Boiler keeps short cycling with only UFH is on

[windsor-tg]

THe UFH is outputting ~ 3.9kw now.
Re rapid cycling, you should be able to extend that, on most Vaillants the anticycle time depends on the target temperature and the set time, it doesn't show this table on your MIs but I'd be surprised if it isn,t.
Check d2, it defaults to 20mins which is actually a anticycle time of 4.5mins at 65C, your anticycle time is 2 mins which suggests a time setting of 10 mins or less, if its at some low setting, suggest setting it to 20mins which should give a actual anticycle time of 4.5 minutes, you can allways increase it to 25 or a max of 30 minutes which gives max actual anticycle time of 6.5 mins at 65C. You could allways consider then installing a 25L (or bigger) buffer on the UFH flow which would give a steady (theoretical) flow temp of 65C for 10 minutes at 3.9kw (1.9LPM) and 5 minutes (4.8LPM)at 10kw UFH output.

You might open the manual by pass another 1/4 to 1/2 a turn, there does seem to be some bypass as the boiler return is ~ 40C with a UFH return of 35C, a bit more might help. Also feel the bypass piping, it should be very hot (65C) if bypassing is taking place.

Also just ensure that the main (red) circ pump continues to run during the anticycle time as should the UFH manifold pump.

thanks @John.g
I just checked my boiler and d2 is set to 20. Do you want me to change this?

I will run some more tests tomorrow. Shall I run it first with only UFH on, and then run it with both, CH and UFH on? Also, before running the system, shall I turn the manual valve anti-clockwise so that the marker which is currently at 12 oclock is at 6 oclock?

I am 99% certain that both, the main circ pump and the UFH pump continue to run during the anticycle time. I will check on how hot the bypass piping is.

With regards to the buffer cylinder, is this something to consider once all the tests are complete?

 

[John.g]

thanks @John.g
I just checked my boiler and d2 is set to 20. Do you want me to change this?

The anticycle time should be 4.5minutes at this 20 min setting, you can monitor this and if only 2 minutes increase it to the max 30 min setting.
You can also check the anticycling time remaining, d.67.

Saw this in the MIs so presume its addressed in setting up the boiler.

I will run some more tests tomorrow. Shall I run it first with only UFH on, and then run it with both, CH and UFH on? Also, before running the system, shall I turn the manual valve anti-clockwise so that the marker which is currently at 12 oclock is at 6 oclock?

Yes, give that another 1/2 turn anticlockwise. Run UFH only first, then both for info.

I am 99% certain that both, the main circ pump and the UFH pump continue to run during the anticycle time. I will check on how hot the bypass piping is.

With regards to the buffer cylinder, is this something to consider once all the tests are complete?

Seems reasonable although I see very little about its merits or otherwise mentioned anywhere.

 

[windsor-tg]

@John.g - before carrying out some further tests in running the UFH, I was asked by someone else on another forum to run tests with just the CH on as they suspect I am not just having issues with the UFH but have a more fundamental system problem. This is because in all my tests so far, they said the boiler return temperature is too low whether or not my system is on rads, UFH or both. It's just that the problem is made much more obvious with just the UFH running. So running these latest tests they believe is going to tell more about the system without UFH distraction.

I gave the manual valve a 1/2 turn anticlockwise so that the marker is at 6 oclock. I was asked to increase the boiler temp to 70 for these next set of results and I started to take readings at generally 10 minute intervals unless where shown below otherwise.

Observations from this were 1) the bypass piping was very hot 2) I could hear the sound of water circulating through the pipework in the airing cupboard 3) after about 50 minutes of the system running, the boiler started cycling every 3-4 mins and flow temp rocketing from 45 to 60-71. It may have been doing this earlier but I was not standing by the boiler all the time. The main pump continued to run.

After 80 minutes of the system running, I then increased the pump speed to 3. Immediately I saw the boiler stopped cycling and the flow temp remained at 70 with the boiler still firing. Some radiators far hotter than others which I assume is due to the TRVs. Radiators furthest away getting much hotter when speed turned to (3). Overall, it appeared the boiler was running better with the main pump speed at (3) with only the CH on.

Based on these set of latest readings and your overall thoughts, can you let me know what tests you would like for me to carry out? d.67 is set to zero so not sure if I need to adjust this along with d.2

 

[John.g]

I would run on UFH only (pump speed 3) with the recirc open as is, 1 full turn open?, we can get a fair idea then by comparing the UFH return temp with the boiler return temp, the UFH flowrate will probably double again but not sure why this should happen as theoretically the pump(s) head should be 4 times greater to achieve this. You will probably revert to the by pass only a 1/2T open as originally but see how it runs at 1T open.

You dont set d67 to anything but you must observe it as soon as the burner trips and the boiler goes into recycle, if you catch it straight away then that is the actual anticycle time, it then counts down to 0 until the next anti cycle. I would increase d.2 the anticycle time to 30mins which is actually 3.5mins at 70C boiler temp, its only 2.5mins at the 20C setting.

 

[windsor-tg]

I would run on UFH only (pump speed 3) with the recirc open as is, 1 full turn open?, we can get a fair idea then by comparing the UFH return temp with the boiler return temp, the UFH flowrate will probably double again but not sure why this should happen as theoretically the pump(s) head should be 4 times greater to achieve this. You will probably revert to the by pass only a 1/2T open as originally but see how it runs at 1T open.

You dont set d67 to anything but you must observe it as soon as the burner trips and the boiler goes into recycle, if you catch it straight away then that is the actual anticycle time, it then counts down to 0 until the next anti cycle. I would increase d.2 the anticycle time to 30mins which is actually 3.5mins at 70C boiler temp, its only 2.5mins at the 20C setting.
View attachment 84903

Thanks @John.g. Just to clarify:

1. shall I keep the boiler temp at 70 or return it to 65 which is what it was from the previous tests & readings?
2. where you say 'pump speed 3 - I assume you mean the main red recirculating pump in the airing cupboard beside the zone valves? If so, this is currently on speed (3) following the last test
3. not sure what you mean by "1 full turn open" - do you mean the manual valve which I turned 1/2 anticlockwise so that the marker is at 6 oclock? Do I need to turn it further anticlockwise?
4. I will look to observe the d.67 reading as soon as the burner trips
5. I will increase d.2 to 30
6. I will only run the UFH with CH off
7. I assume I leave the UFH pump settings as they are?

From the readings I sent earlier today where I tested the pump speeds 1 and 3, with just CH on, is there anything that indicates why the boiler ran ok at circulating pump speed (3) but not at circulating pump speed (1) ?

 

[John.g]

All very strange, the difference in pump heads should only increase the flowby ~ 15% so possibly blockage.
You replied to my post in that other site re reducing d.0 to ~ 7kw to get the boiler flow temp to fall, then increase it in 1kw steps until the flow temp returns to 70C, can then calculate the exact flow rate. Do this on CH only and on main pump red speed3. Close the manual bypass valve fully and then open it to its original setting of 1/2 turn open before starting.

 

[John.g]

Should have said shut the bypass fully and if the boiler keeps running then cary out the test above, ch only on, pump speed 3.

 

[windsor-tg]

Sorry @John.g, I am confused on what you are asking me to do. Are you John Carroll on the other forum (buildhub.org.uk) where you replied to my post? If so, would you mind listing the steps you want me to do?

If you’re not John Carroll on the other forum, if you can outline again what you want me to do, that will be very helpful

 

[John.g]

OK, with CH only on, ensure main red pump on speed 3 (8M), when the boiler flow/return temps are steady and when the boiler flow reaches ~ 70C (its present setting), shut the manual bypass valve fully and if the boiler continues to run continuously again note the boiler flow/return temps. Then reduce d.0 to say 7kw, the boiler flow temp should then start falling, then steadily increase d.0 until the flow temp again reaches ~ 70C, note the flow/return temps, you now know what output (within reason) the boiler/CH system requires, we can then calculate the actual flow rate quite accurately from the above.

When test completed reopen the manual bypass valve a 1/2T again and again note the flow/return temps after another say 5 minutes.

 

[windsor-tg]

Just a follow up question if someone can help.... if I have a system boiler installed with closed couple tees, do I still need an external pump(s)? I am aware that the existing UFH manifold pump is needed for the UFH system.

 

[ShaunCorbs]

Yes for the ufh manifold the pump isn’t rated for the heating pipework from the boiler etc

So you will need two or theee depending if it’s a system boiler/ combi or heat only

 

[windsor-tg]

Yes for the ufh manifold the pump isn’t rated for the heating pipework from the boiler etc

So you will need two or theee depending if it’s a system boiler/ combi or heat only

thanks @ShaunCorbs. Just to clarify, for a system boiler:

1. internal pump contained within system boiler
2. UFH manifold pump - pump already exists
3. is another external pump(s) required for central heating?

 

[ShaunCorbs]

point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes

 

[windsor-tg]

point 2 the manifold pump isn’t rated to pull from the pipework so you will need another one here

Point 3 yes

thanks @ShaunCorbs.

Just to add some context, I am looking to replace my heat-only boiler with a system boiler. I already have an external pump beside each of my zone valves - photos attached of zone valves marked 1 = HW, 2 = UFH, 3 = CH

The external pump is a UPS2 25-80 (photo attached). Any issues with this pump being used for the central heating?

From what you have said, the manifold pump isn’t rated to pull from the pipework so you will need another one here. Can you expand upon where another pump will need to be fitted?

 

[ShaunCorbs]

That pump is fine for the ch you will need a separate 15-5/60 pump for the ufh after the cct if the cct is above the boiler etc

 

[ShaunCorbs]

Eg

 

[John.g]

One of the CCT (or LLH) advantages is that the primary circulating flow can be controlled to give a flow high enough to control the maximum dT which can exceed the boilers max allowable, generally ~ 25C, another advantage is that it helps to get the boiler away without exceeding the flow temp tripping point since all gas boilers fire up at ~ 65% of max output for up to 60 secs before modulation. The disadvantage is that the boiler return temperature increases with reduced demand.
For example if you set the primary flow to 14.3LPM to give a dT of 20C with a 20KW rad demand, the boiler dT will be 20C with flow/return temps of 65C/45C, a rad demand of 9kw will result in a dT of 9C, flow/returns of 65C/56C, a UFH demand of 9KW will result in the same dT of 9C with flow/return of 65C/56C. A rad demand of 2.5KW will give a dT of only (theoretically0 2.5C with flow/return of 65C/62.5C and the same for a UFH demand of 2.5KW.
Of course it will not prevent cycling if the minimum boiler output is greater than the heating demand.

 

[John.g]

Is a CCT/LLH required?.
The above boiler, a Vaillant 438, has a HEX with dP of 4.05M at a flow of 27.2LPM.
If a manual bypass was installed then opening it to give a 10.7LPM permanent by pass will result in a HEX dP of 3.5M (at 25LPM) which leaves 4.5M (8M UPS2 installed) remaining head to circulate 14.3LPM through the rads etc.
You will then end up with (boiler) flow/returns of rads only 65C/53.5C, dT 11.5C, UFH 9kw only of 65C/60C dT 5C, UFH 2.5KW 65C/63.5C, dT 2.5C..

If you could source a true constant curve pump then a ABV could be used to get much improved, lower boiler return temperature and greater boiler efficiency.

 

[windsor-tg]

Eg

View attachment 84994

@ShaunCorbs - thanks for this.

From the diagram you provided, would:

1. the 'heat source' be the equivalent of the new system boiler and the 'primary circulator be the internal pump within the system boiler?
2. where it shows the "secondary circulator along with the 'spring loaded check valve' - would this be for the CH or UFH?
3. and for the "another secondary circuit" - would this be for the UFH or CH?
4. from the two secondary circulator pumps shown on your diagram, where would I use my existing 25-80 pump?
5. similarly, where you mention about the need for an additional 15-5/60 pump for the UFH, where would this be located?

If you were able to provide a quick sketch of how my system should be set up, this will be greatly appreciated.

At the end of the day, what I am trying to achieve is to be able to run the UFH on its own without the boiler continually cycling. When this happens, the room that contains the UFH hardly heats up so the way I get around this is to run the CH at the same time so that the boiler continually runs and the UFH room heats up.

I had a look at the latest Vaillent ecotec plus system 25kW boilers which have a modulation range of 1:10 ecoTEC Plus - https://www.vaillant.co.uk/for-installers/products/ecotec-plus-610-615-620-625-630-635-154880.html. I was wondering if this would reduce the boiler cycling with just the UFH on?

 

[ShaunCorbs]

That would be the rads as it’s the hottest heat load

Then the ufh eg horizontal one

25-80 for the rads

15-60 for the ufh

Same as drawing horizontal set

You want the highest modulation possible eg lowest kw figure of ideally under 1kw if possible

Only just seen you sent me a only sorry will have a look later

 

[windsor-tg]

sorry @ShaunCorbs but I don't fully understand your reply based on the diagram you provided.

From your diagram:

• where it shows the "secondary circuit & circulator along with the 'spring loaded check valve' - is this the CH circuit and the secondary circulator being my existing 25-80 pump?
• where it shows the "another secondary circuit" - is this the UFH circuit, and where the pump is shown, is this the additional 15-5/60 pump you refer to, for the UFH?

 

[ShaunCorbs]

Will draw it out tonight for you

 

[ShaunCorbs]

can do it two ways eg below

 

[windsor-tg]

many thanks @ShaunCorbs. On the Hydronic CCT, stupid question but what do the symbols (attached) denote? From your diagram last week, you showed the position of two CCTs, spring-loaded check valve and purging valve - where would these appear on the Hydronic CCT diagram?

On the System CCT diagram, I just see the 25-80 pump but do not see the 15-80 pump, nor the CCTs, check & purging valves - I they not needed?

Are there any advantages with the Hydronic CCT setup over the System CCT setup, and vice-versa?

 

[ShaunCorbs]

Check valve / swing check

As the boiler is cct you only need one pump ideally a modulating pump is best and no there not m

Both the same

 

[windsor-tg]

Check valve / swing check

As the boiler is cct you only need one pump ideally a modulating pump is best and no there not m

Both the same

sorry @ShaunCorbs - what do you mean by "Check valve / swing check" ?

you comment "As the boiler is cct you only need one pump ideally a modulating pump is best and no there not m" - what is this referring to?

 

[ShaunCorbs]

You will need a check valve to stop reverse circulating as you don’t have a zone valve you could fit a zone valve but a check valve / swing check is cheaper

The purge valves etc arnt needed don’t know why there’s a m on the end tho