Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

replacing an older thermostat for a hydronic system - ideas?

np_mdbr Member Posts: 31
I would like to replace our main thermostat with a digital, programmable model. The old model isn't as accurate as I would like, and it causes the system to cycle too frequently (off 2 minutes, pause for 5 minutes, on again for 2 minutes) when it thinks the target temp has been reached.

I'd like a high-quality replacement but don't know which brand/model to focus on. As mentioned, this is a hydronic system. This is the main thermostat. There are multiple other zone thermostats that are digital. I am pretty sure none of them actually measure floor heat, which isn't great, but the house does stay warm when we need it.



  • mattmia2
    mattmia2 Member Posts: 7,190
    I was sure it was going to be something proprietary from the rest of your system. You could probably adjust the anticipator and get rid of the short cycling. How many wires does it have? Virtually any heat/cool thermostat will probably work but you might have to add wiring to power an electronic thermostat.
  • Jamie Hall
    Jamie Hall Member Posts: 20,467
    The cycling like that is from a misadjusted anticipator. You should be able to get the cycle time out to half an hour or so without too much trouble. If not -- any of the digital Honeywell/Residio thermostats have a heating type or cycles per hour selection in the menu which will do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 18,221
    Do you have radiant floor heat?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,970
    edited December 2022
    You should see something like this inside your existing thermostat. the pointer marked with a red arrow is the heat anticipator setting. Move that to the "Longer" setting to see if you can get the short cycling to stop.
    I'm just wondering, Are you using the cooling portion for your air conditioning? If yes, then do not change the heat anticipator on the cooling side (if there is one)
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • jumper
    jumper Member Posts: 1,911
    Those now illegal GE & Honeywell mercury switched 'stats are keepers.
    What do servicefolk do with them when they replace them?
    Same question for archaic all metal shower heads and premium sinks and toilets?
  • np_mdbr
    np_mdbr Member Posts: 31
    edited December 2022
    @hot_rod Yes, all three floors have radiant floor heat. The slab takes the longest to heat (expected) but I see no indication that there are any measurements taken from the slab itself... just ambient temperature measurements from each thermostat. There is one thermostat per zone.

    @mattmia2, @Jamie Hall @EdTheHeaterMan - I've never heard of an "anticipator", you learn something new every day! That's a great shot of the internals, I am going to pull it apart tomorrow to take a look!

    @EdTheHeaterMan Yes, this thermostat also controls the main blower when we are using the A/C. I'll stay away from the heat anticipator.

    @jumper Hmmm, maybe I should hold on to this one? The scheduling part would be nice, but I suppose that could be done with a controller between the thermostat and the main system, e.g. Raspberry PI or similar.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,970
    np_mdbr said:

    @EdTheHeaterMan Yes, this thermostat also controls the main blower when we are using the A/C. I'll stay away from the heat anticipator.

    You can adjust the Heat Anticipator for the one that does heating. I believe the cooling anticipator on your thermostat is not adjustable, it is usually just a small resistor embedded in the subbase somewhere. I would definitely adjust the one for heating to the longer setting to see if it helps.
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • nibs
    nibs Member Posts: 499
    We managed to find an unused round Honeywell mercury stat and would not change it.
    Our boiler seldom comes on more than once per day, unless we light the fireplace, cook a big meal or have more than 6 or 8 friends in.
    We set a max supply water temp at 110 deg F and set the boiler to ramp up as slowly as poss. Most of the time it runs around at around 100 Deg F with a delta T of about 10 deg F.
    First thing I would do is try turning down the supply water temp.
  • mattmia2
    mattmia2 Member Posts: 7,190
    If this is all radiant, that generally doesn't respond well to setback. it just wants to always be at one temp. the setback could be ok for cooling. that being said, you can replace that with an electronic programmable thermostat. if it is multi stage you will need a multi stage replacement.