Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 39 years


• Article 12 - January, 1994


TECH TIP - A liquid space heating control system problem and its solution©


Last week I was helping a colleague troubleshoot a controls problem with a liquid solar space heating system that interfaced with a natural gas forced air furnace. The space heating liquid to air heat exchanger was located in the cold air return plenum next to the furnace. The problem was intermittent and would occasionally result in the house over heating. The heating would continue until the solar storage tank heat was exhausted. Turning the solar thermostat down had no effect. In fact, disconnecting the thermostat had no effect. The pump supplying heated water to the heat exchanger and the furnace fan would continue to run. This made for some uncomfortable warm days in the house. Of course, there were no circuit diagrams available so we had to take time to trace out the circuitry. After getting an idea as to how the system should operate, we proceeded to try various operating options. Since some of the relays were in the furnace room and the rest were in the crawl space adjoining the furnace room, it was helpful to have both of us working the problem, i.e., one in each area watching the relays. Upon going through several operating options, I noticed once in awhile, one of the relays would not move to its de-energized position when power was removed from its coil. So I had power applied to the coil and gently removed the relay from its socket. This was repeated several times. Some of "the time, the relay contacts would remain in the energized position and some of the time they would move back to the de-energized position. Under normal conditions, the solar thermostat would activate a relay that would turn on the furnace fan and a second relay would turn on the solar to heat exchanger pump. When the pump relay stuck, heat would continually be delivered to the heat exchanger even though the solar thermostat no longer was calling for heat. With no call for heat, the furnace fan relay was de-energized. However, the fan continued to run and ran until the solar tank heat was exhausted. The problem -- since heated air was being blown over the furnace gas flame heat exchanger, the bonnet snap switch low limit thermostat was closed, thereby providing an alternate path for power to the fan. This continued to supply power to the fan until the bonnet air temperature dropped to about 90° F. The cause of the sticking relay is unknown. The contacts seem to be fairly clean. I can't manually get the contacts to stick. I need to talk to a relay manufacturer about this problem.


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