[OpenTRV-dev] TRV control - my problem, and musings so far

Tue Apr 2 21:37:12 BST 2013

One interesting point on OpenTherm that may make it easier than it first seems is that there are (apparently) actually 2 protocols: OpenTherm Lite (OT/-) and OpenTherm Plus (OT/+). The latter (OT+) is what most people mean when they say "OT", and is bi-directional based on voltage control by the room unit and current control by the boiler. Plain old OT/- is actually just a PWM voltage signal from the controller, which ought to be easily doable (the boiler controls current to indicate fault status, but if we're not going for OT certification on the PoC...).

Ask your preferred search engine about "The OpenTherm Communications Protocol".

> From: Thomas Hood <EMAIL ADDRESS HIDDEN>
>To: Closed list for developer discussions <EMAIL ADDRESS HIDDEN> 
>Sent: Tuesday, 2 April 2013, 12:05
>Subject: Re: [OpenTRV-dev] TRV control - my problem, and musings so far
>On Tue, Apr 2, 2013 at 11:53 AM, Kevin Wood <EMAIL ADDRESS HIDDEN> wrote:
>> 4) Integration and control strategy
>> I'd mused that each "zone" could transmit a figure each time it "woke",
>> perhaps based on a comparison between current temperature and set point,
>> indicating the local level of "desperation" for heat. The programmer would
>> sum these and use it as a criteria to determine if it's worth lighting the
>> boiler. In this way, a single zone dropping a fraction below target while
>> the rest are happy wouldn't result in a potentially wasteful short cycle
>> of the boiler, yet a number of zones in such a state would, as would one
>> zone that's well below target or where the set point has recently been
>> increased. Each zone would receive a response from the programmer,
>> indicating if heat is being supplied, and also if a pump overrun condition
>> exists, allowing zones to open and take "waste heat" based on some
>> criteria.
>Writing software to control the boiler(s) optimally and safely based
>on information provided by other system components will certainly be
>one of the biggest challenges.
>Among the commercial systems that I have looked into only Honeywell
>Evohome and Danfoss Living Connect dare to control the boiler and only
>Honeywell can control a modulating OpenTherm boiler. Reading forums
>related to these systems I didn't get the impression that the software
>in those systems has been well tuned yet.
>OpenTRV-dev mailing list
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