Room stat

Discussion in 'Eco Talk' started by Andyrattle, May 30, 2015.

  1. Andyrattle

    Andyrattle New Member

    Hi all not sure if this is correct place but seemed to fit.

    I have a new combi boiler, and also now have a room thermostat switch thing. I already had thermostatic valves on all my rads.
    I'm a bit confused as to the point of the room stat if the rads are already under control.
    Ive currently got it in the living room and put those rads on full and set the stat on 22.
    Then all the other rads in other rooms I've set to taste.
    Is this the best way to do it?
     
  2. First - yep, this is the correct place :).

    You seem to have it sussed really well. The TRVs control the individual rads in each room, but the rad valves in the 'room stat' room should be fully opened or disabled. It is often suggested that the rads in the 'stat room should not have TRV vales on them, but in practice - as you've found - it's good enough to chust have the TRVs fully open. That's pretty much the same as not having them fitted at all.

    There might be a bit of balancing required if you find that the whole system turns off before the rest of the house gets warm enough, but that should only require that the 'other' rads are tweaked open a wee bitty more, or the lockshield of the 'stat' room is tweaked shut a tad.

    Anyways, it usually works out ok...
     
    Andyrattle likes this.
  3. Andyrattle

    Andyrattle New Member

    Cool. All I need to do then is find somewhere the mrs can't fiddle with it.
    ;)
     
    Deleted member 33931 likes this.
  4. MGW

    MGW Screwfix Select

    I also could not work out why a thermostat and TRV's in my house TRV's upstairs to stop it getting too hot and down stairs open plan so controlled with a thermostat. Mother house was old style with doors on every door, originally 4 solid fuel fires, first central heating was not that good really, but it did reduce drafts from the fires, it seems house poorly designed, for solid fuel with an open flue replacement air should be provided close to the fire so it does not cause drafts through the house, these were missing, so central heating with balanced flue was great, not because of heat, but it removed the drafts.

    However with the change of boiler, there was also a need for TRV's, the modern boiler does not simply switch on/off but it has a variable output, not sure if controlled by flow rate or return water temperature, however the TRV which also opens and closes gradually controls the boiler flame height so in winter the boiler just varies output and you get a near hysteresis free temperature control.

    However as summer approaches we reach a point where the boiler can't turn down any more, so it starts to cycle on/off, some do have anti-cycle software, this is designed to reduce how many times the boiler fires up to test if heat is required, again it seems different methods are used, running just the pump and using the back pressure it could test to see if all TRV's are closed without actually firing up the boiler.

    The problem is one the boiler manufacturers don't seem to describe how their boiler flame height and anti-cycle software works, and two for so long plumbers were told don't fit a TRV to radiator in the room with thermostat plus the use of towel rails without any TRV if back pressure was used it would likely not work because of not having TRV's on all radiators.

    So the shore fire way to get it all to work, is to fit WiFi heads to every TRV and have them communicate with a hub which in turn tells the boiler when to run, so if any TRV has a target higher than current boiler will run. Great, but expensive.

    So now we move to the cheaper methods to tell boiler it is no longer required. We are not controlling the homes temperature with the wall thermostat, we are telling the boiler you can switch off, so first thing is at what temperature as summer arrives do we want to close down the central heating? Personally I would say if the house is at 19°C in the morning I don't want to warm up house or by mid day it will be too hot, however 19°C is too cool for a living room, so the thermostat needs to be in a room where 19°C is acceptable, this often means the hall.

    However the hall is often cooled when the front door is opened, so we tend to put a large radiator in the hall, both to reheat it when front door is opened and to dry coats. Throttling back the hall radiator with the lock shield valve is not really going to help it dry coats or reheat hall after the door has been opened. So the answer is duel thermostats, the TRV will open when hall is cold and reheat it quickly, however it is set on the low side so although the hall heats up to 17.5°C quite fast, after than point the TRV slows down the rate of temperature raise, remember the TRV does not switch on/off but gradually closes so works in a range of around 2°C so until the room is at 19.5°C it will not completely close, so if a wall thermostat in the same room is set to 19°C then as summer approaches it will turn off the boiler and stop it cycling.

    However are are some draw backs with this cheap approach, the thermostats are matched to each other, I set mothers by putting the limit pins in wall thermostat at 19°C then slowly increasing temperature of TRV until on a warm day the wall thermostat operates. This means I can turn down the temperature to act as a frost stat when leaving house, then back up on return, but really nothing between the two set limits, the other problem is when using a couple of eTRV's in the two down stairs rooms, there is nothing to ensure the boiler runs as the set temperature changes to a higher temperature on the eTRV's.

    I found two ways around this problem, one is the programmer, having it turn off an hour before the eTRV are due to open ensures boiler starts to run as valves open, the other a legacy from previous attempts at control is I have a programmable thermostat in the kitchen wired in parallel to hall thermostat and programmer so I can program it to keep boiler running for morning boost if required.

    Since I have two eTRV's I can remotely alter the temperature of those two rooms using the phone or PC, however since I can't remotely cause the boiler to fire up, it is of very limited use, it takes hours for rooms to cool or heat as the target temperature is changed so in real terms the WiFi option was not required, and I could have used cheaper eTRV heads.

    Main point is it works, and is cheap to install, however I would be first to admit a proper system like the EvoHome would do a better job. I look at the forum posts about fitting Hive and Nest and scratch my head wondering why, although at least with Nest there is a IFTTT link with eTRV's with a follow command, the eTRV has a much reduced range so it would not really work in the hall with a thermostat in the same room, it relies on the TRV having a 2°C span of operation and the thermostat on wall having a 0.5°C span. If the user can remember just to set two temperatures, and not to try altering to get it just a little warmer or a little cooler then maybe they would work, in essence as a switch, however having seen how long it takes for the temperature to settle after any change, they do seem rather pointless.
     
  5. Can't believe I didn't suggest "down yer troosers." :(
     
  6. Wow - comprehensive. And I understood some of it :)

    In many ways, simply having your boiler on a timer and TRVs on every single rad is the simplest most effective way of providing the right temp to each individual room. Modern regs dictate, I understand, that boilers also require an 'inter-lock' which is a weird name for a system that shuts it down when there isn't demand, rather than allow it to constantly 'cycle'. And the most common 'inter-lock' is a room 'stat.

    Yes, I suspect there's limited advantages in fitting Nests and suchlike to many homes with a basic CH set up. All it is in these cases is a somewhat more sophisticated programmable room stat - at around 5 times the cost. A big plus, I guess, is the ability to remotely control the temp - turning it up when you are on your way home, for example. They also 'learn' from your house's conditions - how long it takes to heat up, etc etc.
     
  7. MGW

    MGW Screwfix Select

    My intention was to get the eTRV's first then at a latter date get "Nest" which can integrate with the IFTTT of the TRV's with either Nest following the TRV's or the TRV's following Nest. However when I added the TRV to hall radiator everything started to work as required, every instruction manual I have read says no wall thermostat where you have a TRV however they worked, and my thoughts are, if it's not broke don't try and fix it, so I am leaving well alone, it works and that's all that matters.
     

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