Seisco Temp Jack-up and Reset Connections

David_24

This is a continuation of the post "Taco 700 Reset - where to put sensor?" I thought the info here-in might deserve it's own post.

The elecric boiler in my system is an early version of the Seisco RA-22 - maximum temp 130. I learned I could not update the board to a SH version to get the additional heat output to make it useable with the convection part of my system. Even if I could upgrade, I was told the max temp would be 145 degrees. I was facing tossing it and buying a "real" electric boiler. Given that, I went for broke to modify it and save the $500 investment. I can not recommend this be tried as I do not know the ramifications yet, but it is presently working great to deliver 160 degree water.

The resistor values in this modification apply only to the Seisco with the older 200 ohm sensors.

The resistance of the sensors (5 of them) in this model drop as the temperature goes up. I decided all I had to do was add some resistance to the sensor circuits to fool the computer into believing that the water temp is lower than it actually is. And by using the "disable" connections on the Seisco as detailed below, it would add a measure of safety against the modification screwing-up the water flow shut-off sensing.

To modify the Seisco, I made 5 "adapters" consisting of 33 ohm 1/2w resistors - then crimp an insulated male spade lug on one end and an insulated female spade lug on the other end of each resistor. I then pulled one of the temp sensor spades from the circuit board on the Seisco, and inserted the adapter in series - the female spade of the adapter went to the vacated circit board connection, and the removed temp sensor wire plugged into the male spade of the adapter. This was repeated on all 5 temperature sensors - the 4 main and one input sensor.

Then I testd the Seisco by circulating water in the boiler loop only (with current meter clamped on an element to determine operation). I started with the temp control reduced, and advanced it until the water temperature reached 160 degrees - SUCCESS! Also - when I turned off the circulator, the Seisco sensed the flow stoppage and shut itself off as usual - great. I'm squeamish about going any higher in temperature as I'm not sure how much heat the polymer case can handle (which might also require larger value resistors in the adapters) and also because the built-in 185 degree high-temp limit switches might kick-in, causing the unit to lock-out.

My mindset at this point is that I had to either make it work or toss it, so I had nothing to lose. I'm posting this for information only - can't recommend someone else do it as it might wreck the heater - but so far, so good here. Also - if a newer Seisco with 10,000 ohm sensors is modified, the adapter resistors would probably need to be 1000 to 1500 ohms (10% of full resistance) - but I have not tested this.

There are 2 undocumented and unused spade connections on this Seisco heater. One pair of spade contacts labeled "Disable" allow the unit to be turned on/off via external normally closed contacts - to make them work, you have to cut the tiny circuit board trace between the contacts. There are also 2 unused spade connectors near the relays - these have 240 volts on them whenever power is supplied. I used the "Disable" for external reset control, and the 240v contacts to fire a 240v relay which I mounting inside the Seisco cabinet - a pair of contacts on that relay signal my external "steering" relays to determine if the dual-fuel electricity is presently on for electric, or else steer the ZVC406 end switches to the propane boiler as outlined in other post.

As mentioned in the other post, I used an external group of 24v steering relays. When the Seisco electricity is on, the ZVC406 "pump" switch is routed, through steering relays, to the Seisco circulator controlled by the SR506. The "boiler" switch is routed through the steering relays to the Seisco "Disable" contacts (be sure trace is cut as indicated above). The circulator stays on as long as a thermostat calls for heat, and the Taco PC700 causes the Seisco output temperature to be reset per outdoor temperature. It works very very well. The taco PC700 can take the propane boiler up to 185 if needed - but if the Seisco is in use at that reset temp, it will float-out at 160 from it's own internal controls - still enough to heat the house, albeit a little more run time. One other word of caution - I'm not totally sure if controlling the Seisco reset using these "disable" contacts is hard on the element relays, or if it is causing a softer shutdown by signaling the Triacs - they don't provide the schematic so I don't know how the "disable' contacts work.

Also - The Seisco has a slightly nasty habit in it's stock form, which this reset connection to the "disable" contacts solved. If water flow is started to the Seisco, then stopped abruptly before it can "float-out" on triac-reduced voltage, it has a tendency to start boiling the water inside the unit before the processor can determine that the flow is stopped - kinda ugly. By using the reset system described on the "disable' contacts, the heat shuts down immediately as the circulator pump stops - preventing the 1 or 2 seconds of "boil-off". So using the "disable" contacts, even if your configuration is much different, may not be a bad idea - protects from someone moving a thermostat then quickly moving it back again.

Again, this mod was make it work or toss it out - nothing to lose for me, except an afternoon of clearing smoke out of the house if a catastrophic failure happened. This is info only - not intended to duplicate without your own risk of damaging the heater, the warrenty, your pride, or the house. Frankly, as someone in the electronics business, I think the combination of sensitive microprocessor circuits controlling high-power devices is always scary - I'm hoping those built-in high-temp limit switches in the Seisco are wired to kill the element relays directly rather than talk to the Micro-P which might find itself brain-faded by an errant lightning strike - if so wired then it should be relatively safe (fingers crossed).