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timmmay

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#119158 22-May-2013 18:33
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I'm looking for a controller for my bathroom underfloor tile heating.

We had an OJ Electronics unit it - well two actually, they replaced the first one. They're really flexible in terms of what temperatures you can set and when, which is great, but they don't seem to follow the program you set. If I set it for 24 degrees it would sometimes only heat up to 22 degrees, and would sometimes get up to 28 degrees. It seems like it had a tolerance of 3 degrees, which is huge. The unit knew exactly what temperature the floor was, it just didn't seem to care.

The electrician put in a WarmupNZ unit. It's set to 24 degrees but warms the floor to 28 or 29 degrees. It's also not very flexible - it's preset to temperature 1 (which you can set) for the morning and evening, and to temperature 2 daytime and night. I want it basically off during the day as no-one's here, and on at night but at a moderate heat.

Can anyone suggest another brand that's flexible and actually works as a thermostat?

 **** NB **** This is a 2013 thread, I've restarted the discussion in 2015 in page two at this post.

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MikeSkyrme
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  #823701 22-May-2013 18:41
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Have a look at devi.co.nz

I have the Devireg Touch model.

It looks like it might accept the Warmup thermostat sensor you have installed,but give them a call to double check.




Michael Skyrme - Instrumentation & Controls

timmmay

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  #823712 22-May-2013 19:04
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I don't have a warmup thermostat, I have one my electrician put in that may have come with the OJ Electronics one.

The Touch model looks to be compatible with the sensor I have. Thanks for the tip, I'll check it out :) Any idea of cost? I know the other controllers are about $200 + GST at trade price, but not sure of what I paid as it was part of the package.

 
 
 
 


DarthKermit
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  #823715 22-May-2013 19:10
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Does your original unit claim to be an accurate temperature regulator? Or is it just approx?




Whatifthespacekeyhadneverbeeninvented?


MikeSkyrme
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  #823721 22-May-2013 19:17
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timmmay: I don't have a warmup thermostat, I have one my electrician put in that may have come with the OJ Electronics one.

The Touch model looks to be compatible with the sensor I have. Thanks for the tip, I'll check it out :) Any idea of cost? I know the other controllers are about $200 + GST at trade price, but not sure of what I paid as it was part of the package.


Sorry, I have no idea re. the controller cost, I purchased a complete kit, with 4 Sq mtrs of mat, the controller and sensor for approx $470 + gst.




Michael Skyrme - Instrumentation & Controls

timmmay

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  #823729 22-May-2013 19:38
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DarthKermit: Does your original unit claim to be an accurate temperature regulator? Or is it just approx?


Not sure of the specifications, but it measures and takes settings accurate to 0.1 degrees. The electrician says one degree is more than he'd expect. Will check the specs when I'm on a PC.

timmmay

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  #823734 22-May-2013 19:54
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I just found the specifications of the original unit.

It says "On/Off differential 0.4 °C", which suggests it shouldn't be more than 0.4 degrees over.

DarthKermit
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  #823740 22-May-2013 20:03
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timmmay: I just found the specifications of the original unit.

It says "On/Off differential 0.4 °C", which suggests it shouldn't be more than 0.4 degrees over.


0.4°C seems a pretty accurate measurement to me.




Whatifthespacekeyhadneverbeeninvented?


 
 
 
 


timmmay

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  #823741 22-May-2013 20:04
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It measures to 0.1 degrees, and it shows it on the screen. It just doesn't take any action when it gets outside the temperature range it's meant to keep to. It's like the thermostat has alzheimer's. A degree each way would be fine.

The electrician initially said he would expect it to stay within 0.3 degrees, so he was pretty close.

Handle9
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  #823764 22-May-2013 20:41
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Disclaimer: I work for a thermostat manufacturer.

Thermostats are by their nature are fairly coarse. A typical switching differential is 2K. If you set the thermostat to 20 degrees the heating will switch on at 19 degrees and off at 21 degrees. 0.3K is not ususal.

Things to check:
Is the floor probe what the control is off? Most thermostats can control from floor temperature or air temperature. You should always control off floor temperature. Generally the floor probe is proprietary which means you are stuck with the brand of thermostat you have installed.

Most thermostats have two setpoints, which is what it appears the one you have does. It is extremely rare for the temperature to fall down to 5 degrees, which is what the night setback setpoint is. It is effectively off.

Gross generalisation now follows: most electricians know nothing about thermostats. You would not believe the number of electricians I have had to explain to that a thermostat does not have a neutral connection (it switches the phase, not the neutral). Get someone who actually sells and installs underfloor heating to check your installation, not just a sparky. They are "generally" useless. YMMV.

List price for our thermostats is $165+GST, floor probes are $35+GST. What installers will charge you is variable obviously.

timmmay

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  #823776 22-May-2013 20:57
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Thanks Handle, much appreciated. There's a probe in the floor controlling the temperature, and the electrician does have it set to use that, not air temp sensing. I could get someone else in but I'd have to pay 'em, the electrician is included with my bathroom setup - this is warranty work now. He did some tests with his meter thingies, I wasn't really watching, but he was on the phone to the technical guy at the supplier at the time.

The Devireg Touch claims to support the OJ Electronics probe. I think that's worth a shot.

Niel
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  #823912 23-May-2013 05:59
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The 0.4 degrees spec will be hysteresis, not controlling the temperature within that range. The tile surface is far away from the temperature probe (5-10mm is a lot) and you have thermal lag plus air flow to consider. And then there is the question about how deep the temperature sensor was installed under the floor surface, and what (if any) insulation is below the temperature sensor.




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timmmay

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  #823925 23-May-2013 07:33
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Thanks Niel. The thing about the OJ controller is it did indicate temperature accurately on the display, which I checked with an IR thermometer. The two were always within half a degree of each other.

The thing it was doing wrong was not paying attention to the user settings. If I set it to 24 between 5pm and 7pm, allowing for it to come up to heat, it would still vary the temperature between 22 and 27. Even on the "temporary override" if I set it to 26 it would miss that by a degree or two. The main thing that bugged me though is sometimes it just went into "thermal runaway", heating up to 6-8 degrees hotter than set.

The problem with it wasn't in the sensor, it was in the hardware or more likely the control software.

Niel
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  #824212 23-May-2013 16:43
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In that case, maybe the heater pad is insufficiently rated or too close/far from the probe. To check if it is the software or the installation you need to monitor if the heating element is turned on at the time of high temperature, or off during low temperature.

The best control software is a self learning PID algorithm which is not that hard to do, which will take a few cycles to learn the thermal lag. But this does not work well when you cycle the temperature and when the environment changes (i.e. weather). A basic PID control loop is not hard to do, but I suspect most just use an on/off control with hysteresis. PID has the advantage of using the rate of temperature change to switch off before the upper set point, and on before the lower set point is reached. The hard part is knowing how long before/after, and this is where self learning comes in. Industrial controllers take a range of temperature probes and usually include a self learning function and is also fully configurable. But they look industrial...

I believe I have a couple of basic analogue OJ controllers in my garage, can't imagine them being PID but will have a look if I find them.

Since we got aircon (set to 21 in Winter) and close all windows/doors, our bathroom floor is not cold. Not warm either, but not cold. We do have a rib raft concrete floor which is a large thermal mass. Our power usage is also 30% lower than a year ago, with essentially no other usage change.




You can never have enough Volvos!


timmmay

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  #824214 23-May-2013 16:50
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Our heat pumps are a bit away from the bathroom. They're not super cold, but the heat transfer when you have bare feel makes them feel cold.

The OJ controller was quite random. Sometimes it would have a setting of 24 and a measured temp of 22, and the heating element wouldn't be turned on. Sometimes it would have a setting of 24, a measured temp of 26, and it would still be heating. It just made no sense.

The probe is between two legs of the mat, on the floor under the controller.

One thing they did is not put coils near the toilet - the floor temp there is lower anyway because it's closer to the outside of the house, but the floor all around the toilet is effectively not heated as they didn't put the element close enough. I ask them to fix it, they said no, we'd have to replace the whole floor. I'm still considering whether to take them to small claims or not, they say it's industry standard.

Handle9
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  #824305 23-May-2013 18:43
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It's a 16A thermostat - it'll be on/off a switching differential (i.e. hysteresis).

In reality PID controllers don't exist for this price point, particularly with this sort of load rating. You are using electric heat, with no way of modulating it as they are either electronic devices with relays, bimetallic or bellows operated. You don't really need them anyway to be honest as the system is so slow that integral or derivative action is pretty meaningless.

In fact derivative action is almost never used in air-conditioning or heating control as it creates a lot of issues, particularly with air-conditioning systems. Rapid changes in temperature for an air conditioning system are really unpleasant, particularly in cooling.

Derivative action is used when you have faster systems with instantaneous shocks that require rapid response - conveyor belts and cruise control is were always the examples used in my controls classes at uni. Typically space climate control has systems with time constants in minutes, in fact when you look at room temperature sensors they will often have a piece of brass in them to slow their reaction time down, and damp the system a bit.

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