Vailant ecotek plus 438 + dab evoplus 80/180m - proportional or constant curve

[ant2]

In winter I set the boiler d0 to 30-38kw, dependent on how cold it is. the pump is set to proportional curve at 8m.

In spring I set the d0 to 15kw as it's only heating the 300l water tank. I noticed the boiler often cycles constantly waiting on the pump and it can take over an hour to get the tank to any decent temperature as its cycling every 30-60 secs at worst.

I've set the pump to constant curve with a 5.5m and this seems to get the water pushed out much better to allow the boiler to stop cycling and modulate. It then heats the tank in 10-20mins

The manuals for the pump are not very DIY friendly, I read proportional pressure works well with TRV's, I presume that's why it doesn't work so well when the rads aren't on and it's only heating the tank.

What would you all recommend, setting constant curve in the spring, alongside the d0 at 15kw and reverse it in winter, or just leave the pump at constant curve with 5 or 8m the whole time?

13 rads, 5 towel rails ,UFH in one large room and 300L hw tank.

Thanks

 

[Johntheo5]

Are you sure the the PP setting is at max, ie 8M?. a 8M PP setting should IMO give very similar head/flowrates as that set to CP 5.5M with the cylinder only being heated so maybe just check that PP setting again?.

You say that a 5.5M CP setting heats the tank in 10 to 20 minutes, say 15 mins, if so and you are heating 300L from say 25C to 60c then that requires 12.2kwh and will require a boiler output of 48.8kw which its incapable of and your max boiler output is set to 15kw so doesn't add up.

 

[ant2]

The pp was definitely set at 8m.

We previously heated the tank 1h in morning, 30 min at noon and 1h at night. It was set at 8M at 22KW D0.
If I was to guess the taps were not more than 45c at that setting. The shower is on 40 degrees in our house but I only have it on for 1min to wash as I take cold showers so I didn't noticed. It was only when I was testing the temperature with a probe at the top that I realised we only get 52c+ on the taps in the winter as I forgot how hot that feels.

I cranked down the heat times to 30 min morning, 20min lunch and 30min evening a few weeks back, which I thought should be enough.
The water was barely 40 degrees and on inspection I noticed the boiler spends over half its time cycling.

When I was watching the pump on proportional for HW only, I noticed I never see the pressure go much above 4M and it was mainly around 3.5M, although it only ran 30 seconds before it cycles. I think the issue is the pump was probably trying to adjust for PP and it never gets time to boost before the boiler cuts off again and shuts pump off? I also thought the issue with HW tanks is there is no resistance change so I am not sure if it will ever boost more than the minimum?

With the CH on, soon as the boiler fires that pump spins up to well over 7< and the boiler is roaring at 30+kw, which is probably why we never see issue with HW in the winter.

Once I adjusted to constant and anywhere after 5.5->8M it doesn't seem to cycle. I worked out 5.5 with 15kw at 65c boiler temperature stopped it from cycling till the water was at 52c at the downstairs tap and around 58 at the tank stat, so I left it on that and moved the tank stat from 60->58, as 52c at the tap seemed hot enough to me.

My timings of 10->20 were from the lunch heat up, so it was probably heating from atleast 40c to complete in 15-20mins.

I'd have to check on the exact heat up time in the morning when it was around your stated 25c and as you say according to calculations that would look to be around 45-60 mins at 15kw, so I am sure you are right.

My question other than the above would be, should I leave it on constant for summer and either increase the KW output to help finish in the times specified (20kw?) or set it to 1h in morning/30 lunch/ 1 hour in evening along with 15kw for most efficient gas usage?

 

[Johntheo5]

I don't think it will make any real difference whether you top it up 3 times/day vs once/day.

Is this a externally mounted circ pump? and does it display the head in M (you quoted 4M & 3.5M above), if so it might also display the power in watts, W, if so you might post these readings for the CP and PP settings.

 

[ant2]

On constant curve-
5.5m
Q= 1.1 m3/h
S=3279rpm
P=58w
H=2723 h

Yes, but is best to have the boiler on longer at reduced kw for longer or to run the boiler at a bigger kw for a shorter time?

 

[Johntheo5]

Its the boiler return temperature that determines its efficiency, on HW duty only the boiler return temperature will be the coil outlet temperature which in turn depends on the dT between the boiler temperature and the HW cylinder temperature and the circ flowrate, so maybe a reduced boiler power will result in a lower return temperature, but overall I don't think so.

Should also have said the the lower the circulation flowrate, the lower the return temperature.

Re the pump settings, I can definitely tell you that based on the values above that the pump on a PP setting of 8M should give almost exactly the same values as the CC 5.5M. If not then there is a problem somewhere in the pump controller.
Can you post the values when running in 8M PP mode.

 

[ant2]

Proportional seems to also now be working, I think increasing the boiler temperature from 65 -> to 67c+ D0 from 22kw -> 15KW has allowed it to stop cycling as much and push the water out more gradual

Before test: Water at tap downstairs circa 50 degrees, tank was last heated at 18:30 and not heated again till 09:30 and had a power shower and small bath in between.
Proportional:
After a few mins: 5.0M / 44W
13 mins: 5.7 / 54W
22 mins-> 38 mins: 6.0-6.1 / 62W
38 mins: boiler cycling constantly - temp at downstairs tap now 53.7c, which is about what I get on Constant Curve before I see constant cycling. When cycling , pump is on about 4.5->5.3M - Video if interested-

It then cycles until it shuts off from the 1h timer I set, I get about 53.9c after 20 mins of cycling i.e 0.2c extra. Although when I put the HW back on, the HW tank stat doesn't signal to call the boiler back on, so it must be heating pretty close to the 59c on the hw tank stat before it cycles.

Bath tap upstairs shows 54.7c and I would have though i'd get a 4->5c loss via pipework from tank, so tank stat is probably correct.
Note: When it's in the cycling state trying to get the last degree I have tried increasing the pump to 8M on CC but it never made a difference to the cycling.

Summary:
Think i'll leave it on 8M proportional and tank stat at 59 degrees all year. Then every summer reduce D0 to 15KW and be done. unless others have any suggestions?

 

[Johntheo5]

The PP8 looks "good" now.

You have some nice info available there now to see if you can fine tune the settings to give the lowest possible return temp without causing the boiler to cycle.
You can see the flow/return boiler temps by looking at d.040 & d.041 which will then tell you the dT through the boiler.
The boiler output is....... flowrate in m3/hr X 16.66 X60 Xboiler dT/860 kw, so (m3/hr X 16.66 X 60 X dT)/860 kw.
You can then see how close you are running to the minimum output of the boiler, whatever that is, 5.5/6.0kw?

 

[ant2]

This evening's run:

Summary: It appears PP/ 15kw/67 degrees boiler flow works fine up until the last 15 minutes. i,e up to 53.9c kitchen / 54.7c bathroom tap
It constantly cycles unless I put the boiler temperature up to 77c + Constant curve 8M head.
This brought the taps up a further 1.8c i.e downstairs: 55.4 and bath tap 56.5. The tank stat then shut it off. Presumably the boiler shuts off when the return is too close to the flow temperature, so pushing the temp up and then pushing it harder via the pump was the only way to stop that.

Conclusion: Do you have any other tips?
Other than setting the pump to CC + 8M and increasing boiler temp from 67->77, I see no way of stopping the cycling for those last few Degrees. Is it worth having the boiler temp so high and the pump on full to prevent the cycling and getting the 1.8C extra on the taps? I'd have thought it would burn a lot more gas.

Results:
Few mins: D40 = 55/56 - D41=42
5 mins: D40=64 - D41=51 - Flow 0.3m3/h - H:4.6M
18 min: D40=65 - D41=59 - Flow: 0.3 - H:4.6M
21 min: D40=66 -D41=60 - Flow 0.3 - H: 4.8M
23 mins - boiler start to cycle, only runs for 10 seconds before pump overrun - D41=61
25 mins - (on pump overrun) - D40: 57 / D41:=57
26 mins - (on fire) D40: 67 (then cut off) - D41 = 56

Set Constant curve to 8M
D40 = 56-67 - D41 - 55/56 (still constantly cycling)

Left several minutes:
Turn up stat to 73, next fire boiler starts to modulate and hold for 4 mins
D40- 69-73 - D41 - 55-63 - Q=8M

Set back to PP as test, held for 3 mins and then started to cycle again - D40 - 74 D41 - 64- H:5.8M - Q:1.0m
On pump overrun on PP - D40:65 - D41 - 63->66

CC set again at 8M - Boiler holds till stat cut off (3-4 mins) D40: 70 -> 74 - D41 - 67-68

 

[Johntheo5]

Why has the flow fallen to less than 1/3rd of its original of 1.1m3/hrs to 0.3m3/h?, that should remain constant, Suggest changing to Cp 5.5M, if the flow was 1.1m3/hr or 18.33LPM then the dT can fall as low as 4.3C and the boiler output would still then be 5.5kw??
Can you see what the min output of that boiler actually is??

Also check and see if the cylinder coil return has a gate (or lever) valve and open this fully.

 

[ant2]

When the boiler was cycling i set it back to pp, it did show 1.0m for flow. It's logged under q. Not sure why the flow shows as lower on initial heat up than what i saw on the previous cc test, but both settings still cause cycling at the end unless the temp is elevated to 77.


How would i see the min output of boiler? I can set d0 to as low as 8kw iirc.

The return doesn't have any gate valves except on the other circuits , that have separate wlmotorised valves on the flow which are shut atm.. and those gates are open anyway.

The only other thing on the return is a prv type thing as per attached.

Wonder if i clock back the boiler kw output further again it will allow it to pump without cycling as it's more gradual.

 

[Johntheo5]

The minimum output is sometimes shown in the users manual, the 938 manual (can't find a 438 manual) gives a minimum fuel input of 7kw which at say ~ 85% efficiency would be around 6kw minimum output, if the demand is below this then the burner will cut out and boiler cycle when the flow temperature reaches its setpoint or target temperature + 5C. You are limiting/controlling the the maximum output which does nothing really once the demand is below this, it obviously does limit the cylinder heat up time while the coil output is greater than this set limit.

If the flowrate is 1.0M3/hr, then (assuming) a 6kw minimum boiler output will mean that once the boiler dT falls below, (6*860/60/1/16.66), 5.16C, it will cycle.
You only need to increase the cylinder temperature ("by law") to 60C once/week for legionella protection so why not settle for say 50/53C normally and use the lowest flow temperature necessary to avoid cycling, but it's important to maintain that 1.0 m3/hr flowrate at all times, if possible.

 

[ant2]

the minimum heat input appears to be the same for the 438- 7.2kw - https://mans.io/files/viewer/492940/40

I experimented again with CC at 5.5 and the Q was between 0.3->0.5 m3/h, I put it on PP and it was the same values.
Either way, the boiler seems to cycle once the tap gets to 54 ish in the bathroom UNLESS the flow temp is set to 75, regardless of CC/PP or heat modifications, all the way up to 8M. I think it was worthwhile spending the time finding that out as I atleast know PP seems to operate correctly and to fix this I can just increase the flow temp.

I tried all temps up to 70 and it constantly cycled. I knew 77 worked from yesterday .I tried 75 today and this allowed the boiler to modulate and kept pushing till the temperature was met by the HW stat. If I set the boiler over 75 then I seem to be able to heat the tank to whatever I want.
I have now set the tank stat to 60c and I get 58.5c at the nearest bathroom tap.

In terms of savings, I wonder how much extra money this costs moving the boiler 65->75c and heating the tank from 54c -> 60c just for the HW. I can't imagine much more than £20 per month, which may well be worth it to ensure legionella is never going to be an issue?

I'll try it again in the winter as I believe I can reduce the flow to 62->65 as long as I set heating to start 20 mins after the HW as this is what I used to do . As the extra heat will get pulled via the radiators flows and hopefully still heat the hw tank.

Unless you have any other ideas ?

 

[Johntheo5]

That seems OK to me, what is the boiler return temp with a flow temp of 75C?, I get a return of 62C (flow assumed at 0.4m3/hr) assuming boiler output is at a minimum output of 6kw towards the end of the heating cycle.

A lot of systems now use hot water priority, whenever there is a call for hot water the boiler automatically switches to DHW heating only and increases the flow temperature to 80C to heat/reheat the cylinder as rapidly as possible, this is relatively in efficient as the boiler return temperature is quite high at maybe 70C or so, as soon as the cylinder is reheated, the heating system reverts to its heating mode and re sets the flow temperature to its heating SP which may be as low as 45C, this then ensures that the boiler is running at its best efficiency for the longest period.

 

[ant2]

My boiler return was around 58-65 iirc at 75 flow.
Shame my boiler hasn't been wired set up to use the hw temp stat as i do have a separate control knob just for hw but its not used and only uses the heating temperature, then I could have split it.

In addition, What i have done is put the timer for hw on for 30 min in morning and 30 in the evening. Sun-friday I.e reducing the time so its likely not going to heat past 55c unless we haven't used the water much (saving a few pennies) and then on a Saturday morning the hw runs for 1 hour in morning and 30 at night as that's always long enough for it to heat till stat cut off, giving us the 60c HW tank and tap bacteria cleanse once per week.