Advice on controlling heating boiler using solid state relay and raspberry pi


Postby andrum99 » Sat Jan 16, 2016 10:53 pm

I am looking to build a Raspberry Pi based "smart" heating controller for my central heating that uses solid state relays to switch the heating on and off according to certain criteria. I would intend the controller to replace the heating timer and room thermostat that i currently have controlling the boiler. The question I have is this: The boiler says it should be connected via a 3A fuse but only draws 114W max. The solid state relays I am intending to use are rated max 2A. Can I just replace the supply fuse for the boiler with a 2A fuse in order to provide adequate protection to the installation? Or should I leave it alone and put a separate fuse in on the high voltage line to my controller? I have assumed that the timer and room thermostat connections on the boiler are switching the entire load of the boiler, since there is no mention in the documentation that this is not the case. It is a Worcester Greenstar 30Si compact.

Also, is 2 solid state relays (or perhaps one) the best way to control the boiler in this way?
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Postby ericmark » Sun Jan 17, 2016 1:11 pm

One major problem when using non standard equipment is to get anything repaired should anything go wrong.

I am no boiler expert but it seems boilers do come in different flavours the very old type simply switched on or off, as they progressed then we got two feeds so after switching off it could run on to cool down then we got a huge step forward the condensing boiler. With this system the temperature of the return water became important, until then only the output temperature mattered.

This resulted in a new system the thermostatic radiator valve (TRV) this controlled the temperature of each room and the boiler did not just switch on and off but throttled back as the return water got warmer and the boilers started to have software installed called anti-cycling which in essence changed how long to wait before retrying to see if heat was required.

There has been a move to use better TRV which include time clocks and WiFi links however it does not matter if the TRV turns on if the boiler is not running then room does not get warm, and the clocks in the TRV upset the boiler anti-cycle software so we move to the central controller.

In essence instead of using water return temperature to control boiler we use a central hub and all the TRV’s link to central hub and tell it when heat is required and this in turn tells the boiler to fire up.

How it does it varies boiler to boiler some are simply turned off and on others have a data link to tell it how much heat is required.

The major problem is to ensure what you add does not mess up the software already in the boiler.

So step one is get the boiler service manual to find out what it already has built in. This is why I have not fitted the systems to my house.
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Postby ericmark » Sun Jan 17, 2016 5:19 pm

I had a look at the boiler and it would seem there is an option to connect items to the boilers which connect direct into the boilers brain not simply an on/off option. However as far as I can see there is only an option to fit units made by Worcester there does not seem to be any option to fit third party devices.

The Raspberry Pi controller as far as I can see will only switch the boiler on/off in the same way as a timer.

As I said in first post before I was able to study the boilers instructions what one has to be really careful of is that your super controller is not in conflict with the boilers own controller.

As to the fuse 3A and 13A are considered as preferred sizes although you can get 1, 2, 5, 7 and 10 these are rarely used. Likely a 1A fuse and contacts will be ample. However some items do have start-up loads specially refrigeration but can’t see this is valid with a boiler, more likely they have selected the preferred size.

Not sure why you will have any high voltage that would only be used for ignition. Most the boiler will be low voltage or extra low voltage with AC 50 volt is the change over point between low voltage and extra low voltage.

To protect a solid state device you need semi-conductor fuses so swapping from 3A to 2A will have no effect as neither will protect semi-conductors so I would not worry at all if using 2 or 3 amp it will have no effect with protection offered.

With control of heating mats I used solid state relays because they were switching maybe 30 times an hour, but with central heating likely only switching 3 times an hour so unlikely to be required, using standard mechanical relays would not be a problem.

In the main “Smart” heating means allowing for hysteresis, the biggest problem is when we put the sensor near to the radiator it shows lower than room temperature but put it remote from the radiator it has a large hysteresis. What the “Smart” unit does is counter these effects either shows a lower than recorded temperature or simple numbers 0 – 6 or slowly reduce the start temperature with time or increase the finish temperature with time to compensate. The simple way was a heater built into the thermostat.

I had problems with my mother’s living room when I first put the wireless thermostat in the room I selected a position away from the radiator which resulted in a large hysteresis but I found putting the thermostat close to the radiator actually worked better one just had to set it a little on the high side.

You can actually get replacement heads for the TRV which have timers built in and it would seem this is really a good idea, except for working out how the boiler will react.

What you are trying to do is emulate the Honeywell evohome WiFi Controller and I can see the point £590 for the controller and 4 TRV heads is expensive. We are talking around £50 each for the heads when a simple head is less than £18 with a timer built in but your major problem is to work out how to get your Raspberry Pi controller to talk to the TRV heads. Talking to boiler is easy, it’s talking to the heads where we get stuck. I looked for a solenoid head to work like a zone valve however just could not find one. Only option seemed to be to buy cheap electronic heads and modify them.

I hope you can find a way to control your radiators. If you do, please tell me. My son came to the same problem, all the heads designed to link seemed to use Wifi, as he said he didn’t really need wifi but all the units seem to be designed for retro fit not as original equipment.
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Postby andrum99 » Sun Jan 17, 2016 7:35 pm

All I want is the ability to substitute a Raspberry Pi in place of the existing controller. The existing controller switches mains voltage but there is nothing in the boiler documentation that I have that says what current the controller needs to be able to switch. The existing controller uses a mechanical relay. I think the safest idea is to use a mechanical relay as the heat dissipation from a solid state relay seems proportional to the load it is switching, whereas the only real heat dissipation in a mechanical relay is probably from the movement coil, or whatever that part is called lol. Using a mechanical relay means I will need a darlington driver to drive the relay probably.
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Postby ericmark » Mon Jan 18, 2016 11:52 am

Not used a darlington driver in years, modern Op-amps have in the main removed the need. But main point don't forget the diode to get rid of back EMF as the supply to relay coil is removed.
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Postby RedMillikan » Thu Apr 21, 2016 6:19 pm

In the main “Smart” heating means allowing for hysteresis, the biggest problem is when we put the sensor near to the radiator it shows lower than room temperature but put it remote from the radiator it has a large hysteresis.
What the “Smart” unit does is counter these effects either shows a lower than recorded temperature or simple numbers 0 – 6 or slowly reduce the start temperature with time or increase the finish temperature with time to compensate.
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Postby ericmark » Thu Apr 21, 2016 10:45 pm

I bought two so called "Intelligent" temperature controllers to brew beer with. There is nothing "intelligent" in them. Allowing for the time delay is hard, switch on say as temperature drops to 19°C and switch off at 19.2°C to drop the temperature to compensate for time delay is not really an option. There is only 0.2°C to play with.

Now start with a room at 10°C and warm to 19°C then that's another story. But most motorised valves are on or off, so one would need to alter the mark/space ratio. Sounds good in theroy, but in practice the boiler would switch off when on a space, so when mark time arrives simply no hot water being pumped around.

This is the problem. Each room is independent so to work we need electrical operated TRV's on each radiator connected to a central control unit which in turns tells the boiler when to fire up. As said sensing needs to be close to the radiators, and the only way to ensure whole room even heated is to circulate the air.

The Myson radiator seems to do this, however there is a major problem, the Myson at least the one I have does not restrict water flow, all it does is switch fan off and on. Hot water arriving back at the boiler will turn boiler off with condensation type, so only way is to pipe in series not parallel, this is the method used with large buildings.

So all electronic control is no longer required. Time switch starts boiler, as the water get hot the Myson switches on its fan until the room is at set temperature when fan switches off, when no Myson's are running then hot water returns to boiler who's anti-cyclic software varies the off time before it re-tries. Yes it works. However a Myson in the living room is one thing, having the fan kick in with a bedroom is not really what I want.

But it only really helps heating up from cold, once warm no real advantage. In real terms the Honeywell Evohome is about the best commercial control system after the Myson radiator. To make your own TRV is not really cost effective. It is the Wifi controller which one could possible use a raspberry pi to replace. So £181 for the Evohome controller. You need to find out how to control the TRV with the raspberry pi and get a touch screen to control it with plus all the smart phone software. Not saying it can't be done. But really more than I would want to tattle.

There are other systems made like the nest and hive, however these are really for a hot air system not a water system. With hot air circulation all rooms are the same temperature. So all you need is one thermostat for whole house. But with water systems each room is individually controlled so the nest and hive don't work. Yes they and switch heating on and off remotely but they can't control the temperature of all rooms. They can only control one room. And most people do not live in one room houses.
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