Here's a more general heating question for you control wizards out there.  I'm designing a number of systems both GARN and Solar heating with LP backup boilers, and I'm looking to disable the boiler end switch till the tank (garn or solar) can't provide heating water at the temperature required by the reset controller mixing for the floor loops (I want the customer to be able to fire the boiler to 190-200 deg, and leave for 3-4 days (trucker) and have the boiler only fire when the Garn tank drops below say 110 when it's 0F out, because that's where it needs to be at that temp.  then when it's 30 deg out, it'll not fire the boiler till the tank gets to about 100 deg.  this isn't as big a deal with a GARN, but with a solar heated tank, the more we can cool that tank off, the more efficient the collectors will be when the sun comes out.  

trather than arbitrarily setting that setpoint at the highest temp the heating system will ever need, we can pull more heat out.  

this doesn't seem to be too much to ask of a simple electronic device, but I can't find one out there that will do this.  I've looked at everthing I can think of in the tekmar line, and honeywell.  is there someone else out there that's making controls that I should look at? 

thanks,
Karl

 You might check with Caleffi to see if they make anything like that in their solar control line. If you call them see if you can get in touch with Bob Rohr.

Sounds to me like you need an if-but-either-or control 

 I guess put a little more succinctly, I need a outdoor reset controlled setpoint controller.  Varies it's on/off setpoint based on outside temp and a slope/curve I program in.  

I'm pretty familiar with Caleffi's controls, as they're made by RESOL, a longtime solar control manufacturer.  they're both differential controllers.  turn on when panel gets 4-16 degrees warmer than the tank, and turn off when the tank gets close to the panel temp.  both avoids losing heat out the panels, and avoids wasting power when there's not enough heat to harvest. 

 Karl

 Could you use a double pole aquastat, either immersionwell type or strap on, lashed up to the appropriate pipe to monitor the temperature? One pole is normally open the other normally closed. When either side moves the other provides the opposite input control to a connected device. I usually install one on primary secondary piping layouts that monitors the Garn temp (primary loop) and controls a circulator on the load side (secondary loop to keep the "backup" heating system from heating the Garn along with the house.

 I could use 3 aquastats, one to watch outside temp (double throw), and switch between the other aquastats, each with different settings to enable the backup heating system depending on the tank temp. 

say, below 15 or 20 degrees outside temp, aquastat 1 is enabling aquastat 2, which enables the backup system when tank temp (primary loop temp?) drops below 130 deg for the furnace fan coil. when the outside temp is above 20 deg, it enables the backup system when the tank temp drops below 120, or wherever it won't satisfy the heating load.    Crude, but it'd work.  Requiring a lot of fiddling to set up though.  good for the tinkerer. 

of course in a simple heating system control scenario, (like furnace fan coil) it'd probably be easiest to use a 2 stage thermostat.  upper setpoint enables garn heat, and disables the burner, 2nd stage disables garn pump and enables the burner. 

boiler controls, multiple zoning is so much more fun. 

Karl