Can someone explain the use of a heater INSIDE a mechanical (bimetal strip) room
thermostat? (It usually just seems to be a resistor placed near the bi-metallic
All it seems to do for me is make the thermostat very inaccurate. If the
temperature is only a bit lower outside than inside, the temperature inside is
what I set the stat to. As the temperature outside drops further, the inside
temperature strays further and further from what I set it to (in the colder
direction). This is presumably because whenever the heating is on, the stat
thinks it's warmer than it really is, because of the internal heater. If I
disconnect this internal heater, the stats functions properly.
I've replaced the offending pile of crap with a digital one anyway, but I wanted
to know why they design them like this.
On Sun, 04 Mar 2012 14:42:26 +0000, Lieutenant Scott wrote:
[ ... ]
It's an anticipator, and should be set so as to introduce exactly the
right amount of heat inside the thermostat casing so that it doesn't have
to wait for the heat to percolate from a radiator across the other side
of the room, thereby tailoring the response so that it stabilises sooner.
It's a bit like trying to throw a Yo-Yo away whilst still holding onto
the string. And yes it's a crap plumbers idea.
The earlier part of the thread is gone from the quoting now.
We were discussing why bi metallic strip mechanical
thermostats have a heater for the bi metallic strip.
They do that to reduce the hysteresis, and they reduce it by
heating the bi metallic strip with the heater when the load is on.
That means that they switch more often than a bi metallic strip
thermostat with no heater would.
That means they switch more, and do take longer to get up to
temperature when starting from cold, because they switch off
before the set point is reached, because the heater heats the
bi metallic strip.
They do however still get to the set point, they just dont stay
on all the time until the setpoint is reached when starting from cold.
That is not what my graphs from when we had a mechanical stat show.
They show an almost constant heating run, until the set point is
reached, then several short rapid boiler runs, as heat is absorbed by
the fabric. Once the fabric is up to temperature the graph settles down
to a more leisurely pace.
There definitely a heater resistor installed and it was working. I
would guess the heat output of the resistor was rather well matched to
what was needed.
Insulated cavity brick. It seems we are quite well insulated, because
the internal temperature, even on a below freezing cold night, rarely
falls more than a couple of degrees over night with heating off.
Not relevant to this particular post but I've just dismantled (smashed
up) my old thermostat. The resistor is located at the back and is
coloured red, red, yellow which is 22Ok (I think) so it's not going to
generate an awful lot of hot air.
I've found that the convection of the room air from the heat source (a radiator)
heats the room very evenly anyway. The stat switches off when the room reaches
the required temperature without that bloody heating thingy!
As you said, it just doesn't work as designed - it's guessing! All that happens
is when the heating is needed to be on more, it thinks it's going to be warmer
than it is, so it doesn't switch it on enough.
Yes, but you will find if you measure it properly that the heated
thermostat does produce less difference between the highs and
lows in the room.
Nope. Its trivial to prove that it does reduce the dead zone/hysteresis.
Fraid not. If that was the case, you would see a bigger swing in the room temps
and you dont in fact see that.
They wouldnt include an extra cost heater in thermostats if there wasnt a reason
Well the one I have in the garage is currently keeping it at the temperature set
on the dial (the heater is on 20% of the time). The outside temperature is
about 10C. When it's 0C outside, the garage is about 4C lower than what's set
on the dial (the heater is on 80% of the time). With the resistor disconnected,
the temperature is what it says on the dial under both external conditions. The
hysteresis is about 2-3C.
Out of nothing more than curiosity, I have checked back through my
house temperature logs. I don't make any particular point of storing
my system just stores weather data and along with that data, the
house's temperature and humidity - every 5 minutes.
At the moment we are running on an electronic wireless thermostat.
Aside from times when an outside door has been open for a while, the
room temperature is held within 0.6 deg. C for the entire day.
Looking back through my log to a little over two years ago, when we had
a wired electro-mechanical stat., in the same circumstances, the
temperature appeared to remain within 0.8 deg C.
Having just checked - there was no noticeable difference, once a
settled temperature was achieved. There were much bigger temperature
swings when a door was opened in cold conditions, but once back to a
settled condition no obvious difference other than the more rapid
firing of the heating.
Agreed! I would expect temperature swings of at a guess 5 deg C, with
no resistor in place.
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