Temperature of a radiator?

As you say @JohnW2 with my oil fired non modulating boiler having a programmer feed a thermostat and thermostat feed a zone valve and zone valve feed boiler and pump is fine.

But with a modulating boiler you don't want to be switching it on/off all the time, so the zone valve is not controlled by the thermostat, it is only controlled by the programmer, so there needs to be a way to switch off the boiler once all zones are satisfied, to stop it cycling all summer, easy way is a thermostat in a cool room which is always heated has no outside doors, or alternative heating normally on the entrance floor, it is set to only switch off the system on warm days. But clearly this room would not be part of the zoned system but heated at all times.

But the ebus thermostat has an added advantage in that it allows the boiler to run cool before starting to cycle, so less heat is lost through the flue, it can also control a sample room rather accurately and be connected to weather monitoring, so it would be better placed in the main living room, and the lock shield valves setting so this room needs heat for longer than any other room in the house, but it clearly can't be a room controlled by a zone unless there is a method to switch to some other thermostat when that zone is off.

Using Wiser, Hive, EvoHome, or Tado each TRV head is a zone, and the thermostat is designed to keep the boiler running if any room wants heat, Hive is an odd one out as it does not use the ebus, but relies on the boilers own algorithms to turn down the output, so with Hive two wall thermostats could be connected in parallel, although not really required, as long as a TRV has sent a demand for heat in last ½ hour in theory it should run, does not matter how hot the room with the thermostat is, but what is the point having block zone valves when every room is it's own zone, kind off defeats the whole idea.

So assuming nothing more than a simple TRV in each room, so TRV only stops over heating of any room, just can't see how two zones valves will work, and once you start fitting programmable TRV heads, then the zone valves become pointless.

ericmark said:

As you say @JohnW2 with my oil fired non modulating boiler having a programmer feed a thermostat and thermostat feed a zone valve and zone valve feed boiler and pump is fine. ... But with a modulating boiler you don't want to be switching it on/off all the time, so the zone valve is not controlled by the thermostat, it is only controlled by the programmer, so there needs to be a way to switch off the boiler once all zones are satisfied, to stop it cycling all summer ....

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As I said, I don't see any problem provided one only uses the zone valves as zone valves - i.e. to control which parts of a central heating system are connected (in a plumbing sense) at at a particular point in time. It's no different from having a series of, say, gate valves which one operates manually to determine which parts of the house will get heated.

Once zone valves have been set in the required configuration, whatever systems you have for controlling radiators and boilers can just function normally, just as if there were no zone valves.

As I said, the problem will arise if you try to use the zone valves to do anything more than act as zone valves, because their intrinsic on/off functionality is not compatible with the concept of a modulating boiler.

Kind Regards, John

I'm a bit late to the party, but in my opinion in depends on a lot.
For touch safety keeping it below 50 would be good.
For comfort the lower the better to reduce air currents.
For heating up time on small radiators higher is better
for boiler/system wear/noise lower is generally better
For efficiency the lower the better

There is a lot of misinformation about return temperatures for condensing boilers and somehow getting a one off payoff at a specific temperature around 55c. The dew point of the flue gas is around 55c, so the boiler will start to condense a tiny bit at 55c. However therefore the reduction in gas exit tempterature 55-50c will have a much bigger effect than 65-60c. If you can get your flue gas to leave at 20c and/or lower than 100% humidity then that would be better still.

I have looked at a 400 x 1200 mm double panel radiator and output is just 1.5 kW at 50°C difference. Since it is the difference that matters 20°C + 50°C = 70°C so 1.5 kW at 70°C and 750 W at 45°C so I can see why one would want to run them hot if small, but as you say if they do the job the colder the better which is the whole idea of the modulating thermostat as the house warms up the radiators cool down giving better economy.

But still two questions, one is there a building regulations or other regulations limit? could the boiler be filled with FRF (Oil) and the radiator heated to 150°C if I wished? Not that I want that, but is it allowed, other than when the radiator is in a box?

And second question is

John D v2.0 said:

For comfort the lower the better to reduce air currents.

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why is reduced air currents considered better? In my old house I had a Myson fan assisted radiator and two standard radiators in the same room, the Myson would kick in as the boiler fired up, as long as room was cool enough and would circulate the air warming the room up fast an even, once the room was warm it would stop, but as far a comfort was considered it was great, we could heat the room within 20 minutes of arriving home, no need for wifi thermostats warming the house up ready, to be fair also had a 4.5 kW gas fire in the room so with that lit, even on coldest day room warm in 10 minutes, but the Myson would circulate that air into other rooms as well.

Down side was noise, late at night watching TV with sound down low, and Myson would kick in, not a lot of noise but would need to turn up TV a touch, but is some was good as one associated the noise with warmth, so as soon as you heard it you felt warm. That one over 25 years old, adjustable speed fan and thermostatic control, both room temperature and water temperature so if pipes not hot it would not run. New iVector model has automatic multi speed fan so it slows down as room heats up. Because they draw air from the floor level the inbuilt thermostat works very well.

The TRV on standard radiator worked better with a modulating boiler than a marl/space boiler, because the modulating boiler tries to run continuous until it reaches the minimum output the TRV slowly opens or closes regulating the temperature of the radiator to suit what the room requires, so there is a continuous thermal flow of air, and the TRV is cooled or warmed by this flow of air and it is likely at bottom of radiator in the best position to monitor whole room, so the hysteresis with a modulating boiler and TRV heads controlling temperature was very low.

However with this house using oil the TRV control is not quite as good as the water temperature varies more between firing of boiler and less as the house is satisfied, it still works well but not as well as it did with a modulating boiler, but as with a modulating boiler it is messed up by the wall thermostat, while wall thermostat is on, the pump is running and the TRV heads do a good job, once wall thermostat is off clearly the TRV can't regulate temperature, so the lower the water temperature the longer the wall thermostat is on for as better it works.

What is really needed is a automated boiler water output temperature control, so it turns up high for first ½ hour when boiler turns on, then turns down again to give that boost when heating first switched on, you get that with modulating boilers, but not with mark/space, however of course this also means with Mark/Space boilers using the wall thermostat to control temperature does not really upset anything, OK it means the pump stops, but that is not so bad, but with modulating boilers if the TRV is set higher than the wall thermostat so the wall thermostat takes control then the boiler works like an old mark/space boiler and you lose all the advantages of a modulating boiler.

ericmark said:

... could the boiler be filled with FRF (Oil) and the radiator heated to 150°C if I wished? Not that I want that, but is it allowed, other than when the radiator is in a box?

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I already suggested that in post #6 - and someone then suggested that one could just stick with water and pressurise the system.

Kind Regards, John

JohnW2 said:

I already suggested that in post #6 - and someone then suggested that one could just stick with water and pressurise the system.

Kind Regards, John

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Where I once worked we made candles and we had a boiler running with fire resistant fluid used to melt the wax, the problem is FRF is carcinogenic so any spillage is rather serious so not really an option with home central heating, but I expected there would be a building regulation limiting how hot stuff could be. But I suppose we are allowed cookers with red hot areas so unlikely to be a building regulation as to heat of a surface.

ericmark said:

.... but I expected there would be a building regulation limiting how hot stuff could be.

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Possibly, but I think that most parts of the Building Regs are, like Part P, devoid of virtually any explicit/detailed requirements.

Kind Regards, John