Smart TRV's - are they worth it to add to a tado smart thermostat and boiler control?

Thanks for sharing. 

I had these recommended but in my opinion they ( any smart TRV) would only be effective if rooms were isolated from each other by having the doors closed.  TRV’s are after all only fitted to do fine controls in local areas where heat gains may be experienced e.g. a particularly sunny room or a room with high output appliances.  General heating in a house should be controlled by a room thermostat fitted in a room unaffected by such inputs and that room, typically a hallway should not have a TRV fitted at all.   Smart TRV’s will only be effective if the doors between adjoining rooms are closed else temperatures will equalise between rooms through draughts.

I very much question the suggested saving's by TADO which are likely based upon certain conditions and not the average home.  I personally doubt that there’s a payback on their high costs.  I have manually adjusted TRV’s on my radiators and have very rarely ever changed their settings.

The positioning of the room (overall) thermostat matters. It is often placed in the hall, however it is usually best placed where the radiator is least oversized (or most undersized). That way the stat should only drop out when all other rooms have reached temperature. The presence of TRVs in rooms will throttle back the water flow (and hence heating) as each room approaches its set-point, forcing more flow (heat) to the remaining rooms.

I'm not sure what the advantage of "smart" TRVs is. You could fit to bedrooms and turn down the heating during the day but the overall savings are probably marginal.

The energy efficiency improvements from adding a smart thermostat such as the Tado, Hive or Nest is again debatable and I would say are often over-stated.

Our house had a stat in the hall (undersized radiator), however the lounge also had problems heating (end of the pipe run and microbore). The result being that the hall was set much hotter than we liked simply to get the lounge to temperature. We fitted a Hive in the lounge (TRV set to maximum), and turned down the radiator in the hall, hence the system now controls on the lounge temperature. Overall savings maybe 7% to 10%.

A neighbour (same design of house) fitted a Tado in the dining room (radiator 40% oversized). This should have significantly reduced fuel consumption (as they don't use or heat the lounge), however their usage is around 15% higher than before (compensated for weather). The monthly notifications that the stat has saved 8% in the month from using weather compensation are therefore meaningless. I have no reason to believe that the Hive or Nest would have performed any differently to the Tado. I would point out that compared to the conventional (bang-bang on-off) controllers, these smart stats attempt to control the flow temperature by adjusting the on/off duration of the boiler, resulting in often warm radiators. This can lead to the comment that its not working.

Just a bit of personal experience over the years. Thermostats and TRVs can fight each other if you get it wrong. To illustrate this if you have a thermostat in a room and turn the TRVs right down, no matter how hard the boiler works it will never heat the room and every other room will get very hot and you will use a huge amount of energy.

A long time ago British Gas fitted a new boiler and also TRVs to my lounge (the only room they did). The house thermostat was also in the lounge but didn't say how to use them together. When you have a thermostat in a room you don't need to use the TRVs in that room and they should be fully open so that the thermostat can control the room temperature.

I then fitted TRVs to all the other radiators in the house and controlled the temperature locally with the TRVs in that room

If you have a room that is occupied and colder (loses more heat) than any of the others then put the thermostat in that room, fully open the TRV in that room and set the temperature of all the other rooms locally with the TRVs. I bought a wireless thermostat just for that reason and it works very well.

Peter

Hey ​@BeePee ​@Peter E 

Thank you both for your input onto this thread! Very helpful information, and I must say personally, has made me realised just where I could do with one or two in my own home!😀 This might fix the issue I’ve been having without even knowing it.

Just to add that TRVs and gas boilers work well together but not with heat pumps because of the way they work. If you are thinking of installing a heat pump you must discuss the issue of using TRVs because they are not good with the early versions of heat pumps but later versions / different manufacturers may be able to cope with them.

The advice seems to be to use caution when using TRVs with heat pumps. However they can cause issues with gas boilers as well, causing them to "short cycle". In essence as TRVs start to close down, the water flow is forced around the remaining radiators, and the overall flow rate falls.

Now gas boilers have a fairly large hysteresis, probably around 15°C. Boiler systems were generally designed for a delta T (difference between inlet and outlet) of around 10°C, so the hysteresis must be larger than this (when the boiler turns off the outlet will rapidly fall to the inlet temperature and you don't want it switching back on straight away). Steady state, with the boiler running at its lowest heat setting, the outlet would be around 10°C above inlet. If the demand is less than the lowest boiler output then the flow will slowly heat up and the boiler will turn off. However when the system is running at these low flows, and the boiler re-ignites at its high burner setting, then the flow temperature rises rapidly, quickly exceeding the boiler set-point, (before it gets a chance to modulate down) and it switches off. This can repeat several times before it finally manages to catch.

It is also worth noting that even if the boiler doesn't "short cycle" the average flow temperature will be somewhere between the setpoint and the lower hysteresis point. A boiler set to 60°C will thus, in these low load low flow conditions, be providing water somewhere between 60° and maybe 45°C. Since the flow spends most of its time cooling rather than heating, the average flow temperature is probably nearer to 45°C.

Although I (and others) have often suggested running condensing boilers at lower temperatures, this can cause issues as described above.

​@BeePee and there is a difference between how the old bang-bang boilers work (full on/off no modulation) and my new modulating Vaillant. My Vaillant runs the pump slowly (no gas) when the temperature heads downwards towards the set point to sample the return temperature and may come on at a low level to head off the temperature decrease if necessary. I'm sure it has the ability to learn just how much heating to apply on each occasion. Not only are there TRVs in my house but it works in parallel with a heat pump, solid fuel stove and sometimes fan heaters when the electricity price is sub 5p/unit where they are doing most of the heavy lifting on all but the coldest days.

As an aside, last night we had a major HV power cut that disconnected over 4,000 houses in our area. We had the woodburning stove to save the day and led floodlight lamps. I'm never going all electric.

The same return/pump sampling happens with heating the hot water tank. Thus dragging half of the hot water out of the tank during the process which is probably why gas hot water heating is only about 55% efficient and why I can replace it with an immersion heater at a break even cost.