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Good spot on the pipe temp measurement. When I put the thermocouples in the boiler on the return next to the sensor they are low by ~5 degC. Going to have to first measure the return in the boiler manually to make sure it's the thermocouples that are wrong and then redo the calibration as some...

I removed the ABV and blanked off the pipework That little grey line at 10 is d.15 Output of PWM pump in %. It's not totally reliably captured and it only has three values that I can see 0%, 45% and 100%. I think the pump only has two speeds so this would make sense and I'm still yet to test...

Well I didn't expect that. Removed the external ABV and the effect this morning was that the boiler flow / return delta went from 8/9 degC to 10/11 degC but the return away from the boiler delta to the boiler return went from 13/14 degC to 6/7 degC My assumption is that the external ABV was...

This may show the difference better. 1 cycling still has demand so boiler flow and return track down towards pipe return temp. Third cycle where the demand is removed and zone valve closes the boiler flow and return reduce slowly as the water is recycled through just the bypass

I think your seeing a data capture timing issue in the bottom one as the capture of flow and return were at different times. I've sorted the issue out in the top one so there should be only a couple of seconds between the measurements. The lines at the bottom of the chart give radiator, UFH and...

You house probably uses a similar amount of energy to mine (10 rads + 2 rooms with UFH) and much of what you said is where I was trying to go. Unfortunately my boiler as an older model and can only modulate down to 12kW which is above the usage of the house most of the time. Once I fix the...

Tried setting d.1 pump anti cycle to 10mins (from 5) and d.2 burner anti cycle to 60mins (11.5mins for 65 flow target) from 20mins It looks like it is cycling over 12 mins period and pump running for 9/10 mins after burner switching off so only 3 min burn at a time This is slightly better than...

Also Vaillant Technical Support has confirmed my boiler can only modulate down to 12kW!

The ABV is one of these Honeywell DU145 ABV The test was with just the radiator demand on with probably most of the TRVs open. The ABV says it is set to 6 bar but the top comes off of it so I've tightened it until I can feel it click which I assume is now at 6 bar

Well wasn't expecting that. Looks like the external bypass is letting the flow through. The temperature just after the bypass is near enough the same as the boiler reported return temperature and not the same as the return temperature from before the bypass about 50cm away from it I wonder if...

Not sure if this will help but we have an similar unvented HW cylinder but we have put an additional old school insulation jacket over it as well as the foam that is on the HW cylinder. At the bottom of the cylinder it looks like it looses 5 degC in 3.75 hrs

That matches the document I have that came with the boiler. I'll try the things suggested to see if it makes it any better. I'm also thinking that a weather compensation controller is a waste of money as when it changes the flow set point the boiler wont be able to modulate below 12kW and will...

No d.15 in my manual. I'll have a look on the boiler tomorrow as it's in the loft

We have an ecoTEC plus 637 VU 376 3-5 R1 GC 41-044-49

Setting d.19 = 1 should mean that everything is on low speed but I'm not sure if there is any way to check pump speed? I have a slight worry about the external ABV and was going to remove it to make sure it wasn't the cause as the pipe the other side of it is hot but that could just be...

That would explain it as they didn't ask how old it was. I didn't know they changed it in later models

I had read about this and was on the list of things to change to see the effect. Consistent with what you said this morning was 3degC outside. Rads, DHW and UFH all had demand at the same time so boiler ran for ~1.5hrs before reaching setpoint 65 + 5degC = 70degC and switched off. Looks like...

The lowest you can set it on d.00 is 12kW for the 637 but the Vaillant technical help line said it would modulate down to 6kW which I wasn't sure was correct. I did wonder if the VRC720 weather compensating controller would be able to set the boiler to a lower output than you can do manually

The boiler cycles horribly most of the time as you can see on the right hand side of the chart due to the boiler being oversized so not exactly efficient and condensing as much as it should. Turned boiler down to 12kW (lowest it will go) but still cycling so I'm trying to understand it a bit...

External bypass has been screwed down to max pressure once I found out there was an internal bypass and I'll check tomorrow but I dont have d.14 just d.19 which is set to 1 slow speed pump I guess the question is "Is it an issue that the internal bypass is constantly letting water through?"