We recently moved in.  The only control system for the heating is the target boiler "high" temperature. This temperature was adjusted until the house was comfortable. It was expected and hoped that water temperatures necessary to heat the house would be 95F or below. As the graph below shows, the actual maximum fluid temperature was around 89F. The pipe coming out of the boiler is lukewarm to the touch! This indicates that the effort that went into creating the radiant floors really paid off. During regular heating, this Buderus GB142/30 boiler is operating at maximum (94%+) efficiency.

The graph also shows how the boiler cycles.  The boiler turns on when the fluid temperature goes about 10F below target maximum temperature. The boiler is on (at about 9kW minimum heat) for about 80% of the time. Note that at the time this data was taken, the chimney flues were open, weatherstripping was incomplete and a ventilation fan was on and off. We expect less heating effort once the bugs are shaken out.

If heat were to be continually applied (e.g. from solar heating during the day) then heat could be injected at a temperature about 2F below 88F.

Closer scrutiny of the above graph reveals:

a) the skinny spikes on the heating graph above are due to hot water heating. When this kicks in (mostly to top-up the temperature in the tank) the boiler increases its output to 160F and diverts its flow. When the hot water is up to temperature, the boiler spills a bit of high-temperature fluid into the regular heating system. This results in a spike in the heating manifold temperature.  

b) When the boiler is off the fluid starts to revert to room temperature, although it doesn't get that far because the boiler will kick in at around 77F.

c) The boiler turn-on temperature and high temperature are subject to a setback - influence from exterior temperature sensor. 

Control System Approach

The benefit of starting off with a really dumb, simple controller is that we get to see its weaknesses and it is obvious where sophistication needs to be added. During the coldest part of the year there wasn't any major problem with temperature control at all, but the heat distribution in the heating zones was not perfect, so that is where the first heating control effort will go. As the winter turns to spring, temperatures go up, and it is clear that the boiler can't be turned low enough to operate in this manner, so we need to add a deliberate on/off control capability. This is also simple to do.