Thermostat difference?

[Tim Green]

I wondered if anyone knew if there were, aside from price, bells 'n
whistles, major differences between thermostats used for home
furnace/air-conditioning systems.

I have a White-Rodgers thermostat and have noticed that if it's 72 degrees
inside, and I set it go go on at 70 (that is, in the on position, but
nothing actually running), the display will, in short order--say, 20-30
minutes--drop a few degrees and the furnace goes on.

If, on the other hand, I leave it in the off position, the temperature may
remain the same for hours, or even rise a degree.

It's happened without fail, over and over, for the two and a half years I've
been here. Even now that I've just replaced the furnace, the phenomenon
persists.

Just curious...

Tim

 

[CWatters]

"Tim Green" <strawblond@sympatico.ca> wrote in message
news:XWykd.25311$km5.1376288@news20.bellglobal.com...

I wondered if anyone knew if there were, aside from price, bells 'n
whistles, major differences between thermostats used for home
furnace/air-conditioning systems.

Yes. see below.

I have a White-Rodgers thermostat and have noticed that if it's 72 degrees
inside, and I set it go go on at 70 (that is, in the on position, but
nothing actually running), the display will, in short order--say, 20-30
minutes--drop a few degrees and the furnace goes on.

Ok so far except that it sounds like your house isn't very well insulated.
eg When you turn off/down the thermostat setting the house cools far too
rapidly.

If, on the other hand, I leave it in the off position, the temperature may
remain the same for hours, or even rise a degree.

So with the thermostat turned down you still see a temperature rise. That
may also be possible.

Possible causes...

a) There is a fault somewhere and the heating is still coming on...

Does the furnace also heat or supply hot water? In the UK boilers (=furnace)
also heat the hot water. They use a valve that diverts the hot water from
the boiler either through the heating system or through the hot water
storage tank. If the valve fails then some water can be diverted to the
heating loop when it's meant to be heating the hot water storage tank loop.

b) There is another source of heat.

For example the fridges, washing machines or simply the sun warming the
house.

c) Thermostat hysteresis (Temperature or Time) exists.

This is what I think you are seeing...

Temperature hysteresis : Old style bimetallic strip thermostats never
switch on or off at exactly the same temperature. The actual switching point
depends on the direction in which they are going. For example if you set it
at 70 it will switch ON at 68 and OFF at 72. This prevents the thermostat
oscillating and the contacts arcing when the air temperature is exactly the
same as the set temperature. It also stops the boiler constantly switching
on and off which is bad for efficiency and its life. It does however allow
some under/overshoot. eg the air temperature can rise to 72 before the
boiler switches off or fall to 68 before it switches on.

Time: Some modern electronic thermostats have a smaller or no thermal
hysteresis . Instead to prevent oscillation they have electronics that limit
the number of boiler cycles per hour. This can be as low as 6 or 10 cycles
per hour. eg When the air temperature is the same as the set temperature it
switches the boiler ON and OFF about every 6 mins (=10 cycles per hour).
This normally results in much less over/undershoot than with the old
bimetallic strip type thermostats. With some types of thermostat you can
adjust the maximium number of cycles per hour but this is very rarely
needed. Sometimes only the installer can change this. Some models are badly
designed and only sample the air temperature or the setting every 6-10 mins.
This means they don't appear to respond when you adjust the set temperature!
The solution is to adjust the setting then wait at least 20 mins and see
what the boiler/furnace does. Making small frequent adjustments to the set
temperature can produce very confusing results with the boiler appearing to
have a life of it's own.

Hope that helps.

Colin

 

[CWatters]

"CWatters" <colin.watters@pandoraBOX.be> wrote in message
news:mRGkd.18830$9Z7.1200264@phobos.telenet-ops.be...

Sorry that should say..

When the air temperature is the same as the set temperature it
switches the boiler ON and OFF about every 6 mins (=10 cycles per hour)
**Maximium**

 

[Rich Webb]

On Wed, 10 Nov 2004 20:07:02 -0500, "Tim Green"
<strawblond@sympatico.ca> wrote:

I wondered if anyone knew if there were, aside from price, bells 'n
whistles, major differences between thermostats used for home
furnace/air-conditioning systems.

I have a White-Rodgers thermostat and have noticed that if it's 72 degrees
inside, and I set it go go on at 70 (that is, in the on position, but
nothing actually running), the display will, in short order--say, 20-30
minutes--drop a few degrees and the furnace goes on.

If, on the other hand, I leave it in the off position, the temperature may
remain the same for hours, or even rise a degree.

It's happened without fail, over and over, for the two and a half years I've
been here. Even now that I've just replaced the furnace, the phenomenon
persists.

Pure speculation here (this IS usenet, after all...) but perhaps with
the switch in the off position the device remains in a power save mode
longer or is in a different power save mode, as it is not responsible
for any machinery control, and so it doesn't update the temperature
display as often. Have you tried hitting it with a warm air stream for a
few minutes when it's on and off (after reaching equilibrium in each
state) to see if there's a difference in the response with a (sort of)
known stimulus?

--
Rich Webb Norfolk, VA