Honeywell battery digital thermostats no longer have freeze protection circuit

The Steam Whisperer

I just got off the phone with Honeywell and apparently they no longer have the back up mechanical circuit for freeze protection in the digital thermostats if the batteries die in a battery powered thermostat.
I remember this being a major problem with the early battery powered thermostats and nobody changed the batteries and the home would freeze up. They added the simple mechanical ( probably thermister) circuit set at around 52F to solve the problem. Apparently this is no gone in the race to the bottom.


BEWARE>>>

mattmia2

It was probably just a fixed thermostat. Fixed mechanical thermostats in a little rectangular metal can are common in electronics. You can buy the same component with some leads hanging off it from supplyhouse.com although I seem to recall them being much more expensive than the component from an electronics supplier.

ChrisJ

I've had several VisionPro 8000's apart and never noticed anything like that.
A thermistor would've still needed power in the circuit.

The Steam Whisperer

Chris J ... HMM Can't the thermistor run off the controlled circuit and not battery power? It seems that generally the construction thermostats ( which I thought were thermistors) are just 2 wire. I am not that savvy on electronics.

HVACNUT

I don't think I've ever read anything about freeze protection in any Honeywell thermostat manual. 
And Honeywell Resideo doesn't have a good ring to it. Bring back Minneapolis. 

Jamie Hall

The Steam Whisperer said:

Chris J ... HMM Can't the thermistor run off the controlled circuit and not battery power? It seems that generally the construction thermostats ( which I thought were thermistors) are just 2 wire. I am not that savvy on electronics.

A thermistor is just a resistor which varies its resistance with temperature. It has to have some kind of electronic wizardry (which can be pretty simple, but it has to be there) to get it to act as a switch (which a mechanical two wire thermostat is). There is a thermistor in all digital thermostats -- but they all have to have either battery power or power stealing to work. They will not provide reliable results without power -- battery or power stealing.

mattmia2

I'm pretty sure the construction thermostats are just fixed bimetal thermostats. You can do it with a thermistor in certain cases but that wouldn't play nicely with the wider array of types of equipment control circuits and there is enough current involved that self heating would be an issue. You can essentially select a termistor that is very low resistance at the setpoint and that will conduct but it would have to be a very low current control circuit to minimize self heating and the control circuit would have to be ok with the control voltage crossing a threshold rather than a distinct on/off.

ChrisJ

I could see a simple bi metal switch closing around 50 degrees working and costing $1 to make.

But, a thermistor needs a little more.

mattmia2

ChrisJ said:

But, a thermistor needs a little more.

It depends on what the circuit it is controlling is. There are lots of places a thermistor is used as a crude time switch, thermistor heats up with the current form the load, resistance rises, thermistor essentially turns off the load, really common for degauss circuits in CRT displays, it is what many solid state small motor starting "relays" are. But for general purpose use for a 24vac HVAC control it would need more. I suppose that more could be a small really with a low power coil if you did the math right.

Jamie Hall

Quite true, @mattmia2 -- if you got the math exactly right. The problem is that you are depending on the properties of the relay coil (pull in current and drop out current) neither of which is particularly constant and you still need a power supply. It's not all that hard to design an analogue circuit using a transistor, a potentiometer, and a thermistor, a few other odd bits, and a stable power supply to get pretty decent switching on temperature... with the transistor switching a relay -- but you have to have the stable power supply, and swapping transistors will force you to recalibrate.

ChrisJ

Really guys?
Did the math right for voltages between 22-28 volts?

Jamie Hall

ChrisJ said:

Really guys?
Did the math right for voltages between 22-28 volts?

Heating power supplies are hardly stable! That's why I mentioned a few other bits up there... rectify and drop the voltage to something handy, like 5 volts or so... use the thermistor as part of a voltage divider with the potentiometer... drive a transistor with that, power side of the transistor operates a relay, and there you go.

Why bother, except as intellectual exercise!

mattmia2

Use a zener shunt regulator. like 6 smd components could be cheaper than a mechanical t-stat both in parts and in labor to assemble. Could pick a thermistor that has a curve that makes it nearly 0 ohm at the setpoint and rises rapidly with temp too.

Jamie Hall

A simple Zener regulator would be the way to go. Choice of thermistor depends somewhat on the temperature range you want to be able to vary the setting over...

And you still need a power supply.

A mechanical bimetal would, indeed, probably be more expensive. And would probably last a century or more... don't have quite that much time experience on solid state stuff yet.

ChrisJ

The bimetal switch I used on my generator for oil temp, and the one on my steam main for the Ecosteam certainly weren't very expensive, $2-3 each to buy online?


Of course.
None of this is helping @The Steam Whisperer

mattmia2

But if you can place and reflow the components for the backup circuit with the rest of the circuit it costs virtually nothing in labor. adding a component that has to be placed by hand adds another step to the process. of course there are also smd thermostats.

The Steam Whisperer

I posted the information as a warning that this problem is now around again for anyone that is using a battery powered T-stat. Talk about going backwards!

JUGHNE

I always always try to run new tstat cable that will include the common for this reason. Not good to tell someone that their $6000.00 or whatever system quit working because of a couple dollars worth of batteries.

If no common wire possible, would some form of freezestat wired across the R and W in the basement act as freeze protection....simple something that closes at 40-50 degrees or so?

Jamie Hall

JUGHNE said:

I always always try to run new tstat cable that will include the common for this reason. Not good to tell someone that their $6000.00 or whatever system quit working because of a couple dollars worth of batteries.

If no common wire possible, would some form of freezestat wired across the R and W in the basement act as freeze protection....simple something that closes at 40-50 degrees or so?

Can't see why not...

mattmia2

I assumed the battery powered t-stats were consume only products.

ChrisJ

mattmia2 said:

I assumed the battery powered t-stats were consume only products.

Eh?

mattmia2

I assumed that a professional would run c to the stat and power it off the control xfmr to avoid this exact situation.

ChrisJ

mattmia2 said:

I assumed that a professional would run c to the stat and power it off the control xfmr to avoid this exact situation.

Ah.
I think they do, unless it's a long old run that is a major pain to run a new wire (probably often the case) or it's a lower end digital thermostat that doesn't offer using a common.

The older Visionpros used to do both, common with or without batteries or battery only. My Prestige is power only, no batteries. But the Prestige is nice in that the thermostat head only needs two wires, period.