The Hydronic Loop Cycler

After a couple day cold snap, my best friend called me up to discuss the upcoming weekend.  It turned out that he and his fiancée had spent a full day defrosting one of the zones on their radiant hot water heat.  To make matters worse, the pipe managed to burst in three places, requiring repair.  Needless to say, I felt very sorry for his ordeal and wanted to help him prevent it from happening again. 

There were several factors that led to the problem.  First, he owns an older farmhouse, which has known insulation problems from the time he purchased it a year previous.  The pipes for this particular zone run through a crawlspace that is rather drafty.  Second, the night that this problem happened, the temperature dropped well below zero (Farenheit) with a stiff wind, allowing the cold to penetrate this crawlspace more easily.  Third, they had been using the woodstove to provide heat in the living room, which essentially relieved this heating zone of its duties for the evening.  This inactivity was only extended when the thermostat automatically set back the temperature at night, keeping the zone off for even longer.

So, what do you do?  Ultimately, you don't want to allow the pipes to cool off to a temperature that will allow the water to freeze.  The solution in this case was to simply cycle the heating loop on periodically to keep it warm.

  Circuit Description

Click here to view the schematic

Basically what I needed was a timing circuit that would just switch on the solenoid from time to time to send 160°F water through the loop.  I could do this easily by just closing the connection that the thermostat normally uses to call for heat.  I also wanted to add a design feature that would active the timer only at temperatures where it was needed.  This would alleviate my friend from having to remember to switch the unit on when needed and off when not.  I accomplished this through the use of a 10k Ohm thermistor available at Radio Shack (Catalog number 271-110) as my temperature sensor.  I also chose to utilize the 24VAC source used to control the servo valves so that I wouldn't have plug the unit into a wall outlet.

The first portion of the circuit is nothing more than my power supply.  A bridge rectifier converts the 24 volt AC line supply to DC.  A 2200µF Electroyltic capacitor does the primary smoothing of the resulting waveform into a more stable DC supply.  I used an LM317 adjustable voltage regulator to drop the 32 Volts DC to about 6.5 Volts DC (remember that 24 Volts AC, is 24 volts RMS, which works out to a peak voltage of about 33.6 volts, then subtract about 1.4 volts lost in the rectifier).   One point to note: The schematic does not show a 1µF capacitor that I added across the output of the regulator to ground.  This is recommended by the manufacturer to improve transient performance.

The second portion of the circuit involves the activation temperature control.  I employed the use of the venerable 741 operational amplifier to perform a voltage comparision between the setpoint and the voltage divider created by the resistors and the thermistor.  Some technical notes here;  While you need to watch out that the resistor values aren't too large that the amplifier's bias currents don't become a significant factor, it's just as important that the current flowing through the thermistor remain as small as possible.  Remember what a resistor does, it dissipates power in the form of heat.  So, if the current flowing through the thermistor is significant, it will begin to heat up, introducing significant error in the temperature being measured.

The pot in the voltage divider sets the threshold voltage where the amplifier changes states.  This voltage subsequently corresponds to a particular temperature threshold as determined by the thermistor's characteristics.  To allow for fine tuning, I used a 15 turn pot so that the voltage can be adjusted at very minute steps.   When the thermistor cools off to where the voltage  drops below the setpoint, the amplifier output goes high, switching on the transistor to power the timer circuit.

Now, there's a switch  leading into the timer circuit.  This switch has a center off position, as well as its two on positions.  One position is the automatic mode, and allows the temperature detection circuit to determine when to activate the timing circuit.  The other position is the manual mode, where the timing circuit is always activated.  A small technical point here.  The voltage sent to the timer from the temperature control circuit is about a volt and a half less than when the switch is in manual mode.  This is due to the headroom required by the 741 op amp and the voltage drop across the transistor.  Reflecting back upon the design of this circuit, I could have probably gotten away with powering the timer directly from the output of the op amp, but wanted to make sure that I wasn't creating a short condition. Originally, I was considering a different switch arrangement that would have called for an open collector type design to prevent this from occuring, thus the transistor.  Regardless, this voltage difference is negated in the way the timer works.  Since the timer works with voltage ratios to the input voltage, the timing remains essentially the same duration at different input voltages as long as the input voltage remains steady and constant during timing.

The third part of the circuit is the timer itself.  Here, I used another very popular IC, the 555 timer.  The timer is basically connected in an astable multivibrator fashion so that it cycles on and off.  It does so REALLY SLOWLY in this case.  The diode is added so that the timer's on duty cycle could be set below 50%.  To aid in setup, I connected a pushbutton across the capacitor so that it could be momentarily shorted out, essentially resetting the timing cycle. Now, the 555 works on the charging and discharging RC time constants of the capacitor and the resistors between the voltage input and the discharge.  Normally, the circuit oscillates at a frequency much faster than what I have setup here.  In such cases, many resistive losses can be ignored.  However, in this case, such losses actually become somewhat significant.  On the discharge side, I have a 4.7M Ohm resistor.  Using the formula, T = 0.7*(RC), this should give me an off time of about 54 minutes.  In actuality, I have an off time closer to 40 minutes.  This can be accounted to resistive losses within the capacitor, as well as possible resistive losses across the printed circuit board (remember, a megohm makes a BIG difference in this case).
The on time is adjustable through the 500K pot.  A 10k Ohm resistor was placed in series so that the minimum on time was at least 10 seconds.

The output drives an indicator LED as well as a small reed relay.  This relay creates a closure in the same way the thermostat would do when it calls for heat, turning the heat zone on.  Note that I have a  recoil diode across the relay.  This is important when driving a relay from an IC, as the relay coil can kick back some nasty transient voltage spikes when it is deenergized.   Also, if a larger relay is used, you might consider placing a transistor in between to help provide a stronger drive for the relay, as well as adding a buffer between the relay coil and the IC. 

With this arrangement, I have the circuit setup to turn on the heating zone for about 2.5 minutes every 40 minutes to prevent it from cooling off to the point at which it will freeze up.

©2004  Ray Meyer, N9PBY