Inconsistant Thermostat Reading

[Microcell]

For some unknown reason, the temperature reading on our thermostat actually increases when the AC comes on. Therefore the AC will never shut off unless we do it manually. At other times, it will show a low temperature when we're burning up and a high temperature when we're freezing. We've tried changing out the thermostat and have had the same problem. In fact, I set the old one on top of the new one and they showed similar readings.

The thermostat is located in a hallway connecting a bedroom, bathroom, basement, living room, and garage.

Any thoughts?

[LazyPup]

In all honesty the dumbest thing the thermostat manufacturers have ever done is to put a highly accurate fast responding thermometer on the display. As a result people are continually looking at the reading and quibbling over a degree or two difference in the readings.

In truth, no matter how efficient your heating or AC system is, if you put 100 accurate thermometers at different locations in the house, no two of them will have exactly the same readings at any given moment.

Why? Because air tends to stratify in differing temperature layers throughout the structure. By the physical laws of nature Hot air rises and Cool air falls. Just the motion of people or pets moving within the structure will then cause shifts in those layers, combine that with drafts that may enter the structure, ceiling fans, Kitchen exhaust fans and applinces such as vacuum cleaners, laundry dryers or cooling fans on the refrigerator coils as well as the physical force of the HVAC blower unit forcing one temperature of air into the room as it draws the opposite out and it quickly becomes apparent that the temperature will never be perfectly steady at any given point.

In the trade the room temperature display on the thermostate is technically referred to as a "Convenience Thermometer". It has no actual bearing on the control circuit of the Thermostate, but rather it is intended solely to offer a convenient estimate of the average room temperature. In the past the convenience thermometers were intentionslly the liquid mercury or alcohol vial type, or slow reacting bi-metal indicators which displayed the room temperature which was averaged over a 3 to 5 minute period.

With the advent of digital progamable thermostates also came instant reading, infinitely accurate thermometers. As a result we now hear people complaining that they have the AC set on 68 but the room temp will not drop below 69 before the unit shuts off. While the room thermometer display is capable of reading 1 deg increments, in truth, exhaustive scientific studies have confirmed that the human body is not capable of consistantly sensing a temperature difference of 3degF. or less.

We must also remember that short cycling HVAC equipment can and will cause serious damage to the equipment, therefor the actual control thermostats are calibrated to be accurate to within +/- 3 degF.

Now to answer your question, why does the temperatute go up when the AC first starts?

When the system is off the air in the room is static, therefor the heat rises to the ceiling. At any given time the actual temperature at the ceiling will be 3 to 8 deg warmer than the temperature at eye level in the room and maybe 10 deg warmer than the air at the floor level. When the AC starts all those thermal layers begin moving and as the hotter air at the ceiling is pulled down to the return air duct it passes the thermotate causing the convenience thermometer to show a temperature increase.

In fact, the air stratication layers are so great that often just raising or lowering a thermosate mount 10" to 12" will make a total difference in the perceived comfort range of a room.

[Microcell]

Thank you for your response.

Upon further reflection, it occured to me that our AC is drawing air into the basement through the open basement door (right by the thermostat). So much so that the door will close by itself if it is only open a small bit. Perhaps it was causing the system to pull hot air from the top of the house down and past the thermostat and into the basement door. I've closed the door and that seems to have helped.

What I'm wondering now is why you'd want return air ducts on the lower level of the house at all? Wouldn't it make sense to pump cool air into the bottom of the house, push the hot air up and draw it off on the top levels?

[LazyPup]

For absolute peak performance a system should be desinged to take advantage of the laws of nature. Given that cold air is heavier than hot air, AC should tecnically be discharged at the ceiling and the returns at the floor. In that manner the heavier cold air is falling down into the comfort zone and pushing the hot air toward the returns.

During the heating cycle the system should be reversed, with the heated air now being discharged at the floor where it will naturally rise up into the comfort zone.

[Hube]

In COOLING mode it is best to have Return air located at the UPPER wall location (the warm air rises)of a room.
High wall and low wall returns can be incorporated within the same stud space. Seasonal control is accomplished by a damper in the low grille. When this low damper is open ,air is blocked to the top grille. When this low damper is closed, air is open to the top grille.
Thermostat error can occur when the air is "stagnant" (not moving)that's why its best to have the units low speed fan running continuously. It will prevent this fluctulation in the t stat.

[Microcell]

quote:Originally posted by LazyPup

Given that cold air is heavier than hot air, AC should tecnically be discharged at the ceiling and the returns at the floor. In that manner the heavier cold air is falling down into the comfort zone and pushing the hot air toward the returns.

It's true that I'm completely uneducated about HVAC so I'm probably wrong but this just doesn't seem plausible to me. If I picture a glass of warm water that is dyed red and imagine pouring cold blue dyed water into the glass, it would seem to me that the blue water would simply fall through the red water to the bottom of the glass, not push the red water down (though it would probably stir it up a bit). Wouldn't the same be true for air?

On the other hand, if I were to pump cold blue water into the bottom of the glass, I could see it forming a cold layer on the bottom and pushing the red warm water higher and over the top of the glass. Eventually I would be left with a glass of cold water and the warm water would be gone.

What am I missing here?

[LazyPup]

We must keep in mind that the temperature will never be constant at any location or level in the room, therefore we strive to maintain the desired temperature in the comfort zone.

If one were to get technically precise the comfort zone elevation also changes by they type of activity in a room. The comfort zone should approximate the elevation of the body core during normal activity, thus in a bedroom the comfort zone would be from 2 to 3 feet above the floor, level of the core of the body while lying on a bed.

In a living room the comfort zone is from 2.5 to 3.5 ft (The elevation of the body core while sitting on a couch or easy chair.

In the kitchen the comfort zone is from 4 to 5 feet (Elevaton of the body core while standing).

If you were designing a home with a separate zone for each room and if you wanted your control system to be technically perfect the center of the comfort zone would be the proper elevation for the thermostat in that room.

For typical residential applications the thermostate is placed at the center of the comfort zone for standing activity in the living room or den area. The Thermostate is not put in the bedrooms because when sleeping the body metabolism is lowered therefore the body is not generating as much internal heat and will tolerate a bit higher air temps. In turn, the thermostat is not put in the kitchen because it would get false readings from heat that is generated during the cooking process and the remainder of the house would then be too cold.

The system is then designed to maintain a constant air flow while keeping the temperature most constant in the comfort zone for that specific room.

Understanding that cold air is heavier, it stands to reason that it would fall. If we were to discharge the cooler air into the room at floor level it would remain close to the floor while warmer air would rise into the comfort zone, thus, even though the AC is working perfectly, the room would still constantly feel hot.

By discharging the cooler air into the room at the ceiling, the cooler air then falls into the comfort zone. In turn the air entering the room is displacing the air that is already in the room. With that in mind, the returns are placed close to the floor.

For each cubic foot of air that the air handler pushes into the room, it also draws an equal amount of air out of the room, thus with the returns at the floor there is a slighly lower atmospheric air pressure at the floor level which aids in pulling the hot air to the return grille and allowing the heavier cold air to drop to the comfort zone.

[Rewired]

We had a very similar problem in our house some years ago. (House is a 1 1/2 storey backsplit).. We found in our case that having a cold air return at the highest part of the house as well as one right at the basement floor cured all our problems with a very hot upper floor and a freezing basement... We leave it set up like that all year 'round. As well we have the furnace fan run 24/7/365. Mind you as it was mentioned above by Lazypup, all the rooms will never be the same temperature.. as is the case here but at least they are pretty close to being "just right"...

A.D

[HayZee518]

I've got a Monitor heater that I use for whole house heating. It uses a cold junction thermocouple to sense the room temperature. All it is is a bimetal junction, iron constantin and copperfused together where they meet. I can move this thing up or down a wall and see temperature differences at each 2 inch distances within a room. Just demonstrating what Lazypup is saying about air stratification at different levels.