This is Hunter Fans Digital Thermostat Model 44360.
It has worked to control my gas furnace for 6 years.
I like it's design and features. Hunters have a bad rep with
service people from what I've read in HAVC forums. But
I like mine and would like to repair it.

Lately its calibration is off by 2 to 4 degrees. I know this
by comparing it with three other thermostats: a new Honeywell
replacement digital and 2 old Honeywell mercury-switched thermos. In
addition, a room thermometer confirms the others' readouts.

As the room reaches warmer temps the Hunter's readout will
become more unstable and gravitate towards the 4-degree
range.

All this may or may not have started after I knocked the Hunter off
the wall. There was no apparent damages.

Looking at the cb there is a protected part that sticks out.
It has a plastic guard around it. The circuit board has the marking
"RT" . It appears to be a very small glass bead or
mini-mini LED type of device. Exhaling warm breath or placing my
fingers around the guard will cause the readout to rise.

So I'm guessing this is some type of themistor. There appear to be no
other marking on it. But it is so very small.
It kind of looks like this:

http://media.digikey.com/photos/Hone...104KAJ-B01.JPG

and more info on these therms at:

http://search.digikey.com/scripts/Dk...me=480-3117-ND

Would replacing this fix my thermostat or is there a bigger problem
with the calibration? If the former how can I id the
replacement?

Thank you in advance.
less

On 2/27/2009 10:58 AM spake thus:

This is Hunter Fans Digital Thermostat Model 44360.

Lately its calibration is off by 2 to 4 degrees.

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


--
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because it generates considerable business for me in consulting and
upgrades. As long as there is hardware and software out there that
doesn't work, I stay in business. Incidentally, my company motto is
"If this stuff worked, you wouldn't need me".

- lifted from sci.electronics.repair

In article ,
kens says...
On 2/27/2009 10:58 AM spake thus:

This is Hunter Fans Digital Thermostat Model 44360.

Lately its calibration is off by 2 to 4 degrees.

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

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.

On 27 Feb, 19:58, wrote:
This is Hunter Fans Digital Thermostat Model 44360.
It has worked to control my gas furnace for 6 years.
I like it's design and features. Hunters have a bad rep with
service people from what I've read in HAVC forums. But
I like mine and would like to repair it.

Lately its calibration is off by 2 to 4 degrees. I know this
by comparing it with three other thermostats: a new Honeywell
replacement digital and 2 old Honeywell mercury-switched thermos. In
addition, a room thermometer confirms the others' readouts.

As the room reaches warmer temps the Hunter's readout will
become more unstable and gravitate towards the 4-degree
range.

All this may or may not have started after I knocked the Hunter off
the wall. There was no apparent damages.

Looking at the cb there is a protected part that sticks out.
It has a plastic guard around it. The circuit board has the marking
"RT" . *It appears to be a very small glass bead or
mini-mini LED type of device. *Exhaling warm breath or placing my
fingers around the guard will cause the readout to rise.

Have you blown the dust out of this area?

Chris

In article , krw wrote:
In article ,
says...
On 2/27/2009 10:58 AM spake thus:

This is Hunter Fans Digital Thermostat Model 44360.

Lately its calibration is off by 2 to 4 degrees.

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

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.



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

In article , (GregS) wrote:
In article , krw
wrote:
In article ,
says...
On 2/27/2009 10:58 AM spake thus:

This is Hunter Fans Digital Thermostat Model 44360.

Lately its calibration is off by 2 to 4 degrees.

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

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.



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.

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), put finger
to keyboard and composed:

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?

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:
http://www.springerlink.com/content/tn758n7285033298/

This document ...

http://www.thermistor.com/references...sign_Guide.pdf

.... 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:
http://www.omega.com/temperature/Z/pdf/z256-257.pdf

- Franc Zabkar
--
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