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