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Managing a CH system fitted with an automatic bypass valve.

Discussion in 'Plumbing and Central Heating' started by Paul Griffin, 22 Feb 2021.

  1. Paul Griffin

    Paul Griffin

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    Hi,
    I have a Drayton MiTime T740R (3 zone) system with two room thermostats, all the radiators have a TRV (Honeywell) and there is an automatic bypass valve.

    I wonder if somebody could advise me on managing the system.

    I've got a good understanding how it all works but I was wondering if it would be OK to simply set both room thermostats to a high temperature (say 25deg) and control the heating using just the TRVs.
    The benefit of this would be that if I want to change the temperature in any room I simply adjust the TRV and the room would warm up, at the moment of course if I open a TRV nothing happens unless the room thermostat is below the temperature of the room it's in.
    I know that it's not considered right for all the TRVs to be closed when the heating circuit is live but does that matter if there is an automatic bypass valve.
     
  2. D_Hailsham

    D_Hailsham

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    A TRV must not be installed on the radiator in the room where the room stat is located.
     
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  4. ericmark

    ericmark

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    Yes likely that would work, with a gas modulating boiler the boiler would modulate first then once at minimum output start cycling, and a oil boiler would just cycle.

    That is what I was told, however it did not in real terms pan out. We were told on/off wall thermostat on ground floor, in a room kept cool, with no outside doors, or alternative heating, the idea is as summer approaches the wall thermostat turns off the central heating and stops it cycling, however there are a few problems with that idea.
    1) Often no such room.
    2) With a modulating wall thermostat it can either be in main room or linked to the TRV heads, putting it in cool room pointless.
    3) In a hall, you want fast recovery when door opened, but slow heating once approaching the target temperature, and a simple lock shield valve simply can't do that, so wall thermostat set to work with the TRV head, and if the target is say 18°C then TRV set to 17°C and wall thermostat to 19°C so as @Paul Griffin says the wall thermostat never turns until summer arrives.

    I know EPH does a master/slave thermostat that can use OpenTherm, the Drayton does have an add on for OpenTherm temp2.jpg but not sure how it works with multi zones? But other wise the whole zone idea seems flawed.

    I spend most of my day up stairs, so does my wife, we have named the rooms craft room, office, dinning room, living room, kitchen, and bedroom 1 & 2 which gives a good idea what they are used for, we also have 4 rooms we call the annex and using motorised zone valves hard wired with micro switches to divide heating into main house and annex makes some sense, but within the main house we want some flexibility as to when and to what level each room is heated.

    So in our case we use the hall as master control area, three controls, the lock shield valve, the TRV and the wall thermostat work together to in theory ensure the boiler runs when required. And the schedule for the TRV and wall thermostat are matched so they work together. There is however a flaw, the hall cools too slowly. Heating up one can control, cooling down is more of a problem, so after midnight when the set temperature is dropped, the hall is still above 17°C so boiler not running, but bedroom has cooled to 15°C but to correct requires a new wall thermostat that can link to the TRV heads.

    Every home is different, late mothers house hall did cool down fast enough, so it did work.

    Learn from my error, the important thing is cooling not heating, you can slow up the heating easy with the lock shield valve, and/or temperature settings, so my wall thermostat increases at 0.5°C per hour so boiler runs at least once an hour, that's OK, it is over night cooling which is the problem.

    Hind sight Hive would have worked better than Nest as I can link Hive TRV heads to the wall thermostat, so I can set more than one rooms minimum temperature. Although Drayton Wiser TRV heads it seems are considered the best, but if I set up a perfect system the cost would never be repaid, so do need a compromise.

    So I have two wall thermostats, TRV's in all but upper bathroom, 4 wifi TRV heads, 5 bluetooth TRV heads and 5 non programmable heads, latter in annex and shower room.

    However I have not really set them as I should, my bedroom at moment set to 18°C but thermometers show 21°C mainly due to sun on the window.

    The biggest problem is a TRV marked *123456 what is the point of that? If I want 20°C I want to set TRV head to 20°C not 1/3 between 2 and 3 and cross fingers.
     
  5. MJN

    MJN

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    It's because a device located next to a radiator and bolted to a hot water pipe is not well placed to measure an absolute temperature of the room. What you therefore need is just a relative measurement (hence the 1 to 6) and you can adjust it up and down until the desired comfort level in the room is maintained.

    Think of it like the volume control on your TV or hifi - we cope quite happily despite most not being measured in decibels.
     
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  7. ericmark

    ericmark

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    I think more due to the large slewing range, a mechanical TRV has a slewing range of around 3°C, i.e. difference between fully open and fully closed, the position may not be prefect, you want to measure the return air, and the radiator causes a thermal circulation, simple like this circulation3.jpg however in real terms items like windows can change it a bit circulation2.jpg this is why as a compromise the wall thermostat was placed on the wall facing the radiator being in the thermal flow but as far from the radiator as practical, the Myson fan assisted radiator worked very well due to the forced circulation, however it is not always good to have whole room at the same temperature, some times it is better to allow pockets of warm or cool air, either where sun shines through a window, or window loses more heat than walls, I lived in a house with hot air central heating before the days of double glazing, and whole house even heat, but heat losses were high so expensive to run.

    The first electronic TRV head I got, IMGP8035.jpg had two sensors, one for water and one for air, and the water one compensated for the direct heat from the radiator, however the built in anti-hysteresis software was a bit OTT, so in the morning set to 22°C for one hour then down to 20°C to counter the OTT anti-hysteresis software, and once the lock shield was set except for where morning sun caused a problem with slight overheating they were spot on, however expensive, the second type I got were far cheaper, EQ3-Straight-213x300.jpg no wifi, although did have bluetooth, so either you can adjust with phone or link to another head so if two radiators in the same room they can work together.

    I got the first type as told they would work with Nest, however when Nest was bought out by Google the support was dropped, so only option now with Nest is set the same schedule. We are told the Drayton Wiser TRV heads like the Nest wall thermostat works out how long it takes to heat a room, so the problem I had with OTT anti-hysteresis software is avoided.

    However all these clever algorithms depend on things like doors being always the same, in spite of the EQ3-Straight-213x300.jpg being cheap, it actually has a window open detect to turn off the heating in the room for a settable time, should a rapid temperature drop be detected, I have the one in kitchen set like that so it switches off while we unload to car and have back door open.

    We have had all sorts of cleaver ideas for thermostats, this one 84067_P.jpg used a mark/space ratio as it approached the target temperature to stop it over shooting, sounds great, and worked well with most oil boilers, however with the gas modulating boiler every time it is switched off externally when it switches on again it does so at maximum output, so the thermostat completely messed up the boilers own algorithms, and caused the boiler to run less efficiently.

    So either we want the wall thermostat to turn boiler up/down, not on/off, or the wall thermostat only stops the boiler cycling in the summer, or there is a link between the wall thermostat and the TRV heads when using a gas boiler.

    Much depends on the make and model of the gas boiler, it is all well and good saying the EPH thermostats can be set as master/slave so work with a number of hard wired motorised valves and OpenTherm, but if the boiler is not OpenTherm enabled that clearly will not work. Like the Mac computer which is very good, but does it's own thing and will not run third party software, so with Boilers we have Bosch which has not allowed third party thermostats to interface with it.

    So there is one odd one out of the smart thermostats, designed to be simple on/off but work with the linked TRV heads, so when working with TRV heads it acts more like a hub. Drayton also seems to have a system to work with hard wired zone valves and OpenTherm, although never worked with it, but clearly a TRV is a zone valve, maybe not hard wired, but never the less it has to qualify as a zone valve, but the problem is the balance between installation cost, running cost, and how well it controls the home.

    So all central heating is a compromise. If my oil costs £500 a year, and very cleaver control can reduce that to £400 a year and the system will likely last 15 years, then break even point is £1500, spend any more and you will not get the money back, specially if you include interest. So back to the compromise, maybe two or three devices to measure temperature and decide if boiler runs, placed in the rooms most likely to be under temperature or critical rooms, and other rooms using stand alone TRV heads which are much cheaper.

    But the whole system has to include what is already installed, all well and good saying rip it all out and installed EvoHome, or Wiser, but then we have the £1500 limit, so it has to integrate with what is already there. No good singing the praises of OpenTherm if the boiler is not OpenTherm enabled.

    The big question with hard wired zones, is would combining the zones together work better? And then use the TRV heads to set up zones? There is no one size fits all answer, likely in most homes you will need more than one thermostat that can switch heating on, be it two hard wired wall thermostats in parallel or a hard wired wall thermostat with wireless link to other thermostats, possibly a TRV head but not all the TRV heads need to be linked.

    Mothers old house two wireless thermostats in parallel IMGP8038.jpg one receiver above consumer unit and one below it, trying to keep some distance between the two with EMC in mind. So one thermostat in kitchen and one in hall, neither ideal positions, but as a pair they worked well.

    Aim of wall thermostat:-
    1) Ensure boiler is running when required.
    2) Stop boiler when we get warm weather.
    It is not to control room temperature, that is what the TRV is for.
     
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