Repair HunterFan digital thermostat?

On 2/27/2009 10:58 AM snipped-for-privacy@gmail.com spake thus:

You've probably already looked for this, but are there any adjustments (like trimmer pots) on the board?

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- lifted from sci.electronics.repair

In article , snipped-for-privacy@but.us.chickens says...>

There is often a "heat anticipator" setting that when whacked will throw the calibration off. It's purpose is to compensate for the thermal mass of the building and look at the rate of the temperature rise so it doesn't overshoot. Some are better than others.

Have you blown the dust out of this area?

Chris

On good thermostats the computer program adjusts the anticipation or really looks at PID. Some also have fuzzy logic.

A thermister uses resistance to set temp calibration and gain to adjust linearity. If any of these variables gets thrown off, they need to be adjusted. They would be a pot of some kind.The thermister is probably OK.

greg

What I meant about the good thermostats, they look at the temperature swing and adjust the programming to compensate. Its an on going process. There is no way of knowing by reading the specs i just did, to know how it opperates, unless it says.

Air currents around the thermostat can be tricky. The best method I have found is to measure the thermostat with an IR gun. But you said you dropped it, and I would suspect miscalibration.

greg

Thanks for all the replies.

There is no anticipator. It's a digital. No pots. No adjustments. There is a coil; a crystal; a relay; a box - not an IC- from what I can tell. Switches that (1) indicate gas or electric; (2) enable or disable (fuzzy logic) temperature swing compensation just before start of the next program. { its purpose to raise comfort level to meet the new setting on next program before the new program starts} And a span range is handled by keypad entry.

Anyway it's not about whether or not the t-stat meets the set programming cycles.

The thermostat is not reading room temperature correctly. I have 5 thermostats nailed to the wall now. Two old style HW round mercury with anticipators, and three digital style. And this one is off by the greatest degree.

At room temp. 70 it reads 72. The others read 70 -71. At 72 , the Hunter44360 reads 75. At 74 it reads 78. And so on.

There isn't much on the circuit board other than what I described and a few caps. I originally thought that the thermistor sensing varying temperatures would produce varying resistance and somehow translate that into the room temp display.

So if it's not the thermistor, what handles the reading of room temp?

On Sat, 7 Mar 2009 12:21:57 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:

The thermistor handles this. It is probably connected as part of a potential divider driven from a regulated voltage source. Maybe there is a factory calibration routine built into the uP inside the "box" ??? How does the thermostat read at lower temperatures? High or low? Is there a possibility that self heating may be affecting the reading? Does the thermostat read lower with its cover removed? Maybe you could "calibrate" your thermostat by shunting the thermistor with a high valued resistor, or by adding a low value in series with it. Or you could take some resistance readings at various temperatures and look for a matching replacement.

The following test results suggest that thermistor accuracy is not affected significantly (less than 0.5 deg C) by aging:

This document ...

... suggests that a replacement thermistor manufactured with the same RT curve should require no calibration.

============================================================== A thermistor can be defined as having an interchangeability tolerance of ±0.1°C over the range from 0° to 70°C. This means that all points between 0° and 70°C, are within 0.1°C of the nominal resistance values for that particular thermistor curve. This feature results in temperature measurements accurate to ±0.1°C no matter how many different thermistors are substituted in the application. ==============================================================

Here are some RT curves:

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