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Old 09-03-2008, 03:43 PM   #1
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Problem with fan coil unit/hydronic system.


My house was constructed about 2 years ago and has a hydronic heating system that uses fan coil units to distribute the heat. I have 5 thermostats distributed around the house. They are all 2-pole line voltage tstats.

If the tstat is "Off" neither the fan operate nor is the boiler activated to heat the water. If the tstat is "On" but below (even well below) the room temperature the fan never turns on BUT the boiler is constantly heating the water and the water is constantly pumped to the fan. This is grossly inefficient as even when the house has reached its target temperature the boiler is still working. Until I noticed this my energy bills were 3 times what they are now!

I would like to install a programmable thermostat to set up a heating schedule but I anticipate this will not prevent the core problem with the boiler always being on.

Is this system working the way it's supposed to? All 5 thermostats cause the problem (i.e. if any of the 5 aren't "Off" the boiler is on). It seems extremely inefficient and I have a hard time believing this is intentional. Since each fan also has a on/off switch on the wall it would seem much better to just leave the fans "on" all the time and have the boiler kick in when the temperature falls below the setting. If I reversed the line/load wires in the thermostat could I accomplish this? Is that safe to do?

The ideal solution would seem that BOTH the boiler and fan be activated when the temperature falls below, and NEITHER is on when the temperature is above. How would I accomplish this? In researching thermostats I've not read about any that seem to deal with this problem.

Thanks all for your help!

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Old 09-05-2008, 03:58 PM   #2
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Problem with fan coil unit/hydronic system.


After having the chance to do more research and disassemble and inspect the wiring I know the following additional information...

My heating is controlled by a single pole breaker in my panel, 15A.
My thermostat has 2 line wires and two load wires (i.e. double pole). Both line wires are wire nutted to THREE black wires coming from the wall.
One load wire is nutted to THREE red wires leading into the wall. I've confirmed the three red wires is the heater/pump load.
The other load wire is nutted to a single black wire. This is the fan load.

By switching the load wires I was able to get the desired behavior that the heater/pump does not start up until the room temp is below the tstat temp. This means the fan is always getting power when the tstat is on, but the fan unit has an aquastat that waits until the water reaches a certain temperature (140F I think) before the fan spins. So this has solved my immediate energy/gas bill problem.

Now to the issue of selecting a replacement thermostat. It would seem to me I could use any 120V double pole thermostat since that is essentially what I have now. My question is, if I circumvented the switch between the fan load and the line by wiring the fan load directly to the line wire (i.e. replace the "line" wire from the tstat used for the fan with the actual fan load wire from the wall) and then used a single pole thermostat to switch the heater/pump load would I encounter any problems (other than the fan always being on)? Would this violate any code?

Hoping someone can offer some advice here, thanks...

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Old 09-05-2008, 04:57 PM   #3
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Problem with fan coil unit/hydronic system.


i'll take a stab at this.... my hydronic heat is slant fin(around the perimeter walls no zones) off a Weil-McClain boiler and pump.i have a Honeywell Chronotherm IV....7-day digital stat when the stat calls for heat the circulator runs and loops the water thru the boiler the internal limit is what brings the gas on to heat the new water from the baseboard.so in short the stat controls the pump and the boiler is cold when the call starts to heat.you need to see if your internal limit control is off the circuit when your zones are not calling.if not your boiler is maintaining the water until the pump call pulls it out...need to verify what is running when the heat isn't called for.on the 5 zones when one calls the other shouldn't be that far behind.the fan coils should only be controlled by the stat seems like you have no "control valves" that their using the fan as the off/on to heat.might want to consider putting a digital stat to control the pump which in turn wil controlthe boiler with water being heated...then let the stats cycle the fans as per zone so you elimnate 5 points controlling the pump.i service commercial systems larger the a residential set-up like that what they do with larger fan coils is the inlet pipe has a temperature KLIXON that closes when the pipe gets to a heating temp and the fan comes on with the wiring run thru a wall stat for heating.if the stat calls and the pipe is cool (no fan) if the klixon closes hot water in the pipe but the stat isn't calling( no fan)
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