My Viessman boiler journey

[Oz07]

3 minutes ago, JohnMo said:

I've just simplified and started to do DHW direct via immersion only. Left all the piping valves etc in place so could revert later, if I wanted, but over the last 3 weeks haven't been shocked by electric usage.

Sounds good what made you do this is it quicker recharge than ashp?

[marshian]

12 minutes ago, JohnMo said:

I've just simplified and started to do DHW direct via immersion only. Left all the piping valves etc in place so could revert later, if I wanted, but over the last 3 weeks haven't been shocked by electric usage.

 

Current winter daily average HW heating is 4.5 kWh at £0.052 per kWh

 

No solar, no battery and std fixed electricity rate at £0.26 per kWh - be a big increase in cost to heat direct via immersion only 

 

We only ever heat direct in summer during heating system improvements (have some more planned for 2026)

[JohnMo]

1 hour ago, Oz07 said:

Sounds good what made you do this is it quicker recharge than ashp?

Mostly a long run of piping, although getting what looks a good CoP, generally 3+ it wasn't really that good, as we got a big slug of cold water hit the cylinder coil and it cooled the vessel. So small recharge needed to add a disproportionate of energy.

 

So will just leave heat pump to tick away, doing heating and cooling. 

[Oz07]

7 hours ago, JohnMo said:

Mostly a long run of piping, although getting what looks a good CoP, generally 3+ it wasn't really that good, as we got a big slug of cold water hit the cylinder coil and it cooled the vessel. So small recharge needed to add a disproportionate of energy.

 

So will just leave heat pump to tick away, doing heating and cooling. 

Atleast you've always got a back up if the immersion fail.

[marshian]

Well Viessmann technical have washed their hands of the boilers behaviour - their conclusion is that it is normal behaviour for a heat only boiler running weather compensation 

 

I'd done a few videos showing the boiler behaviour and shared them with the technical team hoping that this would help to point me in the right direction if it was a set up issue

 

Seeing as they are adamant it's not a boiler issue as such I'll share one here

 

Basically attached a remote microphone to the boiler because the phone itself can't pick up the sound change when it ramps up or down and then filmed the Vicare app on another phone which shows flow temp and modulation level so it's as dull as a dull thing from a content perspective

 

 

The commentary to go with it is in the description but I'll put it below as well

 

9.7 Deg Outside Air Temp - Target Flow temp ~25 Deg C

 

Timelines (so you can skip the boring bits)

 

00:00:20 Boiler Starts (Initial Purge and Burn followed by ramp down)

00:02:12 Boiler at "Minimum modulation"

00:02:47 Running thro the settings (Weather Comp Curve Slope and Level)

00:03:22 thro to 00:15:35 Boiler stable running at min (flow temp slowly rising from initial 27.7 Deg C to 29.2 Deg C)

00:15:37 Boiler modulation increases driving flow temp higher faster

00:17:43 Boiler modulation decreases and then rises again

00:20:18 Boiler modulates back to minimum - flow temps drop to 30.5 Deg C

00:21:45 Boiler shuts down (due to flow temp exceeding target plus 5)

00:21:16 Running thro the settings (Weather Comp Curve Slope and Level)

[JohnMo]

I have a very similar issue with my heat pump and working through it, I think I know what causes it.

 

Mine occurs when the heat pump hasn't really got any work to do or on an upward outside temperature change, with a warm house.

 

The controller wants to keep the heat pump running but it not getting the return temperature doing what it expects. It's trying to pull return temp down, but the floor isn't willing to change quickly. In our situation the floor is saturated with heat and return temperature is very stable. So the logic starts to loose control - acts like a spoilt child. Not willing to switch off, but really has nothing to do.

 

The control logic, being to manage a dT and upper flow temp (with an over temp hysterisis). So return is stable, dT cannot be maintained with reducing target flow temperature, so controller starts to work, counter intuitively and it's normal programming isn't working.

 

He is a plot over 50 mins. 

The previous hour to this outside temp rose a couple of degrees, plus the sun was starting come through from dark and below zero. I have set up a simple thermostat to kill the heat pump, when house hits a given level, about 0.2 above target room temperature. This limits this behaviour quite well.

 

Your radiator system could be doing exactly the same thing, it just cannot shed the heat, so return temperature stays stable. You boiler control system isn't setup by Viessmann to cover this situation? The same as my Hiaer one isn't.

 

This is a problem 99.9% of users will never notice, I monitor mine to death, so see everything, so do you.

 

[marshian]

16 minutes ago, JohnMo said:

This is a problem 99.9% of users will never notice, I monitor mine to death, so see everything, so do you.

 

Yeah 100% this is something I see and most people wouldn't notice it 

 

It's more common between 4 and 8 deg C - after 10 deg it doesn't really happen

 

It hasn't done it for a couple of days since I've changed my strategy for heating and my current WC curves are to to be frank bloody weird......... I'll post them up later but in summary I now have 3 WC curves, one for below zero, one for zero to 10 deg and one for 10 degs and above and I swap between the curves with the target room temp

 

 

[marshian]

This was my curve

 

 

above 10 deg this is my curve

 

Room temp target 13 

 

 

between 0 and 10 

 

room temp target 17

 

below Zero 

 

room temp target 20 

 

 

it’s basically as a result of flattening the heat curve to a very small slope and then because the room temp offset changes the curve just using that manually to compensate for the outside temp