Heat Rejection System Scheme Tested

If you look at the schematic diagram for the system, you will see a heat rejection loop. This was intended to be used if the collector had heated both tanks to their limits.

Instead of using an exchanger coil and a fan, as shown in the Viessmann control scheme, I always intended to turn on a few selected radiant heat zones by overriding their thermostats and driving the zone valves open. Doing this means that I will not have to "waste" the heat.

This has been accomplished by using a temperature probe (S3) to operate a programmable thermostat in the Vitosolic controller (thermostat 1). The thermostat turns on the S3 output, which drives the coil (120 volt) of an "interposing" relay. The contacts of this relay short out across the zone thermostat and open the zone valve. I was able to do this because I "home ran" all the thermostats as well as the wires to the zone valve manifolds.

I wired in the relay today and lowered the thermostat on and off temps below the tank temp and turned the interposing heat rejection relay on and off. The thermostat was overridden and zone valve for the test zone opened and closed.

In order to understand the scheme, you need another piece of information. There is a pump that delivers hot water to the boiler low loss header (P6) on the schematic. Like all the other differential controls in the system, temperature is measured at the source (S3 in the tank), and destination (S6 at the header). When the source is 5 degrees centigrade above the destination, there is adequate heat differential for transfer. Once the P6 pump turns on, the boiler will be suppressed from firing and delivering heat, saving propane.

In order to maximize the benefits of this system during this winter, I will program the relay to open zone valves when the tank is 10 degrees above the low loss header operating temp and deliver heat to the main living area zone. This will take some occasional adjustments depending on the outside air temp, as the Viessmann boiler controls raise the low loss header temperature as the outside air temp drops according to a programmed curve.

Right now it is looking like I will need another vacuum tube array to have enough collection capacity to heat the DHW and effectively heat the house. As we progress into winter, the sun angle will drop clearing the bottoms of the tree foliage that blocks the mid-day sun and also reach the highest efficiency angle respective to the collector inclination, so time will tell.