Hi,
I have a question about a difference in behaviour, when heating hot water (HW) in Boost mode, compared to heating HW in the regular programmed schedule mode. This question has nothing to do with using the Nest Thermostat to heat the radiators in our home; it's all about HW control.
Equipment:
- Viessmann Vitodens 100 26kW Regular Boiler (Heat Only)
- Joule 200 litre unvented cylinder
- S Plan system with 2 x Honeywell zone valves for heating (1 zone) and hot water (1 zone)
- Nest Learning Thermostat Gen 3
- Nest Equipment Settings (in thermostat):
- Other boiler, gas, radiators
- Heat control on/off
- HW control on/off
For the last couple of weeks, I've noticed that when heating HW in Boost mode for, say, 1 hour, the circulation pump runs for the full duration of that 1 hour boost period (+3 minutes overrun), regardless of the water temperature in the cylinder. My expectation is that the pump should stop running as soon as the water in the cylinder is up to temperature (+3 minutes overrun).
When heating HW according to the regular programmed schedule, the circulation pump will stop circulating just as soon as the water in the HW cylinder is up to the target temperature (+3 minutes overrun).
Why is there a difference in behaviour with how the HW Boost mode works compared to how it works in regular programmed schedule mode? Surely requesting 1 hour of HW heating should behave exactly the same as an ad hoc schedule of 1 hour duration?
The problem with this unwanted behaviour is that once the water in the cylinder has reached the target temperature, the circulating pump continues to circulate the central heating water through the boiler thus making the HW cylinder behave more like a thermal store and cooling the water in the cylinder, which then calls for heat to restore temperature lost through excessive circulation. The side effect of this is that the boiler keeps cycling throughout the duration of the boost period to restore the heat lost through excessive circulation, thus wasting gas and electricity. And so it continues until the boost period ends.
Why does HW boost mode not behave the same as an ad hoc schedule and stop circulating as soon as the HW in the cylinder is up to temp?
Thanks,
Steve
