The heat will need to be kept an eye on when the ER is on though, will need to shut it off when it becomes warm and back on when cold, etc..

If you can make it work that way better to do that than risk damaging the entire system to the point it won't work at all. The computerized controls can be frustrating and illogical to work sometimes.

There is also the possibility the "new" batteries don't have a good charge, had that happen just recently with some cheap ones.

It bypasses the heat pump and should heat from the gas burner.

Press "System switch" to select "Heat" mode, then press the "EMER" key.

Good luck!

You the wrong info. If you find that nothing works, pm me and I will tell you what might work. At least it works on all furnaces.

so we are just trying to troubleshoot the stupid thermostat.

Red wire and a white wire. In most heating applications, these control the heat. TURN OFF ALL OF THE POWER TO THE SYSTEM FROM THE CIRCUIT BREAKER PANEL!!! It's 24 volts so the danger is not to you but if you fuck with these wires with the power on you will fry the transformer. You don't need that! Unhook thr red and white wires from the baseplate and twist them together. Then turn the power to the system back on. The boiler SHOULD fire up. IF that works, they will have to either go through the same process to turn it off when it gets too hot or maybe control the temp by opening some Windows. That will at least get them through the night. If you get a replacement stat tomorrow you can install it yourself. The thermostat terminals are each labeled with a letter, B,W,G,R, etc. Write down which color wire goes to each terminal and make sure you re attach the new stat with the same connections. You could have as few as three wires, Red White and green or as many as seven or Eight depending on your system. Probably a good idea to take the whole stat with you when you go to buy a replacement to make sure you get one that works with your system cause not all stats are alike. REMEMBER, ANY TIME YOU FUCK WITH THOSE WIRES MAKE SURE THE POWER TO THE SYSTEM OS OFF!

Best I can do for you without seeing the system but I spent 43 years in the HVAC biz so I'm not just some dumb handy man do it yourselfer.

clean the battery contacts and reinstall batteries
if still getting "BATT" display
verify that the new batteries are good
if still bad
replace thermostat. Do you have an old manual one in a junk drawer?

but impossible to get a skilled person out there on a Sunday- and they have more than 20" of snow, and a subway shut down. Hoping some stores are open in the AM.
Am suggesting they check the battery contacts.

Are you sure the new batteries are fully charged?
Can you test them?
Are you getting one of the "E" (followed by number) Error display?
Did you follow the Error Mode instructions where you remove the batts and hold down any button for several seconds to fully discharge the unit?

Try the emergency heat mode. It shouldn't do any harm to try it.

You might try cycling the power at the main breaker too.

Hmm, I will see what they say.

I second this advice.

Are installed correctly. If either or both batteries are inserted with the terminals in the wrong direction, the thermostat won't work. Proper polarity is essential. That's my guess after reading the manual. It's easy to install a battery with reversed polarity. Very common mistake. The two batteries are not installed the same, either.

Problem solved. I'm not an HVAC guy, but did work as a handyman years ago. I can't count the number of times I fixed something by re-installing batteries in the right orientation. It's always the first thing to check with anything that uses batteries.

Cleaning the terminals is another great idea. I'm glad it worked out.

If you're talking about what I think you're talking about, that situation has never come up.