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Thread: Do you change your target temperature with the seasons?

  1. #1
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    Default Do you change your target temperature with the seasons?

    In my home office I have a new, large vertical rad (9000 BTU I think) with an HR92 set to 19C. It's not behind a sofa or anything but it has huge issues reporting the temperature correctly. Today my desk thermometer says it is 15C but the HR92 thinks is is 20C and has turned off. It seems the moment the rad comes on, the unit heats up and on cold days, the room doesn't get warm.

    Whereas in the summer when the ambient is perhaps 17C, the room was sweltering and I basically turned the valve off... which is the exact opposite of the point of the evohome setup!

    I've given up and ordered a separate thermostat but that's another £70 to make the thing do what it surely is supposed to anyway. It's not like the radiator is stuck behind a sofa, though my heating system does run very hot to get enough BTUs out (traditional victorian house so we need a lot of heat).

    I just wondered if others massage their settings summer/winter, or just leave it on in the summer and find it works smoothly.

  2. #2
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    I increased the target temp to try and compensate but it currently reckons the temp is 21.5C whereas it's actually 16 in the room.

    I don't seem to see this in other rooms - could the vertical rad be a factor? Or could the HR92 have an issue with its shielding... I tried to stick insulation between it and the rad but no help

    I know you can adjust an offset temp in the unit but then I would just have to change this seasonally instread... and considering how the unit heats up within just a few minutes of opening, I don't know it would help.

  3. #3
    Automated Home Legend paulockenden's Avatar
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    A 'normal' TRV would probably have the same problems. At least Evohome accepts that measuring temperature at the rad valve won't work in all situations and so offers the option of using a room stat.

  4. #4
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    Quote Originally Posted by paulockenden View Post
    A 'normal' TRV would probably have the same problems. At least Evohome accepts that measuring temperature at the rad valve won't work in all situations and so offers the option of using a room stat.
    I assume that's true but you just turn it a bit. I didn't have this problem with the old TRV... maybe they are less sensitive. I'd love to know the design of a HR92 how they try to shield it. I even wondered if I can rig a little fan

  5. #5
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    Quote Originally Posted by MrBoy View Post
    I assume that's true but you just turn it a bit. I didn't have this problem with the old TRV...
    I don't understand your reasoning - if you did have to set your old TRV differently in summer and winter then it did have exactly the same problem ?
    maybe they are less sensitive.
    Less accurate for sure. There's a reason why mechanical TRV's are numbered 1-5 not in degrees C.
    I'd love to know the design of a HR92 how they try to shield it. I even wondered if I can rig a little fan
    The HR92 doesn't try shield itself from the heat beside the radiator - neither does a conventional TRV, since those are radially symmetric.

    They sense the room temperature by relying on convection. The heat from the radiator causes convection currents around the room. The air rises through the radiator, travels along the ceiling to the opposite wall and down that wall, then returns across the floor.

    So cold air from the floor - which is below average room temperature - is drawn past the TRV cooling it and largely offsetting the effect of localised heating by direct IR from the radiator panel. Obviously this relies on a good strong convection current forming around the room.

    Anything that reduces or blocks that convection current will cause the temperature reading to be wildly inaccurate. (much higher than the true room temperature - as you're experiencing) Things like radiator boxing in, furniture in the way etc can cause it. Also the reading will be much less accurate on an old fashioned radiator without convecting fins since they rely more on direct IR radiation than convection.

    You say you have a vertical radiator - is it a finned convector, and if so is it actually designed to be operated vertically ? I ask, because one of the new buildings at work has two long vertical radiators installed in the foyer.... which are actually just horizontal radiators turned 90 degrees. This means the convection fins on the rear are running horizontally instead of vertically, more or less eliminating the convective abilities of the radiator... If this kind of mistake can be made on a brand new commercial building it could be made in a residential property.

    Another possible cause is if you have the radiator at one end of the room and an open door to colder rooms at the other end of the room - in this situation temperature sensing beside the radiator is never accurate because there will be a temperature gradient along the room - which can be solved by positioning a remote sensor close to where you are in the room.

    As for whether I tweak my schedules between summer and winter - the short answer is that in all rooms with remote temperature sensors I do not have to make any change to the schedules. A set point of 21C in the Living room is just as comfortable in the middle of winter as spring/autumn or even a cold summer day, because it's measuring the true room temperature a good distance away from the radiator. It also speeds up the initial warm up time because measuring the temperature beside the radiator causes the radiator to be throttled back too soon while only the radiator side of the room is warmed up.

    I've just been experimenting with this issue in the kitchen - previously it used the HR92 as sensor. Unfortunately the radiator is under one of the counter tops and nestled tightly between two kitchen cabinets with the HR92 right in the corner of this recess. Needless to say, it doesn't sense the room temperature very well.

    Two problems - one is that in winter when it says it's 20C it's clearly much colder than that in the room, as the radiator is on an interior wall. Thus the exterior wall side of the kitchen is quite cold, so it gets turned up a bit in winter.

    In warmer weather the opposite is the case - it needs to be turned down, especially around cooking times, and the radiator tends to come on when it doesn't need to. Where it's located it simply doesn't detect the heat from the oven and gas stove top - so the kitchen ends up getting as high as 25C after a while even while the HR92 is still reporting 20C and blasting heat out from the radiator to add to the oven heat. To combat this the kitchen is only scheduled for 18C at cooking times, but then if there isn't any cooking the room stays chilly.

    Getting a bit sick of this I wanted to test the effectiveness of a remote sensor so I've temporarily relocated the Evotouch as sensor to a shelf in the kitchen near where I would install a DTS92 and sure enough it solves all these issues perfectly. Now I can just schedule the room to 20C when it's in use and be done with it.

    In warm weather the radiator doesn't even come on now whereas it would have unnecessarily come on before, in cold weather the radiator is blasting away nicely to reach an actual room temperature of 20C so no need to turn it up. And best of all, literally within minutes of turning on the gas stove top the heat from the stove warms the sensor which is about a metre away from the stove at shelf height, this quickly shuts the radiator off to prevent it from helping to overheat the room - so the room is no longer shooting up to 25C when cooking, more like 21-22C.

    Within 10 minutes of starting to cook the radiator has gone from piping hot to the HR92 being completely closed and radiator cooling down.

    A remote sensor is always better and there are certain situations where it's more than just a luxury and really is needed. As Paul says, while the sensors are expensive, at least with Evohome you are able to fit remote sensors. No such luck with manual TRV's!
    Last edited by DBMandrake; 30th October 2018 at 09:30 PM.

  6. #6
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    In all but the simplest room configurations I use a separate room stat. I think itís pretty obvious that if you have a sensor such as a HR92 mounted on the radiator itís never going to give you the true room temperatures ( at least in the area of the room you occupy) no matter how much you play around with the HR92 compensation offsets.

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