I just bought a house with a central boiler and circulating pump that provides hot water to hydronic heaters with blowers to each room. Each room has a MD26 thermostat which controls the circulator pump and blower. The circulator pump turning on will call for heat from the boiler. All the thermostats join to a TACO SR501 switching relay that is then wired to the circulator and the boiler. The issue is that the thermostats are unable to properly regulate temperature because the blower and circulator seem to operate separately. When the dial is turned right a relay clicks and the blower turns on, however the circulating pump will not come on until the dial is turned further right and there is a second relay click. The way I would like the heating to function is that the blower and the circulator will kick on at the same time instead of having to turn the dial until there are two clicks. With the way the thermostats currently function, the blower will never turn off even though the circulator pump stops and the hydronic heaters go cold, eventually blowing cold air in the room.

Will installing a newer smart 120v thermostat such as Mysa or Sinope in each room solve this issue and kick on and off the circulator and blower at the same time?

  • do you have wiring diagram for us
    – Traveler
    Commented Dec 1, 2022 at 18:43
  • You could invert the blower and circulator wires on the thermostat. So it would enable the circulator first, then the blower if hydronic heating isn't enough.
    – bobflux
    Commented Dec 1, 2022 at 19:17
  • I will attempt to create a wiring diagram based on what I am able to access later today. I just took a look at the TACO relay that I have installed and it seems to have the thermostats, circulator, and boiler connected using the "Typical Wiring" layout as found in this diagram: cimg0.ibsrv.net/gimg/www.doityourselft.com-vbulletin/642x1116/…. Is it possible that the current thermostat behavior described in my post is due to how the thermostats are connected to the TACO, or is it more likely due to the way they are wired to the blowers?
    – Ham Clam
    Commented Dec 1, 2022 at 19:20
  • @bobflux, this seems like a good tip assuming that the order that the blower and circulator turn on is due to the wire configuration on each thermostat. What still might need to determined is how exactly all the thermostats join before reaching the TACO relay as this may be a factor.
    – Ham Clam
    Commented Dec 1, 2022 at 19:22
  • 1
    Hmm. The normal setup for these is the thermostat calls for heat, and a thermo-switch clicks on for the fan when the coil is hot (so it won't blow cold air, and it will continure to blow until the coil cools down when the pump stops.) The fan usually has no direct connection to the thermostat.
    – Ecnerwal
    Commented Dec 1, 2022 at 19:31

1 Answer 1


Normal behavior for 2-pole line voltage thermostats

On those, the two poles act differently.

  • One pole is switched on anytime the knob is moved from the "OFF" position. It is connected at all times except when in "OFF".
  • The other pole operates through a thermostat and connects or disconnects based on temperature.

This is big news to many people including myself; I expected a DPST switch where both legs operated together on thermostat control.


Was this house a "flip"? Was it a "hit and run" sort of deal where no one ever had to actually live in the house with this thermostat?

NEC 110.3. Use according to instructions/labeling.

Now a line voltage thermostat is made for switching high amperage (like 20 amps) to an electric resistance baseboard heater. So I am surprised to hear of one being used for a hydronic system. I mean obviously they're capable of it, but I'd expect low voltage thermostatic controls.

These line voltage thermostats are simply not made for that use, and it violates the wiring instructions to have them control two completely separate things. As discussed, that just doesn't work.

NEC 725.55 separation of low-voltage systems

The other problem here is that the wire which opens the Taco flow valve is low voltage, note the "R" and "W", which a standard 24-volt thermostat shunts to call for heat. The wire which switches the blower is line voltage. Low voltage and line voltage are not allowed to be mixed inside the same enclosure, so they can't be on the same thermostat unless it is specifically made for that, with some sort of divider.

So it appears the very design here is faulty, and probably was not done with a permit. It may be time to call in an experienced hydronic professional and clean up the mess.

  • I just learned something so +1
    – bobflux
    Commented Dec 1, 2022 at 20:43
  • @Harper-ReinstateMonica, thanks for the detailed response. As to whether heating system was installed as part of a flip, I don't believe it was. This is a townhouse built in 2008 (USA) and I have been in another unit and its heating system is set up the same way. However it's totally possible that the current set up is not up to code. If a low voltage thermostat is what should be used, how are they typically set up to also kick on a blower? If the TACO relay I have is low voltage, how is it working with the current t-stat? Is it likely there is a transformer involved somewhere?
    – Ham Clam
    Commented Dec 1, 2022 at 21:32
  • This does indeed seem to be a mess that calls for a pro.
    – Ham Clam
    Commented Dec 1, 2022 at 21:33
  • 1
    @Ham it's a 2-pole thermostat. They put line voltage on one pole and low voltage on the other pole, violating NEC and the device's instructions. The right way is to make the whole shebang low voltage controls, with a relay to turn on a 120V circulating fan. Commented Dec 1, 2022 at 21:35
  • 1
    @HamClam The relay with low-voltage coil and line voltage switched terminals is set up with the appropriate separation between low and line voltages, and that's how it's done.
    – Ecnerwal
    Commented Dec 2, 2022 at 1:00

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