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]

On 19/01/2026 at 20:28, 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 amount 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 hysteresis). 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.

 

Here 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

[marshian]

11 hours ago, JohnMo said:

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.

 

It's definitely something like that - only issue is the boiler doesn't have a return temp it does all the management on the flow temp (and possibly the flue temp) 

 

11 hours ago, JohnMo said:

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.

 

Upward temp changes are also a factor - especially when I'm trying to get extended run times. I was toying with getting a clever mate to make me an interface which blunts the changes (basically reads the outside temp sensor but only sends a change in temp to the boiler once an hour)

[JohnMo]

2 hours ago, marshian said:

Upward temp changes are also a factor - especially when I'm trying to get extended run times. I was toying with getting a clever mate to make me an interface which blunts the changes (basically reads the outside temp sensor but only sends a change in temp to the boiler once an hour)

Not sure that helps, I tried a flat curve, to see what happens and it got a lot worse. The heat loss reduces, so still throws the controller out.

 

13 hours ago, marshian said:

summary I now have 3 WC curves

That sounds like too much intervention and faff. I would be setting to manage coldest outside and then have a thermostat, used in an energy limit mode. So basically a permission for boiler to run, but cut out when curve doesn't exactly match house needs and house starts to get a higher temperature than target.

[marshian]

1 hour ago, JohnMo said:

Not sure that helps, I tried a flat curve, to see what happens and it got a lot worse. The heat loss reduces, so still throws the controller out.

 

That sounds like too much intervention and faff. I would be setting to manage coldest outside and then have a thermostat, used in an energy limit mode. So basically a permission for boiler to run, but cut out when curve doesn't exactly match house needs and house starts to get a higher temperature than target.

 

Nothing is a permanent solution mainly using the opportunity to see if the behaviour can be ironed out 

 

Since 01/01/2026 so 62 Days it's done it's ramp up thing 250 times so as an average 4 times a day 

 

Quite tempted to pull the spare MBU from it's storage and see if I can extract the code - I'm 100% sure it's firmware driven

[marshian]

Ignoring a boiler that won't respect the range rating......

 

Finally got a HW Tank Brand new still in shipping packaging with installation kit for £625

 

Suitable for either ASHP or Fossil Fuel Boiler

 

180 Litres 2.4 m2 Coil Surface area (largest one that I could get without going pass 180 litres)

 

It's a Grant and they seem to be happy for it to be used as vented

 

I had to drive 400 miles to Wales and back to fetch as seller wouldn't ship but he was really good to deal with.

 

Next job try and find a suitable surrey/York flange that will fit the 3/4" BSP Male outlet on the top of the tank - current Surrey flange in the existing tank is 1" BSP so I can't re-use that and it would probably be too big to go into the tank from memory. Both showers are pumped and before I fitted the surrey flange I trashed a couple of pumps in a short space of time due to air in HW supply.

 

 

[SimonD]

23 minutes ago, marshian said:

It's a Grant

 

I like the Grant cylinders, nice to install. They're also very competitive compared to the various others. I'm now gravitating towards these as a standard option.

[marshian]

10 minutes ago, SimonD said:

 

I like the Grant cylinders, nice to install. They're also very competitive compared to the various others. I'm now gravitating towards these as a standard option.

 

It's QRSC180 so older model (build date on it is 2025)

 

In my research I did find out that the new QR2SC180 has a 3.2 m2 Coil surface area but even though my pockets aren't deep enough for that when I searched no-one had any stock.....

[SimonD]

5 minutes ago, marshian said:

 

It's QRSC180 so older model (build date on it is 2025)

 

In my research I did find out that the new QR2SC180 has a 3.2 m2 Coil surface area but even though my pockets aren't deep enough for that when I searched no-one had any stock.....

 

Ah okay, yes there was a quiet update on the coil sizes. I think they still list the coil area as 2.7m2 but when you get the new ones, it's at least 3m2. Special order only, but that works fine for me on my jobs.

[marshian]

Hmm seems really damn tricky to find a suitable Surrey or York Flange with 22mm fittings and 3/4" BSP female connection to the tank.....

 

Did find this Warix Flange

3/4" BSP Male connection to the tank and 22mm compression suitable outlet fittings

I wondered if I can use this 3/4" BSP Threaded Socket

To provide the required connection to both tank and Warix Flange

 

Only downside I can see with the Warix Flange is the non aerated feed for HW comes from the vertical top connection and the bottom horizontal connection feeds the normal circuit (in my case that would effectively be connected to the cylinder vent pipe that finishes above my cold water store in the loft above the HW tank)