Opentherm - calculating boiler control setpoint

Forum about Opentherm, Hvac, House heating, Boilers

Moderator: hvxl

Opentherm - calculating boiler control setpoint

Postby ModbusMan » Sun Dec 13, 2015 8:37 pm

Hi opentherm guru's

I am trying to construct a multi-zone heating system. Basic idea is that in the daytime the boiler would use the standard (Rehema iSense) thermostat in the the living room and in the night time use an alternative temperature sensor (MAX! wall thermostat) in the master bedroom. For those interested, hw consists of a RPI, OTGW with otmonitor and MAX! cube + wall thermostat and radiator valve controls. Using FHEM as domotica sw. Works great but just missing one thing, how to calculate the boiler temperature control setpoint when I am using the bedroom thermostat? Btw, the iSense does a great job in maintaining the room temperature when set to a constant setpoint.

I set the room temperature to a constant 21.5 oC and logged the boiler\thermostat data using FHEM and made a load of screenshots from the OTGW graphs. Trying to detect a pattern here. Easiest when nobody turns the hot water on ;-)

So far as I can see:
1: The (basic) boiler control setpoint calculated by the iSense varies (for a 21.5 oC room setpoint) between 40 and 45 oC. Seems to vary as a sinewave.
2: Periodically the control setpoint is increased to 90 oC for a short period of time. This seems to be triggered when the boiler water temperature falls to below the control setpoint by 5 oC.
3: This 90 oC boost ends when the boiler water temperature rises to about 10 oC above the control setpoint after a minimum boost time of +/- 4 mins.

Unfortunately the above observations do not always hold true. About 20% of the time the boosts seem to be triggered/stopped by something else. Turning on the hot water seems to have an impact, but this does not explain all the deviations. Also how does the thermostat calculate the basic bolier control setpoint.

Would be grateful if anybody could enlighten me further...

Thnx,
ModbusMan
ModbusMan
Starting Member
Starting Member
 
Posts: 15
Joined: January 2015

Re: Opentherm - calculating boiler control setpoint

Postby D_Hailsham » Mon Dec 14, 2015 5:51 pm

Which boiler do you have?
D_Hailsham
Starting Member
Starting Member
 
Posts: 35
Joined: August 2013

Re: Opentherm - calculating boiler control setpoint

Postby ModbusMan » Tue Dec 15, 2015 10:21 pm

Boiler is Remeha Avanta 28c, thermostat is iSense also from Remeha. Both date from Dec. 2009.
ModbusMan
Starting Member
Starting Member
 
Posts: 15
Joined: January 2015

Re: Opentherm - calculating boiler control setpoint

Postby D_Hailsham » Thu Dec 17, 2015 6:51 pm

The flow temperature is calculated according to the heating curve. This is explained in the boiler manual and the isense manual Page 24, Fig 25 of English manual). The four parameters (P1/P29 and P27/P28) can be set directly using the boiler controls or via the iSense. The parameters have to be adjusted for each installation.

As for the strange temperatures you have encountered (90C etc), these are determined by the internal logic of the boiler. All the iSense does is send the boiler a required flow temperature. It's up to the boiler how it meets this requirement.
D_Hailsham
Starting Member
Starting Member
 
Posts: 35
Joined: August 2013

Re: Opentherm - calculating boiler control setpoint

Postby ModbusMan » Sat Dec 19, 2015 12:30 am

Thanks for the info.

I downloaded the iSense installation & Service manual as you mentioned which gives a lot of info and details various settings which can be configured in the iSense to optimize performance/efficiency. I will study these further.
On my system the iSense does (according to the graphs produced by my opentherm gateway) jack the boiler control setpoint up to 90% for 3 or 4 mins whenever the boiler water temperature drops below the control setpoint by more than a couple of degrees.

Looking at the boiler water temperature the boiler too has some kind of control logic. As far as I understand from the opentherm protocol though the boiler control setpoint is always sent from the thermostat to the boiler and is not bidirectional. Unfortunately there doesn't seem to be anyway on this forum to upload a screenshot of what I see.

I will start with the heating curves and try to see if I can construct something for my RPI which can produce something similar.
ModbusMan
Starting Member
Starting Member
 
Posts: 15
Joined: January 2015

Re: Opentherm - calculating boiler control setpoint

Postby Rene » Sat Dec 19, 2015 9:30 am

You can attach images or other files to your posts.
Rene.
User avatar
Rene
Global Moderator
Global Moderator
 
Posts: 1689
Joined: October 2008
Location: Netherlands


Return to Hvac, Heating Opentherm Forum

Who is online

Users browsing this forum: No registered users and 1 guest