Hot water boiler runs an HOUR with new t-stat

JonS

I had a fine digital Honeywell t-stat that expired a week ago. I have tried replacing it with entry-level Honeywell and Hunter t-stats. Both caused the boiler to run 45 min - 1 hr when heating, causing overshooting of set temp. The boiler then sits idle for 2 hrs while the water in the radiators gets lukewarm and then it starts the whole process over again, but only when the t-stat allows the temp to fall a full 1 degree or more below set temp! So temp swings a full 3 degrees up and then down over this 2-3 hr cycle.



In otherwords, the new t-stats are not cycling at all inside of an hrs time, even though I set them on "3 cycles per hr" per instructions for hot water systems!



The last t-stat never acted like this. It kept the temp at exactly the same temp very well. I had a heating contractor over today who says it must be the t-stat. So I am trying a more expensive model: a honeywell 6000 series t-stat at the moment. Does anyone have an explanation or can suggest a t-stat that works well with hot water heat?

JonS

More expensive thermostat works good

All the first thermostats I used were equipped to work with hot water heat, but did not cycle the boiler - as explained above. But glad to report the latest t-stat I put on is working very well!! I just had to spend more on the more expensive model. Why? If the cheaper Honeywell and Hunter models were both made for hot water boilers, why did they not cycle 3 times an hour? They didn't even cycle once an hour.

MikeG

I've used all kinds

Jon,

I've used all kinds, entry level, mid and expensive.  I have three Hunter t-stats in my house. Just the basic 5-2 digital programable using 2 wire.  Just using the on off feature. Three zones with circs thru a Taco circ controller.  Works fine.  My son has three Ritetemp universal t-stats on a three zone system with three zone valves (White Rodgers) which require a three wire.  These were cheap.  We'll see how they hold up.  Otherwise we would have had to use an expensive one that works with a three wire zone valve.  So far they work great.  The Hunter calls it "span setting", the Ritetemp calls it "swing setting" .  This determines the on and off range above and below the setpoint, which affects the cycles.  Had to play with the span/swing settings for each t-stat to get the comfort level and balance depneding on the room characteristics.