I think there is one snag with this. The setup you describe is fine for a given temperature setpoint. Assume we have reached our setpoint. All rooms are now up to temperature. We now wish to go out for the day and knock the temperature back. I think that what will happen is that the rooms without thermostat but with TRVs will be warmer than we want, and with the expected savings in fuel not being achieved. Using the standard way of balancing, to try to achieve an equal rate of heating in each room, is only way I can think getting round this, apart from using zone valves throughout.
Mind you, my sleep deprived brain might have led me to conceptual errors here ;-)
Not necessarily. If the stat is set low, the heating may not come on at all because it will take a while for the house to cool below the lower setting. If it *does* come on, it will only do so sufficiently to bring the room with the stat up to the reduced stat setting. The other rooms will get a bit warm - but probably not sufficiently so for their TRVs to operate - so the fact that the TRVs are set higher than you want under these circumstances is probably not relevant.
The point I was trying to make, though, is that when the house has cooled down to the lower setting, then whenever there is demand it is likely to be with the TRVs at full bore. The energy usage is therefore higher than necessary.
Get TRVs throughout. Replace one head with the decorating cap and swap decorating cap and trv head if moving wireless stat (or just leave TRV on max in room with wireless stat - it needs excercise theough.)
Of course, a decorating cap would leave the radiator full off, unless not screwed on properly. However, many TRV ranges do have a lockshield head that can replace the thermostatic head.
Not at all. It really depends of how far you lower the thermostat and how long it is at that lower setting. Let's assume that the system is set up and balanced such that the room without TRV, and with thermostat is the slowest to reach desired temperature, as described by Christian.
We start at the 'steady state' with all rooms up to temp. TRV's will now be pretty much closed. Maintenance of the setpoint is achieved with flow through the TRV'd radiators being restricted by the TRVs. There is, over a period of time, a given amount, say 'x', of energy required to be output by the boiler.
Now we knock the thermostat back a couple of degrees or so to save gas while we are out. We want to save some gas, but not let the house get really cold. During the period of cooldown you are correct. None of the radiators get heat. Energy usage is nil.
At some point we reach the new (lower) thermostat setpoint and wish to maintain it until our return. During this period, unless we have adjusted the TRVs, there will be a greater flow through the TRV'd radiators in an attempt to maintain the TRV setpoint. The TRV'd rooms will not achieve the temperature reduction we have requested. Overall, therefore, we have higher energy usage than might be expected from our lowering of the thermostat setpoint.
Whether the difference is significant is open to debate. Maybe I'll try some experiments....
It depends on how well set up the system is. If the thermostated room is set only to be marginally slower to heat than the other rooms, then setting the thermostat back will be effective. The other rooms would only get a couple of degrees warmer than the thermostat room.
This is the ideal situation. After all, we don't want the lounge to heat up too slowly, just slowly enough so that the other rooms get a chance to heat up before the stat trips.
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