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carol_3 Member Posts: 397
Check the cycle rate of the thermostat. If you have fin tube radiators, it should be set for 5 or 6 cph, even if the manufacturer says differently for "hot water." When the manufacturer says hot water, they mean big cast iron radiators. If your stat isn't electronic, rather than cycle rate it has anticipator. That's the little dial inside with decimal points in front of all the numbers. For best temp control--no overshoot--match the anticipator setting to amp draw of the load (zone valve?). You can find the amp draw on the end of the zone valve. If the anticipator is set for slower cycling (e.g. 3 cph) you will get temp overshoot when the heat turns off, and your space will feel cold before the heat turns on.


  • johnz
    johnz Member Posts: 3

    Does anyone know of a thermostat that works well with hot water boiler systems? The one that I have seems to allow the boiler to operate for longer than required. The house temp. always increases by 1-2 degrees after the boiler shuts off. Are there more sensitive thermostats available for these systems? Thank you, John
  • John Ketterman
    John Ketterman Member Posts: 187

    I don't know your system but I can speculate. People tend to blame the stat because that's their interface to the heating system, but your problem is probably not with the thermostat. You probably have an old-fashioned cast-iron boiler so the water needs to get very hot in every cycle. Yours may also be vastly oversized---most boilers are. So the water gets quickly heated to 180F or so by the time the stat senses any heat.

    A wall stat can't do a good job of controlling temperature in such a case, because by the time it senses a small rise in temperature and stops the boiler, there is a whole lot of heat already in the water and it keeps heating your house for a long time more.

    There is no free/cheap solution to this. It is one reason why hot water heating has acquired a bad name in the US. Modern condensing boilers (required by law in Europe) do not need to get the water so hot and so provide gentle heating, but of course it'll cost you lots of money to replace your current boiler. With your current boiler, thermostatic radiator valves (TRVs---required by law in Europe) will moderate the overheating and allow room-by-room control, at the cost of several hundred dollars per radiator.

    OK, here's a crude but free solution. Look for the aquastat in your boiler, which sets the maximum water temperature. Turn it down to 150F. You'll feel less overheating. Now on very cold days 150F may not be high enough to keep your house warm, and if so you'll have to turn it back up.
  • Fred Harwood_2
    Fred Harwood_2 Member Posts: 195

    What's the present thermostat?
  • GusHerb
    GusHerb Member Posts: 91

    i personally like the white-rodgers 1f90-371 0r 1f97-1271 for hot water heat systems.
  • JB_2
    JB_2 Member Posts: 68

    Strange- could this be a compter room where tracking temps. is must? Is it the whole house? or just one zone of the house? where is the t- stat located, lots of things could affect this.If you have domestic hot water off this system such as coil, I would not turn down limit controls.most thermostats have an adjustment too match room comfort. more info is needed. 1-2 is not that bad.
  • johnz
    johnz Member Posts: 3

    Thanks for the replies.. More info on my system. New burnham oil fired boiler, old cast iron radiators in the house with Hunter thermostat(doesn't seem to have capability to set "cycles").. also I can't set water temp. lower than 180. Any suggestions for a better stat for this type of system? thanks , John
  • Ken D.
    Ken D. Member Posts: 836

    Any zone valves in the system?
  • adambuild
    adambuild Member Posts: 414

    We use the HONEYWELL VISIONPro 8000 for just about every new install and repair job and have had great success with them. Available wholesale only around here. As Carol mentioned you must set the CPH for your application.
  • awaltiii
    awaltiii Member Posts: 17

    I have basically the same system as you in my home.

    Cast iron radiators run at a much lower temperature than ost people would think - anywhere from 110° F to a high of 140° F.

    The "overshoot" of the thermostat setpoint can be caused by the heat anticipator being set wrong as Carol Frey stated. But, the problem is more likely caused by the boiler temp high limit too high for the outside temperature that particular day. I would assume your problems of overshoot are happening on "warmer" days. As has been stated in replys this is due to the delay in the thermostat finally being satisified and now having "hot" radiators continuiing to radiate heat needlessly and expensively.

    An outdoor reset control would most likely solve this problem. It measures the outside temperature and changes the boiler temperature so as to not overshoot the thermostat setting. If your boiler has a domestic hot water coil or an indirect hot water heater the amount of hot water made will decrease on warmer days (when the boiler water temperature is lower).

    I once monitored the boiler temperature along with the outside temperature in the Hartford Connecticut area while keeping the house at 70° F in the month of December and at no time did the outlet temperature of the boiler go above 110°. I only use the boiler for heating at this time.

    You can buy a Honeywell Outdoor Reset Hydronic Control or one of many solid state (electronic) controls. Using one will solve your problem and save alot of money in the long run. However, you will need a thermowell in the boiler for the boiler water sensor.
  • Plumb Bob
    Plumb Bob Member Posts: 97

    > thermostat(doesn't seem to have capability to set

    > "cycles").

    Yes, it does, or it has a temperature deadband (allowed variation) setting (set it smaller), or it has a setting for steam/hotwater/air/electric (set it to "forced air").

    > also I can't set water temp. lower than 180.

    Yes you can. Look inside the boiler. An outdoor reset control is an add-on to do this adjustment automatically, but every boiler has a way to do this manually.
  • Plumb Bob
    Plumb Bob Member Posts: 97

    > New burnham oil fired boiler

    And I bet the contractor made it even bigger than your old boiler "just to be safe"...right?
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