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Thread: Can't get one zone to heat up

  1. #11
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    Quote Originally Posted by therealfronty View Post
    Anyway, there is something weird going on, when the hr92 shows valve position 58%, if I take the head off I can't actually manually open the valve any more, it is already fully open, so it seems the head is reaching the 100% open point maybe earlier than its expecting (although not sure why it doesn't report 100% in that case)? So I was just having a play, and if I set valve stroke to 1, it then reports the valve position as 100% (when fully open), and also sends that back to the controller, I can see the demand for that zone goes up to 100%.

    When demand is 0, it does fully close the valve regardless of whether I set the valve stroke to 1 or 0, but it seems that on 0 it's not reporting the valve as fully open even though it is. The valve body is a TRV4 and most of my other rads are fitted with these and don't have any problems reporting the fully open valve position, so not sure why this one is different. Have left valve stroke set to 1 for now so at least in system summary I can see 100% when fully open. I need to check the temps in that room again as there might be something else going on if the valve was fully open anyway. All very odd.
    Quote Originally Posted by therealfronty View Post
    Old room stat in the living room is still wired into boiler so that is the only thing that can actually call for heat, but hr92's successfully turn off the rads upstairs during the day (and the conservatory) so all good.

    So whether system status on the evohome controller shows 50% or 100% demand it doesn't affect boiler output as the evohome isn't connected to the boiler, and I don't have the opentherm bridge anyway so even if it was connected via a bdr91 all it could do is turn it on or off. Not sure how much % needs to show on the system status before it actually turns the boiler on, anyone know? Is there a threshold it has to go over? ie if all zones show 0, but one zone goes to 20%, is that enough to fire the boiler?

    I've already tried swapping with another hr92 and checked the adaptor moves freely and made sure the valve is fully open when refitting.

    It's just a bit of a mystery that the valve position on the hr92 display was only showing 58% even though the valve was fully open. Not to worry, setting it to full stroke seems to have fixed it even if it's just a display issue.
    Why are you still using the old room stat to control the boiler instead of Evohome?

    Evohome's algorithms are designed to control heat output of radiators by controlling both the individual water flow through each radiator and by controlling the duty cycle of the boiler, which is derived from the heat demands sent from the individual HR92's in each zone.

    The configuration you have is unsupported and you should not expect the system to work properly in this state. Unless you have a good reason not to I'd strongly suggest you ditch the original wall stat and give Evohome control of the boiler, otherwise you can't expect it to work properly.

    As for the stroke setting - it sounds like your valve bodies are very stiff either by design (not fully compatible with the HR92 from day one, even if they screw on OK) or have become stiff with age (perished rubber washer inside or rusting slide pin inside) and if one radiator needs stroke 1 and all the other radiators use the same valve body design I would recommend setting all HR92's to stroke 1 mode. (I use stroke 1 mode on all my radiators and found I needed to)

    If you do, make sure after changing the setting that you remove each HR92, turn the black wheel anti-clockwise then refit it - this will trigger a valve pin calibration with the new stroke setting so that it can calibrate the fully closed and open positions.

    Regarding heat demands - if you use a BDR91 relay any heat demand over 10% as reported in the system settings page will bring the boiler on, assuming you have load scaling disabled, which is the default. Keep in mind that if you use a relay it's not just full on or full off, it uses TPI - a form of duty cycle modulation so the boiler is switched on for a certain percentage of each cycle period. (default cycle time is 10 minutes) So the higher the heat demand the greater proportion of each 10 minute period the boiler is on, giving a form of proportional control.

    If you use Opentherm I believe any heat demand 1% and above will bring on the boiler and the requested flow temperature is proportional to the heat demand. The flame in the boiler may of course shut off if the requested flow temperature is already reached, leaving just the pump circulating.
    Last edited by DBMandrake; 21st May 2021 at 09:05 AM.

  2. #12
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    Lots of interesting discussion points! :-)

    Currently I have two rads in the living room where the old wall mounted roomstat is, neither currently has a TRV (as you'd expect). The roomstat is still controlling the boiler simply because I haven't gotten around to wiring in the boiler relay yet (I have got one, it's just plugged into a mains socket so I can see the green light come on and off, although nothing is wired into the relay contacts yet - I'm going to reposition it temporarily in the airing cupboard to check the wifi signal is good before wiring it in properly). What I'm intending to do is fit hr92s to these two rads, but I'll have to change the old valves to fit TRVs. That means a drain down (system is due a flush, clean and inhibitor anyway) plus I have a third rad that has an incompatible TRV body so will change that at the same time. My rad in the bathroom doesn't have a TRV so that will become my bypass, I don't have a ABV on this system but it is open-vented, so I was wondering wouldn't the pump just pump water out of the expansion/overflow pipe into the header tank if every radiator was closed? I'll have the bathroom rad acting as a bypass anyway.

    As for replacing the roomstat, that is hard wired into the wall, so what I intend to do is disconnect the roomstat wire in the wiring centre and replace it with the BDR91, then use the roomstat wire to feed DC to the controller, so I need to get a wall mount kit and wire the transformer into the mains up by the wiring centre. Obvs can still use the evotouch in the living room just using it's tabletop mount for now.

    I have a Viessman 100 open vent boiler (which is opentherm compatible) on a y-plan, so the current controller is controlling the pump, 3 port valve, DHW etc. The heating is set on constant already as the roomstat has a timer (to be replaced by Evohome), so the old controller will just control the DHW on a timer, however I do also have the hot water kit so could add that too, but looking at all the wiring I think I'll need to get someone in for that unless I can trace all the wires and figure out what they all do, then use a new wiring centre (I've seen the honeywell one in screwfix that is basically a giant 10 way chocolate block). So hopefully I can then get rid of the old controller and tidy up some of the wiring.

    Good to know about TPI, tbh my Mum has opentherm on her boiler and it does seem that when it's close to the set point the rads don't get very warm and it spends hours with the pump running and firing for short periods - I'd rather it just warm the rads up, dump some heat then shut off, but I know it's all about preventing undershoot/overshoot etc. So if I fit the opentherm bridge it's probably the last thing I'll do.

  3. #13
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    Quote Originally Posted by therealfronty View Post
    Lots of interesting discussion points! :-)

    Currently I have two rads in the living room where the old wall mounted roomstat is, neither currently has a TRV (as you'd expect). The roomstat is still controlling the boiler simply because I haven't gotten around to wiring in the boiler relay yet (I have got one, it's just plugged into a mains socket so I can see the green light come on and off, although nothing is wired into the relay contacts yet - I'm going to reposition it temporarily in the airing cupboard to check the wifi signal is good before wiring it in properly). What I'm intending to do is fit hr92s to these two rads, but I'll have to change the old valves to fit TRVs. That means a drain down (system is due a flush, clean and inhibitor anyway) plus I have a third rad that has an incompatible TRV body so will change that at the same time.
    Sounds like a plan. Before I had Evohome my bypass radiator was the hall way so I had to add a TRV valve body to that one to get HR92's on all radiators, and also added an automatic bypass valve.
    My rad in the bathroom doesn't have a TRV so that will become my bypass, I don't have a ABV on this system but it is open-vented, so I was wondering wouldn't the pump just pump water out of the expansion/overflow pipe into the header tank if every radiator was closed? I'll have the bathroom rad acting as a bypass anyway.
    You definitely don't want the system to be "pumping over" through the header tank as that will oxygenate the water leading to both air bubbles in the system and corrosion in the radiators...

    If there's too much differential pressure in an open vented system (my system is open vented as well) it can cause air to be sucked into the system from the vent pipe, especially during a pump shutdown/start-up transient. (You might hear a gurgling noise)

    Whether it pumps over or sucks in air due to low pressure depends on where the pump is connected in relation to the boiler and vent lines, and not all vented systems have the pipes in the correct configuration.

    See here for some details about both these problems:

    http://www.home-heating-systems-and-...ng-tips-6.html

    In short, no you can't rely on pumping over to get away without an automatic bypass valve on a system with all radiators controlled. If you want to control all radiators (as I do) then you need an automatic bypass valve especially on an open vented system where there is risk of pumping over or sucking in air due to pressure differentials.

    As for replacing the roomstat, that is hard wired into the wall, so what I intend to do is disconnect the roomstat wire in the wiring centre and replace it with the BDR91, then use the roomstat wire to feed DC to the controller, so I need to get a wall mount kit and wire the transformer into the mains up by the wiring centre. Obvs can still use the evotouch in the living room just using it's tabletop mount for now.
    This is exactly what I did - removed the old hard wired wall stat, installed the BDR91(s) in the boiler closet connected directly to the wiring centre, installed the wall mount transformer inside the wiring centre box and re-purposed the wire run to the wall mount for 5v - not installing the transformer behind the wall mount also allowed me to flush mount the wall mount without a back box where the wire comes through the wall which saved me carving out a lot of brick and plaster...

    The only thing to watch out for is voltage drop across a long cable run at the low 5 volts the device operates at - if you have full size 16amp mains flex like mine did you'll be OK, but if the old thermostat was connected via thinner wire you might have an issue. For some inexplicable reason the voltage from the transformer in the wall mount is lower than the table top stand as well - the table top stand puts out 5v while the wall mount is only about 4.6v. (!)
    Last edited by DBMandrake; 25th May 2021 at 03:25 PM.

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