Welcome to 'the public sector' (aka bureaucrats)
I've seen publically-funded facilities assessed for 'performance' almost literally by weighing the amount of paper they produce!Kind Regards, John.
Thermostat fr underfloor heating with high thermal inertia
- Thread starter jonnyhifi
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I've seen publically-funded facilities assessed for 'performance' almost literally by weighing the amount of paper they produce!Kind Regards, John.
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Thinking aloud (holmslaw will love this
) ... if the thermal interia is really as high as you imply ('hours to come up to temperature') then I suspect that no 'direct' control system is ever going to prove satisfactory, because of the enormous amount of hysteresis that would exist. In order to avoid 'overshoots' the 'pulses' you postulate would presumably have to be fairly short and hours apart - such that it could take days to reach target temperature, and that's even assuming that enviromental conditions didn't change during that period.As you and others have perhaps hinted, maybe one has to think more laterally and have a control system programmed to actually estimate/predict what on/off switchery it has to do, on the basis of the prevailing conditions, in order to achieve the desired target temp, without even attempting any control on the basis of real-time temperature sensing. The system could obviously be refined as it was used (what you are calling 'learning', although it could be done by empirical manual adjustments). If it is going to take hours for temperature to reach equilibrium after each and every 'on', 'off' or 'pulse', I really can't see any alternative. However, this can't be a unique problem, and there are some very clever control systems engineers out there.
Kind Regards, John.
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I would agree with BAS as to reaction times. We fitted underfloor heating to just one room in my mothers house to dry the wet room floor. It was a total failure. Once shower is switched on it cools the floor from just cool enough to walk in with bare feet to the lot cooler water temperature. On switching off the shower it takes 1/2 hour to dry the floor. My mother has vacated the room well before that time and only advantage is when returning to room latter.
With the towel rail turned off the room in spite of only just cool enough to walk on tiles is too cool in winter. Maybe without extractor fan it would be enough but just glad there was a large towel rail installed.
The thermostat failed three times. The underfloor sensor also failed and in spite of being in a pocket we were unable to remove it for replacement. After the third time the thermostat failed we gave up. Unlike a central heating thermostat it took the full current required by the mat. And although within the rating it clearly could not take the power.
I have forgot about this when talking about the DRT2 thermostat and clearly it would need a relay to use it as designed for control not full current.
For a house occupied 24/7 it may work but clearly no good for morning heat when getting up then off while at work and evening heat on return. I have looked at the cut away Roman building in Chester with underfloor heating and wonder how it worked back then and why the idea was abandoned for so many centuries after they left. Having seen my mothers modern system I now know why.
Being a bit unfair my son also installed under floor heating (water) in his narrow boat and it worked well. However there was also the heat direct from Aga and radiators. The main use was to stop water boiling he also had option of directing water to radiator pushed hard against the hull to keep water cool in the summer.
The PLC program continually switched the heat on and off. On is referred to as Mark and off is called space. To control the heat the mark/space ratio was changed and that would be only way to really control the heat with electric. Clearly this needs more than a simple thermostat. Using a PLC it was relatively easy. But to get a contractor to switch that often they did not last long so had to use solid state contractors. This also resulted in having semi-conductor fuses as MCB's are too slow.
Cost would be massive. I would guess about £750 for parts and one still needs to write the program. Possible a thermostat like this
will do the job. The PT100 sensor can be placed so as to get a balance between room and floor temp but at £79.15 + £39.10 and that's not including price of enclosure and one still would not be sure it would work I would not really consider it worth the chance.
Looking at the RS Components list of thermostats I am sure there is one which will do the job. One can pay £256.67 just for electronic unit without price of sensors or enclosure. In industry yes we take a chance and we can call on RS to sends experts to advise on best options. At home think I would be looking at alternative heating as electric is not a cheap method anyway.
With the towel rail turned off the room in spite of only just cool enough to walk on tiles is too cool in winter. Maybe without extractor fan it would be enough but just glad there was a large towel rail installed.
The thermostat failed three times. The underfloor sensor also failed and in spite of being in a pocket we were unable to remove it for replacement. After the third time the thermostat failed we gave up. Unlike a central heating thermostat it took the full current required by the mat. And although within the rating it clearly could not take the power.
I have forgot about this when talking about the DRT2 thermostat and clearly it would need a relay to use it as designed for control not full current.
For a house occupied 24/7 it may work but clearly no good for morning heat when getting up then off while at work and evening heat on return. I have looked at the cut away Roman building in Chester with underfloor heating and wonder how it worked back then and why the idea was abandoned for so many centuries after they left. Having seen my mothers modern system I now know why.
Being a bit unfair my son also installed under floor heating (water) in his narrow boat and it worked well. However there was also the heat direct from Aga and radiators. The main use was to stop water boiling he also had option of directing water to radiator pushed hard against the hull to keep water cool in the summer.
The PLC program continually switched the heat on and off. On is referred to as Mark and off is called space. To control the heat the mark/space ratio was changed and that would be only way to really control the heat with electric. Clearly this needs more than a simple thermostat. Using a PLC it was relatively easy. But to get a contractor to switch that often they did not last long so had to use solid state contractors. This also resulted in having semi-conductor fuses as MCB's are too slow.
Cost would be massive. I would guess about £750 for parts and one still needs to write the program. Possible a thermostat like this
Looking at the RS Components list of thermostats I am sure there is one which will do the job. One can pay £256.67 just for electronic unit without price of sensors or enclosure. In industry yes we take a chance and we can call on RS to sends experts to advise on best options. At home think I would be looking at alternative heating as electric is not a cheap method anyway.
Fascinating discussion, thanks guys !
TY for the "spirited" disucssion re stupidity. I have to say I'm never adverse to having pitfalls pointed out, just in case. Though, I had thought about this one. In fact, when we originally made the floor, we sank some (empty) copper pipes in, as pockets, ready to be filled with thermal conducting goo, and whatever in the future appeared a sensible thermocouple/ sensor etc, for when I got around to it (errr about 10 years later er hem).
Having done a quick WIKI search, to get the proper name, a class of controllers called proportional–integral–derivative controller (PID controller), seem like the way to go, (and indeed I think the inverters used with Photovoltaics have a "learning on the job" algorithm, so boxes are commonly built with these kinds of algorithms in them)-- but for heating systems ? I can't beleive such a thing isn't out there, and in fact I read an article a month or so back, about the main original ipod deisgner, who has started a company to produce a ludicrously expensive heating controller that "learns" what to do with the heating... and looking like objet d'art...
anyway the wiki article was
http://en.wikipedia.org/wiki/PID_controller
thoughts guys ?
TY for the "spirited" disucssion re stupidity. I have to say I'm never adverse to having pitfalls pointed out, just in case. Though, I had thought about this one. In fact, when we originally made the floor, we sank some (empty) copper pipes in, as pockets, ready to be filled with thermal conducting goo, and whatever in the future appeared a sensible thermocouple/ sensor etc, for when I got around to it (errr about 10 years later er hem).
Having done a quick WIKI search, to get the proper name, a class of controllers called proportional–integral–derivative controller (PID controller), seem like the way to go, (and indeed I think the inverters used with Photovoltaics have a "learning on the job" algorithm, so boxes are commonly built with these kinds of algorithms in them)-- but for heating systems ? I can't beleive such a thing isn't out there, and in fact I read an article a month or so back, about the main original ipod deisgner, who has started a company to produce a ludicrously expensive heating controller that "learns" what to do with the heating... and looking like objet d'art...
anyway the wiki article was
http://en.wikipedia.org/wiki/PID_controller
thoughts guys ?
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PID control loops, and variants on them, are very well-established in industrial control applications. However, I think that, even with PID, you may well still have problems such as I described if there is (as you have implied) a very serious lag time between any changes brought about by the control system and the consequent temperature change - indeed, the article you cite seems to suggest that PID systems can become problematical, or unstable, in such situations. However, I'm no control engineer!
Kind Regards, John.
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As I said to start with I did write a PLC program to do the job. However £200+ for PLC then thermocouples and 5 - 20ma converter to feed into the analogue input on PLC plus the sensors. Not counting software, connecting leads and PC. Oh yes solid state contractors. Also since I will guess not always connected to PC also a HMI to enter the temperature and times like any other thermostat.
As a hobby project great but as a cost effective project forget it.
To hold the temperature at a set level is not so hard. If temperature rises you increase the space and if it drops decrease the space. But to heat from cold is more complex. Getting the required 100 degrees per hour I needed was again not too hard but getting it not to over shoot the 700 degree limit was hard which is what is required by your system when starting from cold.
With the stress relief we found the common simmerstat as used with old cookers rings worked well and with pre-heat holding the pipe at 150 degrees it worked better than the electronics as the welding heat would mess up the electronic unit.
So if you wanted a consent heat 24/7 then a small motor on a worm drive adjusting a simmerstat would likely correct the temperature as the outside temperature varied. Would be a good project to build one.
But I would think you want to switch on at a set time every day and off at a set time. This is where you run into a problem. You need to not just a thermostat but a thermometer able to tell the system how close it is to set temperature not just when temperature is reached.
Also also a method of not just switching on and off but varying the power. In practice this is done by switching on and off with a mark/space ratio.
So in practice one needs to use something which although not perfect is closer than what you have.
So consider a scenario thermostat set at 18 degrees once that switches off a simmerstat bypasses it until 20 degrees when it will switch off. So it runs flat out to 18 degrees then say 75% for last 2 degrees which would drastically reduce overshoot. Simmerstat will fail from time to time but designed for 3kW so should last a reasonable time and are cheap.
The 75% would be manually altered starting at 10% as autumn approached to 100% with hard cold snap. I think it would work but as with all non standard systems if anything went wrong you would have a problem getting it fixed.
This is the problem with any non standard system. I have seen it again and again. Was great when installed but guys now 80 and no longer able to repair himself and the electrician, plumber or any other tradesman looks at it sucks through their teeth and either want to renew the lot or will walk away. I had the problem with my car. Went to work abroad car electrics had a fault and non of the local garages would repair it. Lucky I was home a week latter but I did 6 month trips so returned it all to standard.
Consider your ill and it breaks down. What then. So although we can work out ways to improve the control unless you can find a standard system which standard tradesmen will understand and can repair then I would not do it.
If you do decide to modify then at least make it as a bolt on extra which can be removed with ease and returned to standard.
Underfloor heating as an extra is great. But as only form of heating it's not so good. If thinking electric some panel heaters which share the job with underfloor would likely give a good compromise.
As a hobby project great but as a cost effective project forget it.
To hold the temperature at a set level is not so hard. If temperature rises you increase the space and if it drops decrease the space. But to heat from cold is more complex. Getting the required 100 degrees per hour I needed was again not too hard but getting it not to over shoot the 700 degree limit was hard which is what is required by your system when starting from cold.
With the stress relief we found the common simmerstat as used with old cookers rings worked well and with pre-heat holding the pipe at 150 degrees it worked better than the electronics as the welding heat would mess up the electronic unit.
So if you wanted a consent heat 24/7 then a small motor on a worm drive adjusting a simmerstat would likely correct the temperature as the outside temperature varied. Would be a good project to build one.
But I would think you want to switch on at a set time every day and off at a set time. This is where you run into a problem. You need to not just a thermostat but a thermometer able to tell the system how close it is to set temperature not just when temperature is reached.
Also also a method of not just switching on and off but varying the power. In practice this is done by switching on and off with a mark/space ratio.
So in practice one needs to use something which although not perfect is closer than what you have.
So consider a scenario thermostat set at 18 degrees once that switches off a simmerstat bypasses it until 20 degrees when it will switch off. So it runs flat out to 18 degrees then say 75% for last 2 degrees which would drastically reduce overshoot. Simmerstat will fail from time to time but designed for 3kW so should last a reasonable time and are cheap.
The 75% would be manually altered starting at 10% as autumn approached to 100% with hard cold snap. I think it would work but as with all non standard systems if anything went wrong you would have a problem getting it fixed.
This is the problem with any non standard system. I have seen it again and again. Was great when installed but guys now 80 and no longer able to repair himself and the electrician, plumber or any other tradesman looks at it sucks through their teeth and either want to renew the lot or will walk away. I had the problem with my car. Went to work abroad car electrics had a fault and non of the local garages would repair it. Lucky I was home a week latter but I did 6 month trips so returned it all to standard.
Consider your ill and it breaks down. What then. So although we can work out ways to improve the control unless you can find a standard system which standard tradesmen will understand and can repair then I would not do it.
If you do decide to modify then at least make it as a bolt on extra which can be removed with ease and returned to standard.
Underfloor heating as an extra is great. But as only form of heating it's not so good. If thinking electric some panel heaters which share the job with underfloor would likely give a good compromise.
Yes - but, as I've been saying, what if you have to wait for a few hours after each increase or decrease in the mark:space ratio to find out what effect it's had on temperature - how good control do you think you could then achieve? If you detected a temperature rise and hence decreased the mark:space ratio, the temperature presumably could continue to rise further for a long time before it (may) start falling as a result of your change - and, of course, if you didn't change the ratio by enough, it might never fall (just carry on rising, but not so quickly).
Indeed - similar, but far more difficult problems, if one has to wait a long time before a change has its effect.
Kind Regards, John.
It was my opinion that there was no way to control "standard" underfloor heating of electric elements in a thermal mass to suit the heating needs of a domestic bathroom. Experience of several systems owned by friends and relatives has proved that opinion to be valid.
They are effectively storage heaters with little if any method to control the rate of release of heat from the store. It can be slowed by a thick rug when the room is not in use.
As a source of background heating to take the chill out of the room by keeping the room around 12 deg C UFH can be effective and it can do this with a relatively simple control system. The 12 deg set point will still only result in the room being some here between 10 and 15 degrees.
Accurate temperature control to 16 degs or other chosen comfortable temperature will only be possible with fast reaction heating equipment acting on its own or in addition to the UFH system.
Underfloor heating systems where the heating elements are not embedded in a large thermal mass are more responsive and more controllable but even these still have to cope with the thermal insulation of the top layer that people walk on. And the temperature may have to be too hot to walk on if the heat losses from the room are significant. Then the extractor fan will be removing warm air, the replacement air could be chilling the room faster than heat can come from the floor.
When deciding on UFH this exercise can be very interesting.
Look at the size of the hot water radiator needed to warm the room. Multiply the area of the radiator by its operating temperature and this will give a value for the area.degree parameter needed to heat the room. Then divide this area.degree value the area of the floor to be heated. The result is the temperature the floor has to be to satisfactorily heat the room. Then add in the delay due to thermal mass heating and cooling.
They are effectively storage heaters with little if any method to control the rate of release of heat from the store. It can be slowed by a thick rug when the room is not in use.
As a source of background heating to take the chill out of the room by keeping the room around 12 deg C UFH can be effective and it can do this with a relatively simple control system. The 12 deg set point will still only result in the room being some here between 10 and 15 degrees.
Accurate temperature control to 16 degs or other chosen comfortable temperature will only be possible with fast reaction heating equipment acting on its own or in addition to the UFH system.
Underfloor heating systems where the heating elements are not embedded in a large thermal mass are more responsive and more controllable but even these still have to cope with the thermal insulation of the top layer that people walk on. And the temperature may have to be too hot to walk on if the heat losses from the room are significant. Then the extractor fan will be removing warm air, the replacement air could be chilling the room faster than heat can come from the floor.
When deciding on UFH this exercise can be very interesting.
Look at the size of the hot water radiator needed to warm the room. Multiply the area of the radiator by its operating temperature and this will give a value for the area.degree parameter needed to heat the room. Then divide this area.degree value the area of the floor to be heated. The result is the temperature the floor has to be to satisfactorily heat the room. Then add in the delay due to thermal mass heating and cooling.
Slightly off-topic, but:
Even if the element doesn't work due to a fault if the floor is tiled and the element is within 30mm of the surface you may still be able to detect the slight variance in temperature and locate the wires. (You can use them to locate wall ties in a cavity wall and see which have failed so I don't see why it wouldn't work.)
If you have access to a thermal imaging camera it is dead easy because the wires will all show up through the floor covering with an accuracy of -1mm per mm of depth. The only thing it won't show is the supply cabling past the last 300mm or so, but that should always be identifiable by locating the end of the heater element and the junction that supplies it.JohnW2 said:
Even if the element doesn't work due to a fault if the floor is tiled and the element is within 30mm of the surface you may still be able to detect the slight variance in temperature and locate the wires. (You can use them to locate wall ties in a cavity wall and see which have failed so I don't see why it wouldn't work.)
Thank you everyone, a very very interesting discussion.
I am coming to the conclusion that some kind of simple but dirty solution is probbaly the way to go, perhaps connecting the underfloor heating simply to the night supply (to keep my cost / kWhr as low as possible) with a thermostat that I tweak as the seasons go by. I see there are some digital ones available that allow programming of up to 3 temperatures a day, so maybe I set it to "medium/ low" for most of the night, and in the last hour or so, give a kick up to a higher temperature, with some empirical fiddling as to times, and temperature settings to make it all work.
Of course, the annoyance iwth storage systems is if you are out all day, they've almost certainly given a lot of heat out from the start, when you didn't want it, and indeed with an underfloor heating storage system, there is no "knob" to turn to slow or speed up the rate of heat delivered, form that which has been stored, as you very incitefully pointed out. The rate of heat delivery is simply proportional to "delta T", so ironically I can see the house will be hottest at perhpas 10--11 am, once the heat has made its way throught the screed and floor topping.
Bummer.
esp. as walking on a warmed floor is soooo nice
I do also agree with the very sage observation of "how's it going to get fixed when I'm 80". Again, a real bummer
I find getting trademsan who even understand "standard" stuff is difficult enough.
I've had a case of that with a project I'm peripherally involved in today.
Early on we'd bought some (plumbing) shock arrestors for a domestic refurb: where a wet room is being put in and another bathroom being refurbed. We've changed the cold system from vented to unvented, and taps are being changed from old rubber washer ones to ceramic disc taps.
During the first phase (bathroom 1) it became apparent that the pressure pulse created when a tap was turned off was significant. (A large column of water whizzzing down the pipe, towards the now closed tap, that consequently needs to be decelerated in a very short time, leading to large forces.) so we fitted the first shock arrestor very succesfully.
The plumber originally working on it, dropped out for personal reasons, and the guys now working on it, (i.e. bathroom 2 elsewhere int he building) said today "I don't know what these are for, you never need these they don't do anything".... Ooooooo where do we start --- Newton's laws ? lol wilfuil ignorance is a big problem. Tomorrow I'm going to have the "I'm a professional I know everything about plumbing" conversation from them -- I've already had the "Engineers always over engineer structure, better if you leave it to us professional builders" conversation. OK.. so they have been lucky in not killing anyone yet....
I find this arrogant ignorance depressingly common.
God help me if I try and do somehting "clever" in my own house !
I am coming to the conclusion that some kind of simple but dirty solution is probbaly the way to go, perhaps connecting the underfloor heating simply to the night supply (to keep my cost / kWhr as low as possible) with a thermostat that I tweak as the seasons go by. I see there are some digital ones available that allow programming of up to 3 temperatures a day, so maybe I set it to "medium/ low" for most of the night, and in the last hour or so, give a kick up to a higher temperature, with some empirical fiddling as to times, and temperature settings to make it all work.
Of course, the annoyance iwth storage systems is if you are out all day, they've almost certainly given a lot of heat out from the start, when you didn't want it, and indeed with an underfloor heating storage system, there is no "knob" to turn to slow or speed up the rate of heat delivered, form that which has been stored, as you very incitefully pointed out. The rate of heat delivery is simply proportional to "delta T", so ironically I can see the house will be hottest at perhpas 10--11 am, once the heat has made its way throught the screed and floor topping.
Bummer.
esp. as walking on a warmed floor is soooo nice
I do also agree with the very sage observation of "how's it going to get fixed when I'm 80". Again, a real bummer
I find getting trademsan who even understand "standard" stuff is difficult enough.
I've had a case of that with a project I'm peripherally involved in today.
Early on we'd bought some (plumbing) shock arrestors for a domestic refurb: where a wet room is being put in and another bathroom being refurbed. We've changed the cold system from vented to unvented, and taps are being changed from old rubber washer ones to ceramic disc taps.
During the first phase (bathroom 1) it became apparent that the pressure pulse created when a tap was turned off was significant. (A large column of water whizzzing down the pipe, towards the now closed tap, that consequently needs to be decelerated in a very short time, leading to large forces.) so we fitted the first shock arrestor very succesfully.
The plumber originally working on it, dropped out for personal reasons, and the guys now working on it, (i.e. bathroom 2 elsewhere int he building) said today "I don't know what these are for, you never need these they don't do anything".... Ooooooo where do we start --- Newton's laws ? lol wilfuil ignorance is a big problem. Tomorrow I'm going to have the "I'm a professional I know everything about plumbing" conversation from them -- I've already had the "Engineers always over engineer structure, better if you leave it to us professional builders" conversation. OK.. so they have been lucky in not killing anyone yet....
I find this arrogant ignorance depressingly common.
God help me if I try and do somehting "clever" in my own house !
Whilst that statement is, indeed, both simple and true, as far as it goes, I suspect the reality is much more complex .... for example, for a constant electrical power input, the temperature of the heating element (hence also Δt) will, in turn, be dependent upon the rate of heat flow away from the element, resulting in negative feedback - e.g. if room temperature rises, Δt decreases, hence heat flow decreases but, in turn, that will lead to an increase in element temperature, hence tending to increase Δt and heat flow again. The extent to which that process 'hunts' (oscillates) before some sort of equilibrium is achieved will obviously depend on the time lags involved in the various steps of the loop.
Kind Regards, John.
Indeed - very clever!
It's the 'may' that would make me a bit nervous about that. If the image were very clear, then fair enough - but, if not ..... !! Mind you, if there is a fault in the buried heating wires, that's probably the least of one's problems
Kind Regards, John.
Whilst you lot are wracking your brains trying to figure this out Danfoss Devi already has the solution.
http://tinyurl.com/c6asnbm
http://tinyurl.com/c6asnbm
Thanks - that's interesting. I obviously don't know how well it works (can't find any reviews) but, in relation to the discussions that have been going on here, it doesn't sound all that 'clever' and, with a price only around £60, quite probably isn't.Whilst you lot are wracking your brains trying to figure this out Danfoss Devi already has the solution.
http://tinyurl.com/c6asnbm
One particularly suprising thing (at least, to me) is that the recommended mode of use/installation for bathrooms is to use only the floor temperature sensor (not the room sensor). I also note that it 'must not be exposed to moisture' - perhaps a little difficult with a bathroom, unless one installs it outside the room!
Kind Regards, John
They are absolutely right.
No engineer has ever been know to spend any time, or money (on computing, for example) to find a way to make things lighter and stronger at the same time, i.e. to try and do more with less...
//www.diynot.com/forums/viewtopic.php?t=7855&start=0
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