is it possible in Node-Red to create a flow that monitor a temperature from a temp sensor, and when the temperature is changed then it should update that to a termostat (TVR) with posting that to a webserver with, http://192.168.1.58/ext_t?temp=measured t°?
Shelly TVR can get temp data from an external temp-senor, this is what the TVR config page say “Enable external temperature corrections endpoint http://192.168.1.58/ext_t?temp=measured t°” (192.168.1.58 is the ip to my TVR
Use an Events:state node to monitor the temp sensor and the make your preparations in the next nodes for sending the info in a http requests node to TVR.
You can use the following nodes (change the settings to match your homeassistant server and entity id)
[{"id":"e05ba4135b143ae9","type":"http request","z":"e763399b.2e2bf8","name":"","method":"GET","ret":"txt","paytoqs":"ignore","url":"http://192.168.1.58/ext_t?temp={{payload}}","tls":"","persist":false,"proxy":"","authType":"","senderr":false,"credentials":{},"x":690,"y":620,"wires":[["a58e2b642d5c09a3"]]},{"id":"e5d782c6193164bb","type":"server-state-changed","z":"e763399b.2e2bf8","name":"","server":"1d707c49.07a204","version":4,"exposeToHomeAssistant":false,"haConfig":[{"property":"name","value":""},{"property":"icon","value":""}],"entityidfilter":"sensor.temperatura_da_casa","entityidfiltertype":"exact","outputinitially":false,"state_type":"str","haltifstate":"","halt_if_type":"str","halt_if_compare":"is","outputs":1,"output_only_on_state_change":false,"for":"0","forType":"num","forUnits":"minutes","ignorePrevStateNull":true,"ignorePrevStateUnknown":true,"ignorePrevStateUnavailable":true,"ignoreCurrentStateUnknown":true,"ignoreCurrentStateUnavailable":true,"outputProperties":[{"property":"payload","propertyType":"msg","value":"","valueType":"entityState"},{"property":"data","propertyType":"msg","value":"","valueType":"eventData"},{"property":"topic","propertyType":"msg","value":"","valueType":"triggerId"}],"x":390,"y":640,"wires":[["e05ba4135b143ae9"]]},{"id":"1d707c49.07a204","type":"server","name":"Home Assistant [mine]","version":2,"addon":false,"rejectUnauthorizedCerts":true,"ha_boolean":"y|yes|true|on|home|open","connectionDelay":true,"cacheJson":true,"heartbeat":false,"heartbeatInterval":"30"}]
If it was me then I would rater create a generic thermostat in HA that controls the TRV since (I assume) these messages can be quite frequent and a lot of them not important at all.
This means you load the network and drain the batteries on the TRVs.
I would rather keep the processing on a mains powered device than on a battery powered device.
So true.
I run my TVRs in NR, but I have a node that round off to a half degree and if it is the same as before then it does not update the TVR.
I have powered my TVRs from a USB charger though, so its not the battery drain that is the issue for me, but instead that fact that it uses RF868 to communicate and that protocol has a limit to the amount of data you are supposed to send so as not to flood the spectrum for other users.
great, will have a look at this 
Would it be possible to share your flow?
My flow does not use a http call in the end and is quite complicated due to many sensors and settings affecting the flow.