How to switch Viessmann DHW off when immersion is auto on

[Tristar]

Hi guys, I have a Vitodens 200, and also a 4Kw PV array on the roof. I intend to fit a Solar iBoost which diverts excess electricity to the immersion heater.
Under normal circumstances the DHW runs at about 60 degrees, and ideally I would like to have a circuit from the iBoost that switches the DHW off when the immersion comes on. Poor experience of talking to Viessmann technical dept. suggests that they are unlikely to be helpful.
Has anyone come across a solution to this

 

[JohnD]

I gather you have some kind of hot water cylinder, is that right?

The immersion heater will take some hours to heat the cylinder, even longer on cloudy or patchy days, so the method you are describing is not ideal. Additionally, the cost of gas to heat a cylinder is in the region of 12p, so it will take you a long time to recoup the cost of your device.

Show us a pic of the thermostat on the cylinder.

Presumably you also have some kind of timer or programmer on the boiler to control the HW on times, right?

 

[ajstoneservices]

I would like to have a circuit from the iBoost that switches the DHW off when the immersion comes on

The Iboost doesn't have such a facility.

The best way to use it would be to heat your cylinder via the boiler once a day ( late evening) during the summer month depending on your water use. That way you will make use of the water heated by the iboost and have hot water for showers/baths etc in the morning.

 

[Tristar]

I gather you have some kind of hot water cylinder, is that right?

The immersion heater will take some hours to heat the cylinder, even longer on cloudy or patchy days, so the method you are describing is not ideal. Additionally, the cost of gas to heat a cylinder is in the region of 12p, so it will take you a long time to recoup the cost of your device.

Show us a pic of the thermostat on the cylinder.

Presumably you also have some kind of timer or programmer on the boiler to control the HW on times, right?

Thanks. Yes Hot water cylinder is a CentreCylinder HE120, with a 27" 2.8-3Kw immersion. Although I take your point about unit costs, my friend who fitted the PV system is in a similar position and is exporting far more than the deemed 50% that they pay for, sometimes 80%, so I would rather use this than give it back for nothing, or even for the 4p odd per Kw that they pay.

I realise that if no present system will allow what ideally I require, I will have to rely on juggling the DHW timer. However I am sure that such an integrated system will eventually become available, after all Immersun and iBoost systems only came about from demand.

Thanks for advice

 

[Tristar]

I would like to have a circuit from the iBoost that switches the DHW off when the immersion comes on

The Iboost doesn't have such a facility.

The best way to use it would be to heat your cylinder via the boiler once a day ( late evening) during the summer month depending on your water use. That way you will make use of the water heated by the iboost and have hot water for showers/baths etc in the morning.

Yep, that's what I'll have to do. Thanks for your help.

 

[Jackrae]

With the 200 series boiler I believe you ned a vititrol 100 wireless system to get external radio input from a room stat. This also permits manual and auto control of the DHW but it doesn't seem to offer a means of getting an external over-ride signal into it.

The tank sensor for the 200W is presumable an NTC type device (at least mine is) which will measure around 2500 ohms (2K5) at 60 degrees. One method might be to "kid" the boiler into thinking that the water was hot enough when you want to use the immersion heater. This might entail a relay system that would switch out the tank sensor and substitute a resistor of say 2K2 ohms which would make the boiler think the DHW temperature was more than adequate.

Not exactly an elegant solution but one which might suit your purpose.

 

[Tristar]

With the 200 series boiler I believe you ned a vititrol 100 wireless system to get external radio input from a room stat. This also permits manual and auto control of the DHW but it doesn't seem to offer a means of getting an external over-ride signal into it.

The tank sensor for the 200W is presumable an NTC type device (at least mine is) which will measure around 2500 ohms (2K5) at 60 degrees. One method might be to "kid" the boiler into thinking that the water was hot enough when you want to use the immersion heater. This might entail a relay system that would switch out the tank sensor and substitute a resistor of say 2K2 ohms which would make the boiler think the DHW temperature was more than adequate.

Not exactly an elegant solution but one which might suit your purpose.

Not being an expert on leccy, but I think I see what you are suggesting. As soon as the iBoost switches on, a sensor would operate a relay which would then insert a rsistance in the DHW sensor cct. Am I correct?

 

[Jackrae]

That's what I'm suggesting. When your IBoost turns on immersion it also drives an independent relay to disconnect tank sensor and switch in a dummy resistor.

Typical 240vAC plug-in relays, bases and resistor are readily available from the likes of Maplin

Providing the relay has to be energised to switch over to the dummy resistor it is effectively "fail-safe" in that if the relay fails to operate for any reason the boiler controls operate as normal.

With respect to Veissmann "technical" I have also had mixed responses.
Their more mature technical helpers seem to be more tolerant to customer questions. Perhaps, being older and wiser, they have better communication skills.

 

[Phill Peck]

Need to know what system and controls u have.

Is it a 4 pipe system or do u have zone valves?

Are all your controls Viessmann or do u have a Honeywell(etc) room thermostat?

With regards to technical line.
On the 200 series a lot can be adjusted in the codes, so they will be reluctant to give out to much info to a diyer. You could make a that mess of the setup

 

[Tristar]

That's what I'm suggesting. When your IBoost turns on immersion it also drives an independent relay to disconnect tank sensor and switch in a dummy resistor.

Typical 240vAC plug-in relays, bases and resistor are readily available from the likes of Maplin

Providing the relay has to be energised to switch over to the dummy resistor it is effectively "fail-safe" in that if the relay fails to operate for any reason the boiler controls operate as normal.

With respect to Veissmann "technical" I have also had mixed responses.
Their more mature technical helpers seem to be more tolerant to customer questions. Perhaps, being older and wiser, they have better communication skills.

Thanks very much, sounds feasible.

 

[Tristar]

Need to know what system and controls u have.

Is it a 4 pipe system or do u have zone valves?

Are all your controls Viessmann or do u have a Honeywell(etc) room thermostat?

With regards to technical line.
On the 200 series a lot can be adjusted in the codes, so they will be reluctant to give out to much info to a diyer. You could make a that mess of the setup

Yes a 4 pipe system and no roomstat, but has a weather compensator fitted, and all controls are from the boiler panel, although I am considering a wireless roomstat as the temp sensing system on the boiler, while no doubt accurate, does not allow the fine tuning required when sitting in the lounge on a winter evening.

I have been through code setting with Viessmann, which is - more or less- explained in the manuals, but I'm still not completely happy with the temp control setup. Incidentally the "Viessmann trained" installer had not much idea how to adjust the codes and spent half a day on the phone to them trying to set it up! As an aircraft engineer, I think I have made a better job.

 

[Phill Peck]

The codes are fairly simple. You just don't wanna be adjusting the wrongs things as u can get in a right mess.

In fairness to your installer even Viessmann own engineers won't know all the codes.

In a 4 pipe system u are gonna be a bit stuck holding it off

 

[JohnD]

remember that the solar iBoost controller can turn on a variable amount of voltage, according to what is spare current. It may provide 25W or 3200W* depending on sunlight and usage by other appliances in the house.

(it limits voltage, which on a simple resistive load like an immersion heater, has the effect of controlling power used)

*because the solar inverter raises the voltage above that from the grid, you may get 250V in your house so during sunny periods, which means the immersion will use more than its nominal 3kW.

On a cloudy day, solar power generated might be very slight, and it would not be sensible to prevent the gas boiler heating the cylinder. Nor would it be sensible to turn the boiler back on every time you boiled a kettle and the controller turned off power to the immersion.

More sensible all round to let the immersion heat the cylinder all day, subject to solar availability, and time the boiler to come on at 5pm or so. It will only run until the cylinder is up to temp as measured by the cylinder stat.

 

[Agile]

It is very easy to do what you need to do.

But first you have to decide just how much you will wait for solar heated water.

During the summer you can get useful heat until after 2000hrs from the solar but by then it will probably already be up to temperature.

As John says, these solar immersion controllers apply proportional power to the immersion element according to the amount of power being exported with the objective of putting virtually all the solar power into the immersion.

Different controllers seem to use different control methods and many probably dont comply with EU CE requirements and probably not approvable by the power supply companies either on account of their equivalent power factor. Burst control and phase control etc.

Most of these controllers probably have external contacts which can be used for holding off the boiler HW heating. That can be achieved most conveniently by adding a resistor of about 2500 ohms in PARALLEL with the cylinder sensor. Thats simpler and more fail safe than switching the cable.

But during the winter and on very cloudy days its probably best to be able to manually disable the boiler hold off as the spare solar power is going to be so small.

Its quite easy with a little electronics knowldege to build your own solar radiation sensor which can disable the boiler hold off on very dull days. That can include further sophistication by comparing the cylinder temperature with your needs.

Because of the unpredictable nature of our weather I would always advise a larger cylinder for solar storage. Ideally of course those designed for solar use with a boiler heating coil for just the upper part with the solar heating from the base.

Tony

 

[Tristar]

As usual, most informative and helpful replies, giving much food for thought. As I have already said, I am sure someone will come along with such a system very shortly.