If you put a zone stat in a hall, it needs to have a rad in that space. Otherwise, the stat will remain in a cold condition and heating will be on continually in the bedrooms, be that limited by trvs if fitted, and if so, putting the boiler in a state of constant bypass as it spends most of its time just heating up the zone circuit.
The alternative solution I can think of for this is to down size the radiator in that room or move the thermostat to other side of the same room. I have another solution explained below to eliminate both thermostats and use digital TRVs. It’s going to be long but might give someone in my situation some foot for thought in future. Please skip to the end to see what I did.
I did some study and I think if found the problem and system is working fine. The installation manual seems to be inheriting many things from older boilers which are not relevant.
This boiler has an automatic pump that keeps the “flow” constant. All information available online and the installation manual talk about a specific temperature drop over radiators. This is only applied to systems with constant head pumps. I.e. those that you set the speed. if we use an analogy to electric circuit, to have constant flow (current) on each radiator, you have to adjust the pressure drop (resistance) when we have constant supply pressure(voltage). Since the heat is proportional to amount of hot water that passes per time unit (flow) we can use temperature drop to measure it relative to other rads.
The pump in my boiler maintains a constant flow (current) automatically. That’s why even if I almost shut all valves, it will ramp up the speed and pressure to maintain the flow. Therefore regardless of valve position, the same amount of flow passes and temperature drop would be the same. Of course this until pump reaches its max limit that is not logical to run it at that all the time.
How the flow is decided? My boiler has a weather competition outdoor sensor that is simple 10k NTC. It also has a “room temperature knob” that according to manual it should set the desired room temperature. It doesn’t explains but what it means is that you could scrap the room thermostat and let the system control everything. It will use delta T between flow and return at boiler to figure out actual room temp instead of a wall sensor and it uses outdoor sensor to predict room temp easier. Think of it as a built-in opentherm thermostat. Then you could use TRV for each individual room. You probably have to over design the system to achieve it.
Unless you use digital TRV with timer, you cannot switch off like downstairs automatically over night. Also they should be wifi enabled otherwise for each change you have to adjust all of them every time. (This is another solution for me)
for my situation, there are two zones. two room thermostats eliminate the benefit of having a system that quickly can adjust its output. Therefore it runs at a constant output and shuts down the system until room is colder than desired. Let’s use another analogy and use cruise control as an example. It’s like you want to travel at 50mph but set the CC at 70mph. Then manually (like my thermostats) cancel the CC when you reach 52mph and resume when speed drops to 48mph and go on. The problem with that is, ECU thinks you want to reach 70 when you resume so car wants to accelerate harder. While when you set the cruise at 50mph, it reduces the throttle before 50mph and keep the throttle adjusted using engine load instead of speedo changes. It runs way more efficient. Unfortunately this is my current setup and it has to change in future.
another alternative solution is to use a multi zone opentherm room thermostat that does the same thing but because I have two, I can stick to mechanical TRVs.
My boiler uses outside temperature and set point at knob to decide about the flow. It also sets the output and return temps to define system output. When I first attempted the balancing, outside was around 3’C and I had knob at 25’C. Using the graph below, it was running at around 70% of its max flow. I thought if I have system balance at highest flow (70%), it will not cope with lower flow rates. I decided to balance it at 30%. I had two ways, drop the knob to 11’C that at 5’c outside will have pump running at 32%. It worked but the burner didn’t kick in because return was 20’C. I found that at 20’C outside, sensor resistance should be 12.5kohm. Replaced the sensor with a 12.5kohm resistor and set the knob to 22’C. Even with low flow, I didn’t need a 20’C drop as explained above, instead , I adjusted the lockshield valves to have the same flow temperature at the inlet of each radiator. This was normally around 2-3’C below boiler output. Interestingly, at the very same position the noise at lockshields were eliminated. Reconnected the outdoor sensor. Set the knob to near max. Had the system run for couple of hours and I noticed the boiler is doing its job. Sometimes on the same radiator delta T was about 20’C and sometimes 2’C. Also the burner is not on all the time when pump is working. I guess unlike normal condensing boiler, a combi would benefit from the exhaust gases at any temperature due to very high flow, high efficiency heat exchangers and because of its control on burner output power, it doesn’t need 20’C, it will drop gas flow until there is enough exhaust heat to be absorbed by return water. I check the flue gasses regularly and there is no fog coming out anymore.
now rooms heat up more uniformly and it can reach temperatures I want. In the past, even if I set room thermostat at 27’C, so long as the knob was set to 22’C it would never reach 27. Because system automatically reduce the output power to minimum when return temp suggests room is at 22’C. Room stat is nothing more than a timer when the knob is not set at max.
