Weather compensating boiler control

[sbrucey]

I'm replacing a combi with a system boiler, I've already had the hw cylinder and zone valves installed and was thinking about a weather compensated boiler since they're apparently more efficient.

I was looking at a Viessmann boiler, as it comes with compensation built in (just add an external sensor) but I see it has separate flow and returns for the DHW and Heating. Unfortunately, I didn't get separate f/r installed for the cylinder and now it's occurred to me that when the boiler flow temp has been lowered by the compensator the cylinder won't heat up as quickly, if at all.

Vaillant do a system that has a hugely complicated clock arrangement that basically seems to override the compensator during the DHW periods, so it essentially runs at max temp. The boiler is in the garage so I don't want to keep going out there to change the time program whenever I want hot water out of the normal periods

Given that I've messed up the pipework I guess the next best thing I can do is have the boiler run at max temp when the DHW is calling and be compensated when doing Heating only. I can achieve this by controlling the Viessmann from the heating zone valve and disabling the compensator with the DHW zone valve but it seems a little Heath Robinson -ish (I know it's more complicated than that but I was a sparky in a previous life, I can manage it).

My question then: Can I get a boiler that will accept two start signals, one to run at max output, one to run with weather compensation? Bonus if it will accept three signals (2 heating / 1 DHW) and run at different outputs (pump speeds?) depending how many zones are calling.

NB Whole thing is only 12KW, so yes, absolutely having two heating zones was overkill, but I could, so I did

Thanks for reading!

 

[Dan Robinson]

When retro fitting like this we use a relay to shut the sensor off from the boiler during a hot water cycle. Not sure how the viessmann would cope with that though.

 

[mysteryman]

The correct procedure is to study the boiler first, and then install accordingly!

If you have two 2 port valves, you will need to make provision for circulation during pump overrun. It is better to use a 2 position diverter valve - not a mid-position valve.

If you have got the boiler already, it can be connected to external valves plus weather comp. Tell us exactly which boiler you have,

 

[sbrucey]

I haven't bought the boiler yet, was hoping to find one that I could make work with the pipework as installed. Disabling the compensator sensor during the HW cycle was my best guess so far, was wondering if there was something better out there.

I've got two port valves, but three of them (2 heating zones, 1DHW). I was planning to use one of those spring loaded bypass valves for the pump overrun, but you'd recommend a 2 position diverter? Presumably with the motorised port as the bypass and activating it when all the others are closed?

I'll give Viessmann a call, I don't know why I didn't think of that in the first place

 

[Dan Robinson]

Atmos Intersystem Or Intergas HRE, Or Atag Q Series.

Honeywell RF pack 5.

External weather sensor wired direct to boiler with relay set to NC and has power applied by the Hot water valve making it go NO.

Sorted.

Rads will run hotter during hot water cycles, but then, as I say to customers, you are wet/naked/waking up/going to sleep, so a little boost to the heating is nice.

MM isn't clear if the Viessmann can accept this form operation, but it works well where we have used it.

In theory you can get full OpenTherm control through Pack 5 as well.

 

[mysteryman]

For the benefit of those who do not follow what I have already said:

You can do this with a Vitodens 100-W Compact boiler.
You need the boiler - 13 16 19 26 or 35kW plus the Z007933 kit.

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[Agile]

The system needs to be arranged with hot water priority so the rads are not powered during the hot water reheat with the higher flow temperature.

Tony