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Many thanks @SimonD and @John Carroll. I don't have too much of a problem in changing the boiler as it is 13 years old. I know I could get a few more years out of it but seeing that it was making a 'kettling' sound earlier last week, there is probably a chance that the heat exchanger on the boiler is partially blocked. Where you say 'get modulating controls', what controls will these replace and/or add? Also, where you say the heating system will probably need balancing, is that adjusting the lockshields on the individual rads to try and get all the rads to heat up equally? What other components would be needed? Regarding connecting the UFH to the primary heating circuit using close coupled tees - would this all be done in the airing cupboard where the circulating pump, and all 3 zone valves are located? Is there anything to be changed with the setings on the UFH pump, blending valve or flow meters? What will the closed coupled tees potentially achieve that a low loss header or buffer cylinder will not? Finally, I have a heating engineer coming back on Wednesday to check out my system. Is there anything specific you think is worth him checking out that may help in validating the problem (e.g. detecting a blockage in the system) and/or possibly resolving the problem?

thanks @John Carroll. I am not sure what this will achieve by doing this? I can only decrease d.0 in 1kW slices. @SimonD wanted these results to see if I have a more fundamental system problem so I think I will wait until he has seen the results.

@SimonD - so I ran the system by first increasing the boiler temp to 70 as requested and taking readings generally at 10 minute intervals unless shown below otherwise. Observations from this were 1) the pipe between the manual gate valve and automatic bypass valve was very hot (pipe shown on the attached photo and circled in yellow) 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 When I turned off the system, I did notice the return temp rising from 62 to 69. dropping down to 62 and then rising up to 67. Not sure if this has something to do with the pump overrun when the system shutsdown. Not sure what the next step is now 😕.

thanks @SimonD. I will look to get these tests run later today and will report back. From the photos I sent of the pipework yesterday along with the rad sizes, did that help?

Will do @SimonD. Just as a matter of interest, since I am having issues with just the UFH, by running these tests on just the CH, what will this possibly prove?

Thanks @SimonD. I will look to run these tests tomorrow. Just to confirm, do you want me to just run the CH on its own, and set the boiler flow temp to 70? initially on main pump speed (1) for say 15 mins (providing readings in 5 minute intervals)? change the main pump speed to (3) and run for 15 minutes providing readings in 5 minute intervals? Yes, I have 21 radiators of which 4 of these are towel radiators. Here are the sizes, etc:

@SimonD - thanks for this. Yes, you are correct in that I do not get this behaviour/issue when the rad zone is open. I do possibly think the heat exchanger on the boiler is partially blocked as the boiler was making a kettling sound earlier in the week. Regarding the pipework on the UFH, I have attached some photos of the pipework in the airing cupboard in the landing where the main pump, 3 zone valves (HW, CH, UFH) and the UFH pipes (2 white plastic pipes) come up from the UFH manifold which is located in the room downstairs. The flow & return pipes come up from the boiler in the garage up to the airing cupboard (photo 'pipework around pump' and labelled '1' and '2'). To your question "where is the UFH connected to the primary pipework" - photos 'Airing Cupboard 8 & 9' shows the UFH zone valve connected to pipework from the CH pipework and then connected to one of the UFH white pipes. The other white pipe is connected to one of the existing pipes which is connected to the pipe going to the boiler. A few of the heating engineers who have visited have said the pipework is correctly installed. The flow & return 28mm pipework runs from the boiler to the airing cupboard is 6-7m in length. The distance of the UFH pipes from the airing cupboard to the UFH manifold is around 8.5m Let me know if it would help for more photos. I did some further readings today and results are 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. Are the readings still strange? Anything I can do to determine if there is a flow restriction in the UFH?

@SimonD - just to run some further tests, I reduced the speed of the main pump to speed (1) from speed (3) (photo attached) and took the following readings with just the UFH on.

Yes, I've heard that a Veismman cam modulate down a lot further but none of the geating engineers I have spoken to have suggested a Veissman

@SimonD - I decided to run a fresh test today 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 also attached photos of the main pump and UFH pump when both, the CH and UFH are on

Thanks @JohnMo, @SimonD, @Lofty718. I have just come back from an evening out and gone through your responses. @SimonD - I do not have any design information of my UFH. The UFH pipework was layed in the screed by by builder and his heating guy installed the manifold, etc relating to the UFH. The UFH system is unbranded and I use Heatmiser as the main control for the radiator & UFH system. When I run the UFH system on its own from cold, it heats up but then starts cycling. With just the UFH on, I can take the same type of readings and report back if that helps? @Lofty718 - I have read elsewhere about close coupled tees. Why did you use them? Can you send a photo of this? From what I have reported, is there anything else I can do and then report back? I have an heating engineer coming back on Wednesday to run some tests. Are there some tests I can make him do and/or get him to check things and report back? With the boiler making kettling sounds earlier this week, I think the boiler is probably coming to the end of its life but I want to make sure I can somehow cure this problem with the short cycling as well as replace the boiler.

@SimonD - so here are the readings below (I also attached a file showing these readings incase the image below did not come through) . As well as taking the flow & return readings from the boiler, I also took readings from the UFH manifold. The blending valve on the UFH is set around 47 degrees. I took the readings across 5 minute intervals. What I found is that when I had both, CH and UFH turned on, the boiler was happily firing (without any sound of kettling which there was earlier in the week). The flow meters on the UFH was showing 2.5 (range 0-5). When I turned off the UFH, the flow meters on the UFH went to 5 and these flow meters stayed on 5 when I turned on the UFH and turned off the CH. After about 10 minutes of the UFH only being on, I could see the boiler short cyling (every 2 mins) and the boiler flow temperature would rocket from 45 to 72 degrees. I hope this helps but happy to do some further tests to get to the root cause of the problem.

sorry @SimonD, back to the one of the questions you asked me "Have any of these engineers mentioned that it's the controllers that govern boiler modulation rather than the boiler itself? E.g. the Vaillant VRC 400 weather compensating control for Ecotec boilers that adjusts flow temperatures according to a heat curve and heat demand? " - only the guy, Szymon from Urban Plumbers, who I spoke to yesterday mentioned the Vaillant Weather compensating control and installing all Vaillant controls. Only Vaillant controls can fully modulate the Vaillant boiler, no non-Vaillant controls can, he said. When I spoke with Vaillant technical team yesterday, I do not recall them mentioning I needed a weather compensating control as well as Vaillant controls. I had another engineer come over last night (recommended to me by a friend). He is going to come back next week to carry out a heat loss calculation (finally!). From what I descibed to him, he said a low loss header is needed, and was very confident that assuming the heat exchanger on the boilre is not blocked, this will solve the problem. The weather looks to be getting cooler next week so I'll carry out those measurements.

thanks @HughF. I just spoke with Szymon; very knowlegable and super busy. From what I described to him, he said that I need a full weather compensation setup with Vaillant controls. Essentially, I need hydraulic seperation (which can be done multiple ways) along with the Vaillant controls so that the Vaiilant boiler can be properly modulated. This, he said, is not cheap 🥴. I will proceed with what @SimonD has suggested but it does appear that I need a hydraulic seperation in place

thanks @SimonD. To your questions: I do not think so. Do you mean something like a kettling sound? One of the engineers said they could here a noise but I could not. If it is a kellting sound, what does this mean? None of the accredited Vaillant and Worcester Bosch installers have suggested a heat loss calc. Even when I spoke with Vaillant technical, they were shocked that none of these installers recommended a heat loss calc. I may have mentioned it earlier but only one of these installers went into each room to see the size of each radiator to make a mental note of the kW each radiator would be, and hence, they arrived at deciding a 30kW boiler is the right size. I have just been onto the phone with Vaillant technical again and they are saying that the reason my boiler is cycling is because the flow rate through the boiler & system is not enough when only the UFH is operating. For a 24kW boiler, the min flow rate needs to be 17.2L/min, and for a 30kW boiler, the flow rate needs to be minimum 21.5L/min. I have no idea to know what the flow rate is for my UFH. Their recommendation is therefore to install a LLH (rather than boiler cylinder which is for AHSP) along with 1-2 additional pumps. Now I am really confused 😩 No. Would this mean I would need to change my controller/thermostats? I am currently using Heatmiser for programming the times the HW, CH and UFH comes on/off, as well as setting the temperature for the CH and UFH.

in the meantime, I have been liasing with another heating engineer. I told them that my concern in replacing my existing boiler with a new boiler will be that when I just turn on the UFH, I will still have the issue where the boiler continues to cycle because the boiler is not able to modulate down far enough. They mentioned that my flow to the UFH manifold is connected to the positive side of the Grundfos pump, and the return is connected to the main return back to the boiler - this is how it should be. My question to them was "how will this prevent the boiler from continually cycling if I just have the UFH turned on". Their response was "it will never be an issue. For instance when thermostats are fitted on radiators they can all shut down. The bypass then opens up so there is still flow around the system. The boiler senses this and modulates down to a low output and shuts off if required. Once valves open back up the boiler fires back up. Same with your underfloor. Your existing boiler isn’t behaving this way because the heat exchanger is blocked". They suggest I contact the Vaillant technical team to put my mind at rest. More than one engineer has suggested the heat exchanger on my existing boiler is partially blocked. Anyway, I just spoke with Vaillant technical and there were not 100% sure but they did agree on what the engineer said about the bypass. I am still none the wiser though so I will look to carry out the tests recommended by @SimonD

thanks @SimonD - I will look to do this when the weather cools down a bit. Otherwise, my house will turn into a sauna which my mrs would love! Would 5 readings in each state be sufficient, i.e. 5 readings with CH + UFH on, 5 readings with CH only on, and 5 readings (or however many readings until the boiler is cyling) with UFH only on?

@JohnMo - are you saying that if I went with a buffer or cylinder, I will no longer need my Grundfos 25-80 pump? I would have thought this is still needed to pump the water around the radiators.

Thanks @SimonD. Where you say run the system fully open, do you mean having the hot water, central heating and UFH all turned ON and then taking d.40 and d.41 readings? How many readings would you say I should take in this state and would 5 minute intervals be sufficient? Should I then turn off the hot water, and then followed by the central heating such that only the UFH remains turned on?

thanks @JohnMo. thanks @SimonD - I will doible check when the engineer is here is a couple of weeks but I am pretty certain the boiler cycles when there is too much temperature differential. The flow temperature rockets up quickly when the boiler fires rather than the temp going up gradually. Assuming this is the case, how would the buffer cylinder be implemented in this instance? Also, I have had some people mention about Low Low Header - how is the LLH different to the buffer cylinder? Are the supply & installation of either of these roughly the same? The engineer who will be visting in a couple of weeks said a LLH would be in the region of £1,000 for installation 😟

thanks @JohnMo. From your description as well as the web link page, am I right to assume this buffer cylinder should be positioned as close to the boiler as possible? How can I determine the exact size of the cylinder needed? When the central heating is ON, will this cylinder be ignored or does it still have an act to play?

thanks @JohnMo & @SimonD. The S.53 message that I got a few years back was resolved by turning down the power of the boiler to 24kW (d0 = 24). Whether this is/was related to a flow/blockage problem, I am not sure. When the UFH was recently turned on (to test what is happening with the boiler), the S.53 did appear again but went away. I checked with my builder who layed the UFH pipework in the extension and he believes each of the 2 loops were around 60m long. I am not sure what this equates to in terms of litres/min. If I do have a low flow rate, is the only solution is to have a buffer cylinder of x litres in size to allow for the UFH to run on its own without the boiler continually cycling?

thanks @JohnMo. I did have a read of that thread you provided but I cannot really tell how this would work in my situation, what size I would need, and how it would be configured 😕

Many thanks @JohnMo. Would you be able to share a link to one of these buffer cylinders? Would it need to be a particular size? Since my boiler is in the garage but all the other central heating pipes, zone valves and main pump are in the airing cupboard, would the best place for the buffer cylinder be in the garage or the airing cupboard (depending on the size of the cylinder)? Having a diagram on how the setup of the buffer cylinder in relation to the boiler would be great 😊 Are you still suggesting I get rid of my TRVs on the radiators or do I not really need to do this if I install the buffer cylinder?