3) Digital technology is wonderful, isn't it? Happens to almost ALL digital thermometers, if you watch close enough.

Here is what happened to me just today on two different tstats:
System 1) Temp set at 75, changed to 74 to force system on for a quick check. Reaches 74 and shuts off faster than I wanted so I change to 70. Will NOT call for cooling, just sits there laughing at me.

System2) Temp set at 76, change to 75 to force sytem on and it now reads current temp as 77. Hit RUN to get back to normal, and it says the current temp is magically 76 again. Note: no air has even blown yet!.

Bottom line: They are great when they work, and quirky always. Changing batteries and/or reseting them seems to fix most things. But that just my opinion, and I'm sticking to it. :_)

[knightofoldcode]

Being in the computer business, as well as the Electrical side, I'd suggest the same as AmigaDude, as in reset it, and this *MAY* be normal.

However I would NOT suggest putting the batteries in backwords. Look up how to reset your thermostat on it's own. Putting batteries in backwords without the thermostat saying to, may void warranty's, and dammage things. *IF* the thermostat is a GOOD quality, this won't be an issue as it would have a diode, however cheap ones dont. Don't risk it, just look up your thermostat's manual and find out how to reset it on that method.

I'm not certain, but I wouldn't be at all suprised (if the thermostat is of good quality) that the tempurature inside might be 76, while it says it's 74, and it only kicks in once it reaches 77, or something similiar, however it SAYS it's 74. I've seen things like this done on other consumer products. Kind of like a "auto graph" or a "Snap to grid" type thing?


Just my $0.02.
Knight.

[theredneckrev]

There should be a reset button inside the t-stat. Take off the face cover and it should be around the display.

[54regcab]

Check and make sure the hole where the wire comes through to the thermostat is plugged up. Air can leak through the hole and affect accuracy.