Thermostat choice for multi-zone in-floor heating


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Old 02-12-12, 06:39 PM
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Thermostat choice for multi-zone in-floor heating

We have a zone per room on the main floor. The 3 bedrooms have their own stats, the foyer bath and laundry share the foyer stat. The stats control zone valves that all share one circ pump.

This season, I turned the mixer that feeds the slab down a little more than usual as a test. I thought this would make each zone run a little longer and provide some overlap for the circ pump.. reducing its on/off cycling a little.

Granted, this is only by 'ear', but it seems to be making no difference. I can hear the circ relay doing its thing just like every other year. That, and the rooms seem slower to react with general temp change. Even taking into account the lower temps into the slab, it doesnt seem right.

Then it dawned on me.. Im using regular cheap tstats. They all have anticipator values. Wouldnt this be turning off my zones before they should ?? Do 'genuine' radiant heat stats (Wirsbo et al) not include an anticipator ? AFAIK, I just want a simple on/off temperature switch.

Id like to finish the other 2 rooms with their own stats, but if the solution is to buy genuine Wirsbo stats at $50 a pop (x7).. maybe I could find some of those old school round stats and rip the anticipator circuit out ?
 
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Old 02-12-12, 07:29 PM
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I don't think turning the mix temp down is gonna do what you think it will.

The boiler is still heating the water to a higher temperature just as quickly.

You still need anticipators... in fact, with slab heat the anticipator is somewhat more important in that it needs to cut the heat off well before the room reaches setpoint in order to keep the rooms from overshooting the setpoint due to the energy that is stored in the slab, and continues to be emitted from the slab after the pump stops.

This cycling problem is the reason to not have too many small zones (called 'micro-zoning').
 
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Old 02-12-12, 07:42 PM
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So a proper anticipator is needed. I see the value of that.
The last time I took one of these cheap lever type stats apart, I figured the anticipator was just a heater with a variable resistance.. basically heating the stat up to trigger early. No real smarts as to how fast the room is heating up (or not). So I figured that my turning the mixer temp down was allowing the rooms to cool more since the stats were 'self triggering' the same as before.
Do some have more appropriate smarts when it come to radiant heating ? (heh, maybe thats why the Wirsbo et al ones ?)
 
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Old 02-13-12, 02:53 PM
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That's really all the anticipator is... a tiny heater inside the stat which as you say will cause it to shut off a bit earlier, before the stat reaches it's setpoint. The early ones were simple indeed, as you say, a resistor (wire) that heats up with the current of the circuit passing through it.

The modern electronic ones have some 'smarts' built in. The majority don't even have an anticipator setting. They 'learn' over a period of time how quickly the room heats up, and adjust themselves to match the conditions.

For slab heating, there are thingys called 'slab sensors'. It's basically a temperature sensing element that is literally embedded into the slab. These need to be put in during construction though. I guess it's possible to chisel or saw a groove into an existing slab but don't think it's worth the work.
 
 

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