Thanks, from the online calculators the room would need about 5700-6000 BTU for the living room
Love to know how they work that out? I selected one, and it asks a load of questions, but one question was missed, how long do you want it to take to heat up the room?
I moved house, from one which took around ½ hour to heat living room, to one taking nearly 2 hours, both homes could maintain the room's temperature with easy, it was the reheat time which changed.
Also, the new home seems to retain the heat better, so typically heating from 16 - 17°C to around 21°C where the old home cooled more so 12°C to around 21°C.
So with simple times, just means I start heating earlier, or use things like the Drayton Wiser TRV which is able to calculate how long it takes to heat the room, so it can start at different times, to ensure warm at the time the room will be used.
However, geo-fencing tends to have fixed point away from home to turn the heating back on, which with new home is too close to the home, so to put it simple geo-fencing only any good at turning off the central heating, the turning it back on bit simple fails.
I see the questions What is below the room? What is above the room? Select type of outside wall: Select type of window: Select number of outside walls: which will clearly affect how much energy is required to maintain a room's temperature, but will have little effect as to if room heated in ½ hour or 4 hours.
Since in this home I do not use all rooms 24/7, the total BTU output of the radiators needs to be around 3 times larger at least to the boilers size, or my boiler will be switching on/off all the time, and not getting a reasonable run time.
With a modulating boiler not quite so bad, but one will want the boiler to not switch off until rooms in use are warm. So if minium boiler output is 6 kW (20472.8 BTU), then the rooms used together need to sink over 6 kW. Which means at 8000 BTU you need to be heating 3 rooms, or more if the TRV is going to control the radiators.
Remember a TRV does not switch on/off, but is analogue, so as the room approaches the target temperature it should be slowly closing down, which should in turn cause the boiler to also reduce output, but if the boiler is already at minium output, all it can do is start a mark/space ratio, of turning the boiler on/off, and that is not as economical as modulating the boiler.
This house the boiler is old, so that does not matter so much, it will always use a mark/space ratio, but with a modulating boiler turning the boiler off externally also is not as efficient. If the boiler turns itself off, it turns back on at minium output, that is good for efficiency, but if turned off by an external control, then it turns back on running flat out, and has to wait for water to return to the boiler before it starts to modulate.
Clearly each make of boiler will be slightly different, and some allow the use of modulating external thermostats, like OpenTherm, but will a modulating boiler you should be able to if all rooms are calling to sink the whole output of the boiler, if not either boiler too large or radiators too small.
OK there is an exception, when using a direct fed shower, old house the DHW boiler was 18 kW, and I had to find large shower heads or the boiler would cut out while having a shower in summer, so 18 kW is more than required for a shower, unless it has side jets, with electric we get away with 7 kW, but some do go to 14 kW so since when delivering DHW the boiler can stop powering the CH, if being used for a shower we are looking at around 16 kW to ensure a large rose can still give high pressure, but not really much more.
So I have a three story house, with some large rooms, 5 of which can be bedrooms, I used only 3 as bedrooms, having a craft room and office, and my 20 kW boiler always cycles, (turns on/off) so never gives full output into the house even on the coldest days. As said my oil fired boiler does not modulate, so always uses a mark/space ratio to reduce output, the burn time seems rarely shorter than 20 minutes, and first thing can run for 1.5 hours, but since room takes 2 hours to reach temperture clearly my radiators are under sized.
