My Viessman boiler journey

[marshian]

26 minutes ago, John Carroll said:

Wonder was it cycling due to the minimum boiler output being more than the rad demand or due to a roomstat switching it off when room temperature reached its SP.

Former yes - Latter no

 

TRV don’t intervene on majority of rooms - WC flow temp and elevated room targets that cannot be hit with WC flow temp and heat loss of the rooms. So cycling was down to the house/rads not getting rid of the energy in the circuit.

 

was first warm night 8.5 deg C so boiler flow temp would have been 26-27 deg C max and heat loss of the house maybe around 1.6 kWh (so on that basis I’d expect the boiler to fire for 15mins in the hour

 

minus temps over night and it’s firing 45 mins in the hour

 

the 9 cycles might be a one off or I may need to tweak the curve a bit more to align (lower) the flow temps at higher OAT 

[John Carroll]

1 hour ago, marshian said:

Former yes - Latter no

 

TRV don’t intervene on majority of rooms - WC flow temp and elevated room targets that cannot be hit with WC flow temp and heat loss of the rooms. So cycling was down to the house/rads not getting rid of the energy in the circuit.

 

was first warm night 8.5 deg C so boiler flow temp would have been 26-27 deg C max and heat loss of the house maybe around 1.6 kWh (so on that basis I’d expect the boiler to fire for 15mins in the hour

 

minus temps over night and it’s firing 45 mins in the hour

 

the 9 cycles might be a one off or I may need to tweak the curve a bit more to align (lower) the flow temps at higher OAT 

Do you know the total T50 rated output of those 9 rads and what room/house temp you are aiming for.

[marshian]

26 minutes ago, John Carroll said:

Do you know the total T50 rated output of those 9 rads and what room/house temp you are aiming for.

 

Yes I do - actually every damn rad dimensions and T50 output is in a spreadsheet

 

I used it when I was working with the heat loss v rad output at various flow temps to work out where I needed to target the flow temp for the weather compensation at -2.4 Deg OAT

 

Some of the individual rads I've tweaked the flow rates from where it was but overall system balance has remained the same.

 

Front Hall, Toilet, Utility and Study remain on scheduled heating (heat loos of those rooms is a lot more because 3 external walls all on north facing side of the house

 

All other rooms 24/7

 

Heat loss is overstated due to the software wanting higher ACH than I was comfortable with.....

 

I also utterly badgered the dining room calc gave it a vaulted ceiling with no heated room above - Ooops

 

True heat loss is absolute max 4.0kWh at -2.4 Deg C OAT (probably 3.6 kWh based on actual energy used if my monitoring is more accurate)

 

Rooms & Rads.xlsx

 

Edited March 7 by marshian
Forgot target room temps

[John Carroll]

I just used 12 rads output (ignored the ones in yellow) and calculated for a 9 rad output, I reckon that the 12 rads have a total T50 rating of ~ 17.6kW so 9 rads = 13.21kW (T50), the required max output seems to be ~ 4.77kW for those 12 rads = ~ 3.58kW for 9 rads.

 

The required flowrate (for 9 rads) is ~ 0.475m3/hr, this then results in

flowtemp/returntemp/dT/rads output. 41.55C/35.1C/6.48C/3.58kW

At a (your) flowrate of 0.4m3/hr and flowtemp of 28.9C gives (my calcs)

flowtemp/returntemp/dT/rads output. 28.9C/26.42C/2.48C/1.15kW

Your data gave a calculated flowrate of only 0.06m3/hr (15% of 0.4m3/hr)

flowtemp/returntemp/dT/rads output. 28.9C/20.4C/8.5C/0.6kW

clearly something amiss somewhere?

 

 

 

 

WC Rooms & Rads Mars Rev1.xlsx

[marshian]

On 07/03/2025 at 20:45, John Carroll said:

I just used 12 rads output (ignored the ones in yellow) and calculated for a 9 rad output, I reckon that the 12 rads have a total T50 rating of ~ 17.6kW so 9 rads = 13.21kW (T50), the required max output seems to be ~ 4.77kW for those 12 rads = ~ 3.58kW for 9 rads.

 

Wrote a long reply and for some reason the forum or I had a blip and it never posted..........

 

Rooms/areas highlighted in are the hallway, stairwell and upstairs landings - they are all supplied by one ground floor rad

 

I've amended the file to clarify which rads are 24/7 and which rads are scheduled (so only heated when needed)

Rooms & Rads.xlsx

 

On 07/03/2025 at 20:45, John Carroll said:

The required flowrate (for 9 rads) is ~ 0.475m3/hr, this then results in

flowtemp/returntemp/dT/rads output. 41.55C/35.1C/6.48C/3.58kW

At a (your) flowrate of 0.4m3/hr and flowtemp of 28.9C gives (my calcs)

flowtemp/returntemp/dT/rads output. 28.9C/26.42C/2.48C/1.15kW

Your data gave a calculated flowrate of only 0.06m3/hr (15% of 0.4m3/hr)

flowtemp/returntemp/dT/rads output. 28.9C/20.4C/8.5C/0.6kW

clearly something amiss somewhere

 

Well I'm not following that and I need to get my head round it

 

One thing I have noticed is on PP3/6 the m3/hr number varies a lot - can be as low as 0.3 m3/hr to as much as 0.6 m3/hr which is also confusing me!!

[John Carroll]

1 hour ago, marshian said:

 

Wrote a long reply and for some reason the forum or I had a blip and it never posted..........

 

Rooms/areas highlighted in are the hallway, stairwell and upstairs landings - they are all supplied by one ground floor rad

 

I've amended the file to clarify which rads are 24/7 and which rads are scheduled (so only heated when needed)

Rooms & Rads.xlsx 12.37 kB · 1 download

 

 

Well I'm not following that and I need to get my head round it

 

One thing I have noticed is on PP3/6 the m3/hr number varies a lot - can be as low as 0.3 m3/hr to as much as 0.6 m3/hr which is also confusing me!!

Will get back re rad outputs later.

 

Re the pump, Are you saying that PP3 gives a flowrate of 0.3m3/hr and PP6 gives a flowrate of 0.6m3/hr? or, is the flowrate fluctuating while the setting is set on any one of the PP3 to PP6 settings?

 

[marshian]

50 minutes ago, John Carroll said:

Will get back re rad outputs later.

 

Re the pump, Are you saying that PP3 gives a flowrate of 0.3m3/hr and PP6 gives a flowrate of 0.6m3/hr? or, is the flowrate fluctuating while the setting is set on any one of the PP3 to PP6 settings?

 

 

On PP3 (out of 6) I've been checking pump parameters and I'm seeing variation from 0.3 m3/hr to 0.6 m3/hr on the same setting (ie unchanged)

 

I maybe expect a smidge more flow when all rads are in circuit and maybe a smidge less flow when it's just the 24/7 rads in circuit but not that level of change.......

 

I need to get my head round the PP element of pump control

[John Carroll]

1 hour ago, marshian said:

 

On PP3 (out of 6) I've been checking pump parameters and I'm seeing variation from 0.3 m3/hr to 0.6 m3/hr on the same setting (ie unchanged)

 

I maybe expect a smidge more flow when all rads are in circuit and maybe a smidge less flow when it's just the 24/7 rads in circuit but not that level of change.......

 

I need to get my head round the PP element of pump control

 

I think I know exactly how PP control is supposed to work. If you look at the attachments, it will become pretty clear.

On the graphs, the red line shows what the actual flow&head is as the pump ramps up in speed, the blue line is the PP setting, on PP2, zero flow is 1.6M & 3.2M is 1.85m3/hr, as the pump speed ramps up the actual flow and calculated PP flow become exactly the same at only one head, where the red&blue lines intersect, you can see more definition in the calculations.

On CP2 you got a flow 0.5m3/hr at @ 3M head & 15W, these all correspond to exactly the very accurate Wilo data for my 6M wilo Yonos Pico so that flow and head are quite correct IMO.

On PP2, using that flow&head, 0.5m3/hr@3.0M, results in a flow/head of 0.403m3/hr@1.95M and on PP3, 0.535m3/hr@3.44M.

IF the flow&head are steady in CP mode then the should also, too, be steady in any PP mode, (a gradual change will take place if say TRVs are closing in etc.)

Note though that the PP2 & PP3 settings  are taken from the 3 standard settings shown in the screenshot but doesn't really matter as there shouldnt be any fluctuating PP settings.

 

Dab Evosta3 PP2 Rev 0.xlsx Dab Evosta3 PP3 Rev 0.xlsx

Edited March 11 by John Carroll

[John Carroll]

This Graph is based on what may be the 6 PP settings, just change the values to see the effect on the head/flow. Dab Evosta3 PP Various Rev 0.xlsx

[marshian]

Not updated this thread for a while but now the heating season is over it's time to consider where I go next - let call this an end of term report.

 

Boiler review - Did I buy the right boiler

 

Yes - unlike the previous boiler purchase. Very happy with it's weather compensation and low flow temp capability and by the time we got to early spring I'd settled on a WC curve of this.

 

 

I still have a little niggle about it's output being a little higher than claimed min but it's close and I am running at lower flow temps.

 

System improvements/changes

 

I've ordered 9 Danfoss RAS-B2 valves to replace most of the drayton EB4 bodies (adjustable flow via selecting one of 6 orifices)

 

Why?

 

1. majority of the rads are running orifices between 1 and 4 and in most of the rooms the difference between one setting and the next one (up or down) is too coarse and I need finer control to stop the rooms being too cool or overheating without TRV intervention.

 

2. I want to get rid of nearly all the wiser smart TRV's and replace with decorators caps and I can do that if I have finer control of the flow thro the rads. Leaving them on the rads just gives me an expensive battery powered room temp indicator (where I need to replace that batteries every 300 days)
 

3. Replacing the TRV bodies means a drain down and that's an opportunity to re-configure the rads from BBOE to TBOE and tweak a little more efficiency out of them (5%) and I think it might help increase the delta across the rads

 

It's quite amusing to think 4 years ago I was

 

1. running a boiler that was capable of producing 5 times the energy compared to the heat loss of the house.

 

2. Running the boiler at a temp that was only ever condensing in the warm up phase

 

3. Micro managing room temps with "smart" TRV's to heat only when needed via scheduled heating (over zoning)

 

to now

 

1. running a WC boiler at a max flow temp at -2.5 of 34 Deg C so is condensing all the time and running at 97% efficiency

 

2. Heating 24/7 with minimal zoning

 

3. Planning to drop nearly all the smart TRV's and just controlling the room temps with WC flow temps

 

Weird journey when you think about it............

[Oz07]

What's happened to the cost or kwh used? Has it reduced?

 

I suspect I could have had my last places boiler and system set up a lot better but this stuff is a bit over my head. How do you know if boiler is condensing or not?

[marshian]

1 hour ago, Oz07 said:

What's happened to the cost or kwh used? Has it reduced?

 

I suspect I could have had my last places boiler and system set up a lot better but this stuff is a bit over my head. How do you know if boiler is condensing or not?

I am using a little more gas but part of that is down to “learnings” during the heating season so up from 8500 to 9500 but 9000 would be a reasonable expectation

 

For a boiler to condense the return temp to it needs to be below 54 deg. The lower the return temp the greater the condensing energy recovery

[Sparrowhawk]

Good read @marshian, and that matches my experiences very closely. My heating curve ended up as Slope: 1.6 and Level: -7.  It's still not perfect, as on days with lots of solar gain it overheats, while on other days, which are warmer outside but with no solar gain, the house is too cold. But I don't think that's fixable until we have better insulation and draft proofing.

 

@Oz07 In my case we left the heating on 24/7 (previous years manually turned on/off) and our usage went from 13,300kWh to 14,500kWh - but the house was always comfortable, whereas in the previous year it had been down to 15C many winter evenings, and we were wearing coats indoors.  This is still below the 16,500 kWh it was in 2021 when we tried to have it warmer with the old boiler.

[marshian]

10 minutes ago, Sparrowhawk said:

Good read @marshian, and that matches my experiences very closely. My heating curve ended up as Slope: 1.6 and Level: -7.  It's still not perfect, as on days with lots of solar gain it overheats, while on other days, which are warmer outside but with no solar gain, the house is too cold. But I don't think that's fixable until we have better insulation and draft proofing.

 

South facing rear of the house with french doors in both Dining Room and Living Room means solar gain is a useful bonus but doesn't really overheat until we are out of the heating season.

 

What has really surprised me is how effective WC is in meeting the heating needs there were only a couple of times when it got caught out with big temp swings in a very short space of time

 

But my manual weather compensation that I was doing before would have been caught out too 🙂 

 

10 minutes ago, Sparrowhawk said:

@Oz07 In my case we left the heating on 24/7 (previous years manually turned on/off) and our usage went from 13,300kWh to 14,500kWh - but the house was always comfortable, whereas in the previous year it had been down to 15C many winter evenings, and we were wearing coats indoors.  This is still below the 16,500 kWh it was in 2021 when we tried to have it warmer with the old boiler.

 

I think that's the benefit of WC and 24/7 - house temps are amazingly stable and as a result you can run slightly lower target room temps and it feels fine. I'm comfortable in temps from 17 to 19 with no drafts (lack of air movement makes a huge difference to comfort levels at lower room temps)

 

Downside is Mrs Alien is slightly more sensitive to room temps and to her "toasty" is 22 and "Freezing" is 19 but she's getting used to the new regime and back end of winter had stopped declaring the house to be "like an ice box" at 20)