My Viessman boiler journey

[JohnMo]

A PHE need approx 1/3 to 1/2 the area of a normal cylinder coil to do the same job. Mainly due to the turbulent floe within the phe compared to almost static flow in the cylinder.

 

When I was doing hybrid heating against a 3m² coil I was heating cylinder to 50, it would overshoot to 52. But boiler flow temp never went above 55.

[marshian]

54 minutes ago, JohnMo said:

A PHE need approx 1/3 to 1/2 the area of a normal cylinder coil to do the same job. Mainly due to the turbulent floe within the phe compared to almost static flow in the cylinder.

 

When I was doing hybrid heating against a 3m² coil I was heating cylinder to 50, it would overshoot to 52. But boiler flow temp never went above 55.

This is exactly my aim - minimal overshoot at the top of the tank - whole cyl heated to almost same temp at as low a flow temp as I can get away with in one burn cycle within the shortest time frame (ideally under 30 mins)

 

If I can achieve that with current 13 year old copper tank (where I know my losses are still higher than a modern tank) then subsequent replacement of the tank for a more modern jacketed tank is hopefully only going to reduce the static losses and the efficiency of the HW heating isn’t going to change for the worse.

 

 

[JohnMo]

5 minutes ago, marshian said:

under 30 mins

I was doing 210L in that time frame

[marshian]

6 minutes ago, JohnMo said:

I was doing 210L in that time frame

 

Blimey!!!

[marshian]

In other news Boiler had it's first service

 

Gas turned off and disconnected from burner

All electrical connections to burner and ignition removed

Burner came out and it and the ignition electrode were cleaned

Heating coil scrubbed with plastic spinny brush on a drill

Burner re-fitted

Condensate trap removed - emptied - cleaned and replaced (refilled with water)

All removed connections checked - he found a loose/poor connection on the outside temp sensor so he cut back and re-did it.

Boiler fired up and he repeated all the commissioning gas checks (hi and low flue gases)

Boiler wasn't quite 1 year old and from arrival to leaving it was about 75 mins and £72 with the vat (he did say his service work is normally busiest in Sept and Oct and right now it’s fairly quiet for anything except new installs, mainly ASHP because of BUS grants) but I had the boiler purchased & fitted in July for the same reason I want any servicing to be outside the busy period.

 

Scores on the doors from the boiler stats for one year

 

Heating Demand Hours 4117 (171 days - I was initially heating to schedule and only moved to 24/7 in Dec 2024)

 

Burner Hours 1917

Burner Starts 3965

 

Hours on CH 1683

Hours on HW 234

 

Average Cycles per day 11 (Summer HW process has driven down the average)

Average Boiler Cycles per day in winter heating period 22

 

 

 

 

[JohnMo]

Just found a trend I posted on another thread

For clarity the boiler and heat pump were running, the boiler sat behind a plate exchanger and added heat to the ASHP flow. ASHP on its own takes nearly twice as long to heat cylinder.

 

[John Carroll]

5 hours ago, marshian said:

 

 

Ideal world I’d like to achieve flow temp of less than 55 deg C and a target tank temp of 50 deg C with 115 litres of water at 25 deg C heated in a maximum of 30 mins max with one boiler cycle (ie it fires once and runs till stat satisfied - not cycle several times to achieve the target)

 

 

 

 

 

In that case above, then a B8X20, almost 0.5m2, will output a minimum of 15kW with boiler flow/return temps of 55C/40C at a boiler flowrate of only 14.5LPM/0.87M3/hr & secondary side inlet/outlet temps of 25C/50C at a flowrate of 8.7LPM/0.52M3/hr, to heat 115L from 25C to 50C in 25 minutes assuming the cylinder stat is located just above the cylinder outlet to the PHEX inlet.

Edited August 9 by John Carroll

[marshian]

Last year at the start of the heating season I was running scheduled heating and manually setting flow temps.

 

I transitioned to full WC flow temps during Dec moving to 23.5/7 heating (0.5 is HW recharge) and refined the curve in Jan, Feb and Mar with a really small tweak in Apr to fix overheats on warmer days (so bottom of the curve stuff).

 

This year I used the same WC setting I was using at the end of the heating season

 

Comparison of kWh/HDD (based on 16.5 Deg C base line) for first 6 days of this house heating season

 

Day	Last year	This year
Day 1	2.3	0.6
Day 2	4.9	1.8
Day 3	3.5	4.1
Day 4	2.7	2.0
Day 5	5.5	2.4
Day 6	7.6	2.3
Avg	4.4	2.2

[marshian]

All of this post is related to CH only - not HW

 

OK since I've had this boiler installed it's always had a little niggle where even range rated down to min (19 out of 100 so should be 3.2 kW but is actually realistically 4.0 kW due to low flow temps I'm running) it occasionally ramps up to 20% to 30% just before shutting down or even sometimes in the middle of a burn cycle resulting in an early shut down for over temp....  

 

It's definitely worse on warmer days. If I'm in the kitchen and the boiler is on a burn cycle I can hear the tone change as it ramps up. 

 

Viessmann replaced both Main Board and WC Temp Sensor in March this year to eliminate those as a reason for the behaviour even though I wasn't convinced by this action. It improved it enough that I didn't notice it happening so much but it definitely still happened.

 

I've tried to understand what is driving this issue but not been able to (so I've actually put it down to a "characteristic" and recently I increased the range rating to 30 out of 100 (roughly 6 kW) - logic being the boiler is ignoring it anyway so might as well give a bit more head room back 

 

Strangely and I hadn't heard it do it since - then on Sat it did it again and then again on the next cycle. I went and checked the circulation pump - I'm running it at 0.5m3/hr on CH and 0.6m3/hr on HW and the pump was 0.4m3/hr flashing with 0.5m3/hr a little lower than normal but all 4 of the rads where TRV's can and do intervene were closed.

 

(Difference in flow rate is accounted by the lack of resistance thro the coil as opposed to 13 rads with flow controls/restrictors fitted to all of them)

 

I bumped the pump speed from 1 to 2 and it was rock solid on 0.6m3/hr

 

It didn't do it for the next two cycles

 

I put the pump back down to 1 and it did it on the next cycle.

 

I went back to the manual - I'd made the assumption that the boiler flow rate requirement would be at peak output and running a 16kW boiler range rated to 6 kW I could comfortably run at 0.4 to 0.5 m3/Hr

 

I dislike system noise so really want to run the pump on the slowest speed but it looks like the boiler likes a little more flow.

 

Early days but I really hope that's resolved the niggle - I might range rate it back to minimum and see if the issue comes back

 

 

[marshian]

On 13/10/2025 at 15:37, marshian said:

Early days but I really hope that's resolved the niggle - I might range rate it back to minimum and see if the issue comes back

 

Well it hasn't 😞 so I'm going to put my thoughts down and maybe a proposal for consideration from the great minds of Build Hub

 

The "niggle" now only occurs between 6 deg OAT and 12 Deg OAT 

 

So lets cover the data I can provide

 

Pump

 

Pump Speed 0.5 to 0.6 m3/hr set on Constant Curve

Watts 15

Head 3.1 m

 

Rads

 

I have 13 Rads all with TRV's in the house table below explains what they are room sizes Heat loss at -2.5 Deg C OAT and how they are set up (Because there are issues with getting the flow rate low enough for some rads and solar gain with some rooms)

 

 

I regard the circuit as well balanced both upstairs and downstairs

 

I have a pressure relief by-pass but it's fully closed as the circuit never shrinks enough to need it.

 

Boiler

 

Viessmann 100-W "Heat Only" 16kW set up with Weather compensation and DHWP (Using an outside temp sensor and HWD box) It's piped up as X Plan

 

It's running a delta between flow and return at the boiler of 6 to 7 deg C at  Weather compensated flow temps

 

Weather Compensation Curve

 

 

When it's not in the OAT temp range where I see the niggle this is how the boiler behaves

 

Good Days

 

On CH the boiler fires up when the flow temp drops "X" deg below WC Target (Gut feel is X = -7 Deg C below target temp)

 

It will do an initial start at 58% modulation and then over 90 secs drop down to the lowest modulation level that enables the boiler to maintain the target flow temp typically 10.8%

 

It will then run happily at that level until the flow temp exceeds the target by "Y" Deg (Pretty sure "Y" is 5 Deg C above the target temp)

 

Bad Days

 

When it's in the "Niggle" window its behaviour is different during the burn

 

Even if the Boiler is range rated to min (19 out of 100 in the menu = 10.8% or ~4.0 kWh) the boiler ignores the range rating

 

Initial start up is identical to above but very quickly into the period after the start it starts to miss-behave - modulation swings between 10.8% and 30%

 

This frequently results in the Target temp + "Y" being reached during the higher modulation and the boiler shutting down before it's fully refreshed the water in all the rads that were open for heat.

 

My thoughts are that the return temp is changing as some rads have quite a large flow and a decent delta between flow and return and some other rads have much smaller deltas and the boiler is trying to maintain the target flow temp so modulating up and down to provide a stable output.

 

If you read all that - thank you.

 

Proposal

 

So I had a thought what would happen if I set all the rads to an elevated flow rate said sod the balancing control - leave the TRV's to manage the room temps and flows and bring back into play the by-pass (just in case all the rooms hit target temp and TRV's shut down the flow).

 

Part of me says I should just try it and see what happens 

 

Part of me says the TRV's will end up managing the flows as they close and the flow rate in the system and the return temps will be the same so it won't change a damn thing.

 

Anyone think anything else will happen?

Edited October 24 by marshian
Spelling

[marshian]

It didn't change a thing (well all that did happen is the room that needed the highest flow lost temp because it was being shared out to other rooms and it's the last in the circuit)

 

So I reverted it to a dumb boiler and manually set the flow temp - still did the same thing ramping up randomly mid burn cycle - often causing + 5 deg trip.

 

Then I needed to do HW cycle so manually increased the flow temp

 

First thing boiler was only slowly increasing temp (because range rated to 4 kWh - ahh bugger need to undo that setting - Increased range rating to Max because it's one button press in the menu.

 

Then boiler reached my previously set max flow temp cap of 45 deg.

 

So that got changed for 70 Deg and then the boiler froze as I was trying to exit the menu - it was doing HW so I just walked away and left it to heating water (it's happened once before and the solution was to turn the boiler off at the main switch)

 

So when it finished the HW I powered down the boiler (made a cuppa and then switched it back on) house needed a top up heat wise so I range rated it back to Min and waited for it to fire.

 

Guess what happened next?

 

It fired - ramped up to 58% and over the course of 2 mins modulated down to min 10.6% and sat there at min for 17 mins and 30 secs - used 0.103 m3 of gas which by my calcs is 3.98 kWh....... 

 

It didn't miss a beat for the rest of the day.

 

This morning however it was back to it's old tricks - starting to really get on my wick now 

 

So I switched off the boiler waited 5 mins and then waited for it to fire for the next cycle and it respected the min range rating again.

 

So now every time it starts to play up I have a solution to the issue - it's not pretty and I don't believe it's acceptable but I have some information now to go back to Viessmann with to point out that it still has the same problem it had before (and crucially what resets it back to how it should operate)

 

It doesn't seem to matter what pump speed I use Constant Curve 1 or Constant Curve 2 the only difference that makes is the DeltaT at the boiler is 9 deg C with CC1 and 5 Deg C with CC2

 

Pump in CC1

5.5 w

1.1 m head

0.4 m3/hr

 

Pump in CC2

16 w

3.1 m head

0.6 m3/hr