Boiler Reported Return Temperature

[webbm000]

I get the same, the model number may give a clue, VU GB 376/5_5 A for the 7.1/6.4kw one.

View attachment 337640

We have an ecoTEC plus 637 VU 376 3-5 R1 GC 41-044-49

 

[Johntheo5]

Setting d.19 = 1 should mean that everything is on low speed but I'm not sure if there is any way to check pump speed?

I have a slight worry about the external ABV and was going to remove it to make sure it wasn't the cause as the pipe the other side of it is hot but that could just be conduction along the pipe? Something else to add to the list of things to try

Its labeled "Actual pump speed", parameter d.015, in my MIs.

 

[webbm000]

Its labeled "Actual pump speed", parameter d.015, in my MIs.

No d.15 in my manual. I'll have a look on the boiler tomorrow as it's in the loft

 

[Johntheo5]

Looks like you havn't got the pump speed parameter.
Also looks like your minimum output is 12.0kw, probably why your boiler cycled after running for 1.5 hrs with everything on this morning, not much you can do about that except to get all settings as best you can to get it to refire without constant cycling and get a continuous run for 3 or 4 minutes at least as this has only a very small effect on boiler efficiency.
The attached is the boiler you have, I think?.

 

[webbm000]

Looks like you havn't got the pump speed parameter.
Also looks like your minimum output is 12.0kw, probably why your boiler cycled after running for 1.5 hrs with everything on this morning, not much you can do about that except to get all settings as best you can to get it to refire without constant cycling and get a continuous run for 3 or 4 minutes at least as this has only a very small effect on boiler efficiency.
The attached is the boiler you have, I think?.

View attachment 337690

That matches the document I have that came with the boiler. I'll try the things suggested to see if it makes it any better.

I'm also thinking that a weather compensation controller is a waste of money as when it changes the flow set point the boiler wont be able to modulate below 12kW and will just cycle more rather than gaining any extra efficiency

Thanks for your help

 

[webbm000]

Well wasn't expecting that. Looks like the external bypass is letting the flow through.

The temperature just after the bypass is near enough the same as the boiler reported return temperature and not the same as the return temperature from before the bypass about 50cm away from it

I wonder if it was broken when we had a power flush done recently?

 

[Johntheo5]

Interesting, so the return after the external bypass at 61C is a mixture of the system (rads+UFH?) water at 49C mixed with external bypassed water at 69C?. and since the ebus boiler return temp is also 61C then absolutely no internal bypassing which probably makes sense since the circ pump is on its low(est) setting so pump head not enough to open the internal bypass.

What is the external bypass (ABV) setting?

 

[webbm000]

Interesting, so the return after the external bypass at 61C is a mixture of the system (rads+UFH?) water at 49C mixed with external bypassed water at 69C?. and since the ebus boiler return temp is also 61C then absolutely no internal bypassing which probably makes sense since the circ pump is on its low(est) setting so pump head not enough to open the internal bypass.

What is the external bypass (ABV) setting?

The ABV is one of these Honeywell DU145 ABV

The test was with just the radiator demand on with probably most of the TRVs open. The ABV says it is set to 6 bar but the top comes off of it so I've tightened it until I can feel it click which I assume is now at 6 bar

 

[webbm000]

Also Vaillant Technical Support has confirmed my boiler can only modulate down to 12kW!

 

[Johntheo5]

The ABV is one of these Honeywell DU145 ABV

The test was with just the radiator demand on with probably most of the TRVs open. The ABV says it is set to 6 bar but the top comes off of it so I've tightened it until I can feel it click which I assume is now at 6 bar

Its probably knackered or some crap under the seat, I would install a manual bypass with it or without it as the boiler internal bypass is quite adequate IMO.
Have you increased the anticycle time to try and get the boiler away after refiring?. or does it run for a few minutes as is after refiring?

 

[webbm000]

Its probably knackered or some crap under the seat, I would install a manual bypass with it or without it as the boiler internal bypass is quite adequate IMO.
Have you increased the anticycle time to try and get the boiler away after refiring?. or does it run for a few minutes as is after refiring?

Tried setting d.1 pump anti cycle to 10mins (from 5) and d.2 burner anti cycle to 60mins (11.5mins for 65 flow target) from 20mins

It looks like it is cycling over 12 mins period and pump running for 9/10 mins after burner switching off so only 3 min burn at a time

This is slightly better than the 7 min cycle with a 2 min burn for the d.1 = 5 min d.2 = 20 min settings

Changed d.1 to 20 mins to see what effect that has as d.2 is at it's max. Will post that when I have results

 

[Johntheo5]

d.1, pump overrun time, probably won't make any difference as its only used when the room and cylinder stats are both satisfied or programmed off. Once you sort out that external bypass then more water will pass through the rads and should result in lower flow/return temperature after recycling, they seem to be ~ 47C/42C ish now at the end of the recycle time?.

 

[david3456]

The boiler cycles horribly most of the time as you can see on the right hand side of the chart due to the boiler being oversized so not exactly efficient and condensing as much as it should. Turned boiler down to 12kW (lowest it will go) but still cycling so I'm trying to understand it a bit more before justifying the cost of converting to a modulating controller as these apparently modulate down to 6kW

Plumbers fitting oversized boilers seems to be a general problem. I have an Ecotec 424 which can only modulate down to 6kW but the setting goes in 1kW steps up to "Auto". Strange yours has a lowest setting above its minimum power. Anyway, like you, I put a temperature monitor on it. I discovered that it doesn't behave properly on Auto and set it to 8kW. Once the house is warm, the highest average power output required this winter has been 3.5kW. I have 12 radiators but maximum sustained power with all rads fully open at F=70, R= 50 is about 10.5kW so a higher power setting is nugatory. In practice, my system is usually satisfied before F reaches 70.

Concerning your cycling. As the boiler approaches maximum set temperature it will first turn the modulation down. Cycling occurs when minimum boiler power is greater than the power the radiators can dissipate at the set maximum temperature. Four possibilities. 1) You don't have enough or big enough radiators to dissipate 6kW; 2) you have TRVs and some are shutting radiators down before the area around the room thermostat has warmed up enough to cut demand; 3) your balancing is over aggressive so that water flow into the radiators is restricted; 4) your pump speed is too low.

When the boiler is running, two things are happening - the radiators are dissipating power proportional to their temperature and the water is changing temperature. When temperature is rising, radiator power output is less than boiler power and the surplus power is heating the water. When a stable condition is reached, then radiator power output must equal boiler power output and no power is being used to heat the water.

Radiators are rated at 70 degrees or 50 above room temperature. With F=70 and R=42 in your graph, the mid point is 56. So if your radiators have a combined rating of "10kW", then at 56, they have a potential maximum of 7.2kW. Except we see the boiler has come back to 6kW but the temperature is still rising so it must be less than that. Increasing pump speed to get return up to 55 will increase the mid point to 62 to give an 8.4kW potential. Not a big difference. Plot out F-R difference to get a better view of changes than trying to make sense of the separate curves.

Having tried this and doing what you can to your thermostat, then move onto balancing (oh what a can of worms). I have Hive TRVs and I don't balance the system - I believe in removing as much obstruction as possible from water flow when everything is cold. Try unbalancing your system and see if it changes boiler behaviour (first, note down the original positions). Then, if rooms are not heating up as you want them, gently tweak down the most active radiators. Don't get hung up on equalising the speed that radiators heat up or temperature across them but if you're not happy about this, return to original. But take into consideration that balancing is only about heat distribution and nothing to do with doing favours to the boiler.

This is a 24hr plot from my boiler taken today. It always starts up at 6kW then ramps up after 5 minutes. This is easy to see on the green line. At 6.00 is a water heating cycle. At 7.35 there is 3 minute dip from a thermostat quirk, not a boiler cycle. At this point, return appears to level out in the 6kW region so this may have been a stable point at the instantaneous TRV settings (the TRVs try to find an average point and never swing quickly). Between 7.05 and 7.36 the green line shows that the F-R difference is completely steady, even though F and R are rising. If I change pump speed the green line shows a very clear effect.

 

[webbm000]

Plumbers fitting oversized boilers seems to be a general problem. I have an Ecotec 424 which can only modulate down to 6kW but the setting goes in 1kW steps up to "Auto". Strange yours has a lowest setting above its minimum power. Anyway, like you, I put a temperature monitor on it. I discovered that it doesn't behave properly on Auto and set it to 8kW. Once the house is warm, the highest average power output required this winter has been 3.5kW. I have 12 radiators but maximum sustained power with all rads fully open at F=70, R= 50 is about 10.5kW so a higher power setting is nugatory. In practice, my system is usually satisfied before F reaches 70.

Concerning your cycling. As the boiler approaches maximum set temperature it will first turn the modulation down. Cycling occurs when minimum boiler power is greater than the power the radiators can dissipate at the set maximum temperature. Four possibilities. 1) You don't have enough or big enough radiators to dissipate 6kW; 2) you have TRVs and some are shutting radiators down before the area around the room thermostat has warmed up enough to cut demand; 3) your balancing is over aggressive so that water flow into the radiators is restricted; 4) your pump speed is too low.

When the boiler is running, two things are happening - the radiators are dissipating power proportional to their temperature and the water is changing temperature. When temperature is rising, radiator power output is less than boiler power and the surplus power is heating the water. When a stable condition is reached, then radiator power output must equal boiler power output and no power is being used to heat the water.

Radiators are rated at 70 degrees or 50 above room temperature. With F=70 and R=42 in your graph, the mid point is 56. So if your radiators have a combined rating of "10kW", then at 56, they have a potential maximum of 7.2kW. Except we see the boiler has come back to 6kW but the temperature is still rising so it must be less than that. Increasing pump speed to get return up to 55 will increase the mid point to 62 to give an 8.4kW potential. Not a big difference. Plot out F-R difference to get a better view of changes than trying to make sense of the separate curves.

Having tried this and doing what you can to your thermostat, then move onto balancing (oh what a can of worms). I have Hive TRVs and I don't balance the system - I believe in removing as much obstruction as possible from water flow when everything is cold. Try unbalancing your system and see if it changes boiler behaviour (first, note down the original positions). Then, if rooms are not heating up as you want them, gently tweak down the most active radiators. Don't get hung up on equalising the speed that radiators heat up or temperature across them but if you're not happy about this, return to original. But take into consideration that balancing is only about heat distribution and nothing to do with doing favours to the boiler.

This is a 24hr plot from my boiler taken today. It always starts up at 6kW then ramps up after 5 minutes. This is easy to see on the green line. At 6.00 is a water heating cycle. At 7.35 there is 3 minute dip from a thermostat quirk, not a boiler cycle. At this point, return appears to level out in the 6kW region so this may have been a stable point at the instantaneous TRV settings (the TRVs try to find an average point and never swing quickly). Between 7.05 and 7.36 the green line shows that the F-R difference is completely steady, even though F and R are rising. If I change pump speed the green line shows a very clear effect.

View attachment 337953

You house probably uses a similar amount of energy to mine (10 rads + 2 rooms with UFH) and much of what you said is where I was trying to go. Unfortunately my boiler as an older model and can only modulate down to 12kW which is above the usage of the house most of the time.

Once I fix the broken external bypass valve I should be getting much cooler return temperatures (dT of 20+) so can then look at the best route to reduce the cycling. I think increasing the anti cycling time has helped as the pump is on longer after the burner stops so the flow is still going round the rads keeping them warm and reducing the return temperature

This morning the pump was on continuously as the house warmed up with 11.5min boiler anti cycling periods between. 11.5mins is the max at 65degC target so I may try reducing the target temp to 60 to get the anti cycling up to 17mins but need to fix the ABV over easter first

 

[Johntheo5]

There is a very marked difference in the two trends, the one immediately below is exactly as I would expect on recycle, ie the flow&return temps merge shortly after the recycle starts but the bottom trend (from post #26) is different with the flow&return temperatures falling in steps over time?
The parameters difference is I think that the bottom trend is with d.2, the recycle set time set to 60 minutes and the top trend additionally has d.1, the pump overrun time extended from10 minutes to 20 minutes, is this correct??

My understanding is that the pump runs continuously during the recycle period (with pump overrun having no effect) and the pump runs for the overrun time and then stops when all zones programmed off or all stats satisfied.

You might try it with d.1 returned to its original setting and see does the trend then revert to looking like the lower one.