Where to position your themostats?

[TerryE]

I don't really use an environment thermostat as an active component in my CHS. The following graph might help to explain why.

 

I've got temperature sensors all over my system, and plot shows the  two sets of readings: one set for the Ground-floor toilet which also houses my SunAmps, manifolds and Willis; and one set for the average return flow temperature for the slab.   These are for two days: the 4^th and the 6^th Nov.  So some explanation:

 

• The reason for the big humps (peeking at 7am and 7pm) are that my CH calcs put in a primary heating pulse in during the E7 off-peak period (00:00 - 07:00 for us), and if needed an extra top-up in the late afternoon.  The outside temperature leading up to the 4th was really cold here and the top-up was needed, but it had warmed up by the 6th and so it wasn't. 
• The small saw tooth is because I use the on-the-hour temperature as an accurate slab temperature.  I hae found by trial that if I run the UFH pump, then the return thermometers stabilise after 6 mins or so, so once an hour (at 54mins to the hour), I turn on the UFH pump for 6 mins and then use the on-the-hour reading as reference.  This 10% duty cycle is enough to get a good slab reading and reflow any local solar gain around the slab.
• The toilet thermometer reads slightly higher than the other rooms because there is a little residual heat loss from the SunAmps, etc.  The ripple over the day here is about 1¼°C where it is nearer ½°C elsewhere.  When we are not in the house, this reading is pretty stable (as on the 4th when we were visiting Bristol).  But when we are in the house then I see big occasional drops, and the drop is aligned to one of us leaving the back door open to go out to hand out washing or to get something from my shed.

 

My point here is that I have real problems using local air temperatures as an estimator for overall house temperature.  These are just far too responsive compared to the actual overall house temperature.  If you put the thermo in the hall then just opening an external door dumps enough heat to drop the measured temperature a degree or more.  Any cooking does the same in the kitchen as does 3 or more people just sitting in a room.   Using a single wall mounted thermo as environment estimator is just too noisy for me to use sensibly:  I don't want my UFH system turning on just because I've opened the back or the front door for a minute.   One option would to add a low -pass filter (putting my sensor in a box, or behind the plasterboard), but at the moment I don't bother and do everything based on the slab profile.  

 

As a footnote, this is more of a geeky debate,  hence my posting this here rather than in one of the heating forums.  However, if the mods feel it fits better elsewhere, then please move it.

 

 

 

Edited November 10, 2018 by TerryE

[MikeSharp01]

Control theory 101 great explanation. Need to mull it over I do.

[ProDave]

There are many ways to skin a cat.  One thing that is clear in our house is the temperature will never change quickly.  A cold night with the heating off is barely noticable, and when the UFH comes on, it only warms up slowly.

 

I am finding (contrary to others  experience) that an ultra simple mechanical thermostat set at 20 keeps the room comfortable all the time.

 

I also have mine on a standard central heating timer so I can set the on off times of heating and hot water easily.  That is partly so the house is dead quiet over night (the most noisy part of the heating is the UFH circulating pumps*) and partly for when we eventually fit solar PV to try and do as much of the heating in daylight hours as possible.

 

* When  I get around to it I am going to try swapping one of the UFH pumps for a different make to see if I can make it quieter

[Jeremy Harris]

I've not found any problems with using a hall mounted thermostat at all.  This might be because it's virtually in the centre of the ground floor, so opening outside doors some distance away has virtually no effect (I've never seen the under floor heating or cooling come on from just opening a door). 

 

I do have our air cooling thermostat mounted on the landing on the first floor.  I moved it up there because the upstairs was getting to be around 1 or 2 degrees warmer than downstairs in very hot weather, so there is an advantage in having the air cooling triggered a bit earlier, especially as it's neither very powerful and because it's the only means of cooling the upstairs when the outside air temperature is high.

 

I think a great deal depends on the layout of the house, and whether or not a thermostat is placed somewhere where it might be susceptible to a draft when a door or window is opened.  Having a wall thermostat near the centre of the house, in a location well shielded from drafts, seems to work very well for us, and may well be a good general solution for those using off-the-shelf controls.

[lizzie]

I have ufh thermostats in every room, each room is a separate zone and has a separate thermostat.  Got the heatmiser system so can control from an app on phone or from wall stat.  App works....most of the time.

 

Our UFH takes a time to warm up, we have probes down in the slab so we are not reliant on air temp which is generally kept around 22 by mvhr.  Find I need to set ufh at about 25 to get it on and keep it on for any length of time.

 

My ufh manifolds are split into two - living wing is in plant room and bedroom wing is in my bedroom and even now it is inside a cupboard I am still aware of pump noise when its on.  Wish I had put it in one of the other bedrooms so it could be noisy without disturbing me I am a very light sleeper.

[ProDave]

An important "feature" of an air tight house, is if you open one window or door, you get very little in the way of a draught even if is blowing a gale outside. So unless a thermostat is right by the door, I would not expect the temperature around a thermostat to drop quickly. 

[TerryE]

33 minutes ago, ProDave said:

An important "feature" of an air tight house, is if you open one window or door, you get very little in the way of a draught even if is blowing a gale outside.

 

Out rear utility room is basically a corridor with the toilet on one side.  with the door to our central hall closed it is still like a fish tank with one side missing: open the back door and there can be a 10-15°C temperature drop across the doorway.  The hooter air will spill out of the top of the door and the external cold air flood into the room at ground level.  The mixed air in this fairly small volume will still be a degree or more colder as a result.  

 

@lizzie, your approach doesn't really work for our house: it is basically like a Thermos flask:  all of the insulation is on the outside so if you don't have open doors, then the temperature is amazingly uniform both spacially and in time: If I turn the heating off entirely at the moment then the house as a whole cools by ½-1°C per day.   

[lizzie]

My house is the same @TerryE uniform temp etc its like a big storage heater or thermos as you say. I have the zones because I wanted the ability to have some areas warmer if required. e.g I like a cooler bedroom  and a warmer living area... I probably have my house warmer than most of you. 

 

I dont like being cold or wearing jumpers and socks in the house I grew up in a cold old house went to live in warmer climes as an adult now cant bear to live in a ‘cold’ house, like my living at 26ish and my bedroom at 22 ish..... like my bathroom hot too LOL

 

My motivation for this type of build was not lower cost of heating and running house that is just a bonus I wanted clean air, no draughts and a very warm house..... we certainly were very warm in the summer with no shading on the windows 30 was cool, my stats were showing  35 most of the time, too hot even for me, Window film now on so hopes for next summer being better controlled temps.

[newhome]

28 minutes ago, lizzie said:

like my living at 26ish and my bedroom at 22 ish..... 

 

Eek! I’m a 20 degree living room and 16 degree bedroom girl! It would be like walking around in a sauna at those temperatures! 

[joe90]

I am very interested in this thread as because of programming problems it looks like I have to use a separate room stat in the hallway rather than the controller in the cloakroom. Re opening doors, we have a porch around the front door and the back door inside the conservatory, my reasoning, both act as an air lock to reduce heat loss on opening doors (I  just have to train the wife to close doors before opening others ?). I have a floor probe for the room stat but dismissed it after reading @JSHarris blog on the subject. I too would like to up the temp in the lounge slightly (as it’s where we tend to sit doing nothing) and hoped to do this by upping the flow from the manifold???

[TerryE]

@lizzie we spend 3-4 months a year on one of the Greek islands (when we're not busy building houses that is), so we also keep our house warm -- typically a base temperature of around 22°C.  This is about 1°C warmer than when we first moved in last Dec, but we find that we are comfortable with this.  Even using our little Willis (and mostly E7), the cost of the extra heating for this °C increase is around 15p /day for 6 moths a year say or £27, so it's in the noise and this little hike means that Jan can wander around bare-footed and in a tea shirt all year round.  (Whilst this £27 might seem a relatively high increment, our overall energy costs are so low that we are still debating whether we can make the numbers work to justify the capital and installation costs of installing an ASHP.)

 

The issue comfort variations is an issue -- so like you when sitting quietly in the living room in the evening we find that we prefer the room to be a degree or two warmer, but at the moment our answer is to put on a thin jumper in the evenings.  We do have a level of natural zoning in that our 1st floor is about 1°C cooler than the ground floor (we have no heating at all upstairs), but this suits us.

 

@joe90  In my case it's not just Jan that I have to train; it's myself.  If I am going to the shed to pick up a tool, then I know that I should close the back door behind me as I leave the house and reopen it 1 minute later when I return.  Leaving it ajar for a minute is just sloppy, but I already use the utility as an air lock so I probably lose less than a 1p's worth of energy -- life is just too short; I find it easy just to accept that the overall temperature in the utility/ toilet will drop by a degree or so and recover over then next 15-30 mins. 

 

IMO, there is nothing wrong with having a room stat so long as you are aware of the potential limitations resulting from placement, and put it somewhere that represents a sensible aggregate environment -- such as an inner corridor, and avoid putting it in a room with an external door or a room where you will be generating excess heat such as by cooking.

 

There is also a fundamental difference between my and Jeremy's approaches for slab measurement: he used spot probes and I use the averaged temperature of the UFH returns which effectively provides an aggregate temperature averaged across the whole slab.  If you zoom into my graph above then you will see that there is 2 min sampling quantisation noise, but apart from that the reading is stable to better than 0.1°C

[Gone West]

13 hours ago, lizzie said:

 

like my living at 26ish and my bedroom at 22 ish..... like my bathroom hot too LOL

 

Wendy's with you on that .

[ProDave]

Ours is the traditional UFH setup with 1 thermostat per room so for example in the kitchen I want the heating to go off when there is heat from cooking etc.

[Jeremy Harris]

4 minutes ago, ProDave said:

Ours is the traditional UFH setup with 1 thermostat per room so for example in the kitchen I want the heating to go off when there is heat from cooking etc.

 

The problem for us in doing that is that the slab carries on giving out heat for many hours after the UFH turns off.  Any sort of individual room control just doesn't work well, due to the long thermal time constant.  Boosting the MVHR tends to get rid of heat reasonably well, as the heat recovery efficiency drops a fair bit on boost and we have three extracts from the kitchen/dining, utility and ground floor WC all pulling air through from the entrance all into the kitchen/dining room.  When it's not too hot outside, then just opening a window in the kitchen works OK; it just unbalances the MVHR a lot and pulls cooler air in from outside.

[MikeSharp01]

The more I read through this thread the more I wonder if a deep learning classifier might be able to work this all out - eventually. We still seem to be applying classical scientific theory to it while other methods might work a little better in the massed data era. Have a look here for a broader view of these ideas. My musings are that in a well insulated house all the time series thermal events will have data characteristics that might be classified, mostly just slopes of temperature but when tied in to other events around the systems such as power consumption in / gas flow to the kitchen (With rates of consumption here it would be possible to get ahead of the cooling / heating systems) perhaps, the burglar alarm being disarmed = people coming in,  it should be possible to classify the events and then propose control schemes that can cope - although naturally deep learning can do little but work with the laws of physics. Anyway just some musings.

[TerryE]

@MikeSharp01, Mike I find myself wanting to raise a mild dissent with your post. 

 

For those not familiar with what this is all about and have some mathematical toleration, try the Youtube Computerphile videos by Mike Pound on this general topic.  Amazing stuff.  However,  I see this is more of a classical control systems issue rather than a deep learning one.  Deep learning requires huge amounts of training data across a catholic set of scenarios, and this just the case for the thermal control of a passive class house. 

 

In another topic @Ed Davies has given a working definition of time constant and in our case at a ΔT of 15°C our house loses about ½-1°C/day with the heating off -- let's round this at the worst end, and this gives a time constant of ~15 days.  IMO, even if it was a third of this, it would still make no sense to me to try to control it with a bang-bang system that can react in seconds.    Yes, with some careful crafting you might be able to get this to work, but my thought is really why on earth should I bother?  At the moment, my Node RED system runs a set of calcs at 17:00 each day to work out how much heat is needed for the next 24 hrs and then sets up a simple two period ON/OFF execution plan to add this heat. (though it is going to have to get about another 3 or 4°C colder for the second period to be used).

 

I do have a roomstat now -- or at least a DS18B20 sitting inside one of the internal partition walls against the backside of the plasterboard and measuring the average room temperature, which shows a ripple of about ½°C over the day.  My day-to-day 17:00 slab temperature does wonder slightly -- I have been refining the algorithm and the 1-σ is currently about 0.3°C; with this stable performance and the cost of the entire H/W control system a few hundred quid, I am not sure how deep leaning will help me refine this.  The system can also be trivially adapted for ASHP heating the slab directly, so future-proofing for this isn't an issue either.

Edited November 13, 2018 by TerryE

[dpmiller]

what happens if there are marked daily swings with your system, @TerryE? Say ambient is  15c one day, 7c the next or v/v, as we're seeing locally at the mo...

[Jeremy Harris]

If it's anything like our house, then the answer is very little changes.  Worst case for us was in the summer, where we had 34 deg C outside during the day and around 16 deg C overnight on clear nights.  The temperature inside the house barely changed by more than 1/2 deg, due to the high decrement delay structure slowing the rate of transfer of heat in/out of the building and the overall long thermal time constant of the interior.

[joe90]

I am not sure I want our house at a constant temp!. I am only just getting my heating going and learning how it works/suits us. With the solar gain we get during the day (mostly) and being active. I am sure we need no heating but when tired in the evening we need a warmer envoironment. The same goes for first thing in the morning, nothing worse than a cold bathroom (reason for electric towel radiator and UFH) or cold kitchen for breakfast. I was pleased/relieved when @JSHarris found a simple room stat was better than his complicated programming etc (as I have no idea how to do that). ?

[Ed Davies]

3 hours ago, dpmiller said:

what happens if there are marked daily swings with your system, @TerryE?

 

From what he said previously I think @TerryE's system looks at the forecast in working out how much heat to put in overnight. I think that, in general, projecting ahead like this is key to managing household energy efficiently.

 

Most of the systems I see react only to the current conditions (e.g., the battery's nearly charged, start diverting to hot water) rather than looking ahead and making more balanced decisions (it's forecast to be sunny tomorrow and the battery's already half full which gives us plenty of energy for this evening and night so we can start heating hot water immediately). Obviously this sort of thing matters more if you have multiple variable sources and sinks for energy but even with quite simple systems it can make a difference.

 

Sometimes the forecast will be wrong and you'll wind up using more energy than you could have (e.g., @TerryE's system doing a bit more heating in the afternoon or leaving the slab a bit warmer than it needed to be) but generally they're pretty good for at least 24 hours these days.

[TerryE]

@dpmiller, I will expand Jeremy's comment:  If we have a Δt of 8°C and don't compensate for it, the time constant of the house will mean that the house will drop about a ¼°C as a result.  There is also a decrement delay through the walls;  I saw this in my modelling scenarios: if you and a step function in the external temperature, it takes a couple of days for this to make its way through the stone skin and walls.  If the slab temperature drops (or rises) from the computed set point then the daily algo tops up or trims the daily heating amount.  At the moment, I have this step capped at 1hr, so it would take 3 days to step from 5 hrs heating to 8 hrs or v.v. but I find that this is quite responsive enough.

 

The system also takes in the weather forecast from the met office each night and I could plug this is as a feed-forward term.  Next month, I am adding a thermometer in the air-gap between the outer stone skin and the timber-frame and might use that instead if it is a cleaner estimator.  Using this sort of data as a feed-forward term could drop the variability even more but this gets me into further characterisation, Kalman filtering etc. and I have other priorities at the moment. 

 

@joe90, this is a case of personal preference, and one that can only be developed from experience.  Jan and I started out thinking along the same lines as you, but we've come to really like the stable temperature.  OK, we end up working in T shirts is we are doing something a bit physical, but that's what we do in the summer anyway: you just have to get used to the fact that the house internally is like the summer all year round -- and we have.  You say that there is nothing worse than a cold bathroom in the morning, but that's my point we don't have any cold rooms any time.   The house was at 22.6°C last night at midnight and at 23.0°C this morning when we got up.  After my old draughty and hard to heat farmhouse, this is just heaven.   Yes, we could have the whole house a couple of °C cooler and save maybe £50 p.a. on our Willis heating costs but our personal preference is that we'd rather pay the extra £50 a year and be able to wander around bare footed and in T shirts. 

[TerryE]

@Ed Davies, sorry -- I've just cross posted.   As I said I do collect and log the daily local met office forecast, but we have found so far that we don't need to use it.  If I do see evidence of our simple algo not tracking well enough then I will add a feed forward term.  I used to do this sort of control stuff on some of my work projects, so find it interesting but I am also a bit time constrained at the moment.  It's the sort of refinement that I can make any time of the next few months when I have the free time.

[Ed Davies]

Ah, useful to understand that you don't use the forecast in the control loop yet. Still, it's nice to have the option.

[TerryE]

Like Jeremy, I have a lot of instrumentation one my system, but this is largely to enables me to characterise how my house as a system behaves. the thermometers are daisy chained on a One-wire bus and cost pin money.  All of the smarts are done in one RPi.  Most people find using software-based systems daunting, but I've used dozens of languages over 50 years of programming so this is second nature to me.  And the nice thing about software base control is that you don't have to start buying extra control bits and rewiring stuff.  In Node RED, you mostly drag and drop inside a browser window.

 

I've pretty much got to the point where I can write all this up properly for others.  Most aren't going to be interested in the journey to where I've ended up where we are, but more just as simple an explanation as possible of the end point and some understandable supporting data on how the system works.

[joe90]

I take my hat off to @TerryE and the like who can do this stuff, I have trouble programming the tv remote ?. The older I get the more I become a Luddite.