The pros have turned this into a discussion about their favorite
thermostats, and I'm not sure they were addressing whether these are power
stealing or will work with the System 2000.
I'm an electrical engineer, not an HVAC tech, so let me try to give you a
different perspective.
I just had a System 2000 installed in August, I think you've made an
excellent choice, but the System 2000's digital manager is probably not too
familiar to most techs in the field. They see many more plain mechanical
aquastats and relays. I just went through this power stealing thermostat
issue on mine, replacing one of my thermostats with a power stealing model
after the contractor completed the installation (their installation didn't
include new thermostats, I didn't inquire and they didn't offer, so none
were part of the job and they just left my existing ones). I already had a
battery-powered (not power stealing) Honeywell programmable upstairs, but
only a mercury switch mechanical for my finished basement. I decided to
replace the mechanical with a Honeywell CT500 digital non-programmable power
stealing thermostat (didn't need programmability for the basement, but
wanted better temperature control and lower temp range since I don't use
this space much).
Post by HKEKQuestion #1a: If a thermostat can get its power by "stealing" it from
the traditional 2 leads used by passive thermostats, why do
manufacturers even bother to make hardwired models? Question #1b: What
advantage do they offer? Either one will be upset when the power goes
off.
Someone else partly answered this, power stealing thermostats only work with
some types of systems (including most 2-wire 24V systems). The advantage is
there are no batteries to change or go dead. The disadvantage is power
stealing thermostats are not compatible with some systems. My CT500
remembers its setting if the power goes out, and since the system can't run
while the power is out, I don't see a problem with this. I'm not sure if
the clock on power stealing programmables keeps running while the power is
out, though.
Post by HKEKI have an Energy Kinetics System 2000 boiler that was installed last
spring. I have been using the same 2 thermostats that were installed
with my old boiler. One is a simple bimetallic coil spring unit and
the other is an electronic unit with limited programmability. The
electronic unit is "power stealing".
I want to upgrade to 2 programmable thermostats such as the Honeywell
T8600 Chronotherm IVs. Initially, my thought was to get the "power
stealing" versions. I called the tech support person at Energy
Kinetics and he stressed that I should get battery powered thermostats
because the others do not work as well with the System 2000. Huh? Hope
I haven't voided my warranty!
If your contractor left your existing power-stealing thermostat in place I
would think they would be responsible for any damage that it caused.
Post by HKEKMy system seems to be working fine with what I currently have and the
installer never cautioned me about my existing thermostats. Question
#2: Are the System 2000 folks simply trying to get me to buy their
thermostats?
You didn't even say the System 2000 folks tried to sell you a thermostat,
just told you not to use a power stealing one. This is where it gets
interesting. I didn't call them, but did check the installation manual that
my contractor left. Page 17 says "A dull yellow light is common with a
power-stealing thermostat. It is not a call for heat", and says this is
"Normal Operation".
I took this to mean I could use a power stealing thermostat, so I installed
one. It turns out the people you talked to were right, it didn't work well.
Power stealing means the thermostat draws a small current to power the
electronics even when it's not calling for heat, and this caused the System
2000 to occasionally think it saw a call for heat, and to switch on
momentarily then turn off again. I got mine to work correctly by adding an
isolation relay, but otherwise you might be better to avoid them. Whether a
particular power stealing thermostat works will depend on how much power it
draws while it's off. If yours also starts and runs the circulator
periodically for no reason I would recommend you not use the thermostat that
is causing this. See below if you want to know more about this.
Post by HKEKQuestion #3: Do the battery powered units actually run off the
batteries or are the batteries only used as backup?
My Honeywell CT3600A runs off the batteries, they have to be changed every
year or so. It will eventually flash "Lo Batt", and if you don't change
them, I think it will stop working. I also previously had a power stealing
thermostat (with my old heating system) that had a backup battery used only
when power was out, but this was one of the first programmables and hasn't
been made in years.
Post by HKEKQuestion #4: Which method of thermostat power is ultimately "the best"
for this system, regardless of cost and the time required to run
additional wiring.
A battery operated thermostat is easiest, and will work well, provided you
don't mind having to remember to keep good batteries in it to keep the heat
on.
More about power stealing thermostats and isolation relays:
Your HVAC contractor may know about isolation relays, or may not. I re-used
an isolation relay that came with my first programmable thermostat for my
new Honewell CT500.
In the beginning, thermostats were just simple "on-off" switches, and they
connected through the coil in a relay on the burner controls to switch the
relay on and off to operate the equipment. When the thermostat is off,
there is no voltage across the relay coil and it turns off. When the
thermostat is on, there is 24V across the relay coil and it activates.
This simple 24V on-off interface has been modified to add various features.
The first modification was actually with mechanical thermostats that have
heat anticipators. These add a resistance heating element to make the
thermostat turn off early. With these, "on" is not really quite fully on,
there may be a few volts drop across the thermostat while it's on (to
operate the small heating element), this is subtracted from what the relay
coil sees. Instead of seeing 24V, the relay may only see 20 or 22V. This
is normally still enough to activate the relay. In fact, relay specs even
account for this, a 24V relay may be guaranteed to turn on with as little as
19V, for example.
Power-stealing thermostats take advantage of this for a different reason,
they still need power while "on", so they also may "steal" a few volts from
the relay coil, expecting it will still operate, so of the 24V supplied by
the power transformer, the thermostat may keep 2 to 4V, and the relay will
see the other 22 or 20V. Like the case of the mechanical thermostat with
the heat anticipator, the relay still activates.
Power stealing thermostats go beyond this, though. They also need power
when they're not calling for heat. To get this, they draw just a little
current while off, but not enough to activate a relay. A 24V relay may be
guaranteed to be off if the voltage across it is less than 7V, for example.
Power stealing thermostats take advantage of this, allowing just a little
current to pass, but not enough to raise the voltage seen by the relay coil
above this threshold. The thermostat gets the power it needs, but the relay
stays off and the heating equipment it controls stays off.
Digital power-stealing thermostats may actually do one more thing, they
assume they'll be connected to a relay, and take advantage of the fact that
relays are mechanical devices, and the thermostat may pulse "on" for just
milliseconds to steal some power, then turn back off before the relay
mechanical components are able to overcome inertia and move to the "on"
position, or while "on" calling for heat they may pulse off for
milliseconds, again to steal power, but remaining on most of the time,
depending on inertia in the relay to keep the contacts from falling open.
This all works, with the assumption that the thermostat actually connects
through a relay with the assumed characteristics that less than 7V is off,
greater than 19V is on, and the relay arm can't move fast enough to respond
to millisecond pulses.
Enter now your System 2000 digital manager. Your thermostat is no longer
operating a relay. It's connected to digital electronics inside the digital
manager. The digital manager may not respond in the same way as a
mechanical relay to the "mostly on" and "mostly off" characteristics of a
power stealing thermostat, and if the thermostat pulses on and off, this may
also confuse the digital manager, because it may not have quite the same
response to this behavior as a mechanical relay.
In fact, this appears to be the case with my power stealing digital
thermostat, when connected directly to the System 2000 it caused the
thermostat light on the digital manager to flicker dimly. I think this
means the power the thermostat is stealing may be pulses, or may be
fluctuating, and every so often this exceeded the digital manager's call for
heat threshold, causing it to turn on. It would then almost immediately
turn back off.
Your heating contractor may be able to supply you with an isolation relay,
so the thermostat is still powered through a relay, and the relay contacts
in turn switch on and off the thermostat connection on your System 2000
digital manager.
In my case, I used a 24V relay not supplied by an HVAC contractor (it came
with an old model power stealing thermostat to be used for precisely this
purpose, but many other 24V relays are compatible). You might be able to
get just a relay from an electronics supply house or HVAC supply house.
You would need a relay with a coil rated for 24V AC. One that should work is
an Omron MY2-AC24(S) that you can get from digi-key (an electronics supply
house), www.digikey.com, as their part number Z184-ND; I'm sure you can find
other sources too. This is a plug-in relay, Omron makes sockets for it if
you can find them (digi-key doesn't seem to carry the convenient screw
terminal ones) but I just soldered wires to the terminals of mine, and used
it without a socket and wired it to a barrier terminal strip that I got from
Radio Shack to use as my wiring junction point. Unless you're comfortable
with soldering, you may have difficulty building your own and making it
mechanically secure with these parts. You can ask your HVAC contractor if
he can help. If you feel competent to try this yourself, read on...
You need to wire from the 24V transformer through the relay coil, through
the thermostat, and back to the other side of the transformer to power the
thermostat and activate the relay. The relay contacts then are used to
connect to the System 2000 digital manager's thermostat connection to supply
it with a clean on-off signal instead of the sloppy "almost on", "almost
off" that the power stealing thermostat provides. The relay's job is to
convert these "almost on", "almost off" inputs to clean on-off signals.
This should be safe, but it's up to you to decide if you want to modify the
wiring to your system 2000 and take the risk that if something goes wrong it
could be your fault and void your warranty. Turn off the power while you do
this, and make your work mechanically secure so there are no dangers of
short circuits, and check your work before powering on.
Disconnect the thermostat wire from A1 and T1 (or T2, etc. depending on
which zone you're modifying) on the System 2000. Connect a wire from A1 on
the System 2000 to one side of the relay coil (pin 13 if you use the above
Omron relay). Connect one of the wires from the thermostat to the other
relay coil pin (pin 14 on the Omron relay), and the other thermostat wire to
A2 on the System 2000 (it doesn't matter which thermostat wire goes to
which). Connect a wire from A1 on the System 2000 to one side of a normally
open relay contact (pin 9 on the Omron relay, or since pin 13 on the Omron
relay also connects to A1, you can also just connect relay pin 9 to pin 13).
Connect another wire from the other relay contact (pin 5 on the Omron relay)
to the System 2000 thermostat input T1 (or T2, etc.). Check your work, and
turn the power on, and your power stealing thermostat should work normally,
and your System 2000 lights should be either fully on or fully off depending
on call for heat, with no dim lights while off.
The above Omron relay has a coil rated 46mA at 24VAC, this is about the same
as the relay I'm using and my thermostat works fine. A direct connection to
the System 2000 would supply about 73mA to the thermostat (I measured the
above with an ammeter on my system). Honeywell doesn't publish specs on how
much current their thermostats require (I imagine it's a fairly wide range
to accomodate different systems), but I'm using a relay that's very similar
to some used in other heating equipment e.g. zone relays, so I think it
should be fine. Mine has been working for a couple of months now.