Viessmann 100-W Heat Only and Weather Compensation

[Mark Payne]

We are a few months into our Viessmann Vitodens 100-W 19KW Heat Only boiler and it is working fine.
We replaced an old 1999 Baxi. Our house is a 2 bathroom 4 bed detatched, built 1999.

The engineer did not really know the Viessmann boiler and I am not convinced his approach of “turn everything up” was the right one! I have experimented with flow temp and pump flow rates and have a reasonable feel for that now.

Anyway, I am after some advice on weather compensation. It is clear that when it is colder (like when we had the cold snap), I have to up the flow temps into the 70 degree area... then when it is milder (like now today on Christmas Eve) I can be at 65 degs. Maybe lower... I see flow returns of around 12 degrees below the send temp. 65 degs send definitely buys me a circa 50 degree return.

Of course I am trying to keep flow temps as low as possible for efficiency and I do monitor send and return temps using little probes at the boiler.

We are using the Honeywell T6R-HW to programme room temp schedule and geofencing. This also does the DHW schedule.
Remember this is a Heat only version of the boiler. It does not support Opentherm.

So

1. If I fitted the outside weather compensation sensor and changed the boiler mode to weather compensation (having chosen a suitable heating curve and level)… How would we schedule central heating on/off with the Honeywell T6R? Can we do that? Would there be confusion? The T6R wants to cycle the command for heat based on room temperature and in weather compensation, the boiler is trying to follow a curve. How do you actually programme alternative heat targets for day and night as we do now? How would you turn the heating off when everyone is away as we do now with geofencing. Can the T6R and WC co-exist?

2. How does the DHW switching work in WC? Is it the same? … IE will the existing schedule for DHW in the T6R still work? Can a higher hot water priority flow temperature be set when we are calling for hot water?

It is really hard to find explanations of this stuff in the Viessmann manuals and I have read them all!

Thanks in advance
I hope you can help.
Mark

 

[ShaunCorbs]

You can’t weather comp / you shouldn’t as you have a cylinder the only way you can but the heat only doesn’t support this is a 4 pipe system eg heat gets piped to the system boiler and cylinder gets also piped to the system boiler then you can have different flow temperatures

Sorry to say you need to remove / sack the weather comp off it and stick to on / off / optimised room temp as you need the flow out of the boiler set at 65dc else you could get legionella in your cylinder

 

[Mark Payne]

You can’t weather comp / you shouldn’t as you have a cylinder the only way you can but the heat only doesn’t support this is a 4 pipe system eg heat gets piped to the system boiler and cylinder gets also piped to the system boiler then you can have different flow temperatures

Sorry to say you need to remove / sack the weather comp off it and stick to on / off / optimised room temp as you need the flow out of the boiler set at 65dc else you could get legionella in your cylinder

Thanks Shaun.
I never got hold of the weather comp sensor anyway. Have just been thinking about it and you are explaining that I should not do it. Understood.

My lowest flow (as set now) is indeed 65 degrees. When its super cold I need to go up (70 degs) to keep the radiators doing the job. I am surprised there is not a way of automating this a little.

Mark

 

[Enthalpy]

Viessmann sell a Weather Compensating Kit – DHW for Vitodens 100-W Type B1GA: Heat Only Weather Compensating with domestic hot water cylinder:

Vitodens 100 B1GA Heat Only Weather Compensating Kit - DHW - https://viessmanndirect.co.uk/Catalogue/Accessories/Vitodens/Vitodens-100-W-B1GA-Heat-Only-to-32kW/Boiler-Accessories/Vitodens-100-B1GA-Heat-Only-Weather-Compensating-Kit-DHW-ZK07124

‘This is a simple accessory kit to upgrade an open vented boiler that is connected to a traditional S- or Y-plan system to a more efficient “4-pipe” system.’

Instructions here:

https://viessmanndirect.co.uk/files//de36168a-6a60-42b9-b0e4-ada60112a767/WB1B%20Compact%20Outdoor%20Sensor%20Instructions%20-%20Mod.pdf

The Cylinder demand control box requires an extra cable between the wiring centre (usually in the airing cupboard) and the boiler to tell the boiler when the demand is for the hot water cylinder.

Good luck finding a Gas Safe Engineer who is familiar with this. You could ask Viessmann.

 

[Enthalpy]

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The products that qualify for a lower rate of VAT are:

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[ShaunCorbs]

Viessmann sell a Weather Compensating Kit – DHW for Vitodens 100-W Type B1GA: Heat Only Weather Compensating with domestic hot water cylinder:

Vitodens 100 B1GA Heat Only Weather Compensating Kit - DHW - https://viessmanndirect.co.uk/Catalogue/Accessories/Vitodens/Vitodens-100-W-B1GA-Heat-Only-to-32kW/Boiler-Accessories/Vitodens-100-B1GA-Heat-Only-Weather-Compensating-Kit-DHW-ZK07124

‘This is a simple accessory kit to upgrade an open vented boiler that is connected to a traditional S- or Y-plan system to a more efficient “4-pipe” system.’

Instructions here:

https://viessmanndirect.co.uk/files//de36168a-6a60-42b9-b0e4-ada60112a767/WB1B%20Compact%20Outdoor%20Sensor%20Instructions%20-%20Mod.pdf

The Cylinder demand control box requires an extra cable between the wiring centre (usually in the airing cupboard) and the boiler to tell the boiler when the demand is for the hot water cylinder.

Good luck finding a Gas Safe Engineer who is familiar with this. You could ask Viessmann.

If the op has the newer colour display model eg b1ga that’s the kit to turn it into a 4 pipe yes