Puzzled ~ Thermostat Question (heater, bedrooms, gas, upstairs)

[ecam222]

Hi. This home was built four and a half years ago and this is the fifth winter here. I have two heating zones (upstairs and downstairs). For some reason, the downstairs thermostat seems to be working as it always has, but this year, it seems that the only way for the upstairs heat to kick on is if I crank the downstairs at around 72. The upstairs will be around 62 to 64...sometimes goes lower if I don't do it (given the icy cold temps lately). It used to be where I could turn them on, independent of the other. Any ideas as to why that has changed? This has been happening even when it's not terribly cold and I just want to get the chill off. Thanks.

[Charles]

Is it in a different mode? Like some sort of programmed mode?
Is either thermostat being thermally affected in some different way, like for some reason the sun can shine one one, or one is near a space heater?
Does either have a battery backup which may be out of charge?
Can you simply switch thermostats to see what affect that might have?
Could one thermostat somehow be out of calibration (reads 64F when it is actually 70F)?

[ecam222]

Originally Posted by Charles

Is it in a different mode? Like some sort of programmed mode?
Is either thermostat being thermally affected in some different way, like for some reason the sun can shine one one, or one is near a space heater?
Does either have a battery backup which may be out of charge?
Can you simply switch thermostats to see what affect that might have?
Could one thermostat somehow be out of calibration (reads 64F when it is actually 70F)?

I don't believe it is in a different mode. Both thermostats are a different company name. The downstairs one is made by Robertshaw and the upstairs one is a Honeywell. There is no sunshine or space heater nearby. The batteries are still fine on the Robertshaw one, but I'm afraid to force the Honeywell to see if I can get it apart. I'm sure I have the manuals for everything when this house was built. I will have to check into that. No, I can't simply switch thermostats, but I appreciate the suggestion. I suppose it is quite possible that one might be out of calibration, as you've suggested.
Here's how I have them set:
Downstairs: Robertshaw, rectangular (battery powered)
Settings show: System: ON - OFF (I have set to ON)
Auto - ON (I have set to Auto)
Upstairs: Honeywell (Round)
Left Side setting says: COOL-OFF-HEAT (I have it on HEAT)
Right Side setting says: Fan On .. AUTO (I have it on Auto)
I don't have a fan. It's baseboard heat in the upstairs bedrooms. For what it's worth, the boiler is a Slant/Fin, Victory II, Gas Fired and uses propane. I'm sure it's something very simple to figure out.

[North Beach Person]

You might have a bad upstairs zone valve.

[ecam222]

Originally Posted by North Beach Person

You might have a bad upstairs zone valve.

Ok, thanks, I'll look into that.