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timmmay

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  #824355 23-May-2013 19:44
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I feel like I've just stepped into a lecture about two grades above what I understand!

MikeSkyrme
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  #824397 23-May-2013 20:34
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timmmay: I feel like I've just stepped into a lecture about two grades above what I understand!


That's ok... I'm wondering how to get a proportional output to a single heater element....

Out of curiosity, do you have the installation drawings? Is it a single element? Where is the element located?

I would still say the fault lies at your existing controller, but this is without doing any fault finding of my own.




Michael Skyrme - Instrumentation & Controls

 
 
 
 


timmmay

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  #824402 23-May-2013 20:38
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There were no installation drawings. I have a photo is all. It's one heat pad or coil over a room that's probably around 8 square meters. The element is under the tiles, but not sure how far down. I think it's floor, some kind of insulating material, leveling compound, heating element, sealer, then tiles, but I could be wrong.

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  #824474 23-May-2013 22:59
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Okay, I've read up a bit on installations and controllers as my background is more in process control and agricultural technology product design. I see the "better" floor heating controllers have functions like compensating for ambient temperature and lowering the setting by 5 degrees during the night.

With what you describe I'd say the controller is doing what it was programmed to do.

Regarding the price point, if there is already a micro to drive a display/timer/settings then there is very little cost difference between PID and a relay. The difference is in redesign cost (instead of repackaging an old design). Our electric blankets do proportional control, compensate for ambient, have a timer and LCD, just does not sense blanket temperature - and no relay.




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Handle9
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  #824492 23-May-2013 23:54
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You're driving a couple of hundred watts with your electric blanket - the smallest relay on most thermostats are 1440W (6A) and some are 3.8kW(16A). Once you get to that size with solid state you get some heat issues plus there are some cost issues - relays are dirt cheap and more reliable. Obviously PWM with relays is a really bad idea so you can't really PID a relay.

At the end of the day when you are doing a coarse application like underfloor heating you are working with a very blunt instrument so it doesn't matter - you are never going to get close control so why waste money on trying to achieve it. We do use pulse width modulate electric duct heaters through solid state relays but it's generally in a commercial environment and it's never great. Electric heat is a bit scary in that you can cause a fire if it all goes horribly wrong.

The best type of temperature control is using water and valves, but even with hydronic underfloor heating they are just on/off thermic valves which are incredibly slow (it can take 7 minutes for some thermic valves to crack open from when they get a control signal). You are dealing with big slow systems so it really doesn't matter though. If you have to heat several tons of concrete it's going to take a long time anyway.

The way night setback works is generally it doesn't let the temperature fall below either 5 degrees or 8 degrees. This is a cold climate thing and it is sometimes called "frost protection." NZ is really unusual in the amount of electric underfloor heating we use. In the northern hemisphere it's usually radiators or hydronic underfloor heating, possibly passing demand signals back to a boiler.

Going back to Timmays application you shouldn't really get more than 2 degrees over setpoint (these thermostats do seem to have a 0.4K switching differential). Electric underfloor heating stops heating as soon as you switch it off so it shouldn't overshoot much at all. It can go a bit under set point but once the mass is warm you aren't pushing all that much energy into it. I think from my memory of some of his other posts we're talking about an old villa so its not even a concrete floor so it should be reasonably responsive. Something is pretty clearly not right.

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  #824520 24-May-2013 06:37
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Handle9: You're driving a couple of hundred watts with your electric blanket - the smallest relay on most thermostats are 1440W (6A) and some are 3.8kW(16A). Once you get to that size with solid state you get some heat issues plus there are some cost issues - relays are dirt cheap and more reliable.

PWM of AC is a bad idea unless you need to like a fan speed controller.  Cycle skipping is cheap and efficient.

PID works for any system irrespective of the system speed, but setting it up correctly is too hard for installers with far too many variables.

Timmay does not have a concrete slab so low thermal lag.  He says the element is still on when the controller is showing it is above the set point so the conclusion is the controller is likely using other information to override the set point.  Or the controller has a faulty relay(!).

A couple of years ago I've bought 2 heated towel rail timers (in-wall, turns on for 4h every 12h), known brand from a good supplier.  Both failed within 2 months.  It had a counterfeit mains capacitor, we happened to have the identical (same brand) capacitor at work and the ones in the timers were clearly counterfeit.  Replaced the (8c) capacitors and the timers are still working fine.  Timmay's controller can be faulty and turn the element on when the microcontroller is not driving the relay (unless it actually shows heating while showing over temperature, then it is a setting issue).




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timmmay

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  #824531 24-May-2013 08:10
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The OJ controller I had (and it's replacement) is very configurable. I had different temperatures set for different times of the day, rather than letting it just turn down overnight. There was an adaptive function, but it was turned off, as there was no documentation about what it did.

My floor is wood, not concrete. It has a bit of mass, as it's wood, under floor insulation, up to 5cm of levelling compound, insulation board, and tiles - but a lot less than a concrete floor.

The WarmupNZ thermostat seems to control the temperature fine, it's just not configurable enough. It would waste power keeping the floor at the same temperature during the day as overnight, which I won't accept as I need it warm overnight but want it off during the day. This thermostat makes a big click when it turns on or off, so it's a relay, the OJ one was much quieter - either a better relay or solid state.

Thanks for all the thoughts everyone.

 
 
 
 


timmmay

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  #831775 6-Jun-2013 18:26
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Ok, some results.

My electrician called OJ in Denmark. OJ electrical to keep the temperature within 2 degrees of what it's set to, so the relay doesn't wear out. It's a quiet relay, and can switch up to 16A, but we have it only switching 800W (3A or so). In practice it seems to be within 3 degrees. Very configurable, very flexible, you can have four individual periods with different temperatures each day if you want. It just can't keep the temperature accurately.

Devi and WarmupNZ have only two temperatures - the "comfort" temperature, and the "all other times" temperature. That's not very useful if you want it nice and warm during the evening for showers, a bit cooler overnight for bathroom trips, then off during the day. The WarmupNZ one I have in right now keeps the temperature you set very accurately, but has a very loud click when it switches on and off.

My choices are:
- Live with the OJ Electrical thermostat, which really isn't good at keeping the temperature
- Live with the WarmupNZ thermostat
- Use the WarmupNZ thermostat, but have a new circuit wired in with a digital timer on the mains board. That would completely shut off power to the thermostat during the day.

Thoughts? Other options? Yes I know I'm a fussy bug*er ;)

timmmay

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  #1284471 15-Apr-2015 09:37
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Revisiting this topic... I ended up using the WarmupNZ thermostat, along with a timer at the circuit board that turns the power off to it between 7am and 5pm. It keeps the floor temperature accurately, within 1 degree. This worked fine for a while, but it's started "forgetting" the time, which suggests the backup battery inside isn't up to the task of being cycled each day.

Has anyone found an under floor heating controller that can maintain the temperature approximately as set and gives you more than two temperature options? Comfort, economy, and off are what I need. I could try another OJ thermostat but given how poor the temperature control was in the two units I tried I'm reluctant.

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  #1284903 15-Apr-2015 16:29

Seems you might as well just make an Arduino based underfloor controller. At least you will have no problem getting it to accurately maintain set temp. As you could get it to measure floor temp, room temp and outside temp. And for overkill you could also get it to measure humidity. So it can calculate "wet bulb" temperatures. And compensate for different humidity levels.

And a "quick and dirty" way to get 1/2 power output from an element. Is to get a relay to switch a diode in series with the element.





timmmay

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  #1284935 15-Apr-2015 16:40
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That sounds like way too much effort, I just want to buy something flexible that works. I don't think it's unusual or unreasonable for someone to want to have the under floor heating shut off during the day to save power, is it?

nickb800
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  #1285001 15-Apr-2015 18:01
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timmmay: Revisiting this topic... I ended up using the WarmupNZ thermostat, along with a timer at the circuit board that turns the power off to it between 7am and 5pm. It keeps the floor temperature accurately, within 1 degree. This worked fine for a while, but it's started "forgetting" the time, which suggests the backup battery inside isn't up to the task of being cycled each day.

Has anyone found an under floor heating controller that can maintain the temperature approximately as set and gives you more than two temperature options? Comfort, economy, and off are what I need. I could try another OJ thermostat but given how poor the temperature control was in the two units I tried I'm reluctant.


Can you get your sparky to re-wire things so that the thermostat has constant power applied, but the actual element is controlled by the thermostat AND timer in series - so both need to be on for the element to heat. Might require an extra cable between switchboard and thermostat, or the timer colocated with the thermostat

n4

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  #1285037 15-Apr-2015 18:50
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timmmay: That sounds like way too much effort, I just want to buy something flexible that works. I don't think it's unusual or unreasonable for someone to want to have the under floor heating shut off during the day to save power, is it?


We have Devireg controllers. I think depending how low you set the 'comfort' level, you can achieve the system being off during the day unless the weather is really cold or your house insulation is really poor. I'm in Auckland however and this may not apply further south :-)




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timmmay

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  #1285063 15-Apr-2015 19:55
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I checked with devi, they only have two heat settings when I asked a year or two ago.

n4

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  #1285070 15-Apr-2015 20:01
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timmmay: I checked with devi, they only have two heat settings when I asked a year or two ago.


True. What I meant was you could send the comfort level at a point such that it is warm enough to stop the floor freezing overnight, but cool enough that day time temperatures would be high enough to keep the thermostat off.




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