Boiler keeps short cycling with only UFH is on

[Ben-gee]

Even though I have a 38kW boiler, it has been capped at 24kW as I was getting an S.53 display message on the boiler (flow & return temp differential too large) so reducing the power of the boiler (d.0) to 24kW eliminated the message. A 30kW boiler is what is being suggested but I’m going to find it hard to find a boiler that can modulate down to around 1.5kW when I just have the UFH on.

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.

 

[windsor-tg]

@John.g - I just ran a fresh test and the readings are shown below. The boiler temp is set at 65 degrees. When the UFH is off, the flow meters are 0 so I must have taken a wrong reading yesterday. Also, on T+40, the flow temp on UFH was either 40 or 50. I wrote down 50 at the time but I think that was wrong and it should have possibly been 40.

Are the readings still strange? No kettling sound from the boiler when I was running these test readings.

I have attached a photo of the main pump as well as the UFH pump so that you can see what lights are lit up on each of the pumps. I will send a short video on the main pump in a few mins so that you can possibly hear the sound of the pump when it is running with both, CH and UFH on.

Let me know if more tests and/or photos/videos will be helpful.

 

[windsor-tg]

video of main pump running with both, CH and UFH on

 

[windsor-tg]

also attached photo of UFH pump when UFH is running

 

[John.g]

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.

 

[windsor-tg]

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]

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]

@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]

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]

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)?