Zones with central heating how does it work?

Discussion in 'Eco Talk' started by MGW, Jul 20, 2018.

  1. MGW

    MGW Well-Known Member

    I am looking to move house, and I am well aware of heating costs, so read up on how modern central heating works, ideal system using fan assisted radiators is so expensive to install it is very rarely used, so in the main we use TRV to control heat to each room.

    There are two ways to use a modulating boiler, one uses the return water temperature, the other has a modulating thermostat fitted, the latter more expensive to install but means the boiler is cooler when switched off so less energy lost to outside.

    However in both cases it works by the TVR and/or boiler slowly opening or closing and flame height lifting or lowering to match, i.e. an analogue control.

    However motorised valves have set positions, closed, centre, open, there is not analogue control it is open, closed or half way, I note with interest central heating systems with modulating boilers and TRV plus motorised valves creating zones or selecting domestic hot water and/or central heating, and I just can't work out how the digital zone valve i.e. on/off is matched to an analogue boiler?

    Is there a way? or did the installer not have a clue what they were doing?
  2. Allsorts

    Allsorts Well-Known Member

    By their very nature, some controls are 'digital' - like zone valves. I cannot see any benefit in making them 'analogue' - variable - as this is down to each individual radiator TRV (which is analogue).

    What zone valves are really good for is to divide the home in to areas that need heating at different times, the best example being bedrooms which you would normally only wish to be heated just before bed time, and then first thing in the morning. So it makes perfect sense to place all the bedrooms on its own zone, with a digi prog stat located either in one of them, or on a shared landing. I'd say the main purpose of that DPS is timing rather than temp control.

    Boilers are analogue in the sense - as you say - of them having modulating burners which adjust to suit the demand placed on them. It really doesn't matter that they are controlled by a 'digital' on/off stat as when the initial demand is made on the boiler from a cold rad system, the boiler will fire up fully to cope. As the heating demand has is met, the boiler will almost certainly modulate down to 'tick-over' to keep the rads supplied. When the DPS goes off, the boiler will shut down - personally I wouldn't lose any sleep over the waste of the residual heat left in the boiler.

    If you want more control than this, then I understand some DPS - like the current developments - have a learning feature where they will monitor the room temps and note how long the rads take to get them to the desired temp. They will then turn on your CH not when you've 'told' it to, but at the right time to enable the rooms to be at the required temp when needed. Eg, in cold weather the boiler will almost certainly be made to come on earlier.

    You are gong to tell me now that I've totally missed the point of your post... :oops:

    Oh, I understand that underfloor heating works more like what you presumably want - these seem to be continuously analogue and use a thermostatic blender in order to supply the correct temp of water to the heating coils. Is this what you mean - if there was a system that continually thermostatically blended or controlled the actual flow so that it was optimum?

    I'd love it if someone could explain this further to me :)
    MGW likes this.
  3. MGW

    MGW Well-Known Member

    In the home I am in now the eTRV heads are set to alter the temperature at different times of day, however to be frank that much heat is stored in the fabric of the building it is a little pointless.

    We have a curtain on the stairs to stop heat going up the stairs, however in the main the upstairs TRV's never turn on, the heat from down stairs is enough, only on the very coldest day do the valves upstairs ever open.

    Set to 16C over night and 24C first thing at 6 am and 20C at 8 am if set to 20C then the anti hysteresis software reduces the output so it is 11 am before it hits 20C but that 2 hour boost means by 8 am it is at 20C. However at 6 am with it turning to 16C at 9:30 pm the house is usually still well above 16C so I set time clocks to turn off the heating completely then back on at 6 am. This also ensures the boiler is running when the TRV's open.

    Under normal operation in the winter the wall thermostat does not turn off, it is set on the upper range of the TRV setting, however as spring arrives it does operate stopping the boiler from cycling, not ideal way to control, but found my boiler is not OpenTherm enabled so to use a modulating thermostat I would have to use Bosch's own which will not link to the eTRV heads so really no point.

    I will admit it took some time to set the system up, but now it works very well, however I will be leavening this house and so likely will have another house with doors on the rooms, so likely will take the eTRV heads with me and add some more. So I can select when each room is heated, so it would seem any zone valves will simply be set open at all times, seems rather pointless having them?

    The problem is how to stop the boiler cycling, that is all the wall thermostat does, however by fitting two or more wall thermostats one can get around the problem.

    However I must be missing something, why fit zone valves, as far as I can see they do nothing, all control is done by the TRV.
  4. Allsorts

    Allsorts Well-Known Member

    Phew... :(

    Ok, a few things I can try and pull out of that lot... :)

    "...however to be frank that much heat is stored in the fabric of the building it is a little pointless." It surely isn't pointless, since - if the room's heat demands are already met due to the fabric of the building being warm - then the TRV won't open. Which is most of the time. But, if the room cools, they will open. Ergo - they are doing their job. Just because they are shut 90% of the time doesn't mean they are 'pointless' - or am I missing something?

    You seem to have fully-programmable TRVs fitted - ie they do temp and time? In which case I'd agree - the zone valves are fairly redundant. It would appear that you have ultimate control over your CH with each room controlling its requirements independently of the rest?

    Except - as you point out - this requires that the boiler is always 'on', ready to supply. The only override to this is the room stat - I think it's referred to as an 'interlock'? - which all modern systems should have for precisely the reason you say; to prevent the boiler cycling.

    A conundrum indeed. Two basic ways to control your CH - your way which is to have every single rad independent, but this has the drawback of them not being able to communicate with the boiler to actually shut it fully off when there's no demand, or the more conventional setup which is to have zones, in which case your control is not so finite, but at least the boiler IS told to shut off when not needed.

    I know which I would choose.

    In your new house, I'd recommend you fitting the best of both worlds - a zone valve and wall-mounted prog stat in each room for each radiator...
  5. MGW

    MGW Well-Known Member

    I will admit I made a mistake, my eTRV use IFTTT and can be linked to a wall thermostat such as NEST using a follow command, but all that means is the wall and radiator thermostat are set to same temperature, which kind of defeats the whole point of turning off rooms which are not in use.

    However as long as boiler is Opentherm enabled it should work.

    Had I instead used some thing like EvoHome then could set rooms for different times and temperatures and still use the "wall thermostat" to control boiler. Not really a thermostat it's a central hub, but it's called a thermostat. Does not really matter if Opentherm enabled or simple on/off it will still work, but Opentherm is clearly better.

    But any zone valve is open/closed and micro switch is also open/closed so even if the boiler is Opentherm enabled there is no way for the devices to tell boiler what output is required. Or am I missing something?

    I suppose the micro switches could connect to a device which tells the boiler run at 50% with one valve open and 100% with two open, but as yet not seen such a device.
  6. Allsorts

    Allsorts Well-Known Member

    Missing something? No idea, but I clearly am; IFTTT, Opentherm et al mean nothing to me. :)
  7. MGW

    MGW Well-Known Member

    IFTTT = If this then that. It is a protocol used to connect devices to internet, so for example I can set it so if the weather forecast says it is going to freeze then the central heating will fire up. However some one has to write the apps, so you can only do some thing for which some one has written an app, however these are increasing every week, so because you can't do it today does not mean you can't do it tomorrow.

    OpenTherm is a open source format where third party thermostats can be used to control a boiler.

    In the old days we had just two options with a boiler, on or off, however when condensating boilers came out, they needed to run at a rate where the return water is cool enough to cause the water in the flue gases to condensate out, this was done by having a variable flame.

    Early boilers did not have an ebus connection, they simply used the return water temperature, however this means by time it reaches minimum flame height it has also reached maximum return water temperature, so when it reaches minimum flame it then starts to cycle, but it is turning off when boiler is at maximum temperature so all that heat is lost through the flue.

    By giving access to the ebus in the boiler however thermostats can control the water output temperature so warmer thermostat gets the lower the output temperature, so now when the boiler turns off there is less heat lost out of the flue.

    Bosch do a "Wave" thermostat which connects to the ebus, but as far as I am aware this does not connect to the eTRV so it only senses the temperature at one point, where Honeywell EvoHome thermostat takes the temperature from the eTRV and works out how much output is required from the boiler measuring the temperature at multiple points. EvoHome and for that matter Nest can work either on/off or gradually reduce flame height on OpenTherm enabled boilers.

    Because my boiler is Bosch which does not support OpenTherm all I can do electrically is turn it on/off or use a single thermostat for whole house, but temperature of rooms vary with amount of cloud cover, so I need to control boiler from more than one point, so two thermostats are wired in parallel, one switches off when front of house is warm the other when rear of house is warm thus ensuring there is hot water circulating when the eTRV heads open the valves.

    In real terms I can forget about upstairs, the TRV upstairs rarely opens, so just use cheap ones upstairs, eTRV heads only used on the ground floor, since most of the heat upstairs arrives there from down stairs it would be pointless having a zone to switch off upstairs.

    However although this house works OK, my wife wants to move, so looking at starting all over again getting a house to balance between expense of running to expense of installation, having upstairs switch off may work, but switching off downstairs is completely different.

    The problem is if down stairs is switched off and up stairs left running over night, in the morning when down stairs kicks in with up stairs already warm it would now over heat. What one needs is not on/off but increased or decreased temperatures, so heating down stairs from 16 to 20 OK, but 10 to 20 and upstairs gets too hot.

    I say 16 to 20 however when I used that setting it never cooled to 16 over night, in real terms 3 degrees C is the maximum you can allow a room to heat and cool, over that and it over shoots, or takes so long as to be silly, I had to set the eTRV head to 24 degs C for 2 hours then back to 20 degs C in the morning as the anti-hysteresis software in the heads was too good.

    But back to original question, if a zone valve stops a room heating which has a eTRV head in it connected to a central thermostat it will stop the boiler switching off. If the zone valve connects to the central thermostat all well and good, but only seen valves for under floor heating with any connection to a central control thermostat, it seems the are just not made.

    In theory best central heating would use fan assisted radiators, fast warm up time, and can also be used for air conditioning to cool the room, but the building management system costs an arm and a leg. So in real terms maybe one in the kitchen in the kick plate but rest of house standard radiators.

    But it is unlikely I will move into a house with no central heating, so I need to know what work is required to make it work in a reasonable way, and how much it will cost, I can see how a zone valve front to back can help, but all seen seem to be up down which seems pointless, hence question.
  8. Cwt

    Cwt New Member

    Use 2 zones valves to split upstairs and downstairs, fit the etrvs if you think they are worth it.
    Get a boiler with a outdoor weather sensor and internal opentherm programmable room thermostats, this will set the boiler flow temperature taking into consideration outdoor temp, indoor temp, temperature required and the time it thinks it will take it to reach it. Say you set it at 20deg it might only keep boiler on upto 18deg knowing there will be a 2deg overun from the rads.
    Personally I'd spend the money on insulation rather than expensive complicated controls, the energy they save is minimal with long payback times.
    It can also be a pain setting the heat curve for optimum performance

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