Atag boiler seems to ignore set flow temp

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20 Jun 2017
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Hi all,

I've got an atag IR24 conventional gas boiler. Recently due to the warmer weather, I turned down the set CH water temp to 70⁰C rather than the 80⁰C we set it to for the winter (might seem high but we need it fairly high as the rads are on microbore pipe and don't get hot quickly enough if it's much lower), however I've noticed over the past couple of days the flow temp (A0) goes way past the 70 mark.

The boiler has a parameter P0 that can be adjusted which sets the maximum flow temp allowed (Tmax) for the system, in our case this is set to 85⁰C, the maximum temperature possible, which is in fact the factory setting. The instructions go on to say the boiler will shut off when the temp gets 5⁰C above the T-set temperature. This seems to be exactly what it is doing, it gets to 90⁰C before it shuts off, and does this reliably. The controls also show the calculated T-set temperature (A3) which will always max out at 85⁰C. So in this respect it seems to be working as it should.

However changing the main setting of the CH water temp seems to have little effect on the maximum temp the flow gets to. I would have thought that based on the instructions it should max out and shut off at 75⁰C if its set at 70⁰C, but this isn't happening. It is instead getting all the way to 90⁰C before shutting off. So it seems like it is taking the parameter P0 as the CH water temp rather than the actual set CH water temp. I haven't got any fault codes showing for the flow or return sensors and indeed they seem to be working perfectly fine. Any ideas what could be causing this issue please?

I've played around with the settings a little bit setting the CH water temp to different levels, for example setting it to 60⁰C means it will cut out at around 73⁰C, setting it to 50⁰C means it cuts out at 60 ish, it seems quite random. Obviously during these there has been a constant demand for heat. In any case it certainly isn't cutting out at the set CH water temp, or any constant value above it.

Hope you can make sense of my post, and thanks for any replies!
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