[Greenbuilding] Triac Thermostats

[Corwyn]

On 12/19/2011 9:28 AM, John Straube wrote:

> So, for example, a baseboard of 1500W output may turn on for 10 minutes,
> then turn off and the room cools down for 20 minutes, and then the cycle
> repeats. The temperature falls to, say, 68F, the heater turns on, the
> temperatures rises over the ten minutes to, say, 71 F, and the switches
> off again.
> With a a modulating control like a Triac, a very smart thermostat would
> attempt to adjust the output of the heater to 500W continuously (because
> 500 Watts for 30 minutes delivers the same energy as 1500W for 10
> minutes every 30 minutes). This means the average current is lower, the
> electric heater runs at a lower temperature, and the room temperature
> stays more constant.

This sounds great on paper (phosphors), but I don't see how it helps in 
real life.  If your thermostat is only accurate to 1.5 degrees 
(hysteresis of 3 degrees as you baseboard example) then the temperature 
is going to rise until it reaches 71 and then adjust; same as the full 
bore baseboard.  It is not like it KNOWS it will need 500W in advance.

If the thermostat is more accurate, then you might as well use it on the 
full bore (presumably cheaper) baseboard.  With a really accurate 
thermostat, you could probably get down to 1 minute on out 3.  Does 
anyone really think that isn't good enough?  That they need modulation 
by the second?  That the thermal mass of a baseboard heater even 
responds that quickly?

I guess there could be an advantage of using a triac over a relay if the 
MTBF is significantly longer (more cycles).  Is that the case?


Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
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