Calling all Opentherm gurus, question ? Max CH flow temp

Opentherm compatible boiler CH top end flow temperature

  • 70c

    Votes: 0 0.0%
  • 80c

    Votes: 0 0.0%

  • Total voters
  • Poll closed .
2 Jan 2014
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United Kingdom
Hi I have an opentherm compatible boiler and controller / room stat.

Question is when setting up the stat I am prompted to enter max flow temperature for CH. (this can also be done directly on the boiler )

Boiler is capable to going up 80c

I set it to 70c with the following thinking

1) my boiler will never run at full throttle (protect components)
2) running at 70c greater chances of return flow being closer to 55c for condensing purposes
3) it's a 26kw combi and my house isn't a mansion further more we've not seen artic temps yet. E.g. -10
4) the boiler does modulate from 70c down all the way 30c when much heat is not required
5) on the whole my lounge where the stat is holds 14c when standing cold overnight. So not much of jump required to get to 19c

Is my thinking and setting to 70c best course of action or does anyone beg to differ and importantly why ?

Thoughts answers theory all welcomed
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Set it to 80.

The whole point of Opentherm is that it can regulate the flow temperature for best efficiency, so if 80C is judged to be more efficient, it'll use it.

It's a gas burner. Running it flat out won't do it any harm at all.
I like your point but lack of data stops me from making the change. I need facts and data to convince me.

Don't get me wrong it makes sense, but it also depends on how you use the heating system. I did some testing and for folks who only switch the system On, for as andwhen use. 80c makes less sense. Hear me out.

You make the point that opentherm regulates the flow rate for best efficiency. (Agreed) but I also believe it regulates the flow rate for the Situation.

For now we don't schedule the heating for different set points across the day.

If we did I don't think the boiler would ever go to 80c as it could make the set point temps in say 1-2 hours and run efficiently.

As we ONLY use the heating when we need it. Say in evening or morning when the room temp is 14 and we want it get to 19 it will push out 80c for as long as possible to reach 18 as quickly as possible. Then modulate down to prevent overshoot. Then modulate further down to maintain 19c in the lounge.

So the efficiency can only be realised once set point has reached or near reached.

I GUESS if we start using set points for the day the boiler could work a lot differently and 80c could be a front runner. For now 70c is still king.

Thoughts ?????
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If your lounge goes down to 14C overnight that's a big heat loss; I've never seen lower than 17C. It might be worth improving the insulation.

Do you have a weather compensation sensor on the outside wall, or is the OT thermostat set up for internal compensation? (Which stat do you have?)
This big patio door window combo don't help. But we specked thermal lined curtains when we decorated. So that helps.

I have a nest 3rd gen. I didn't want to get into manufacturer specifics but the nest has a feature called true radiant that calculates time to reach set point based on previous records and outside temperature. So this negates the need for weather compensation well at least a physically wired sensor.

And it's this feature I guess would kick in and set the flow temperature even lower if the thermostat knew it had a few hours to reach the temp. Rather a straight out request for heat. On demand which would make the boiler scream to 80c straight away.

Now does the 14c overnight temp seem explained, or possible.

Last edited:
You seem to be missing the point on flow temps!

The max flow temp that YOU need is that which, based on your rad size, will reach the target temp of 21 C when it is -1 C outside.

During periods of colder temps outside, you might need to increase the flow temp a little. But normally the rad size is significantly larger than the minimum required. But some installers do skimp!

So if the system has been designed according to the current design parameters then 70 C max will be totally adequate and ensure boiler efficiency is going to be high.


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