Following all the usual recommendations for heating controls, I have TRV's on all radiators except for the one in the downstairs hallway which also has the overall demand thermostat in the same (quite large) space.

 

What I notice is that from cold everything heats up quite well but as the rooms approach the desired temperature, the rate of heating slows down. At this time the return temperature to the boiler is going above 55^oC and the (condensing) boiler is losing efficiency. The flow temperature is set to 65^oC, and the boiler may cycle on and off a few times before the setpoint is finally reached.

 

So the boiler is modulating down to its lowest power level for the last push to reach the overall setpoint measured in the hallway and many TRV's have all but closed by then. It seems to me that just by having the radiators correctly balanced and no TRV's, it might not spend so long at minimum modulation. Seems rather fundamental but everyone appears to think TRV's are a good thing.

Thinking on a bit more, what's the alternative? If a rad is in a room with solar gain then the room may well overheat with a fixed flow. So bring back the TRV! The problem is more indicative of having a buffer to separate the work the boiler has to do from the work required to get every room up to temperature. But the radiators themselves represent a large volume of water so why isn't this enough to buffer the demand? Could it be that the flow rate is too low I wonder?

Not sure it's anything to do with the buffer. If the trvs are closed the radiator volume is excluded from the heating circuit, so the flow route closed.  Then you are trying to dump xxkW through a single radiator and a buffer.

 

Ideally you would reduce the flow temp, the heating would take longer to heat a room, but you Trvs would remain at a more open position as there would be less likely hood of overshoot.

 

The alternative is to balance the system to reduce overall flow through the radiators so you get a larger delta T and radiators give less heat to the room, trv remain open unless you get solar gain.  Or do weather comp.

5 minutes ago, JohnMo said:

The alternative is to balance the system to reduce overall flow through the radiators so you get a larger delta T and radiators give less heat to the room, trv remain open unless you get solar gain.  Or do weather comp.

 

Yes, my options are a bit limited in that area until I get to grips with the glowworm eBus protocol. I've got a Flexicom that can be controlled via that interface but it's something I've been putting off as I know it'll be like heading down into yet another rabbit hole. But without doing this I'm stuck with a minimum flow of 65^oC to achieve DHW of 55^oC as I only have manual control over one flow temp.

You need load compensation, to reduce the flow temperature before you hit the room setpoint temperature. Reducing overshoot and minimising cycling as you approach the target temperature.

 

That typically requires a boiler with a very wide modulation range, or at least one that isn’t massively oversized.