Homeowner Forum / Repair / February 2010

On Feb 22, 5:07 pm, Glenda Copeland <gscopel...@Use-Author-Supplied-
Address.invalid> wrote:

You have to turn the mounting of the coil spring that the mercury bulb
is connected to.  I don't have that type of thermostat any more, so I
can't give you detailed instructions.  But, I'm sure others here will,
give them a few hours.

It's probably easier to just replace the mercury thermostat with a digital.
It'll be more accurate and have less temperature fluctuation. The little
pointy thing is supposed to be aiming at whatever current amount the device
it's controlling is drawing. It may be a relay or a zone valve, so you'd
have to know this, then find the thing, then find the label on it where it
give it's current draw. You could just point the thing far to one side, then
see how it affects the temperature, then far to the other side, and make
whatever adjustments you need. Keep in mind also, that  Honeywell T-87 yo-yo
thermostats must be level as well. There are marks on it, that you hold a
plumb bob up to ,to properly level it.

You can change it to digital therm. on your own.  I did it so anyone can do
it.  Just don't allow the wires to fall back into the wall.  Seeing that
winter is half over, I'd wait until you don't need the heat.  Then if you
mess up, it won't matter so much.

Tell your husband that just because it's pointed to 85, it doesn't mean the
air temp is 85.  Get a room thermometer and set it near the thermostat.  
Then show him the thermometer's reading of the air temp.  And tell him to
mind his own business.... he he.

I don't have that particular unit, but on mine, it looks like the is a nut
behind the metal thermo spring that could be turned a bit to calibrate it. What
you need is to turn it in the direction that makes the mercury switch less
level - just a little. If such an adjustment is not evident, bending the metal
where the switch attaches to the thermospring could do it.

The copper pointer at the bottom probably adjusts the difference between turn on
and turn off.

1st check that it is level (across the top 2 posts that hold the trim
ring on).

MikeB

Hi,
First thing first.
Did you have this problem all the time or it is acting up lately?
Sounds like good cleaning and calibration is needed.

First, make sure the thermostat is level. If it's
tilted, it won't work right.

On Feb 22, 11:07 pm, Glenda Copeland <gscopel...@Use-Author-Supplied-
Address.invalid> wrote:

The problem you have is called hysteresis.  When the temperature rises
the 'stat switches off & the temp. falls but the stat does not close
at the same temperature as when it was rising.  The hysteresis should
only be a couple of degrees but now it's obviously a lot more. This is
due to ancient technology and probably wear and tear and lack of
lubrication.  The mercury bulb is indestructable but the rest of the
'stat wears. Get a new themostat.  It will pay for itself in a year.
http://en.wikipedia.org/wiki/Hysteresis

On Feb 22, 11:07 pm, Glenda Copeland <gscopel...@Use-Author-Supplied-
Address.invalid> wrote:

I don't know if such things are available in America but in Europe you
can get an "intelligent thermostat."  There is an additional outside
sensor and the heating is switched on earlier in cold weather and
later in warmer weather.  The exact amounts depend on the insulation
of your house (nonexistant I expect in America).  The thermostat
determines this for itself after the first run.  Dependant on your
lifestyle, between 10 and 20% can be knocked off your heating bill.
Some of these devices also compensate for wind .
A digital thermostat can save 10%.  It's important that the thermostat
is correctly sited.  In Europe every room is independently controlled
in new buildings.

I'd yell at you too.  Who needs 75?   Set at 78 and be happy with 68.
Wear a sweater.

Don't pull that cover off. There are electical forces there that will
make your cold cream run.

In that case you could risk bending the spring or some other part that
is holding the mercury capsule, but two major concerns.  1) Being a
Mercury Capsule, it might head for earth orbit.  2) A good chance
you'll bend it in the wrong direction (it's confusing) or bend it in
the right direction but so much that the offset pointer won't be able
to correct it.  And you only get to bend it once or twice and soon it
will break off. Plus you probably need TWO needlenosses to bend it.

You can try to tip the house while holding the thermostat in place.

As to buying a new thermostat, people here are often ready to buy new,
to throw money at a problem.

As to accuracy, what does accuracy matter when people don't know if
they want 74.4 or 74.7, or 75.3?  And much moreso is it not important
when they want something around 75 and their husband wants something
around 68.  Improved accuracy isn't going to help.

Temperature stabilty is adjustable iirc on the round Honeywell
thermostats.  It's called cycle delay or something.  But one can't
lower it too much because of the furnace, not because of the
thermostat.  It's not good for many furnaces to turn off and then turn
on again when the temp drops 0.1 degrees.   This is all the more true
for ACs which need time for pressure to go down where it the
refrigerant is compressed.  The thermostat comes set for a reasonable
value and it can be lessened if wanted with most furnaces.  If it is
also an AC thermostat, either there are two cycle delay adjustments,
the cycle delay applies to the furnace only, or it can't be lowered
more than is safe for the AC.

Sounds to me like your husband is dumber than a doornail.  Maybe one
of those big 20 inch round thermometers that people put outdoors would
convince him that the thermostat numbers are wrong.  Or maybe he's
just too stupid, and you should re-marry someone like me.  (I am
available).

Your mercury thermostat is most likely not level.  Remove the cover
and you should see lines that say "level" on them.  Or may not say
anything, but there are horizontal lines that are supposed to be
levelled.  Loosen the mounting screws and level it.  No need to remove
the wires, just turn it on the mounting screws until it's level and
retighten the screws.

On the other hand, the thermostat might be worn, and just not accurate
anymore.  Modern digital ones can be gotten for as little as about $15
these days.  Even Walmart has them.  They do not need to be level, and
are easy to install.  It might be good to replace it, but go ahead and
level the old one first.  If you do replace it, so NOT toss the old
one in the garbage.  Mercury is toxic.  Recycle it.

The copper pointer is called the "anticipator".  It adjusts the amount
of rise and fall temperature.  In other words, how often the furnace
cycles (turns on and off).  One settings of that pointer makes it turn
on at ex: on 63, off 70. (With thermostate set to 67)  Or, if the
furnace runs more often, it will come on at 66 and go off at 69 (With
thermostate set to 67).  It does not change the accuracy of the
settings, just how often the furnace runs to maintain somewhat close
to the set temperature.  That settings is adjusted by the current draw
of the furnace, and if you dont know that, just change it until the
heat is relatively even without cycling on and off every few minutes.
This takes a lot of trial and error, so it's better to determine the
actual current.

LM