Weather compensation

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This query is prompted by an earlier thread about unvented cylinders.

What do you do if you’re installing outdoor temperature compensation with a conventional system – heat-only or system boiler with diverter valve and HW cylinder?

As the boiler flow temperature is reduced by the outdoor temperature compensation it could be too low to heat the HW cylinder, so having both CH and HW together, using a mid-position valve (Honeywell system Y) doesn’t work. (Unless more features are added e.g. allow mid-position operation when the boiler flow temperature setpoint (via the outdoor controller) is say 5° higher than likely maximum cylinder stat setting.)

On a new installation, am I right thinking you would use an either/or diverter valve, with HW preference?

In a case of retrofitting outdoor temperature compensation, if the system has a mid-position valve, how would it be handled?

As I believe mid-position diverter valves are nowadays much more common than either/or, and the latest regs require outdoor temperature (and even weather, via the internet) compensation, I would guess this situation has been considered.
 
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Yes hot water priority, depends on boiler but I normally use a relay to short or open circuit or change resistance seen by boiler to override weather comp.
 
Yes hot water priority, depends on boiler but I normally use a relay to short or open circuit or change resistance seen by boiler to override weather comp.
Thanks. So I would assume mid-position valves (Honeywell V4073 etc) are on the way out, at least for new installations and major upgrades.
 
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