System Temps and Fine Tuning the System

DHW Temps: The heating of the domestic hot water (DHW) by the controller (vitosolic) is working great. I am heating the DHW to 50C. I have a tempering valve which mixes cold supply water with the hot tank water to keep the hot water from being at excessive temps. I also have a hot water circulating loop. This is a common system used to stop a long wait for hot water at a tap in large homes. The problem is that the water circulating is not tempered, but at the actual tank temp, so the water from the tap starts out at full temp (50C). Not only that, but the higher temp in the circulating loop (even though it is insulated), will result in higher heat loss. All that, plus the PEX tubing used in my house plumbing will not tolerate much higher temps, so that is why I am limiting the DHW tank to 50C.

Heat Collection: The default differential settings for circulating glycol through the collector and into the storage tanks are starting 5C (9F) and stopping 3C (5F). With these default settings, the pumps only cycle for a short time, on the average only a few minutes. The short cycle time comes from converging temperatures. As the cooler fluid from the tank being heated is pumped to the collector, the collector temp drops, and drops faster than the tank temp rises.

To lengthen the cycle time, I will raise the starting temp. I will not lower the stopping differential temp because doing so will likely result in inadequate differential for heat transfer to occur. I plan on starting with raising the starting differential to 7C. The collector temp comes up quickly when solar gain is good (from 10AM to 3PM), so time between cycles should not be much longer. I am betting that the increased length of time between cycles will be matched by an increase in the time length of the cycles.

Radiant Heat Delivery: Another important operating aspect of the system is delivery of heat to the radiant floor system header. The pump (P6 on the schematic), starts and stops on a temperature differential between the tank (S5), and the return line temp of the Low Loss Header (S6).

The tank must be at a high enough temp to match the temp that the boiler delivers to the header to provide radiant heat to the floors. This is tricky because the boiler control system modulates the header temp based on the outside temp, and a day/night settings where at night the header is ran at a lower temp than during the day.

The radiant heat storage tank temp can only be set at one temp, so I have set it's max temp at the top of the outside temp/boiler temp heating curve. So far, my collection capacity is not enough to hit the tank limit. What is available is the differential temp that the header return (P6) can be set at. I have been monitoring the header temp during operation, and it has become evident the the header temp drops when a zone calls for heat. After measuring the Low Loss Header (S8 in the schematic, not connected to the Vitosolic) and the return (S6), I have generally seen a 30 degrees F (about 16 degrees C) differential during operation. Based on this, I have set the S5/S6 differential to turn on the P6 pump to start a 16C, and turn off at 12C.

I will be monitoring the effectiveness of the new settings and posting the results in a week.