Boiler keeps short cycling with only UFH is on

windsor-tg · Sep 16, 2023

John.g said:
First I would check the setting of the manual by pass valve with the red handwheel, put a mark on the top of the handwheel at "12 oclock", then turn the hand wheel very slowly clockwise and carefully note the portion of a turn or the full turns required to close it fully, (should be ~ 3 to 4 full turns between fully open and fully closed) then return it initially to the same setting, you may find a 1/2 turn or so of play before the valve starts to reopen so allow for this, (of course if the valve has been left fully (unlikely) open then you will get 3 or 4 full turns to close it)
The main pump is running on full speed (3) which is 8M, I would change that immediately to speed 1 which is 6M and still more than adequate IMO, the 8M setting is probably the reason for the UFH flowrates doubling when UFH running alone. To change the speed, just press the setting button briefly a few times to highlight I (solid green), that may still be too powerful at 6M but we'll see.
The manifold pump is running on minimum speed in fixed speed (CC) mode so leave that as is even though constant pressure (CP) mode is recommended for UFH.

THe UFH seems to be emitting a fair amount of heat, at T+15, CH+UFH on, a total flow of 5LPM with flow/return of 44C/28C gives a output of 5.59kw and at T+50, UFH on its own,, a total flow of 10LPM with flow/return of 44C/34C gives a output of 6.98kw.
You might run a few more tests after changing the pump speed and checking the bypass opening. Maybe just run the UFH only for 15/20 minutes or so to speed things up., you can then run on both once you post the UFH findings.

View attachment 84853

Thanks @John.g, I will look to do this tomorrow. Just to clarify, for the ABV, are you just wanting to know how many turns it will take to close the valve? I will see how many turns it will take to close it before applying the same number of turns anti-clockwise to return it back to its same position. Unfortunately I do not have the manual for the ABV.

On the main pump, how do I know if the pump is running at fixed speed operation vs proportional pressure control? I’m pretty certain that I have to keep the button pressed before I can then change the speed setting. By doing this, I’m not sure if I am changing the pump from fixed speed to proportional pressure, or vice-versa. Do I need to have it at fixed speed or proportional pressure, whilst being on speed (1)?

At the time I had the UFH installed last year and adding a couple of more radiators, I had the main pump changed to this one. I do recall the installer setting it to speed (3) to ensure the radiators furthest away from the boiler heated up sufficiently.

Once I carry out the UFH tests for 15/20 mins after changing the main pump speed, is it worth then changing the UFH pump to CP mode and noting down the results for a further 15/20 mins?

John.g · Sep 16, 2023

Don't touch the ABV, I'm refering to the manual valve with the red handwheel
As per pump instructions, the green LED is solid on any fixed speed mode and a flashing green in any PP mode.
Will get back to you re UFH manifold pump setting.

windsor-tg · Sep 16, 2023

@John.g - I got a chance to do this. On the manual valve, I placed a mark on the 12 o'clock position on the valve and turned it to 5 o'clock position before the valve closed. I returned it back to the 12 o'clock position.

i changed the main pump speed to speed (1) - photo attached.

I took the readings below with just the UFH on. The boiler was pretty much cycling from the start.

John.g · Sep 16, 2023

Well, the average flow/return of those 4 readings is 43.25C/29C which gives a output of 5.06kw at a flowrate of 5LPM, the boiler's minimum output is, I think, 6.5kw so it has to cycle.
Before you run it on CH+UFH will you just check that manual valve again, just shut it fully and then confirm that it is fully shut by fully opening it by counting ~ 3 to 4 full turns then close it again and reopen it to 12 oclock.

windsor-tg · Sep 17, 2023

Thanks @John.g - one thing I forgot to do was to close the manual value and re-opening it before running the last set of tests. So I did this afterwards.

Do you want me to fully shut it, fully open it and then reopen it to 12 o’clock before running tests with CH+UFH or run the tests again with UFH only?

I assume I leave the main pump at speed (1)?

John.g · Sep 17, 2023

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 · Sep 17, 2023

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

windsor-tg · Sep 17, 2023

Ben-gee said:
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 · Sep 17, 2023

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 · Sep 17, 2023

windsor-tg said:
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

View attachment 84867

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 · Sep 17, 2023

John.g said:
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 · Sep 17, 2023

windsor-tg said:
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.

windsor-tg said:
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.

windsor-tg said:
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.

windsor-tg said:
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 · Sep 18, 2023

@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 · Sep 18, 2023

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 · Sep 18, 2023

John.g said:
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:

shall I keep the boiler temp at 70 or return it to 65 which is what it was from the previous tests & readings?
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
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?
I will look to observe the d.67 reading as soon as the burner trips
I will increase d.2 to 30
I will only run the UFH with CH off
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 · Sep 18, 2023

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 · Sep 18, 2023

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 · Sep 18, 2023

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 · Sep 18, 2023

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 · Sep 22, 2023

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 · Sep 22, 2023

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 · Sep 22, 2023

ShaunCorbs said:
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:

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

ShaunCorbs · Sep 22, 2023

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 · Sep 22, 2023

ShaunCorbs said:
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 · Sep 22, 2023

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

Boiler keeps short cycling with only UFH is on

windsor-tg

John.g

windsor-tg

Attachments

John.g

windsor-tg

John.g

windsor-tg

windsor-tg

ShaunCorbs

Super Moderator

John.g

windsor-tg

John.g

windsor-tg

John.g

windsor-tg

John.g

John.g

windsor-tg

John.g

windsor-tg

ShaunCorbs

Super Moderator

windsor-tg

ShaunCorbs

Super Moderator

windsor-tg

Attachments

ShaunCorbs

Super Moderator

Similar plumbing topics