Boiler keeps short cycling with only UFH is on

[John.g]

Some gate valves can be a bit sticky so you want to ensure that it is ~ a 1/2 turn open, before doing any tests just shut it fully then reopen it fully, should take 3 to 4 full turns, then shut it again and reopen it to 12 oclock, then repeat the UFH only test and post findings, then do the UFH+CH tests.
Leave the pump on speed 1. It will bve interesting to see what the UFH flows are then.

Also let both UFH & CH on long enough to enable the boiler flow temperature reach its target temperature of 65C or there abouts.

 

[windsor-tg]

thanks @John.g
I did what you suggested with the manual valve. It took quite a few turns from the closed position to the open position and vice-versa. It is now back in the 12 oclock position.

Latest readings shown below. General observation is that when the UFH is only on, boiler continually cycling. When CH turned on, majority radiators luke warm with rads furthest away cold. Only when the boiler flow temp went up 60+, did the radiators start heating up. Not sure if this is due to the main pump speed being turned down to speed (1). When the main pump speed was (3), all the rads definitely get heated pretty quickly (with the furthest rads taking longer).

When both, the CH and UFH were on, and the boiler flow temp got into the 60s, I noticed the higher temps on the UFH manifold readings

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[windsor-tg]

boilers that can modulate 10:1 , I doubt you need 30 kw.
18kW is probably enough if you just time the hot water to heat at a time you don’t require heating- then that gets you down to 1.8kw for when you only want ufh.

I’m no expert but I’ve been told that just to get a rough idea, apply 1.5kW to each rad. Since I have 21 rads, that would make the total approx 31kW excluding the demand from the UFH. I can’t see how an 18kW boiler is enough or am I missing something?

 

[ShaunCorbs]

Nope a rough ideal would be if they heat the room fine look up the size and type that will give you a rough output then you can add them all together

 

[John.g]

thanks @John.g
I did what you suggested with the manual valve. It took quite a few turns from the closed position to the open position and vice-versa. It is now back in the 12 oclock position.

Latest readings shown below. General observation is that when the UFH is only on, boiler continually cycling. When CH turned on, majority radiators luke warm with rads furthest away cold. Only when the boiler flow temp went up 60+, did the radiators start heating up. Not sure if this is due to the main pump speed being turned down to speed (1). When the main pump speed was (3), all the rads definitely get heated pretty quickly (with the furthest rads taking longer).

When both, the CH and UFH were on, and the boiler flow temp got into the 60s, I noticed the higher temps on the UFH manifold readings

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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.

 

[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.

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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

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[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.

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[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.

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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) ?