[OpenTRV-interest] Adaptive comfort

Simon Hobson linux at thehobsons.co.uk
Wed Jul 13 10:19:18 BST 2016

Lisa Ann Pasquale <lisa.pasquale at six-cylinder.co.uk> wrote:

> Humidity is a metric which is more relevant for controlling ventilation than heating.

It still has relevance to maintaining "comfort".

> When the humidity rises, you need to ventilate to remove the moisture. Increasing the temperature only disperses it and adjusts the dewpoint - it doesn't remove the moisture, which is a risk of condensation and mould problems within the home - regardless of comfort. Tweaking the temperature is unlikely to alleviate discomfort caused by high or low humidity levels as efficiently or effectively as ventilation.

Mostly true. But humidity does have a part to play in perceived comfort - or at least perceived "is the temperature OK". From my own observations ...
As a very crude generalisation, high humidity gives a perception of higher temperature, and low humidity gives a perception of low temperature - with a fairly wide band of humidity in the middle where the effect is minimal. It's very non-linear.
Thus someone (but not me !) may feel quite comfortable with temperatures in the high 20s when humidity is low, but feel "stiflingly hot" in the mid 20s with high humidity. I suspect some of that isn't so much whether our body is too hot or not, but the discomfort of feeling "clammy" from the sweat that isn't evaporating quickly when the humidity is high.

Of course, air movement affects this. If the air is moving, it clears away that boundary layer of moist air and allows more sweat to evaporate - hence a fan making us feel cooler even though it might have zero effect on actual temperature.

Quite complicated ! And probably far more complicated than is justified - or even productive.

Damon Hart-Davis <dhd at exnet.com> wrote:

> Note that the rather nice humidity and temperature sensor that we use (SHT21) is more expensive than and larger than and has tougher power supply requirements the temperature-only device that we otherwise use (TMP112).  There is a significant cost implication of using RH (maybe even a fiver at retail) so it needs to pay its way.

Which is a very important point. Unless the devices are wired to a power supply, more power == shorter battery life == more cost and inconvenience for the end user. Any benefit needs to be enough to outweigh that cost - and personally I very much doubt that it would in most cases.
Bear in mind that a very large proportion of people (again, from observation) don't grasp the concept of a thermostat. And at the risk of coming across as sexist, my observation is that women are the worst offenders in this. My observations come from having responsibility for office heating and cooling for some years at my last job - and the constant resetting of controls on AC systems.
The cycle would typically go like this ...
I'd reset the system, on it's timer, with a "reasonable" setpoint - and explain how the setpoint can be turned up or down by a degree or two. Bear in mind that some of these users would come to work in midwinter wearing a light summer outfit and be most offended at the suggestion that if they know they are likely to be cold at work, they may wish to reassess what they wear ! This brings up another factor in perceived comfort - if they are coming into the office cold from the commute, then they need the office a bit warmer than if they arrive warm.
So first thing in the morning, someone would turn the controls up to max (30˚C) rather than just turning it up a degree or two. Needless to say, with a 14kW AC unit blasting away in a not too big office, it's soon "quite warm" and the unit just gets turned off.
Then in the afternoon, things are still getting warmer - so the AC gets turned on and turned down to min (18˚C). 14kW of cooling soon gets things chilled down, so it gets turned off again.
Then next morning, the heating hasn't been on - and somehow it's *MY* fault that the office is cold :-/

So once again I reset the system, and explain YET AGAIN that this thing has temperature control and the trick is to just nudge it up or down a degree or two as needed. That might work for a day or two, then the cycle repeats, and once again it's MY fault that they can't use the system.

Eventually, as the offices were expanded, we went with hidden units, with networked controls (which I had modified to work around some of the control issues) so there was nothing for users to fiddle with. I'd still get the occasional "can you turn it up/down a bit ?" requests, but once they weren't able to fiddle with the controls, most users learned to live with it.

So when designing a system, the sort of people discussing it here on this list aren't the ones you need to think about. There's that joke about it being hard to make things foolproof because fools are so ingenious - but there's a grain of truth in that. As above, the majority of people who these things are aimed at will be the type that struggle with the concept of "if you are a little cool then turn the heat up a little", the sort of person where the wall thermostat is either on max or min and is used as a switch (ie the heating is either "on" or "off").

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