MX024 - MIAC Controlled Underfloor Heating System

[Benj]

Article: https://www.matrixtsl.com/resources/get ... php?id=435

Please discuss this article here.

[Benj]

Here is a beta version of the full MIAC control program for my heating system.

MIAC_Underfloor_v1.fcf

(62.4 KiB) Downloaded 1203 times

The green menu button accesses the main menu that allows you to set up the device. Not all the functionality is there at the moment but if anyone is struggling with a menu system etc then this demo may be a good starting point.

I will post the final version of the program up when it is completed along with some photos of the installation.

[Benj]

Here is the latest version of my thermostatic heating controller program for the MIAC.

MIAC_Underfloor_v1.1.fcf

(166.39 KiB) Downloaded 1247 times

It now has a lot more functionality and allows you to save up to 40 programmable events, direct control of the system, fill and drain modes, temperature setting for each zone, back-off temperature setting and a lot of other bits and pieces.

I decided to simply use thermistors for each zone for now rather then create a CAN network of sensor nodes mainly because of time limitations on the project. The thermistors I used (Rapid 61-0410) are simply connected between 12V and the MIAC input terminal and the lookup table provided in the file works to translate the readings to degrees C.

Here are some pics of the installation process.

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[echase]

Looks really good. Sorry I missed the Elektor article.

With a 5k thermistor connected like that the change in voltage at the MIAC input must be pretty small, say from 8 to 9V over 0-100C. So how do you measure that with the A/Ds to get enough resolution, as you are only using 10% of the total input range?

The problem is similar with many IC sensors like the LM61as their output range is about 0.5V-1.5V.

[Benj]

Hello,

The upside to less sensitivity is that your temperature lookup table is smaller and not taking a large chuck of memory. I basically created a lookup that would at least cover every degree from 0 up to around 45. This seems to work really well in practice but does not give you a amazingly accurate temperatures because the closest we can sense the reading is down to whole numbers. For more sensitivity the thermistor could be replaced with the 47K variety and the lookup table would need to be updated appropriately.

[echase]

1 degree resolution is enough. There is a lot of nonsense in commercial temperature meters where they show the temperature to one decimal place and ignore the fact that calibration and non linearity means that it’s often 1-3 degrees out anyway. E.g. thermistors are quite variable sample to sample.

Have you thought to emulate the abilities of commercial building controllers where the on time is made earlier if it is cooler outside, e.g. in winter come on at 6am but in spring 7am because it takes much longer to warm up the house. I think some commercial building controllers may account for the sun as well as solar input through windows can make it a lot warmer, as it does in my house. Also could put a thermistor on boiler output and adjust boiler temperature by season/external temperature rather than wind the boiler’s stat up and down with season.

I fear though that code space would be a problem especially if one adds BST and leap year adjustment as well.

[Benj]

Hello,

I hadn't thought about that but it would be a good improvement. I think there is probably room on the MIAC to allow this functionality to be added as well. My probably comes from the fact that I have written huge programs for the MIAC and i'm sure my heating program isn't as big as some of these monsters. If not then i'm sure some portion of the code could be optimized to save on memory usage. Eg removing some of the strings etc.

Leap year adjustments would be fairly minor, I have done this before and it doesn't add much to the program.

[echase]

I suppose you could account for wind speed too as high wind also slows down warm up.

Did you put the on/off times, etc. into EEPROM as that saves loosing them if power fails? But adds code space.

[Benj]

Hello,

I think the on/off times went into EEPROM but its been a while since I looked at the code. Ill load it up and let you know.

[Benj]

Had a quick look and it looks like I simply hard coded the initial timing presets. These could easily be moved into EEPROM, read back at startup and saved into EEPROM when ever the values are changed.

I did however use the EEPROM to store my initial required temperature settings and it looks like these are updated in the EEPROM when the values are changed so this could be a basis for doing the timing presets.

[echase]

In an “ideal” controller you would set the time at which you want a certain temperature to be reached, e.g. you set the on time to when you get out of bed and not say 1 hour before. Then leave the controller to work out how long before that it needs to turn on the boiler to achieve the set temperature.

My guess is that time to heat up is proportional to (internal - external temperature) X wind speed factor X sunshine factor X boiler set temperature factor. There are no doubt other factors, e.g. whether windows are open, whether it is raining and how many radiators/rooms are turned off. But the predominant factor is internal - external temperature and that is easy to measure. (Edit: I realise that that formula is actually incorrect but the general idea is there).

For off times you can do similar which allows for the fact that it takes some time to cool down. E.g. allow for a 1 degree cool down by bedtime and calculate off time as time before this that achieves 1 degree drop.

Similar for a church controller, especially as building may be unheated Mon - Sat. In really cold weather it might take 12 hours to heat up but only 1 hour in mild weather. So ideally one would set the service start times and let the controller calculate the on times.

[Enamul]

Hi Ben,

I was looking through the Flowcode for Underfloor heating system to extract your software RTC portion for other project. I have found that 183.105Hz TMR0 interrupt frequency for 40MHz crystal has been dealt with tmr_count and tmr_count2 registers where tmr_count has been declared as INT and tmr_count2 as BYTE whereas tmr_count can be maximum 288 and tmr_count2 can be upto 1000. So, tmr_count2 should be declared as INT otherwise this correction of time will not work.

Enamul

[hstone]

Excellent, although i have never seen construction like that before with the floor joist almost sitting on the ground, there had to be a ventilation into that space for it to preserve the boards so close to the ground. It also looks like the original building is very old. I have a crawl space in my house, will this system work with a wood-fired stove system?

[Benj]

Hello,

Many thanks

Yes the house is over 100 years old and there is no crawl space under the downstairs floor.

This technique could be used for almost any floor system and could certainly be used with a wood fired stove to replace the gas boiler being used in mine.

As long as you have the thermostatic valve and a pump as shown in my diagram to mix the hot water from the stove and the cooler water from the pipes then it should all work really well.

Unfortunately I already had my system in place before I bought my wood burning stove or I would have been tempted to use this instead of the expensive gas boiler.

I suppose the downside to this is you would need the stove to be burning all the time. Maybe a small gas boiler, a solar heater or a ground source heat pump would be a good addition so the system can tick over while the wood stove is off or heating up.

[echase]

My first house was a Victorian terrace with a floor like that. The previous owner had replaced all the floorboards beautifully using brass screws, as he was a boat builder. When I moved out the new owner was quickly on the phone asking us if we knew the floor was riddled with dry rot. We did not, as a carpet covered it all, but could believe it was possible as there was no or very little underfloor ventilation and the ground was known to be damp as it was a partial basement.

Don’t share your enthusiasm for wood burner economics. Unless you have a source of free wood I am not convinced that bought logs are cheaper than gas and ther eis all the hasstle of cutting, storing and carrying them and cleaning up the ash. Never seen an authoritative economic argument put though. I have a wood burner.

[Benj]

Hello,

Thankfully I have air bricks under my floor so hopefully the dry rot is not too much of an issue. I do keep checking that everything is tickety boo but I'm by no means a wood expert.

Also regarding the sourcing of wood I simply get old / broken pallets from work and then chop them up using a hand saw, keeps me fit and better then paying to go to the gym Agreed that buying wood is not the future. Buying coal isn't too bad and it burns clean in a modern stove but I expect the prices will go up and up. My parents live next to a forest so they have a fully fledged wood store, lucky

The Ash that is produced from burning should be high in nitrates and therefore good as compost for growing food, again I'm no expert on the matter and usually just dump the ash in the bin.

Really I only use my stove when its cold outside and the underfloor either isn't quite cutting it or to try and save a bit on gas to drive the boiler.