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)

 

 

[marshian]

Water Heating Optimisation

 

TLDR (Sub title - I really should get a better tank and until then I’m going to see if I can replace an annoying tank stat and experiment to see if heating once daily to a higher temp is more or less economical than heating twice a day to a lower target temp)

 

OK Background

 

so it’s a ~115 litre copper tank from 2012, it’s spay foam lagged - heating coil is prob a 3kW (so not very large)

 

Summer it gets heated to 50 deg once daily and between the two of us we use all of the available hot water on a daily basis Tank losses are 0.5 deg per hour in summer but higher in winter so winter we heat a little higher temp and use DHWP to heat water quickly at higher temp so CH can get back to heating house at low flow temps.

 

Tank temp control is a Drayton tank stat.

 

Boiler flow temp is 62 Deg C and 45 mins of heating is scheduled per day (stat declares satisfied at 50 Deg C so actual heating time ranges from as low as 31 min but typically 40 mins is the norm and sometimes the schedule stops the heating cycle before target temp is reached but it’s close enough not to matter

 

 

above is last 7 days peak temp from stand alone temp monitoring (consistent yes?)

 

better still comparing summer water heating with previous years I’ve reduced the average daily water heating energy usage from 4.8 kWh daily to 3.8 kWh daily

 

trouble is the tank stat min is 50 deg and normally once a week Mrs Alien likes a bath - a long bath and she likes a hot bath and she likes to top it up 

 

50 deg C doesn’t cut it for the repeated top ups over a couple of hours so I do a legionnaire's cycle at the same time - manually increase the tank stat setting to 65 deg C and increase the boiler flow temp to 80 deg C - 30 mins and tank is ready.

 

My issue is that finding the 50 deg setting again after is bloody tricky - the 50 deg position is so close to “off” it normally results in a cold shower day after bath night - The Drayton Tank stat is just so “hair trigger” between 50 and off it’s annoying - add to this it requires a small flat head screwdriver to adjust it and it’s bloody annoying…….

 

Tonight I’ve replaced it with a Danfoss one that has a working range of 30 to 90 and has an adjuster that can cope with my fat fingers 

 

This got me thinking………… (this is never good)

 

Is it more economical to heat a tank to 50 deg once a day or to heat it to 45 deg twice a day (ignoring bath night)

 

I can understand that the cooler the tank temp the lower the losses between heating cycles but gut feel says once a day would be the most economical approach……

 

I feel a small trial is needed to prove it one way or another……

 

 

 

[JohnMo]

6 hours ago, marshian said:

The Drayton Tank stat is just so “hair trigger”

 

I just fitted a EPH Digital Cylinder Thermostat (EDBS), you can program to do a legionaries cycle time and temperature and it also a wide range of hysterisis adjustment (reason I bought it).

 

6 hours ago, marshian said:

gut feel says once a day would be the most economical

Pretty much the same as a heat pump, your boiler likes cooler return temps to get best performance. Plus your standing losses decrease. Fine line between efficiency, cost and having to answer the question - why have we run out of hot water?

 

I have moved from heating twice a day, almost need it or not, due the old thermostat being a rubbish design. To setting a hysterisis of +0 -10 degs and heating mostly once, occasionally twice, odd day not at all.

 

Best solution 

20-30 plate PHE, bronze pump, bypass existing small coil, tap into cold inlet  and hot outlet, set flow temp to about 60-70, thermostat to around 50 wife runs bath, boiler fires up you almost have a endless water, she switches off tap, cylinder is recharged.

[marshian]

On 05/08/2025 at 08:13, JohnMo said:

I just fitted a EPH Digital Cylinder Thermostat (EDBS), you can program to do a legionaries cycle time and temperature and it also a wide range of hysterisis adjustment (reason I bought it).

 

Sounds perfect- I'll have a look at those - thanks for the tip (New Danfoss Tank Stat seems to have a similar  +0 -10 deg hysteresis - annoyingly initial usage seems to indicate that the temp scale is not aligned very well to the tank stat switching point (50 Deg C results in ~47 deg tank temp. (Plus is that it has a range below 50 Deg C that isn't just "Off" but I've only got two days of data.

 

 

On 05/08/2025 at 08:13, JohnMo said:

Pretty much the same as a heat pump, your boiler likes cooler return temps to get best performance. Plus your standing losses decrease. Fine line between efficiency, cost and having to answer the question - why have we run out of hot water?

That's the aim - lower boiler flow temp and reduce standing losses

 

The "why have we run out of water" in this house is normally "I do not like a cold shower at any time - sort your shit out"

 

On 05/08/2025 at 08:13, JohnMo said:

I have moved from heating twice a day, almost need it or not, due the old thermostat being a rubbish design. To setting a hysterisis of +0 -10 degs and heating mostly once, occasionally twice, odd day not at all.

 

I'm just trying to find a summer time balance - heat water in tank to low 40's deg twice a day (with reduced standing heat losses) v heat water in tank to just over 50 deg once a day (with higher standing losses)

 

One will result in lower energy usage - just not sure which one.

 

My 45 min schedule and boiler 62 Deg C flow settings gets the tank to 40 deg in 15 mins - it takes a further 30 mins to get to 50 deg. However the first 15 mins the boiler is heating up the circuit, is running a higher modulation before ramping down so the difference may not be as great as the times suggest.

 

On 05/08/2025 at 08:13, JohnMo said:

Best solution 

20-30 plate PHE, bronze pump, bypass existing small coil, tap into cold inlet  and hot outlet, set flow temp to about 60-70, thermostat to around 50 wife runs bath, boiler fires up you almost have a endless water, she switches off tap, cylinder is recharged.

 

I'm really warming to this idea as a way to establish the effectivity of a PHE in terms of tank heat up times. (I'm finding it quite hard to find any real world data on PHE performance)

 

My original thought was get a "Heat Pump" compatible tank 3m2 coil surface area but for a 120 L tank it's a lot of coin. Modern insulated tanks with no coil and two immersion points are much much cheaper (It appears the reduction in tank cost would more than cover purchase of a PHE)

 

Long term I don't want to do it with my existing tank - I could do PHE as step one - evaluate the process - if it works then great.

 

If it doesn't I'll have just a PHE to sell on rather than both a PHE and a Modern Tank which is only suitable for Direct heating only.

 

As a stage two replace the tank with a newer Direct heating (with a lower heat loss level) and hook up to PHE knowing the results will be as expected

[marshian]

I'm thinking this kit (20 Plate 15kW unit rather than the 30 Plate 20kW) because the boiler is 16kW - I'm not sure how much benefit I'd get from the 30 plate version but it's only £60 plus vat more

 

https://beetbg.com/products/dhw-plate-loading-20xplate-kit?srsltid=AfmBOoodTI_0ue5CDsk2tfjQHhl6sCoT7GMV-jrUXRcS5F1xDFg7Ym7l

 

DHW Cylinder Plate Loading 20 x Plate Kit - For up to 15kW heat load Heat Pump, Boiler £450 plus the vat

 

Consists of the following

• 1x 20xPlate SWEP E8TH Heat Exchanger
• 1x EPP Hard Shell Insulation Jacket
• 4x 22mm x 3/4" Flat Faced Unions
• 1x DAB Bronze Pump
• 2x 22m Pump Valves
• 1x 22mm 

[JohnMo]

4 minutes ago, marshian said:

not sure how much benefit I'd get from the 30 plate version but it's only £60 plus vat more

The more surface area your heat exchanger has, the closer the approach temperature is (basically the difference between input and output temperature). Not done the maths, but 20 plate say 6 degs, and 30, 3 degs approach or a similar ratio, so 4 compared 2 etc. SWEP have an online calculator where you can put in all the figures and actually check. If I did it I would use the heat exchanger I have, which a large 40 plate (11m²).

[marshian]

20 minutes ago, JohnMo said:

The more surface area your heat exchanger has, the closer the approach temperature is (basically the difference between input and output temperature). Not done the maths, but 20 plate say 6 degs, and 30, 3 degs approach or a similar ratio, so 4 compared 2 etc. SWEP have an online calculator where you can put in all the figures and actually check. If I did it I would use the heat exchanger I have, which a large 40 plate (11m²).

 
my boiler seems to work on CH with a delta between flow and return of 7 Deg - if the delta narrows it will cycle so maybe 20 plate will be more suitable - just trying to get my head round it so thanks for your contributions

[JohnMo]

5 minutes ago, marshian said:

 
my boiler seems to work on CH with a delta between flow and return of 7 Deg - if the delta narrows it will cycle so maybe 20 plate will be more suitable - just trying to get my head round it so thanks for your contributions

Lots of dT's at play. Boiler dT, dT between boiler flow and the water the other side of the heat exchanger (approach temp). dT from boiler is driven, by boiler algorithm, so flow rate and also how quickly whatever you are heating (air from radiators or water in cylinder) can take that heat away. You know from experience for example, that to stop your boiler from cycling when doing CH while wanting to do low flow temps, you had to up the size of some/all your radiators.  Your cylinder heat exchanger is exactly the same. Big HE allows low flow temps AND allows boiler to manage the boiler dT. A smaller HE demands a higher flow temp to enable boiler to manage its own dT with cycling.

[John Carroll]

On 06/08/2025 at 11:52, marshian said:

I'm thinking this kit (20 Plate 15kW unit rather than the 30 Plate 20kW) because the boiler is 16kW - I'm not sure how much benefit I'd get from the 30 plate version but it's only £60 plus vat more

 

https://beetbg.com/products/dhw-plate-loading-20xplate-kit?srsltid=AfmBOoodTI_0ue5CDsk2tfjQHhl6sCoT7GMV-jrUXRcS5F1xDFg7Ym7l

 

DHW Cylinder Plate Loading 20 x Plate Kit - For up to 15kW heat load Heat Pump, Boiler £450 plus the vat

 

Consists of the following

• 1x 20xPlate SWEP E8TH Heat Exchanger
• 1x EPP Hard Shell Insulation Jacket
• 4x 22mm x 3/4" Flat Faced Unions
• 1x DAB Bronze Pump
• 2x 22m Pump Valves
• 1x 22mm 

 

I'd definitely agree that a E8TH x 20, approx 0.6m2 heating surface would be pretty useless in getting a close approach temperature for you, I don't know why the manufacturers specify these (20)kW etc ratings which only apply at extremely high dts at something like 40C.

As stated above the bigger (within reason) the better.

Here is one that should give 15.0kW output with a approach temperature of ~ 2C, its a B85H x 50, 3.13M2. which might suit your conditions, if you select price list you can calculate the heating surface for any required PHEX, they also seem to suggest a B28H x 76, at 4.75M2. Its probably best to email your requirements to them and possibly to Nordic as well as UK Heat Exchangers.

 

 

 

 

 

 

Edited August 8 by John Carroll

[marshian]

14 hours ago, John Carroll said:

 

I'd definitely agree that a E8TH x 20, approx 0.6m2 heating surface would be pretty useless in getting a close approach temperature for you, I don't know why the manufacturers specify these (20)kW etc ratings which only apply at extremely high dts at something like 40C.

As stated above the bigger (within reason) the better.

Here is one that should give 15.0kW output with a approach temperature of ~ 2C, its a B85H x 50, 3.13M2. which might suit your conditions, if you select price list you can calculate the heating surface for any required PHEX, they also seem to suggest a B28H x 76, at 4.75M2. Its probably best to email your requirements to them and possibly to Nordic as well as UK Heat Exchangers.

 

 

 

 

 

 

I’m slightly confused (nothing unusual there!!!)

 

water flow temp to the PHE has to be above the target tank temp no mater how efficient or how much surface area it has

 

so if I want water at lets say 45 deg C then the flow temp needs to be above that

 

I’m not looking to heat the radiator circuit with a PHE - the boiler copes nicely with a flow temp from 25 to 35 for the rad circuit (it doesn’t mind a delta of anywhere between 5 & 7 deg as long as the circuit is unrestrictive (ie full circuit available to it)

 

needed to heat water today quickly this morning - tank temp start 25 deg (flow temp ramped to max 80) had a 10 deg delta at the boiler at the end of the 30 min cycle) and had the usual issue of a massive overshoot of tank temp (stat set to 50 deg) top got to 57 before bottom got to 50 and it used 4.88 kWh of gas 

 

heating with a lower flow temp (62 Deg C) stat would have switched off at 50 with virtually no difference between top and bottom of the tank and used 1 kWh less energy (this is how I do my water heating in the winter to shorten the cycle time and get back to CH before the house temp drops)

 

My existing tank coil I think is maybe a 3kW one - certainly looking at similar tanks for sale the coil surface area is going to be maybe 0.5 m2 if that.

 

I’d like to get away from high flow temps but still heat in a shorter time period - my thought was either new tank with heat pump compatible coil size 3m2 surface area or PHE with similar surface area and bronze pump to circulate

 

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)

 

If I tried to do that with current set up I estimate that HW cycle would need to be 90 mins and the boiler would fire maybe 3 or 4 times with ever shorter burn lengths and still wouldn’t achieve the target tank temp although it would probably be close)

 

 

[marshian]

Oh and I appreciate what I’d like and what is actually possible might be very different (ie impossible)

[marshian]

Stats from this morning (all Deg C) peak/end temps taken just before cycle ended

start

 

Tank Top (start) 31.3

Tank Bottom (start) 26.2

 

flow temp peak 77.5 

return temp peak 66.4

 

Tank Top (near end) 55.2

Tank Bottom (near end) 47.6

 

my temp monitoring records all temps on the hour every hour (you can see current temp at any time but the data is only logged at hourly intervals) Does mean I have a really clear knowledge of my tank temp losses over time)