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Dual purpose thermostat?

Headymask
Headymask Member Posts: 5
in Radiant Heating

Hello all,

I recently had my home boiler replaced from an old cast iron that ran on oil to an IBC SFC Combi on natural gas. Originally, there was no zone valve for the garage that needs to be heated and instead whenever the boiler was on, a loop provided constant heat to a ceiling mounted heater with a fan. The fan kicked in depending on what temperature was set on the 120V thermostat.

There is now a zone valve that controls when those pipes get hot water via a low voltage thermostat (ecobee). The issue I'm running into is that it's nearly impossible to get both thermostats to sync up. I have to keep the temperature in there above 5C to prevent pipes from freezing.

Is there a thermostat that could serve both functions? Or maybe even a device that allows both thermostats to sync?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 25,191

    "Nearly impossible" is rather an understatement. There is no physical reason why the heating demand of the two zones should or could be in sync. Completely different heat loss profiles and heating capacity.

    Probably the simplest bet would be to have the garage thermostat simply open the zone valve for the garage, and have an aquastat on the fan in there to blow warm air when there was heat.

    The other possibility is to have either zone valve turn the boiler on for heat as required. That is the more normal arrangement when there are multiple zones, and so long as neither of those zones is too small the boiler won't cycle too badly.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 23,738

    If you have two zone valves, you need two separate thermostats

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Headymask
    Headymask Member Posts: 5

    There's only one zone valve that's controlled by the low voltage thermostat. The other thermostat in the garage controls the fan on the ceiling heater. My problem is those thermostats are never in sync…when the zone valve opens and hot water is pushed through the pipes, the thermostat that controls the fan may no be reading the same and either turns on much later or prematurely.

    I really like the suggestion of the aquastat. How would I be able to incorporate the aquastat to the 3/4" copper lines going to the blower heater?

    PXL_20250109_185833342.jpg
    PXL_20250109_185838363.jpg

  • hot_rod
    hot_rod Member Posts: 23,738

    if you want economical control of those areas, you need two zone valves and two thermostats

    No sense in running hot water to the garage, even if the fan is controlled off, you still have heat going out the coils of the unit heater.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Headymask
    Headymask Member Posts: 5
    https://forum.heatinghelp.com/discussion/comment/1833356#Comment_1833356

    I have to run heat to the garage to prevent domestic water lines from freezing.

    It's not two separate areas, rather a single attached garage with the two thermostats as described above. I agree that running hot water without the fan engaging is a waste.

  • pecmsg
    pecmsg Member Posts: 5,476
    https://forum.heatinghelp.com/discussion/comment/1833363#Comment_1833363

    why

    The heat still radiates to the garage.

    JUGHNE
  • Headymask
    Headymask Member Posts: 5

    It would have to stay on for an extended period to radiate enough heat to keep the garage above 5C in an Ottawa winter.

    Also, the pipes run along the ceiling to get to the heater mounted in the corner.

  • Jamie Hall
    Jamie Hall Member Posts: 25,191

    To go back… you need a strap-on aquastat on the inlet line to that unit heater. Amazon has them. Piece of cake.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    bburdHVACNUTHeadymask
  • hot_rod
    hot_rod Member Posts: 23,738

    This is one type that is commonly used. Find an adjustable model in the temperature range you want.

    Screenshot 2025-01-09 at 3.29.37 PM.png

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Headymask
  • HVACNUT
    HVACNUT Member Posts: 6,442

    As @Jamie Hall mentioned, a strap on aquastat like the Honeywell L6006C1018 mounted on the return pipe is typical for that application. Eliminate the line voltage thermostat completely.

    Headymask
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,765

    With the garage I would have left it as is. Why?

    Because you want to keep the pipes from freezing. If the water is circulated and heated the pipes will not freeze unless you have a power failure unless the circulator and /or boiler fail.

    Now adding a zone valve and a thermostat adds to the complexity and are more failure points. If the pipe is insulated its not going loose that much heat. Stat goes wacky=pipes freeze. Zone valve fails=pipes freeze.

    If you leave the zone valve in place with a thermostat add a strap on aqustat wired in parallel with the thermostat to have the aqustat open the zone valve and start boiler/circulator if the pipe gets cold. You can leave the aquastat set at the lowest temp it can be set at probably around 100 degrees.

    Headymask
  • Headymask
    Headymask Member Posts: 5
    https://forum.heatinghelp.com/discussion/comment/1833416#Comment_1833416

    I appreciate all the responses. You guys definitely gave me enough insight to solve this problem.

    I completely agree with your advice and the design of the old system supports exactly what you're saying. The house was built around having continuous hot water supplied to the garage with the fan kicking in at the set temp. However, this was when oil was probably cents per liter and the old cast iron boiler operated on an aquastat. Now with the modulating boiler being "on demand", it technically wouldn't work.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,765

    Bottom line is you have to keep the pipes from freezing either by antifreeze and a heat exchanger or moving water with some heat in it. The more complex it gets the more failure points.

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